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Article

Limited Advantages of Green Manure Planting on Soil Nutrients and Productivity in Intensive Agriculture: A Case Study of Wheat–Maize–Sunflower Rotation in Hetao Irrigation District

by
Na Zhao
1,2,
Jun Zhang
3,
Xiaohong Li
2,
Jun Ma
2,
Jufeng Cao
2,
Hanjiang Liu
2,
Xiquan Wang
1,
Lanfang Bai
1 and
Zhigang Wang
1,*
1
College of Agronomy, Inner Mongolia Agricultural University, Hohhot 010019, China
2
Bayannur Academy of Agricultural and Animal Sciences, Linhe 015400, China
3
Inner Mongolia Academy of Agricultural & Animal Husbandry Sciences, Hohhot 010031, China
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(1), 100; https://doi.org/10.3390/agronomy14010100
Submission received: 5 December 2023 / Revised: 21 December 2023 / Accepted: 28 December 2023 / Published: 30 December 2023
(This article belongs to the Section Innovative Cropping Systems)
Browse Figures
Versions Notes

Abstract

:
Crop diversification has been proposed as a promising strategy for transitioning towards sustainable agricultural systems. The Hetao Irrigation District faces multiple challenges in ensuring food security and farmer livelihood. A diversified green manure inclusive cropping system was designed to evaluate the influences of hairy vetch (Vicia villosa Roth) incorporation on soil nutrients, yield, and economic benefit compared to the existing wheat–maize–sunflower rotation system in the Hetao Irrigation District. The specific method was as follows: spring wheat (Triticum-durum L.)–vetch rotation in the first year, maize (Zea mays L.)/vetch intercropping in the second year, and sunflower (Helianthus annuus L.)/vetch relay intercropping in the third year. Over a 6-year experimental observation, green manure incorporation significantly increased soil alkaline hydrolyzed nitrogen and exchangeable potassium levels. Although the effect of green manure incorporation on the yield of rotated spring wheat and intercropped maize was not prominent, it improved sunflower yield in the second rotation cycle by 11%. Consequently, only the sunflower led to an increase in income of 235 CNY ha−1 in the second rotation cycle. This indicates that the soil-property-building and yield-enhancing effects of green manure incorporation require time to accumulate. However, the benefits derived from increased yield were not sufficient to offset the production costs associated with green manure cultivation, resulting in less-than-desirable economic returns.

