Effect of Chinese Milk Vetch on Zinc Content and Zinc Absorption of Rice in Purple Tidal Mud Soil
1
Hunan Soil and Fertilizer Institute, Hunan Academy of Agricultural Sciences, Changsha 410125, China
2
Department of Quality Management, Hunan Biological Electromechanical Vocational Technical College, Changsha 410127, China
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(9), 1997; https://doi.org/10.3390/agronomy14091997
Submission received: 24 July 2024
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Revised: 27 August 2024
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Accepted: 29 August 2024
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Published: 2 September 2024
(This article belongs to the Section Soil and Plant Nutrition)
Abstract
:Rice is a staple food crop that feeds billions globally. Addressing Zn deficiency in rice is crucial for improving nutrition and food security. Zn deficiency in rice is a widespread issue, especially in purple tidal mud substrates, which often exhibit low Zn availability. The objective of this two-year pot study was to explore the relationship between Zn content, yield components, and Zn absorption in rice grown in purple tidal mud substrate with varying amounts of Chinese milk vetch (Astragalus sinicus L.) incorporation. The experimental design consisted of seven treatments: an unfertilized control, a Chinese milk vetch control, a chemical fertilizer control, and four treatment variations incorporating Chinese milk vetch alongside chemical fertilizer applications. The results indicated that planting and applying Chinese milk vetch improved the grain yield of rice in purple tidal mud substrate, and the yield increased with higher levels of Chinese milk vetch applied. The increased grain yield resulted in higher Zn absorption in rice grains. The application of Chinese milk vetch, both solely and in combination with chemical fertilizers, had varying effects on zinc uptake and grain zinc formation efficiency in early and late rice, with the control and low-level Chinese milk vetch treatments generally exhibiting the highest performance across the two-year period. By introducing Chinese milkvetch following the use of chemical fertilizers, the Zn content in rice grains increased starting from the second year. The treatment with Chinese milkvetch applied at a rate of 2.25 t/hm2 showed the best results in increasing the Zn content in rice grains. The increase in Zn content and Zn uptake by the rice plants gave rise to a lowering of the DTPA-extractable Zn content in the purple tidal mud substrate. Sole Chinese milk vetch application and using Chinese milk vetch following chemical fertilizer application both increased Zn content extracted by DTPA in purple tidal mud substrate.
1. Introduction
For normal human development, zinc is a necessary trace nutrient [1]. It serves as a component and activator of many enzymes, playing a pivotal part in processes like protein synthesis, carbohydrate metabolism, auxin metabolism, and regulation of gene expression in plants [2]. Zinc deficiency affects the health of 17% of the global population and 19% of the population in Asia [3]. The main reason why the human body experiences zinc deficiency is inadequate zinc intake among populations whose diet primarily consists of grains, as the zinc content in grain seeds is generally low. Two-thirds of the global population relies on rice as their staple food, and it is an important staple crop in China [4]. An economically viable and feasible approach to improving zinc intake among populations whose main dietary staple is rice is to increase the zinc content in rice [5].
The primary driver for the variation in zinc content in the rice grains is the zinc content in the soil. The variation in soil zinc content is influenced by factors such as the underlying geology, soil characteristics, management practices, and agricultural systems [6,7], and its accessibility directly impacts plant absorption and utilization. The proportions of zinc-deficient paddy soils in the northeastern, Yangtze River Delta, middle Yangtze River, southwestern, and southern regions of China are 75.0%, 52.3%, 31.9%, 53.2%, and 10.4%, respectively [8]. Purple tidal mud substrate, which forms on calcareous lacustrine deposits, covers an area of 164,000 hectares in Hunan Province and is concentrated in the northwestern Dongting Lake region, specifically in the Changde, Yueyang, and Yiyang areas, exhibiting a deficiency of available zinc [9]. In Nanxian County, Hunan Province, the available zinc concentration in purple tidal mud soil falls within the range of 0.005–0.277 mg/kg, with an average of 0.138 mg/kg [10]. Referring to relevant research [11] as an evaluation standard, this is considered an extremely low level (<0.30 mg/kg). It has been found that the zinc concentration in rice grains is directly proportional to the available zinc content in the soil [12], indicating that factors influencing the soil’s zinc bioavailability will also affect the zinc level in rice grains, although the zinc availability in the soil may not be the sole factor limiting grain zinc content [13].
Proper green manure application can enhance soil fertility, amend soil properties, conserve soil and water, and increase soil microbial biomass and enzymatic activity. Additionally, it can help mitigate soil-borne diseases, control pest infestations, and suppress weed growth [14]. Studies have shown that integrating leguminous green manure into wheat crop rotations can mobilize soil zinc and elevate the zinc concentration in subsequent harvests [15]. Leguminous green manure with lower C/N ratios has lower zinc residue, and the stored zinc in the plants can be rapidly released into the soil. As green manure plants grow and decompose, they release more soluble organic acids like oxalic, malic, and citric acid, leading to a reduction in soil pH and an increase in soluble organic carbon in the soil. These organic substances can dissociate the zinc fixed by soil particles through chelation, increase the zinc content in soil solution, and be absorbed and utilized by subsequent crops, thereby increasing the zinc content in subsequent grain harvests [16,17,18]. Liu et al. [15] believe that black kidney bean and huai bean, which have higher biomass, produce more organic acids during growth and decomposition and have a stronger activation effect on soil zinc, which is an important reason for the increase in zinc content in subsequent wheat crops. Incorporating green manure crops into the rotation can mobilize soil zinc and subsequently elevate the zinc content in following wheat and rice harvests [18,19]. The incorporation of diverse green manure crops, encompassing red clover, sunflower, safflower, and sorghum, into low-zinc calcareous soils has been found to enhance the bioavailability of the soil’s zinc. This practice also results in the buildup of soluble organic carbon and amino acids within the rhizosphere, thereby further facilitating the uptake of zinc into the grains of subsequent wheat harvests. The application of non-leguminous green manure may increase grain zinc content, but this often comes at the cost of reduced crop yields [18]. Regularly incorporating Sesbania aculeata, Crotalaria juncea, or Vigna unguiculata as green manure prior to rice transplanting substantially influences the zinc concentration in rice grains and straw, as well as the overall zinc uptake by the rice crop [20]. Relative to conventional nitrogen fertilization, a 40% reduction in nitrogen input during the rice season, achieved through the planting and utilization of Chinese milk vetch as green manure, does not compromise rice yield while concurrently enhancing the zinc content in rice grains [21]. Chinese milk vetch (Astragalus sinicus L.) is the most important winter green manure crop in the rice-growing regions of China. It can increase the total nitrogen content in the plowed layer, slowly raise the organic matter content of the soil, activate soil phosphorus, improve soil physical structure, increase soil porosity, improve rice quality, and increase yields. Chinese milk vetch plays a positive role in improving cultivated land quality, reducing fertilizer application, and improving the agricultural ecological environment [22,23,24,25].
However, studies examining the integration of Chinese milk vetch have predominantly centered around its impacts on crop productivity, soil nutrient status, soil microbiome composition, and enzymatic function [26,27]. Additional investigation is warranted to explore the extent to which the planting and utilization of Chinese milk vetch influence the relationship between zinc content, yield components, as well as zinc uptake and utilization in rice grown on purple tidal mud substrates. This study used purple tidal mud substrate collected from Nanxian County as the research subject. Via a two-year continuous pot study, this study examined the zinc content in rice grains, as well as dry matter accumulation, yield components, and the uptake and utilization of zinc under the conditions of winter-planted Chinese milk vetch and varying application rates of green manure. The aim is to provide a reference for enhancing both the yield and zinc content of rice grown in purple tidal mud substrates, thereby meeting the food demands and improving the zinc nutrition of the population through agricultural practices.
2. Experimental Materials and Procedures
2.1. Soil Characteristics
The experimental substrate was collected from a cultivated rice field located in Nanxian County, Hunan Province, China (112°20′17″ E, 29°20′11″ N). According to the Chinese soil classification system, the soil is categorized as calcareous paddy soil, also known as “purple tidal mud soil”, which is representative of the Hunan Province region. The pot trial was executed within the plant protection house at the Hunan Soil and Fertilizer Institute. The main properties of the soil are as follows: pH 7.97; available nitrogen 106.37 mg/kg; available phosphorus 14.30 mg/kg; available potassium 95.86 mg/kg; total nitrogen 0.85 g/kg; total phosphorus 0.67 g/kg; total potassium 19.40 g/kg; organic matter 10.53 g/kg; total Zn 94.98 mg/kg; DTPA-Zn 0.68 mg/kg.
2.2. Experimental Crops
Green Manure Variety: Xiangzi-1
In 2020, the zinc content of the incorporated Chinese milk vetch (on a dry basis) was 75.4 mg/kg, with a moisture content of 91.03%. In 2021, the zinc content decreased to 38.1 mg/kg, with a moisture content of 89.71%.