1. Introduction

Enhancing biodiversity in agroecosystems has the potential to promote agricultural sustainability [1,2]. Crop rotation and intercropping are common practices that contribute to crop diversification either temporally or spatially. Previous studies have demonstrated that appropriate crop rotation enhances the provisioning of various ecosystem services in agroecosystems [3,4]. For example, crop rotation can improve soil quality, reduce the occurrence of pests and weeds, and enhance nutrient cycling efficiency [5,6,7]. Furthermore, well-designed crop rotation and intercropping systems can increase the resilience and adaptability of crops, thereby improving the stability of agroecosystems. Diversification through the inclusion of green manure crops in cropping systems represents a critical strategy for enhancing soil nutrients and productivity [8,9]. Additionally, incorporating nitrogen (N)-fixing legume species as a green manure crop can improve the N nutrition of the succeeding main crops, increase the organic N pool in the soil, and reduce the need for external N inputs. Consequently, the green manure crops can contribute to more sustainable agriculture by alleviating weed and crop nutrition issues. Despite the advantages, green manure crops are not widely adopted by farmers due to their additional costs and labor requirements. Moreover, the effects of green manure crops on productivity, crop nutrition, and weed control are variable and depend on crop species, soil type, climate, and cropping system complexity [10,11,12]. This indicates the need for specific experimental verifications to determine the effects of green manure cropping. However, to date, only a few replicated randomized field experiments have been conducted to test the effects of green manure crops in diversified rotation systems.
Adjusting and optimizing crop rotation and intercropping practices require careful consideration of specific factors such as geographical environment, climate conditions, crop varieties, and market demands. The Hetao Irrigation District (HID) is a typical irrigation district where spring wheat (Triticum-durum L.), maize (Zea mays L.), and sunflower (Helianthus annuus L.) are the predominant crops [13]. The three crops cover an area of more than 500,000 hectares. Because of continuous industrial fertilizer use, the unscientific use of cropping systems can lead to crop failure. Sunflower possesses strong drought tolerance, salt–alkali resistance, and adaptability to poor soils, making it economically valuable and widely cultivated by farmers. However, sunflower is not suitable for continuous monoculture: its yield may significantly decline after monoculture for three years. Considering the unsuitability of sunflower for continuous cropping, a diversified rotation system, namely wheat–maize–sunflower, has been established in the region. Notably, the HID faces multiple challenges in ensuring food security, including soil secondary salinization and high inputs of irrigation and fertilization [14,15]. Further optimizing the existing cropping system, such as through the introduction of leguminous green manure, represents a potential pathway towards achieving sustainable agricultural development in the region. Hairy vetch (Vicia villosa Roth) is a common green manure crop that offers advantages such as N2 fixation through symbiosis and soil quality improvement. With the development of agriculture in the HID and the increasing attention to sustainable agriculture, more farmers may consider incorporating leguminous green manure crops into their cropping systems. Numerous studies have highlighted the ability of green manure crops to significantly reduce nitrate leaching by capturing soil mineral nitrogen (SMN), thus providing services such as mitigating nitrate water pollution and recycling N through the release of residual mineral N acquired by green manure crops to the subsequent cash crop after incorporation into the soil [16,17]. However, whether and how the use of hairy vetch in a wheat–maize–sunflower system influences soil nutrients and the main crop production are still poorly understood.
Green manure cropping has been successfully implemented in various intensive agricultural systems worldwide, such as grain crop rotations in northern Europe, soybean (Glycine max L.)–maize rotations in North American, wheat–hay rotations in arid regions in Australia, and low-input grain systems in Africa [18,19,20,21]. China has a long history of applying green manure crops in crop rotation and intercropping. This cropping system is still widely used today, including, for example, the use of Chinese milk vetch (Astragalus sinicus L.) as green manure in rice-based cropping systems [22]. However, the effects of green manure crops vary significantly among studies due to the differences in agricultural practices, soil types, and climates. Previous studies have demonstrated that planting green manure crops after spring wheat harvest is feasible based on local temperature and light conditions. However, a two-year experiment showed limited effects on soil nutrients and the subsequent wheat yield [23]. Maize intercropping with green manure is a promising strategy to enhance maize production while reducing fertilizer inputs, but the effects vary widely depending on green manure species and experimental year [24]. The symbiotic periods between the main crop (i.e., maize and sunflower) and the intercrop (i.e., hairy vetch) is another factor influencing the quantity and quality of green manure [25], which further affects the potential benefits for intercropped or subsequent crops [18]. Therefore, different environments and management practices contribute significantly to this variability and have important implications for the response of the main crops to green manure. It is generally recognized that green manure crops achieve maximum biomass during their flowering stage, which is enhanced when green manure is incorporated into the soil. Considering the different optimal growth seasons and durations of various main crops (e.g., spring wheat, maize, and sunflower), it is necessary to consider the symbiotic period to maximize the biomass of the green manure. Specifically, the symbiotic period between hairy vetch and sunflower is shorter than that with maize to efficiently utilize light and temperature resources. Therefore, a three-year cropping system of spring wheat–maize–sunflower was designed with and without hairy vetch planting to investigate its influences o soil nutrients and productivity.
Therefore, the purposes of this study were to evaluate: (1) the influences of green manure planting on soil organic matter (SOM), alkaline hydrolyzed nitrogen (AN), available phosphorus (AP), and exchangeable potassium (EK); (2) the response of grain yield of spring wheat, maize, and sunflower to green manure planting and rotation cycles; and (3) the economic feasibility of incorporating annual green manure planting into the wheat–maize–sunflower rotation system. We hypothesized that green manure planting enhances soil nutrients while simultaneously increasing crop yields and economic benefits.

2. Materials and Methods

2.1. Site Description

The experiment site was located in the Yuanzi Drainage Experimental Station (37°27′–37°47′ N, 116°19′–116°24′ E, altitude 1035 m), Academy of Agricultural & Animal Husbandry Sciences Bayannur City, Inner Mongolia, China, and the experiment was conducted from March 2015 to October 2020. The average annual air temperature was 3.7–7.6 °C in this region, with a midtemperate continental monsoon climate zone. There were 3100–3300 h of sunshine and 126 days without frost on average per year. The mean annual precipitation and evaporation were 188 mm and 2030–3180 mm, respectively [26]. During the spring wheat growing season in 2015 and 2018, the precipitation was 24.6 mm and 34.0 mm, respectively, and during the subsequent vetch growing season, it was 48.7 mm and 105.8 mm. In 2016 and 2019, the maize growing season had precipitation of 116.9 mm and 89.6 mm, respectively, while the intercropped vetch growing season received 52.6 mm and 62.6 mm of rainfall. In 2017 and 2020, the sunflower growing season experienced precipitation of 168.0 mm and 151.2 mm, respectively, with 54.1 mm and 83.6 mm during the intercropped vetch growing season (Figure 1). The soil at the experimental site was irrigated and wrapped, and the soil was silt loam. Table 1 describes the basic soil nutrients in the 0–20 cm and 20–40 cm soil layers.