Rice Varieties:
In 2020, for the early rice variety Zhuliangyou 39, the zinc content in the dry basis of rice seedlings (moisture content: 91.47%) was 40.9 mg/kg. For the late rice variety Y-liangyou 911, the zinc content in the dry basis of rice seedlings (moisture content: 88.19%) was 39.5 mg/kg.
In 2021, for the early rice variety Lingliangyou 268, the zinc content in the dry basis of rice seedlings (moisture content: 88.44%) was 75.0 mg/kg. For the late rice variety Shenyou 9586, the zinc content in the dry basis of rice seedlings (moisture content: 85.66%) was 70.9 mg/kg.
2.3. Experimental Setup
The study utilized a pot trial approach with seven experimental treatments, each treatment condition replicated in triplicate in a completely randomized design. The specific treatment conditions were as follows:
C: Control group with no fertilizer application;
1.5A: Chinese milk vetch incorporation control, incorporating 2.25 t/hm2 of Chinese milk vetch (93.75 g/pot);
F: Chemical fertilizer control group with no Chinese milk vetch incorporation;
AF: Application of chemical fertilizer with 1.50 t/hm2 of Chinese milk vetch (62.50 g/pot);
1.5AF: Application of chemical fertilizer with 2.25 t/hm2 of Chinese milk vetch (93.75 g/pot);
2AF: Application of chemical fertilizer with 3.00 t/hm2 of Chinese milk vetch (125.00 g/pot);
2.5AF: Application of chemical fertilizer with 3.75 t/hm2 of Chinese milk vetch (156.25 g/pot).
The pot experiment was set up on 9 October 2019. Chinese milk vetch seeds were sown on 21 October 2019. On 31 March 2020, the Chinese milk vetch was incorporated. On 15 April 2020, basal fertilizer was applied, and early rice seedlings were transplanted. On 22 April 2020, topdressing was applied to the early rice. The early rice was harvested on 17 July 2020, and basal fertilizer was applied for the late rice. On 20 July 2020, late rice seedlings were transplanted. Topdressing was applied to the late rice on 24 July 2020. The late rice was harvested on 12 November 2020, and Chinese milk vetch seeds were sown again on 18 November 2020.
On 8 April 2021, the Chinese milk vetch was incorporated. On 23 April 2021, basal fertilizer was applied, and early rice seedlings were transplanted. On 30 April 2021, topdressing was applied to the early rice. The early rice was harvested on the morning of 27 July 2021. In the afternoon of the same day, basal fertilizer was applied to the late rice, and late rice seedlings were transplanted. On 3 August 2021, topdressing was applied to the late rice. The late rice was harvested on 27 October 2021.
Each pot contained 10.0 kg of air-dried soil that had been screened using a 2 mm sieve. Following sowing of the Chinese milk vetch, no additional fertilizer was applied. The sowing rates for the 1.5A, AF, 1.5AF, 2AF, and 2.5AF treatments were all 30 kg/hm2, computed using the pot area of 0.07 m2, which had a diameter of 30 cm on the inside. This equated to a seed quantity of approximately 0.21 g, which was equivalent to around 60 seeds per pot.
The pots were amended with fresh grass 15 days before the early rice was transplanted, and the grass was kept moist with shallow water to enable decomposition. Before the new Chinese milk vetch was incorporated, any Chinese milk vetch previously in the pots was harvested. The original pots contained an insufficient quantity of Chinese milk vetch for the experimental design’s needs, requiring the harvesting of all the existing Chinese milk vetch. The insufficient amount was remedied by harvesting more Chinese milk vetch of the same variety from another location. According to the experimental design’s specified quantities, the harvested Chinese milk vetch was cut into around two-centimeter pieces, thoroughly mixed, and added to the pots. Finally, to achieve a uniform dispersal, the Chinese milk vetch segments were then thoroughly mixed throughout the soil in the pots.
The fertilizer application rates for rice were as follows: For the early rice, the nitrogen (N) application rate was 150 kg/hm2, and for the late rice, it was 180 kg/hm2 (70% as basal fertilizer and 30% as topdressing). The phosphorus pentoxide (P2O5) application rate for the early rice was 75 kg/hm2, and for the late rice, it was 45 kg/hm2 (applied entirely as basal fertilizer). The potassium oxide (K2O) application rate for the early rice was 75 kg/hm2, and for the late rice, it was 90 kg/hm2 (50% as basal fertilizer and 50% as topdressing). The specific fertilizers used were urea (46% N) for nitrogen, calcium superphosphate (12% P2O5) for phosphorus and potassium chloride (60% K2O) for potassium.
2.4. Sample Collection and Analysis
During the rice maturity stage, samples of rice plants and soil were collected to assess the biomass and zinc content of the rice straw and grains for both the early and late rice crops. Furthermore, soil samples were analyzed to assess the DTPA-extractable zinc (DTPA-Zn) content.
2.5. Analytical Methods
The plant samples were digested using a mixed acid solution of nitric acid (HNO3) and perchloric acid (HClO4) in a 10:1 ratio. An inductively coupled plasma optical emission spectroscopic (ICP-OES) technique was utilized to measure the zinc content. In the DTPA-extractable zinc determination, 15 g of soil particles finer than 20-mesh were blended with 30 mL of DTPA reagent and extracted at 25 °C for 2 h. The filtered extract was then subjected to atomic absorption spectroscopy analysis [28].
2.6. Calculation Methods
Harvest Index (%) = Grain Biomass/Total Aboveground Biomass × 100;
Rice Zinc Uptake = Grain Yield × Grain Zinc Content/1000;
Rice Straw Zinc Uptake = Straw Yield × Straw Zinc Content/1000;
Aboveground Zinc Uptake = (Grain Yield × Grain Zinc Content + Straw Yield × Straw Zinc Content)/1000;
Zinc Harvest Index (%) = Rice Zinc Uptake/Aboveground Zinc Uptake × 100;
Rice Zinc Utilization Efficiency = Grain Zinc Content/Aboveground Zinc Uptake × 100, representing the amount of zinc in rice grains formed per 100 g of zinc nutrient absorbed by the aboveground portion of the rice plant;
In the formulas above, yield and biomass are measured in kg/hm2; zinc content and rice zinc utilization efficiency are measured in mg/kg; and zinc uptake is measured in g/hm2.
2.7. Data Analysis
SPSS 20.0 software was utilized to analyze the collected data. The data underwent both analysis of variance and correlation analysis. To explore the differences across the treatment groups, one-way ANOVA was employed, and Duncan’s new multiple range test was applied for multiple comparisons of the means. A significance level of p = 0.05 was established. The correlation analysis involved the use of Pearson’s correlation coefficient to determine the correlation coefficients among the variables. The redundancy analysis (RDA) method was used for analysis, and RDA analysis and plotting were performed using the R language (R 4.3.2) with the vegan package.
3. Results and Analysis
3.1. Effect of Chinese Milk Vetch in Modulating Yield of Various Rice Plant Parts in Purple Tidal Mud Substrate
In terms of grain yield per rice season, the sole addition of Chinese milk vetch significantly increased the early rice yield but reduced the late rice yield in 2020. The yield increase for late rice in 2021 was not significant. When Chinese milk vetch was incorporated after the application of chemical fertilizers, it increased the early rice yield and partially improved the late rice yield in some treatments. However, late rice yields experienced a decrease in the AF treatment in 2020 and in the 2AF and 2.5AF treatments in 2021. Considering the total yield across the four rice seasons over two years, the sole addition of Chinese milk vetch significantly increased grain yield, with the 1.5AF treatment showing a 110.0% increase relative to the C treatment. When Chinese milk vetch was incorporated after the application of chemical fertilizers, it increased grain yield, and the yield augmented alongside the amplification of Chinese milk vetch. Yields increased by 9.7% in the AF treatment, 12.8% in the 1.5AF treatment, 18.2% in the 2AF treatment, and 25.5% in the 2.5AF treatment, all in comparison to the F treatment (Table 1).
In terms of straw yield per rice season, the sole addition of Chinese milk vetch increased the early rice straw yield. The increase in straw yield for late rice was not significant in 2020, but it reduced the straw yield in 2021. When Chinese milk vetch was incorporated after the application of chemical fertilizers, it increased the early rice straw yield and partially improved the late rice straw yield in some treatments. However, the straw yield decreased in the AF treatment in 2020 and in all treatments in 2021 for late rice. Considering the total yield across the four rice seasons over two years, the sole addition of Chinese milk vetch significantly improved straw yield, as the 1.5AF treatment saw a 97.9% rise over the C treatment. When Chinese milk vetch was incorporated after the application of chemical fertilizers, straw yield saw a significant increase as well, with the yield rising commensurately with the quantity of Chinese milk vetch applied. Yields increased by 13.5% in the AF treatment, 23.2% in the 1.5AF treatment, 28.0% in the 2AF treatment, and 36.4% in the 2.5AF treatment, all relative to the F treatment (Table 1).