2.2. Experimental Design

The field experiment was conducted in a randomized block design with three replicates, including two treatments: (i) the wheat–maize–sunflower rotation system from 2015 to 2020; (ii) green manure (hairy vetch,) planting with spring wheat-vetch rotation in the first year, maize/vetch intercropping in the second year, and sunflower/vetch relay intercropping in the third year (Figure 2). Green manure crops were grown until the full-bloom stage, and then plants were cut into 2–3 cm pieces before being incorporated back into the field using a rotary tillage machine. Sole-cropped green manure biomass reached 2339 kg ha−1 and 2663 kg ha−1 in 2015 and 2018, respectively, after spring wheat harvest. However, when intercropped with maize, the biomass decreased to 470 kg ha−1 and 458 kg ha−1 in 2016 and 2019, respectively. For relay intercropping with sunflower, the biomass was 1344 kg ha−1 and 832 kg ha−1 in 2017 and 2020, respectively. The average nutrient contents in the green manure, including C, N, P2O5, and K2O, were 61.6%, 3.8%, 0.35%, and 3.03%, respectively.
Each plot was 40 m2 (10 m × 4 m). The temporal and spatial arrangement of cropping systems are shown in Figure 1 and Figure 2, respectively. Spring wheat was sown with a row spacing of 15 cm, using a sowing rate of 375 kg ha−1. Maize was sown with a plant distance of 24 cm within a row, at a density of 75,000 plants ha−1. Sunflower was sown with a plant distance of 50 cm within a row, at a density of 33,000 plants ha−1. Both maize and sunflower were grown with plastic film mulching. Residues of spring wheat, maize, and sunflower were completely removed from the field. Plough tillage was performed before irrigation the previous autumn, and rotary tillage was conducted before sowing. The fertilization rates in the wheat field were 225 kg N ha−1, 120 kg P2O5 ha−1, and 90 kg K2O ha−1; maize and sunflower were fertilized with 270 kg N ha−1, 120 kg P2O5 ha−1, and 90 kg K2O ha−1. The fertilizers were urea, diammonium phosphate, and potassium sulfate, respectively. In each case, 30% of the N fertilizer, 100% of the P fertilizer, and 70% of the K fertilizer were applied a basal fertilizer, while the remainder was applied as topdressing before irrigation at the jointing stage of spring wheat and maize, or at the squaring stage of sunflower. The spring wheat variety used was Yongliang 4, while Ximeng 568and SH363 were the varieties of maize and sunflower, respectively. Surface irrigation was applied with volumes of 382.5 mm, 472.5 mm, and 315.0 mm for spring wheat, maize, and sunflower, respectively. The dates and volume of each irrigation, as well as the sowing and harvest dates and other field managements practices, are provided in Table S1.

2.3. Sampling and Analytical Procedures

2.3.1. Soil Nutrients Data Measurement

On 10 October, 2015–2020, soil samples were collected from the 0–20 cm soil depth between two rows of crops using a soil auger, following the harvests of all crops. Five subsamples were pooled and fully mixed as one soil sample for each plot. The soil samples were naturally air-dried and manually sieved (<2 mm) for subsequent determination of soil nutrients. SOM and AN were measured via K2Cr2O7-FeSO4 oxidation and alkaline hydrolysis diffusion methods, respectively [27]. AP was extracted with NaHCO3 (0.5 mol/L), and quantified using a continuous flow analyzer (Skalar + Analytical, Breda, Netherlands). EK was extracted with NH4OAc (1 mol/L) and determined using flame photometry (BWB Technologies, Newbury, UK).

2.3.2. Yield Measurement

Grain yield, aboveground biomass, and harvest index were measured from the whole area of each plot at the maturity of wheat, maize, and sunflower. Specifically, plants were cut at the ground level and air-dried until they reached a constant weight. Then, all the plant samples were separated into grains and residues, and aboveground biomass and grain yield (air-dried) were measured. Harvest index is the ratio of grain yield to aboveground biomass at maturity and is calculated as:
Harvest index = Grain yield/Aboveground biomass
For each plot, a random sampling approach was employed to select 20 wheat plants, 5 maize plants, or 5 sunflower plants for yield component analysis, including grain number and grain weight. During each growing season, a comprehensive yield assessment was conducted for the entire field at the full-bloom stage of the vetch green manure. Additionally, 10 vetch plants were sampled separately to determine their moisture content, which was used to calculate the dry weight of the vetch green manure biomass.

2.3.3. Economic Analyses

In this study, the total input includes seed, fertilizer, herbicide, film, irrigation, machinery, and human labor. The net income was calculated as the difference between output and input, where the output represents the gains of wheat, maize, and sunflower. The prices of all inputs and outputs were collected and are shown in Section 3.3.

2.4. Statistical Analyses

The Shapiro and Levene tests were used to check the normality and homogeneity of all data, respectively. The effects of green manure planting and experimental year on soil organic matter, alkaline hydrolyzed nitrogen (N), available phosphorus (P), and exchangeable potassium (K) were tested with two-way analysis of variance (ANOVA). Two-way ANOVA was also used for grain yield and yield components considering green manure planting and rotation cycle in combination with Fisher’s least significant difference (LSD) test at a significance level of p < 0.05. The Pearson’s correlation of grain yield and yield components, yield traits and soil parameters were performed using SPSS version 21.0 (IBM SPSS Software Inc., Armonk, NY, USA). Statistical analyses were performed using Data Processing System (DPS) software version 7.05 [28].

3. Results

3.1. The Effects of Soil Nutrients

The SOM, AN, AP, and EK of the system with GM were all higher than those of the system without GM in 2018–2020. The introduction of hairy vetch as a green manure in the wheat–maize–sunflower rotation system showed a tendency toward increasing the SOM content, although no significant differences were observed until the sixth year of the experiment (Figure 3a). The AN content in the soil with green manure incorporation significantly increased over the six-year experimental period, from 71.1 mg kg−1 to 95.8 mg kg−1 (Figure 3b, p < 0.001). The use of green manure significantly affected the soil AN content (p < 0.05). In 2020, the soil AN content with green manure was 19% higher than that in the system without green manure. The application of green manure did not have a significant effect on AP, even after six years of incorporation (Figure 3c). However, the planting of green manure resulted in 12% and 14% increases in soil EK by in 2019 and 2020, respectively (Figure 3d, p < 0.05).

3.2. Grain Yield and Yield Components

The application of green manure and its interaction with the rotation cycle significantly influenced the grain yield of sunflower (Figure 4c, p < 0.05). In the first rotation cycle, sunflower intercropped with green manure produced a similar grain yield to the system without green manure. However, in the second rotation cycle, the sunflower intercropped with green manure exhibited an increase of 11% in grain yield compared to the system without green manure. On the other hand, both spring wheat and maize demonstrated similar grain yields in the two rotation cycles, regardless of the presence or absence of green manure (Figure 4a,b). As a result, there was no significant increase in the wheat-equivalent yield of the wheat–maize–sunflower system (Figure 4d).
In the second rotation cycle, the number of heads in the wheat–maize–sunflower system was found to be 4% lower than in the first rotation cycle (Table 2, p < 0.01). Additionally, there was a negative correlation observed between the number of heads and the grain yield of sunflower (Table 3, p < 0.05). Furthermore, intercropping sunflower with green manure resulted in a 14% increase in the 100–seed weight and a 6% increase in the harvest index (Table 2, p < 0.05).

3.3. Economic Analyses

The implementation of the wheat–maize–sunflower rotation system with green manure required additional inputs in terms of seeds and machinery (including sowing, rotary tillage, and soil rolling) for vetch planting (Table 4). Consequently, the total input costs of green manure incorporation in the spring wheat rotation, maize intercropping, and sunflower relay intercropping were 3075, 2325, and ¥325 CNY ha−1 higher, respectively, compared to those of the system without green manure planting. As a result of the limited yield advantages achieved with green manure planting, the net incomes in spring wheat and maize seasons decreased by 2755–3075 and 1695–1845 CNY ha−1 respectively. However, in the second rotation cycle, the sunflower grain yield increased, leading to a net income decrease of 1725 CNY ha−1 in the first rotation cycle but an increase of 235 CNY ha−1 in the second rotation cycle.