Solely incorporating Chinese milk vetch markedly elevated the overall biomass yield of early rice per rice season yet reduced the entire biomass yield of late rice. When Chinese milk vetch was incorporated after the application of chemical fertilizers, it increased the entire biomass yield of early rice and partially improved the entire biomass yield of late rice in some treatments. However, the overall biomass yield of late rice declined in the AF treatment in 2020 and in the 2AF and 2.5AF treatments in 2021. Considering the entire biomass yield across the four rice seasons over two years, the sole addition of Chinese milk vetch significantly increased the entire biomass yield of rice, with the 1.5AF treatment showing a 102.9% increase relative to the C treatment. When Chinese milk vetch was incorporated after the application of chemical fertilizers, it also significantly increased the entire biomass yield of rice; correspondingly, the biomass exhibited a gradual increase alongside the heightened amounts of Chinese milk vetch incorporation. Yields increased by 11.9% in the AF treatment, 18.9% in the 1.5AF treatment, 24.0% in the 2AF treatment, and 31.9% in the 2.5AF treatment, all in comparison to the F treatment (Table 1).
The rice harvest index remained consistent across the four growing seasons over the two-year period, with no significant differences observed among the treatments. The treatment with the sole addition of Chinese milk vetch had the highest harvest index, while all four treatments that combined Chinese milk vetch incorporation with prior chemical fertilizer application were lower than the treatment with only chemical fertilizer application (Table 1).
3.2. Effect of Chinese Milk Vetch in Modulating Zinc Content of Rice Grain and Straw in Purple Tidal Mud Substrate
In 2020, for the early rice, the zinc content in the grain was higher in the F treatment compared to the late rice, while in other treatments, the grain zinc content was higher in the late rice compared to the early rice. In 2021, the grain zinc content was higher in the early rice compared to the late rice across all treatments. The sole addition of Chinese milk vetch led to a reduction in the zinc content of rice grain. Across the four rice seasons, the relative decrease in the zinc content of rice grain for the 1.5AF treatment compared to the C treatment was 15.2%, 1.6%, 9.4%, and 5.7%, respectively (Table 2).
The variations in the zinc content of rice grain for the AF, 1.5AF, 2AF, and 2.5AF treatments relative to the F treatment were as follows: for the early rice in 2020, it was −27.6%, −28.6%, −31.3%, and −24.6%; for the late rice in 2020, it was −12.2%, −12.2%, −11.0%, and −12.8%; for the early rice in 2021, it was −3.0%, 1.1%, −11.1%, and −7.8%; and for the late rice in 2021, it was 1.7%, 5.6%, 4.1%, and 4.2%. Following the use of chemical fertilizers, the introduction of Chinese milk vetch caused a preliminary reduction in the zinc concentration of rice grain in the initial year. Conversely, over the course of the second year, there was a subsequent increase observed in the zinc content of the rice grains. The treatment with an incorporation rate of Chinese milk vetch at 2.25 t/hm2 demonstrated the greatest enhancement in the zinc content of rice grain. As time progressed, the variations in the zinc content of rice grain for the AF, 1.5AF, 2AF, and 2.5AF treatments relative to the F treatment exhibited an increasing trend (Table 2).
The zinc content in the rice straw was higher compared to the late rice in the early rice during the two-year maturity period. In 2021, the zinc content in rice straw during the late rice-growing season was higher across all treatments with fertilization or Chinese milk vetch incorporation compared to the late rice in 2020. The zinc content was higher in the C treatment in rice straw during the maturity period in 2020 compared to 2021. In the treatments without chemical fertilizer application, the zinc content in rice straw during the maturity period was higher in the early rice in 2021 compared to 2020. In the 2.5AF treatment, the zinc concentration in the rice straw during the maturity period was greater in the early rice in 2021 compared to 2020. However, in the other treatments with chemical fertilizer application, the zinc content in rice straw during the maturity period was higher in the late rice in 2020 compared to 2021. The sole addition of Chinese milk vetch led to a reduction in the zinc concentration of rice straw. The relative decrease in zinc concentration in rice straw during the maturity period for the 1.5A treatment compared to the C treatment was 22.0% for the early rice in 2020, 25.4% for the late rice in 2020, 36.1% for the early rice in 2021, and 4.9% for the late rice in 2021. For the early rice in 2020, the variations in zinc content in rice straw during the maturity period for the treatments of AF, 1.5AF, 2AF, and 2.5AF compared to the F treatment were −22.1%, −19.0%, −19.5%, and −30.3%, respectively. For the late rice in 2020, the variations were −27.1%, −29.4%, −25.5%, and −26.5%. For the early rice in 2021, the variations were −3.8%, 0.2%, −20.5%, and −1.2%. For the late rice in 2021, the variations were −22.0%, −30.0%, −8.3%, and −26.6%. Applying Chinese milk vetch following chemical fertilizer application led to a reduction in the zinc content of rice straw during the maturity period, with the exception of the 1.5AF treatment in the early rice of 2021, where it was slightly higher than the F treatment (Table 2).
3.3. Effect of Chinese Milk Vetch in Modulating Zinc Uptake of Rice in Purple Tidal Mud Substrate
Sole application of Chinese milk vetch led to increased zinc uptake in the grain, straw, and aboveground parts of the early rice across the two-year period, whereas it led to a decrease in zinc uptake in the grain, straw, and aboveground parts of the late rice. The incorporation of Chinese milk vetch following the application of chemical fertilizers resulted in increased zinc uptake in the straw and, in certain treatments, in the grain and aboveground parts of the early rice in 2020. In contrast, it led to a decrease in zinc uptake in the grain, straw, and aboveground parts of the late rice in 2020. In 2021, the incorporation led to increased zinc uptake in the grain, straw, and aboveground parts of the early rice but decreased zinc uptake in the straw and aboveground parts, as well as in the grain of the late rice in certain treatments. In 2020, the AF, 1.5AF, and 2AF treatments exhibited a decrease in grain zinc uptake, while the AF treatment showed a decrease in aboveground zinc uptake. All treatments led to a decrease in grain, straw, and aboveground zinc uptake in the late rice in 2020. In 2021, all treatments resulted in an increase in grain, straw, and aboveground zinc uptake in the early rice, whereas the late rice exhibited a decrease in straw and aboveground zinc uptake, as well as in the grain in certain treatments. Across the four rice seasons, the overall zinc uptake in the grain, straw, and aboveground parts of rice was lowest in the C treatment, followed by the 1.5A treatment, and highest in the 2.5AF treatment. Compared to the F treatment, the variations in grain zinc uptake were −5.8%, −1.3%, −1.0%, and 9.6% for the AF, 1.5AF, 2AF, and 2.5AF treatments, respectively. For the straw zinc uptake, the variations were −5.0%, 5.0%, 8.5%, and 10.1% for the AF, 1.5AF, 2AF, and 2.5AF treatments, respectively. Similarly, the variations in aboveground zinc uptake were −5.2%, 3.5%, 6.3%, and 10.0% for the same respective treatments. Across the four treatments involving Chinese milk vetch incorporation following chemical fertilizer application, the zinc uptake in the grain, straw, and aboveground parts of rice increased with higher amounts of Chinese milk vetch incorporation (Table 3).
Over the four rice seasons during the two-year period, there were no significant differences in the zinc harvest index among the treatments for rice. For the early rice, the grain zinc formation efficiency was highest in the C treatment, with the 1.5A treatment being the next highest. In the late rice of 2020, the grain zinc formation efficiency was highest in the 1.5A treatment, followed by the C treatment. In late rice of 2021, both the C and 1.5A treatments had the highest grain zinc formation efficiency. The grain zinc formation efficiency in all treatments with fertilizer application was lower than that in the C and 1.5A treatments. The C treatment had the highest grain zinc formation efficiency over the four rice seasons of rice, followed by the 1.5A treatment, and the treatments with fertilizer application showed no significant differences in the grain zinc formation efficiency (Table 3).
3.4. Effect of Chinese Milk Vetch in Modulating Yield Components of Rice in Purple Tidal Mud Substrate
Over the four rice seasons, the sole application of Chinese milk vetch increased the number of effective rice spikes by 4.8 × 104/hm2, 9.5 × 104/hm2, 112.3 × 104/hm2, and 6.0 × 104/hm2, respectively. It also led to an increase in the number of filled grains per spike in the early rice of 2020 and both early and late rice of 2021, with increases of 55.2 grains/spike, 14.3 grains/spike, and 14.3 grains/spike, respectively. In addition, it increased the 1000-grain weight in early rice of 2021, with an increase of 0.6 g (Table 4).
The addition of Chinese milk vetch subsequent to chemical fertilizer application led to increases in the number of effective spikes in the early-season rice for the 1.5AF, 2AF, and 2.5AF treatments, as well as in the late rice of 2020 and 2021 for the AF, 1.5AF, and 2.5AF treatments. Compared to the F treatment, the increases in the number of effective spikes in early rice were 66.6 × 104/hm2, 66.6 × 104/hm2, 104.7 × 104/hm2, and 104.7 × 104/hm2 for the AF, 1.5AF, 2AF, and 2.5AF treatments, respectively, in 2020. In 2021, the increases were 42.7 × 104/hm2, 38.0 × 104/hm2, 66.3 × 104/hm2, and 71.0 × 104/hm2 for the same respective treatments. In late rice, the changes were 9.5 × 104/hm2, 14.3 × 104/hm2, 28.6 × 104/hm2, and 33.4 × 104/hm2 for 2020; and 33.3 × 104/hm2, 14.7 × 104/hm2, −19.0 × 104/hm2, and 0 × 104/hm2 for 2021. The impact of Chinese milk vetch incorporation subsequent to chemical fertilizer application on the number of effective spikes was more pronounced in the early rice compared to the late rice (Table 4).
The addition of Chinese milk vetch subsequent to chemical fertilizer application led to a decline in the number of filled grains per spike in early and late rice of 2020 in all treatments, as well as early and late rice of 2021 in the 1.5AF and 2AF treatments. In 2020, the decreases in the number of filled grains per spike in early rice, compared to the F treatment, were 8.2 grains/spike, 9.6 grains/spike, 6.7 grains/spike, and 1.9 grains/spike for the AF, 1.5AF, 2AF, and 2.5AF treatments, respectively. In the late rice of 2020, the corresponding decreases were 1.0 grains/spike, 2.5 grains/spike, 12.8 grains/spike, and 9.1 grains/spike for the AF, 1.5AF, 2AF, and 2.5AF treatments, respectively. The changes in early rice of 2021 were 3.6 grains/spike, −13.1 grains/spike, −3.4 grains/spike, and 1.7 grains/spike; and in late rice of 2021, the changes were 3.6 grains/spike, −13.1 grains/spike, −3.4 grains/spike, and 1.7 grains/spike (Table 4).
The addition of Chinese milk vetch subsequent to chemical fertilizer application led to a reduction in the 1000-grain weight of early rice in 2020 of all treatments, as well as in 2021 of the 1.5AF treatment. Additionally, a similar decrease in 1000-grain weight was observed in the late rice of 2021 in the 1.5AF and 2.5AF treatments. In 2020, compared to the F treatment, the AF, 1.5AF, 2AF, and 2.5AF treatments resulted in decreases of 1.1 g, 0.4 g, 1.1 g, and 0.2 g, respectively, in the 1000-grain weight of early rice. Conversely, the late rice of 2020 exhibited increases of 0.9 g, 0.7 g, 1.2 g, and 1.2 g in the same treatments versus the F treatment. For the early rice in 2021, the AF, 1.5AF, 2AF, and 2.5AF treatments exhibited changes of 0.3 g, −0.9 g, 0.9 g, and 0.4 g, respectively, compared to the F treatment. Similarly, in the late rice of 2021, the changes were 0.1 g, −0.4 g, 2.5 g, and −1.0 g for the same set of treatments (Table 4).
3.5. Relationship between Rice Yield Components and Factors Related to Zinc Uptake in Rice
A redundancy analysis (RDA) was performed with rice zinc uptake-related factors as the response variables and rice yield components as the explanatory variables, as shown in Figure 1a,b. The explanatory variables in the model presented in Figure 1a accounted for 91.887% of the variance, with RDA1 and RDA2 explaining 85.69% and 6.197% of the variance in rice zinc uptake, respectively. Similarly, in the model shown in Figure 1b, the explanatory variables explained 91.638% of the variance, where RDA1 and RDA2 were responsible for 86.06% and 5.578% of the variance in rice zinc uptake, respectively.
From Figure 1a, it can be observed that the aboveground biomass, grain yield, effective spikes, and filled grains per spike in early rice are positively correlated with rice zinc uptake, while the 1000-grain weight and zinc content in rice are negatively correlated with zinc uptake. The harvest index and aboveground zinc uptake in rice show little correlation. The aboveground biomass and grain yield are two important factors influencing rice zinc uptake. In addition, the aboveground biomass, grain yield, effective spikes, filled grains per spike, and harvest index are positively correlated with zinc uptake in rice, while they are negatively correlated with zinc content in rice. The 1000-grain weight shows little correlation with both zinc uptake and zinc content in rice.
The C treatment is situated on the far left of the graph, while the 1.5A treatment is positioned slightly to the left. Conversely, the treatments F, AF, 1.5AF, 2AF, and 2.5AF are located on the right side of the graph. There is a clear separation between the three groups, but the F, AF, 1.5AF, 2AF, and 2.5AF treatments are not distinctly separated from each other on the graph. This indicates that the influence of rice yield components on zinc uptake-related factors is found to differ between the Chinese milk vetch addition and the C treatment in early rice. There are also differences between the application of chemical fertilizer followed by the addition of Chinese milk vetch and both the C treatment and the treatment involving solely the addition of Chinese milk vetch. However, the differences between the chemical fertilizer treatment and the addition of Chinese milk vetch subsequent to chemical fertilizer application are not significant.
From Figure 1b, it can be observed that grain yield, effective spikes, aboveground biomass, and filled grains per spike in late rice are positively correlated with rice zinc uptake, while the 1000-grain weight and harvest index are negatively correlated with zinc uptake. Grain yield, effective spikes, and aboveground biomass are three important factors influencing rice zinc uptake. Furthermore, aboveground biomass, grain yield, effective spikes, and filled grains per spike are positively correlated with zinc uptake in rice, while they are negatively correlated with zinc content in rice. The harvest index shows a positive correlation with zinc uptake in rice, while it shows little correlation with zinc content. The 1000-grain weight exhibits a negative correlation with zinc uptake while demonstrating a positive correlation with zinc content in rice.
On the graph, the C treatment and the 1.5A treatment are positioned on the left side, whereas the treatments of F, AF, 1.5AF, 2AF, and 2.5AF are located on the right side. A clear separation can be observed between the two groups; however, the C treatment and the 1.5A treatment do not exhibit a distinct separation. Likewise, the F treatment does not display a clear separation from the treatments of AF, 1.5AF, 2AF, and 2.5AF. This suggests that the impact of rice yield factors on zinc uptake-related factors does not exhibit significant differences between the addition of Chinese milk vetch and the C treatment in late rice cultivation. Differences can be observed between the chemical fertilizer treatment and both the application of chemical fertilizer followed by the addition of Chinese milk vetch, as well as the treatment involving solely the addition of Chinese milk vetch. Nonetheless, between the chemical fertilizer treatment and the treatment involving the addition of Chinese milk vetch subsequent to chemical fertilizer application, no significant difference is observed.
There is a positive association between rice zinc uptake and rice grain yield, while a negative correlation exists between rice zinc uptake and zinc content in rice. Additionally, rice grain yield demonstrates a negative correlation with zinc content in rice.
3.6. Effect of Chinese Milk Vetch in Modulating DTPA-Extractable Zinc Content in Purple Tidal Mud Substrate
Over the course of the two-year rice cultivation study spanning four growing seasons, the DTPA-extractable zinc content in the purple tidal mud substrate fluctuated within a range of 0.26 to 0.73 mg/kg. The 1.5A treatment exhibited the most pronounced fluctuations in DTPA-Zn content, whereas the C treatment displayed the smallest degree of variation. During the late-season period of 2021, only the 1.5A treatment (0.69 mg/kg) and the 2.5AF treatment (0.73 mg/kg) exhibited DTPA-Zn levels exceeding the initial value of 0.68 mg/kg. Over the course of the rice maturity stage spanning the four growing seasons, the 2.5AF treatment exhibited the highest DTPA-Zn content, except during the late season of 2021 when the AF treatment recorded the maximum level. During the early season of 2020, the F treatment exhibited the smallest DTPA-Zn concentration, whereas in the remaining three seasons, the C treatment recorded the minimum levels, followed by the F treatment. Consistently across each year, the DTPA-Zn levels in the soil were higher during the late-season harvest compared to the early-season harvest. During the early-season harvest of 2020, the addition of the Chinese milk vetch treatment was the sole factor that induced a decline in DTPA-Zn concentration within the purple tidal mud substrate. In contrast, the Chinese milk vetch treatment led to increased DTPA-Zn levels in the other three growing seasons. Throughout the rice harvest period spanning the four growing seasons, the application of Chinese milk vetch following chemical fertilizer treatment resulted in increased DTPA-Zn levels within the purple tidal mud substrate. Relative to the F treatment, the DTPA-Zn content increased by 11.94%, 20.75%, 1.89%, and 37.74% under the AF, 1.5AF, 2AF, and 2.5AF treatments, respectively, by the conclusion of the experiment. In contrast to the C treatment, the 1.5A treatment resulted in a 94.37% increase in DTPA-Zn content. In other words, the application of Chinese milk vetch, both as a standalone treatment and in combination with chemical fertilizers, resulted in increased DTPA-Zn levels within the purple tidal mud substrate [29] (Table 5).