4. Discussion

This study focused on evaluating the impact of incorporating green manure into a diversified crop rotation system on soil nutrients, crop yield, and economic benefits over a six-year field experiment. We found that different soil nutrient indicators exhibited varying response trends to green manure planting. The soil-nutrient-building effect of green manure incorporation showed cumulative effects over time, as some indicators only showed significant differences in the fifth or sixth year of the experiment. Crop yields in the diversified crop rotation system exhibited significant variations in response to green manure planting. In this particular pedoclimatic region, green manure planting had no significant effect on spring wheat or maize yields, while it significantly increased the yield of sunflower. Due to the increase in production inputs associated with the planting of green manure crops and the fact that not all crop yields significantly increased, green manure planting was not economically beneficial.

4.1. Effects of Green-Manure-Inclusive Diversified Rotation on Soil Nutrients

Through a continuous six-year study of a diversified crop rotation system with the addition of green manure, it was observed that the SOM and AP contents in the soil increased, while the soil AN and EK significantly improved. Changes in SOM require long-term processes. In this study, the SOM in the diversified crop rotation without green manure remained relatively stable, while the SOM in the diversified crop rotation with annual green manure incorporation increased each year. This may have been due to the relatively short duration of the experiment, where significant differences in SOM were not observed. Green manure cropping has a significant advantage in increasing annual carbon inputs into soils, which may result in higher soil C levels over time [20]. However, there is often a time lag in soil nutrient changes, and the effects of cover crops on soil health may take several years or even longer to become evident after adoption [29,30]. Furthermore, green manure crops do not always increase SOM in semiarid environments, even when supplementary irrigation is used, which is consistent with the findings of this study [20]. While this is less applicable to regions in Europe and North America, which have young and fertile soils, it is relevant to the highly weathered soils of the southern Australian cropping zone, where SOM accrual is often limited by nutrient availability [20]. This perspective also explains the nonsignificant increase in SOM observed in this study after six years of green manure planting.
Regarding AN, different types of green manure have different nutrient effects. Legume green manure more significantly increased nitrate and hydrolyzable nitrogen, while nonlegume green manure more markedly increased available potassium, showing that in the subsequent maize growing season, green manure crops increased soil mineral nitrogen levels by an average of 45 kg N ha−1 compared to fallow [31,32]. However, sufficient biomass accumulation is required for green manure crops to influence soil nitrogen availability [33]. In this study, compared to other years, maize intercropped with green manure in 2016 and 2019 had the lowest biomass, resulting in the smallest changes in soil AN after incorporation. In prior controlled field experiments, researchers have paid less attention to the AP and EK in the soil after green manure incorporation. Some studies showed that the long-term application of green manure can improve soil phosphorus availability and exchangeable potassium [19]. Green manure incorporation provides exogenous organic matter to the soil, which, after decomposition, promotes the mineralization of organic matter, conversion of insoluble nutrients, and nutrient cycling in the soil. However, further research is needed to understand the long-term effects of green manure incorporation on soil nutrients and how diversified crop rotation affects the fertilizer effect of green manure.