3.7. Correlation Analysis of DTPA-Extractable Zinc Concentration in Purple Tidal Mud Substrate with Rice Zinc Content and Zinc Uptake
The DTPA-extractable zinc concentration in the purple tidal mud substrate is significantly and negatively correlated with the zinc concentration and zinc uptake in rice grains, with correlation coefficients of r = −0.269 * and r = −0.228 *, respectively.
It is also highly negatively correlated with zinc content in straw, zinc uptake in straw, and aboveground zinc uptake (correlation coefficients of r = −0.731 **, r = −0.456 **, and r = −0.428 **, respectively). The uptake of zinc from the soil by rice plants during cultivation leads to a decline in the DTPA-extractable zinc concentration in the purple tidal mud substrate (Table 6).
4. Discussion
4.1. Interconnection among Rice Productivity, Grain Zinc Levels, and Zinc Assimilation
The findings of this study demonstrate the positive impact of incorporating green manure crops on rice yield and zinc content. Previous studies have shown that even with no reduction or a 20% reduction in fertilizer application, incorporating Chinese milk vetch as a winter cover crop can increase rice grain yield by 6.53% and 4.15% [30]. Long-term field trials have also confirmed that incorporating green manure generally increases rice grain yield [31]. Additionally, research in Turkey by Gunes et al. [32] found that intercropping wheat and chickpea on zinc-deficient soils led to increased wheat yield and grain zinc content.
The current study found that the cultivation of summer green manure species, including Sesbania aculeata, Crotalaria juncea, or Vigna unguiculata, not only increased grain and straw yield but also enhanced the zinc content in Basmati rice [33]. Of the green manure crops studied, Sesbania aculeata had the greatest impact on increasing zinc concentration and accumulation in both rice grain and straw, while Crotalaria juncea substantially increased yield, zinc content, and uptake in the rice [20].
Consistent with previous research [34,35,36,37], this study identified a negative correlation between rice grain yield and zinc content, which may be explained by a “dilution effect” where rapid plant growth outpaces zinc accumulation [38,39]. However, contrary to the yield dilution hypothesis, other studies have found that the decrease in grain zinc levels is not always directly proportional to the increase in yield [40]. The application of clover green manure improved wheat biomass and yield but did not result in a significant reduction in grain zinc concentration, likely due to the release of organic acids and nitrogen during residue decomposition, which facilitated better wheat zinc uptake [41].
In summary, the incorporation of green manure crops can effectively increase both the yield and zinc content of rice. Further research is needed to fully elucidate the complex mechanisms underlying the relationship between crop yield, zinc uptake, and green manure management.
4.2. Correlation between Soil Zinc Availability and Zinc Concentration, as Well as Zinc Uptake, in Rice
A study by Asif et al. [42] in Turkey found that the increase in wheat yield on zinc-deficient soils was primarily due to an increased number of spikes, but grain zinc content decreased significantly due to the “yield dilution” effect. However, sufficient soil zinc can effectively alleviate the negative effects of this “yield dilution” [42]. Additionally, research has shown that in zinc-sufficient soil conditions or with the addition of zinc fertilizer, the incorporation of leguminous green manures like clover can enhance the zinc accumulation in wheat [18]. Organic acids released by different plant root systems in intercropping and rotation systems can also activate soil zinc and form organic acid–zinc complexes, thereby increasing crop zinc uptake [43,44].
Consistent with these findings, the results of this study showed that incorporating Chinese milk vetch green manure was capable of elevating the zinc acquisition abilities of the rice plants, with a positive correlation between the amounts of Chinese milk vetch added and rice zinc uptake. Notably, Singh et al. [19] highlighted the finding that cultivating green manure crops can lead to an increase in both soil zinc content and zinc levels in rice plants, with the two variables exhibiting a statistically significant positive correlation. The long-term incorporation of green manure following the application of chemical fertilizers was also found to significantly increase the total zinc content present within rice grains, likely due to the combined effects of elevated soil zinc content and EDTA-extractable zinc concentration [45].
Contrary to the established research, this study determined that the DTPA-Zn content in the purple tidal mud substrate exhibited a significant negative correlation with the zinc concentration and uptake in rice grains, as well as the zinc content and uptake in rice straw and aboveground portions of the plant. This suggests that as rice plants absorb zinc from the soil throughout their growth, the increase in zinc content and accumulation within the rice results in a corresponding decline in the DTPA-extractable zinc levels present in the soil. Nonetheless, the findings from this investigation demonstrate that both the incorporation of Chinese milk vetch and its incorporation following the application of chemical fertilizers can serve to increase the DTPA-Zn concentration present in the purple tidal mud substrate.
4.3. Impact of Green Manure on Rice Zinc Content
The introduction of leguminous crops like clover onto zinc-deficient calcareous soils can significantly increase the accumulation of zinc within the grains in the subsequent wheat crop [17]. Similarly, intercropping grasses and dicotyledonous plants have been shown to enhance crop zinc nutrition, as both plant types can accumulate relatively high zinc levels [46]. Crop rotations involving wheat and leguminous crops, safflower, sunflower, or sudangrass have also been found to increase wheat grain zinc content to varying extents [15,16,41,47,48]. Specifically, sole sunflower green manure can increase rice grain zinc to 31 mg/kg, while red clover green manure can raise it to 54 mg/kg [49]. Therefore, the incorporation of green manure is considered an effective biofortification strategy to boost wheat grain zinc [17].
However, in contrast to these findings, the sole incorporation of Chinese milk vetch in this experiment decreased the zinc content in rice grains and straw. The reduction in rice grain zinc may be attributed to the relatively low zinc concentration in the incorporated Chinese milk vetch, where the stimulation of rice growth outpaced the enhancement of zinc uptake. Additionally, the application of leguminous green manure has been shown to only increase grain zinc in soils with higher available zinc, likely due to the effect of biological nitrogen fixation [46,50]. Since the soil tested in this experiment had low available zinc, this may explain why Chinese milk vetch failed to augment rice zinc content in the initial year.
Compared to the sole fertilizer treatment, the varying rates of Chinese milk vetch incorporated subsequent to chemical fertilizer application (AF, 1.5AF, 2AF, 2.5AF) exhibited different effects on rice grain zinc over the two-year period. While the initial treatments reduced grain zinc, the 1.5AF treatment led to a slight increase in the second year’s early rice, and all the treatments increased zinc in the second year’s late rice, with 1.5AF being the most favorable. Furthermore, the 1.5AF treatment (2.25 t/hm2) improved both yield and zinc content in the second year’s early and late rice. However, the final zinc concentrations did not reach the targeted range of 41.67–62.5 mg/kg [51,52], and further studies are needed to determine if longer-term applications could increase rice grain zinc to the desired levels.
5. Conclusions
This pot-based study demonstrated that incorporating Chinese milk vetch (Astragalus sinicus L.) into purple tidal mud substrate improved rice grain yields, with the yield increasing proportionally to the amount of Chinese milk vetch added. The impact was more pronounced on early rice than late rice. Zinc uptake in rice grains was positively correlated with grain yield but negatively correlated with grain zinc concentration. The application of Chinese milk vetch, both solely and in combination with chemical fertilizers, had varying effects on zinc uptake and grain zinc formation efficiency in early and late rice, with the control and low-level Chinese milk vetch treatments generally exhibiting the highest performance.
The incorporation of Chinese milk vetch following chemical fertilizers enhanced zinc content in rice grains starting from the second year, with the 2.25 t/hm2 treatment being the most effective. These findings have important implications for improving rice productivity and nutrition in regions with purple tidal mud substrates.
Future research should evaluate the effects of green manure on a single rice cultivar across multiple growing seasons to more conclusively determine the impact on grain zinc biofortification.
Author Contributions
Conceptualization, Z.Y. and J.X.; methodology, Z.R. and H.L.; software, X.Z. and Z.L.; validation, Z.R. and H.L.; formal analysis, Z.Y. and Z.L.; investigation, Z.R. and X.Z.; resources, X.Z. and Z.L.; data curation, X.Z. and H.L.; writing—original draft preparation, Z.Y. and J.X.; writing—review and editing, Z.Y. and J.X.; visualization, Z.Y. and Z.L.; supervision, Z.Y. and J.X.; project administration, H.L.; funding acquisition, Z.Y. and Z.R. All authors have read and agreed to the published version of the manuscript.
Funding
Funding for this study was provided by the Joint Funds of the National Natural Science Foundation of China (grant number U21A20294) as well as the General Project of the Hunan Provincial Natural Science Foundation of China (grant 2019JJ40168).
Data Availability Statement
Readers interested in the data that substantiate the conclusions of this research may obtain them from the corresponding author upon submitting a reasonable request.
Conflicts of Interest
The authors confirm they have no conflicts of interest.
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Figure 1.
Redundancy analysis of rice yield factors and zinc absorption related factors for early rice (a) and late rice (b). Note: The blue arrow lines represent explanatory variables, and the red arrow lines indicate the response variables. GrY—grain yield; Bm—biomass; HI—harvest index; TGW—thousand grain weight; SpN—spike number; GrN—grain number per spike; GrZnC—grain Zn concentration; StZnC—straw Zn concentration; GrZnU—grain Zn uptake; ShZnU—shoot Zn uptake; ZnHI—Zn harvest index; GrZnE—grain Zn formation efficiency.
Figure 1.
Redundancy analysis of rice yield factors and zinc absorption related factors for early rice (a) and late rice (b). Note: The blue arrow lines represent explanatory variables, and the red arrow lines indicate the response variables. GrY—grain yield; Bm—biomass; HI—harvest index; TGW—thousand grain weight; SpN—spike number; GrN—grain number per spike; GrZnC—grain Zn concentration; StZnC—straw Zn concentration; GrZnU—grain Zn uptake; ShZnU—shoot Zn uptake; ZnHI—Zn harvest index; GrZnE—grain Zn formation efficiency.
Table 1.
Yield of various parts of rice during maturity (kg/hm2).
| Rice Season | Treatment | Paddy (kg/hm2) | Straw (kg/hm2) | Total Biomass (kg/hm2) | Harvest Index (%) |
|---|---|---|---|---|---|
| Early rice in 2020 | C | 1070.5 ± 137.4 e | 1120.0 ± 142.3 d | 2190.5 ± 266.8 d | 48.86 ± 2.01 a |
| 1.5A | 3489.1 ± 310.2 d | 3679.0 ± 506.2 c | 7168.1 ± 218.0 c | 48.78 ± 5.54 a | |
| F | 4588.6 ± 169.8 cd | 5677.6 ± 690.7 b | 10,266.2 ± 796.4 b | 44.82 ± 2.55 ab | |
| AF | 5003.8 ± 662.3 bcd | 7739.5 ± 473.8 a | 12,743.3 ± 912.7 ab | 39.19 ± 3.14 b | |
| 1.5AF | 5118.6 ± 1438.1 abc | 8024.8 ± 1110.4 a | 13,143.3 ± 2498.9 ab | 38.47 ± 4.04 b | |
| 2AF | 6288.6 ± 355.7 ab | 9538.6 ± 1890.8 a | 15,827.2 ± 2013.4 a | 40.07 ± 4.34 b | |
| 2.5AF | 6625.3 ± 1564.9 a | 9349.5 ± 1738.8 a | 15,974.8 ± 3289.4 a | 41.33 ± 1.46 b | |
| Late rice in 2020 | C | 979.0 ± 303.8 b | 1565.2 ± 449.3 b | 2544.3 ± 523.4 b | 38.86 ± 9.92 a |
| 1.5A | 927.9 ± 155.0 b | 1602.4 ± 203.1 b | 2530.2 ± 218.5 b | 36.71 ± 5.49 a | |
| F | 2655.3 ± 545.4 a | 4003.3 ± 441.6 a | 6658.6 ± 939.6 a | 39.64 ± 3.27 a | |
| AF | 2597.6 ± 184.3 a | 3733.3 ± 359.4 a | 6331.0 ± 455.3 a | 41.08 ± 2.41 a | |
| 1.5AF | 2891.0 ± 472.3 a | 4135.3 ± 383.1 a | 7026.2 ± 855.3 a | 41.01 ± 1.66 a | |
| 2AF | 2791.9 ± 388.9 a | 4159.5 ± 30.0 a | 6951.4 ± 412.3 a | 40.03 ± 3.24 a | |
| 2.5AF | 2989.1 ± 727.5 a | 4245.7 ± 497.8 a | 7234.8 ± 1008.6 a | 41.06 ± 5.63 a | |
| Early rice in 2021 | C | 378.6 ± 107.2 d | 1009.5 ± 64.4 f | 1388.1 ± 147.5 e | 26.98 ± 5.24 b |
| 1.5A | 1742.9 ± 85.8 c | 3157.2 ± 295.2 e | 4900.0 ± 380.4 d | 35.62 ± 1.07 a | |
| F | 2080.9 ± 128.9 bc | 4281.0 ± 138.7 d | 6361.9 ± 21.8 c | 32.71 ± 2.07 ab | |
| AF | 2304.8 ± 172.8 bc | 4985.7 ± 457.8 cd | 7290.5 ± 513.5 c | 31.66 ± 2.27 ab | |
| 1.5AF | 2576.2 ± 178.6 b | 6128.6 ± 1458.0 ab | 8704.8 ± 1563.1 b | 30.04 ± 4.00 ab | |
| 2AF | 3207.1 ± 735.8 a | 5633.4 ± 164.9 bc | 8840.5 ± 882.4 b | 35.96 ± 4.77 a | |
| 2.5AF | 3533.4 ± 598.1 a | 6980.9 ± 701.3 a | 10,514.3 ± 1239.6 a | 33.50 ± 2.20 ab | |
| Late rice in 2021 | C | 971.4 ± 93.7 c | 1133.4 ± 16.5 c | 2104.8 ± 97.2 c | 46.09 ± 2.37 a |
| 1.5A | 981.0 ± 35.9 c | 1114.3 ± 71.4 c | 2095.2 ± 91.8 c | 46.84 ± 1.52 a | |
| F | 3047.6 ± 507.1 a | 3838.1 ± 54.1 a | 6885.7 ± 557.7 a | 44.06 ± 3.82 ab | |
| AF | 3661.9 ± 90.8 a | 3742.9 ± 71.5 ab | 7404.7 ± 45.9 a | 49.45 ± 1.06 a | |
| 1.5AF | 3366.6 ± 100.3 a | 3642.8 ± 124.5 ab | 7009.5 ± 216.3 a | 48.03 ± 0.45 a | |
| 2AF | 2338.1 ± 757.2 b | 3457.1 ± 398.2 b | 5795.2 ± 889.0 b | 39.73 ± 7.99 b | |
| 2.5AF | 2376.2 ± 95.1 b | 3700.0 ± 175.6 ab | 6076.2 ± 86.1 b | 39.13 ± 2.08 b | |
| Four crops of rice from 2020 to 2021 | C | 3399.5 ± 315.3 e | 4828.1 ± 385.0 f | 8227.6 ± 238.4 f | 41.34 ± 3.96 a |
| 1.5A | 7140.7 ± 336.8 d | 9552.8 ± 683.5 e | 16,693.6 ± 352.0 e | 42.81 ± 2.92 a | |
| F | 12,372.4 ± 469.9 c | 17,800.0 ± 123.2 d | 30,172.4 ± 356.0 d | 41.00 ± 1.08 a | |
| AF | 13,568.1 ± 556.1 bc | 20,201.4 ± 753.9 c | 33,769.5 ± 284.9 c | 40.19 ± 1.85 a | |
| 1.5AF | 13,952.4 ± 1861.3 abc | 21,931.4 ± 2370.0 bc | 35,883.8 ± 3839.9 bc | 38.85 ± 2.35 a | |
| 2AF | 14,625.7 ± 1339.0 ab | 22,788.6 ± 1672.0 ab | 37,414.3 ± 2051.6 ab | 39.09 ± 2.83 a | |
| 2.5AF | 15,523.8 ± 1311.6 a | 24,276.2 ± 1192.2 a | 39,800.0 ± 2462.3 a | 38.97 ± 1.00 a |
Note: C: control group with no fertilizer application; 1.5A: Chinese milk vetch incorporation control, incorporating 2.25 t/hm2 of Chinese milk vetch; F: chemical fertilizer control group with no Chinese milk vetch incorporation; AF: application of chemical fertilizer with 1.50 t/hm2 of Chinese milk vetch; 1.5AF: application of chemical fertilizer with 2.25 t/hm2 of Chinese milk vetch; 2AF: application of chemical fertilizer with 3.00 t/hm2 of Chinese milk vetch; 2.5AF: application of chemical fertilizer with 3.75 t/hm2 of Chinese milk vetch. Lowercase letters that differ suggest significant variations across treatment groups at the 5% alpha level, n = 3.
Table 2.
Zinc concentration in grain and straw of rice cultivated in purple tidal mud substrate with varying amounts of Chinese milk vetch (Astragalus sinicus L.) incorporation (mg/kg).
Table 2.
Zinc concentration in grain and straw of rice cultivated in purple tidal mud substrate with varying amounts of Chinese milk vetch (Astragalus sinicus L.) incorporation (mg/kg).
| Rice Season | Treatment | Paddy | Straw |
|---|---|---|---|
| Early rice in 2020 | C | 22.9 ± 1.4 ab | 63.3 ± 3.2 ab |
| 1.5A | 19.4 ± 0.5 b | 49.4 ± 1.7 b | |
| F | 27.5 ± 7.0 a | 72.5 ± 12.2 a | |
| AF | 19.9 ± 0.5 b | 56.5 ± 10.5 b | |
| 1.5AF | 19.6 ± 1.2 b | 58.7 ± 3.8 ab | |
| 2AF | 18.9 ± 1.3 b | 58.3 ± 8.1 ab | |
| 2.5AF | 20.7 ± 0.4 b | 50.5 ± 9.5 b | |
| Late rice in 2020 | C | 25.5 ± 0.2 a | 47.0 ± 3.4 a |
| 1.5A | 25.1 ± 1.2 a | 35.1 ± 3.3 b | |
| F | 25.7 ± 2.2 a | 45.5 ± 8.0 a | |
| AF | 22.6 ± 1.2 a | 33.2 ± 1.5 b | |
| 1.5AF | 22.6 ± 1.6 a | 32.2 ± 4.5 b | |
| 2AF | 22.9 ± 2.3 a | 33.9 ± 2.6 b | |
| 2.5AF | 22.4 ± 2.3 a | 33.5 ± 3.5 b | |
| Early rice in 2021 | C | 27.2 ± 0.1 a | 84.1 ± 5.3 a |
| 1.5A | 24.6 ± 1.0 bc | 53.8 ± 10.6 b | |
| F | 26.8 ± 2.5 ab | 55.9 ± 12.3 b | |
| AF | 26.0 ± 2.3 abc | 53.8 ± 10.3 b | |
| 1.5AF | 27.1 ± 0.8 a | 56.0 ± 8.5 b | |
| 2AF | 23.9 ± 0.9 c | 44.5 ± 2.5 b | |
| 2.5AF | 24.7 ± 0.8 bc | 55.2 ± 15.0 b | |
| Late rice in 2021 | C | 22.9 ± 1.9 a | 42.6 ± 2.9 ab |
| 1.5A | 21.6 ± 0.6 ab | 40.5 ± 1.8 ab | |
| F | 19.6 ± 1.0 c | 48.2 ± 6.8 a | |
| AF | 20.0 ± 0.6 bc | 37.6 ± 3.8 ab | |
| 1.5AF | 20.7 ± 0.6 bc | 33.8 ± 2.3 b | |
| 2AF | 20.4 ± 0.8 bc | 44.2 ± 12.5 ab | |
| 2.5AF | 20.5 ± 1.9 bc | 35.4 ± 4.9 b |
Note: C: control group with no fertilizer application; 1.5A: Chinese milk vetch incorporation control, incorporating 2.25 t/hm2 of Chinese milk vetch; F: chemical fertilizer control group with no Chinese milk vetch incorporation; AF: application of chemical fertilizer with 1.50 t/hm2 of Chinese milk vetch; 1.5AF: application of chemical fertilizer with 2.25 t/hm2 of Chinese milk vetch; 2AF: application of chemical fertilizer with 3.00 t/hm2 of Chinese milk vetch; 2.5AF: application of chemical fertilizer with 3.75 t/hm2 of Chinese milk vetch. Lowercase letters that differ suggest significant variations across treatment groups at the 5% alpha level, n = 3.
Table 3.
Zinc uptake in rice cultivated in purple tidal mud substrate with varying amounts of Chinese milk vetch (Astragalus sinicus L.) incorporation.
Table 3.
Zinc uptake in rice cultivated in purple tidal mud substrate with varying amounts of Chinese milk vetch (Astragalus sinicus L.) incorporation.
| Rice Season | Treatment | Grain Zn Uptake (Zn g/hm2) | Straw Zn Uptake (Zn g/hm2) | Shoot Zn Uptake (Zn g/hm2) | Zn Harvest Index (%) | Grain Zn Formation Efficiency (mg/kg) |
|---|---|---|---|---|---|---|
| Early rice in 2020 | C | 24.4 ± 2.1 c | 71.1 ± 11.2 b | 95.5 ± 13.3 b | 25.7 ± 1.5 ab | 24.4 ± 4.4 a |
| 1.5A | 67.7 ± 6.3 b | 182.1 ± 28.6 b | 249.8 ± 22.9 b | 27.3 ± 4.5 a | 7.8 ± 0.7 b | |
| F | 126.8 ± 36.7 a | 412.1 ± 92.8 a | 539.0 ± 71.7 a | 24.0 ± 8.7 ab | 5.2 ± 1.7 bc | |
| AF | 99.6 ± 13.5 ab | 436.5 ± 81.1 a | 536.1 ± 94.6 a | 19.0 ± 1.0 ab | 3.8 ± 0.7 c | |
| 1.5AF | 100.7 ± 29.2 ab | 561.5 ± 53.1 a | 570.1 ± 81.4 a | 17.3 ± 2.9 b | 3.5 ± 0.5 c | |
| 2AF | 118.6 ± 1.6 a | 469.4 ± 170.9 a | 680.1 ± 170.1 a | 18.0 ± 4.4 ab | 2.9 ± 0.7 c | |
| 2.5AF | 136.9 ± 29.5 a | 461.4 ± 19.8 a | 598.3 ± 24.9 a | 23.0 ± 4.4 ab | 3.5 ± 0.2 c | |
| Late rice in 2020 | C | 24.9 ± 7.6 b | 74.6 ± 26.1 c | 99.5 ± 26.5 c | 26.0 ± 8.7 a | 27.1 ± 8.5 a |
| 1.5A | 23.1 ± 3.0 b | 55.8 ± 2.0 c | 78.9 ± 3.0 c | 29.3 ± 2.9 a | 31.8 ± 2.7 a | |
| F | 67.5 ± 8.9 a | 180.0 ± 13.9 a | 247.5 ± 7.8 a | 27.7 ± 4.2 a | 10.4 ± 0.7 b | |
| AF | 58.7 ± 5.2 a | 123.8 ± 11.1 b | 182.4 ± 8.4 b | 32.0 ± 3.5 a | 12.4 ± 1.2 b | |
| 1.5AF | 64.9 ± 6.8 a | 132.1 ± 8.7 b | 197.0 ± 10.1 b | 33.0 ± 3.0 a | 11.5 ± 0.9 b | |
| 2AF | 63.5 ± 4.8 a | 141.0 ± 9.6 b | 204.4 ± 5.8 b | 31.0 ± 2.6 a | 11.2 ± 0.9 b | |
| 2.5AF | 66.2 ± 10.8 a | 143.1 ± 29.7 b | 209.3 ± 34.9 b | 32.0 ± 4.6 a | 10.9 ± 2.0 b | |
| Early rice in 2021 | C | 10.3 ± 2.9 e | 84.9 ± 6.9 d | 95.2 ± 6.3 d | 10.7 ± 3.2 b | 28.6 ± 1.9 a |
| 1.5A | 43.0 ± 3.3 d | 169.7 ± 37.9 cd | 212.7 ± 40.9 c | 20.7 ± 3.1 a | 11.8 ± 1.8 b | |
| F | 55.6 ± 2.0 cd | 240.4 ± 59.8 bc | 296.1 ± 61.6 bc | 19.3 ± 3.5 a | 9.2 ± 1.1 c | |
| AF | 60.2 ± 9.6 c | 265.2 ± 28.9 abc | 325.5 ± 24.8 bc | 18.7 ± 3.8 ab | 8.1 ± 0.9 c | |
| 1.5AF | 69.9 ± 4.8 bc | 343.6 ± 99.4 ab | 413.5 ± 97.9 ab | 17.3 ± 4.5 ab | 6.8 ± 1.7 cd | |
| 2AF | 77.0 ± 20.5 ab | 250.2 ± 6.4 bc | 327.2 ± 16.5 bc | 23.3 ± 5.0 a | 7.3 ± 0.1 cd | |
| 2.5AF | 87.6 ± 16.9 a | 384.4 ± 110.0 a | 472.0 ± 101.9 a | 19.3 ± 19.3 a | 5.4 ± 1.2 d | |
| Late rice in 2021 | C | 22.1 ± 1.0 d | 48.3 ± 3.9 c | 70.4 ± 3.2 d | 31.7 ± 2.9 abc | 32.6 ± 3.9 a |
| 1.5A | 21.2 ± 1.1 d | 45.0 ± 1.2 c | 66.2 ± 1.9 d | 32.0 ± 1.0 abc | 32.6 ± 1.1 a | |
| F | 59.5 ± 7.7 bc | 184.9 ± 23.7 a | 244.4 ± 17.8 a | 24.7 ± 4.6 c | 8.1 ± 0.7 c | |
| AF | 73.1 ± 3.4 a | 140.9 ± 16.7 b | 214.0 ± 13.6 b | 34.3 ± 4.2 ab | 9.4 ± 0.9 bc | |
| 1.5AF | 69.8 ± 3.0 ab | 123.2 ± 12.8 b | 193.0 ± 15.8 bc | 36.3 ± 1.2 a | 10.8 ± 0.7 bc | |
| 2AF | 47.5 ± 14.3 c | 151.3 ± 35.1 ab | 198.8 ± 20.9 bc | 24.7 ± 10.0 c | 10.3 ± 0.8 bc | |
| 2.5AF | 48.5 ± 3.1 c | 131.6 ± 24.1 b | 180.1 ± 26.9 c | 27.0 ± 2.6 bc | 11.4 ± 0.8 b | |
| Four crops of rice from 2020 to 2021 | C | 81.8 ± 5.7 d | 278.9 ± 18.6 b | 360.6 ± 17.5 c | 22.7 ± 2.3 a | 28.2 ± 0.8 a |
| 1.5A | 155.0 ± 7.6 c | 452.6 ± 57.6 b | 607.6 ± 57.2 b | 25.7 ± 2.5 a | 21.0 ± 0.4 b | |
| F | 309.5 ± 37.2 ab | 1017.4 ± 58.9 a | 1326.9 ± 40.6 a | 23.0 ± 3.0 a | 8.2 ± 0.2 c | |
| AF | 291.6 ± 17.0 b | 966.4 ± 96.6 a | 1258.0 ± 109.5 a | 23.3 ± 1.2 a | 8.4 ± 0.2 c | |
| 1.5AF | 305.3 ± 39.5 ab | 1068.2 ± 123.4 a | 1373.6 ± 146.4 a | 22.3 ± 2.3 a | 8.2 ± 0.4 c | |
| 2AF | 306.5 ± 14.6 ab | 1104.0 ± 212.7 a | 1410.5 ± 210.9 a | 22.0 ± 3.0 a | 7.9 ± 0.3 c | |
| 2.5AF | 339.2 ± 15.5 a | 1120.5 ± 46.9 a | 1459.7 ± 60.1 a | 23.3 ± 0.6 a | 7.8 ± 0.4 c |
Note: C: control group with no fertilizer application; 1.5A: Chinese milk vetch incorporation control, incorporating 2.25 t/hm2 of Chinese milk vetch; F: chemical fertilizer control group with no Chinese milk vetch incorporation; AF: application of chemical fertilizer with 1.50 t/hm2 of Chinese milk vetch; 1.5AF: application of chemical fertilizer with 2.25 t/hm2 of Chinese milk vetch; 2AF: application of chemical fertilizer with 3.00 t/hm2 of Chinese milk vetch; 2.5AF: application of chemical fertilizer with 3.75 t/hm2 of Chinese milk vetch. Lowercase letters that differ suggest significant variations across treatment groups at the 5% alpha level, n = 3.
Table 4.
Yield-related traits of rice grown in purple tidal mud substrate with varying amounts of Chinese milk vetch (Astragalus sinicus L.) incorporation.
Table 4.
Yield-related traits of rice grown in purple tidal mud substrate with varying amounts of Chinese milk vetch (Astragalus sinicus L.) incorporation.
| Rice Season | Treatment | Spike Number (×104/hm2) | Grain Number per Spike (Grain/Spike) | Thousand Grain Weight (g) |
|---|---|---|---|---|
| Early rice in 2020 | C | 190.5 c | 22.7 b | 26.2 a |
| 1.5A | 195.2 c | 77.9 a | 23.7 ab | |
| F | 214.3 bc | 86.2 a | 24.0 ab | |
| AF | 280.9 ab | 78.1 a | 22.9 b | |
| 1.5AF | 280.9 ab | 76.6 a | 23.6 ab | |
| 2AF | 319.0 a | 79.5 a | 22.9 b | |
| 2.5AF | 319.0 a | 84.4 a | 23.8 ab | |
| Late rice in 2020 | C | 123.8 c | 41.8 b | 19.2 a |
| 1.5A | 133.3 c | 29.1 b | 18.6 a | |
| F | 171.4 b | 84.3 a | 18.4 a | |
| AF | 161.9 b | 83.4 a | 19.4 a | |
| 1.5AF | 185.7 ab | 81.9 a | 19.2 a | |
| 2AF | 200.0 a | 71.6 a | 19.6 a | |
| 2.5AF | 204.8 a | 75.2 a | 19.7 a | |
| Early rice in 2021 | C | 64.0 d | 22.1 a | 20.7 a |
| 1.5A | 176.3 c | 36.5 a | 21.3 a | |
| F | 238.3 b | 40.8 a | 19.4 a | |
| AF | 281.0 a | 44.4 a | 19.7 a | |
| 1.5AF | 276.3 a | 27.7 a | 18.4 a | |
| 2AF | 304.7 a | 37.3 a | 20.3 a | |
| 2.5AF | 309.3 a | 42.5 a | 19.7 a | |
| Late rice in 2021 | C | 123.0 c | 22.1 a | 23.2 ab |
| 1.5A | 129.0 c | 36.5 a | 21.8 ab | |
| F | 190.3 ab | 40.8 a | 21.3 ab | |
| AF | 223.7 a | 44.4 a | 21.4 ab | |
| 1.5AF | 205.0 ab | 27.7 a | 20.9 ab | |
| 2AF | 171.3 bc | 37.3 a | 23.8 a | |
| 2.5AF | 190.3 ab | 42.5 a | 20.3 b |
Note: C: control group with no fertilizer application; 1.5A: Chinese milk vetch incorporation control, incorporating 2.25 t/hm2 of Chinese milk vetch; F: chemical fertilizer control group with no Chinese milk vetch incorporation; AF: application of chemical fertilizer with 1.50 t/hm2 of Chinese milk vetch; 1.5AF: application of chemical fertilizer with 2.25 t/hm2 of Chinese milk vetch; 2AF: application of chemical fertilizer with 3.00 t/hm2 of Chinese milk vetch; 2.5AF: application of chemical fertilizer with 3.75 t/hm2 of Chinese milk vetch. Lowercase letters that differ suggest significant variations across treatment groups at the 5% alpha level, n = 3.
Table 5.
Soil DTPA-Zn (mg/kg).
| Treatment | 30 June 2020 | 12 November 2020 | 7 July 2021 | 27 October 2021 |
|---|---|---|---|---|
| C | 0.34 ± 0.006 d | 0.42 ± 0.061 c | 0.32 ± 0.010 d | 0.36 ± 0.012 f |
| 1.5A | 0.30 ± 0.015 e | 0.61 ± 0.071 ab | 0.44 ± 0.010 b | 0.69 ± 0.012 b |
| F | 0.26 ± 0.006 f | 0.52 ± 0.023 bc | 0.33 ± 0.010 d | 0.53 ± 0.017 e |
| AF | 0.34 ± 0.006 d | 0.60 ± 0.038 ab | 0.47 ± 0.010 a | 0.59 ± 0.021 d |
| 1.5AF | 0.36 ± 0.006 c | 0.63 ± 0.021 a | 0.37 ± 0.010 c | 0.64 ± 0.010 c |
| 2AF | 0.39 ± 0.006 b | 0.64 ± 0.049 a | 0.38 ± 0.010 c | 0.54 ± 0.017 e |
| 2.5AF | 0.46 ± 0.012 a | 0.67 ± 0.079 a | 0.38 ± 0.006 c | 0.73 ± 0.010 a |
Note: C: control group with no fertilizer application; 1.5A: Chinese milk vetch incorporation control, incorporating 2.25 t/hm2 of Chinese milk vetch; F: chemical fertilizer control group with no Chinese milk vetch incorporation; AF: application of chemical fertilizer with 1.50 t/hm2 of Chinese milk vetch; 1.5AF: application of chemical fertilizer with 2.25 t/hm2 of Chinese milk vetch; 2AF: application of chemical fertilizer with 3.00 t/hm2 of Chinese milk vetch; 2.5AF: application of chemical fertilizer with 3.75 t/hm2 of Chinese milk vetch. Lowercase letters that differ suggest significant variations across treatment groups at the 5% alpha level, n = 3.
Table 6.
The correlation between DTPA-Zn concentration in purple tidal substrate and zinc content and zinc uptake in rice.
Table 6.
The correlation between DTPA-Zn concentration in purple tidal substrate and zinc content and zinc uptake in rice.
| Correlation Coefficient | Zinc Concentration in Rice Grain | Zinc Concentration in Rice Straw | Grain Zn Uptake | Straw Zn Uptake | Shoot Zn Uptake |
|---|---|---|---|---|---|
| DTPA-Zn | −0.269 * | −0.731 ** | −0.228 * | −0.456 ** | −0.428 ** |
* p < 0.05, ** p < 0.01.
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MDPI and ACS Style
Yang, Z.; Rao, Z.; Li, H.; Long, Z.; Zeng, X.; Xie, J. Effect of Chinese Milk Vetch on Zinc Content and Zinc Absorption of Rice in Purple Tidal Mud Soil. Agronomy 2024, 14, 1997. https://doi.org/10.3390/agronomy14091997
AMA Style
Yang Z, Rao Z, Li H, Long Z, Zeng X, Xie J. Effect of Chinese Milk Vetch on Zinc Content and Zinc Absorption of Rice in Purple Tidal Mud Soil. Agronomy. 2024; 14(9):1997. https://doi.org/10.3390/agronomy14091997
Chicago/Turabian StyleYang, Zengping, Zhongxiu Rao, Hailu Li, Zedong Long, Xianjun Zeng, and Jian Xie. 2024. "Effect of Chinese Milk Vetch on Zinc Content and Zinc Absorption of Rice in Purple Tidal Mud Soil" Agronomy 14, no. 9: 1997. https://doi.org/10.3390/agronomy14091997
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