4.2. Effect of Green-Manure-Inclusive Diversified Rotation on Crop Yield

In this study, the incorporation of green manure and its impact on spring wheat and maize yields in a wheat–maize–sunflower rotation system was minimal. However, compared to the no-green-manure treatment, the incorporation of green manure significantly increased sunflower yield by 11.1% in the second rotation cycle. Regional and national-level meta-analyses have shown inconsistent effects of green manure on wheat yield in northern China [31] or no significant yield benefits in temperate wheat systems in China [34]. The variation in yield effects is mainly associated with regional climate characteristics, particularly annual precipitation levels. Green manures utilize soil water during the summer months, and this water depletion may negatively impact spring wheat production in ecoregions with limited water resources. For example, similar results have been observed in regions with climatic similarities, such as the southern Loess Plateau of China [35], southern Great Plains of America [21], and southern semiarid region of Australian [20], where wheat grain yields were reduced in response to green manure compared to summer fallow during dry years, but no differences were observed during wet or normal rainfall years.
In the case of a legume green manure, such as hairy vetch, which can provide a significant portion of the N required by the subsequent crop, late termination of the manure crop is usually recommended to allow for higher nitrogen accumulation in the biomass and better synchronization of nitrogen release during decomposition and subsequent crop uptake [36,37]. However, early termination of the green manure crop may be appropriate in situations where rainfall is limited and the depletion of soil moisture reserve by the manure crop is a concern [20,38]. Furthermore, the benefits derived from legume planting are greatly influenced by soil fertility, fertilization practices, and rotation complexity [39]. In this study, the conventional crop rotation exhibited a high level of crop diversity, which to some extent reduced the yield effects of legume green manure. For example, in the United States of America and Canada, legume green manure increased crop yield by 37% when no nitrogen fertilizer was applied, and this benefit decreased with the application of nitrogen fertilizer [40,41].
Numerous studies have demonstrated that intercropping enhances yield per unit land, net returns, and reduces environmental impact on a large-scale and long-term basis [14,39,42]. However, the yield advantage of intercropping is achieved within the same land area unit, and, when compared to sole cropping, intercropped crop yields are often lower due to interspecific competition. In this study, intercropped maize with green manure achieved comparable yields to monoculture, which may be attributed to interspecific complementarity. The planting of hairy vetch improved soil nitrogen supply through nitrogen fixation with rhizobia, thereby compensating for the adverse effects of interspecific competition for light on maize. This finding is supported by the results that the negative effect of nitrogen reduction on maize grain yield was compensated for by intercropping common vetch green manure in northwestern China [24]. Early establishment through under sowing can improve green manure growth and plant nitrogen accumulation, but competition with the main crop should be minimized [33]. We found that the incorporation of green manure significantly increased sunflower yield by 11.1% in the second rotation cycle. This is partly since sunflower is more susceptible to continuous monoculture than spring wheat and maize, resulting in a significant yield increase after the introduction of green manure rotation. Additionally, early under owing can improve green manure growth and nitrogen accumulation, and a well-established legume-based green manure can help stabilize crop yield over time [33]. Therefore, the yield benefits of green manure require time to accumulate, and, in future research, long-term monitoring of the effects of green manure incorporation is needed to strengthen our understanding of its long-term impact.

4.3. Effect of Green Manure Inclusive Diversified Rotation on Economic Return

While the promotion of green manure crops has gained significant attention in China in recent years [34], understanding the factors influencing the adoption of farming practices is crucial for effectively promoting green manure cropping among farmers and harnessing the conservation and ecosystem service benefits it provides. Among the numerous influencing factors, the availability and amount of financial incentives were found to have the strongest impact, accounting for 83.9%, followed by investment in cover crops [43]. In our study, compared to the conventional wheat–maize–sunflower rotation without green manure, the increased costs associated with green-manure-inclusive diversified rotation were mainly attributed to green manure seeds and field management practices such as sowing and tillage. The inclusion of green manure in the diversified rotation led to a minor impact on yield and revenue, resulting in lower net income compared to not growing green manure. When compared to monoculture and postwheat reseeding of green manure, the intercropping planting model required increased investment in seeds, labor, and machinery costs, while the postwheat reseeding of green manure reduced economic benefits compared to those of monoculture wheat. In the conventional planting model, the integration of green manure crops resulted in increased costs associated with labor and other inputs such as sowing and incorporation. In addition to production costs, cash crop yield is a crucial factor influencing the willingness to adopt green manure cropping [44]. However, except for sunflower, we did not observe a significant yield increase in the main crops with the inclusion of green manure. Nevertheless, the soil fertility improvement effects associated with the inclusion of green manure may promote long-term stability in main crop production and enhance resilience to climate fluctuations [29]. Although the incorporation of green manure increased costs in our study, it played a beneficial role in soil improvement and the intercropping with green manure increased biodiversity, making it more environmentally friendly. Considering the relatively low economic profits, green manure can be used for purposes such as fodder. It is worth investigating the impact of reducing the amount of green manure returned to the field on the main crops. By harvesting a portion of the green manure without affecting yields, the overall economic benefits of the system could be increased.

5. Conclusions

Limited advantages were observed in soil nutrients and productivity through the use of green manure planting in an intensive agriculture system. In this study, we investigated the effects of green manure incorporation on soil nutrients and crop yield in the wheat–maize–sunflower rotation system in the Hetao Irrigation District. The incorporation of green manure had a positive impact on soil nutrients. Over the course of a 6-year experimental observation, green manure incorporation significantly increased soil AN and EK levels. Although the effect of green manure incorporation on the yield of rotated spring wheat and intercropped maize was not significant, it significantly improved sunflower yield in the second rotation cycle. This indicates that the soil-nutrient-building and yield-enhancing effects of green manure incorporation require time to accumulate. However, the benefits derived from increased vied were not sufficient to offset the production costs associated with green manure cultivation, resulting in less-than-desirable economic returns. In future research, it is necessary to strengthen the study of the long-term effects of green manure incorporation on soil nutrients and yield. Additionally, more attention should be given to the assessment of ecological service benefits in order to better balance the trade-off between increased crop production and ecological service provisions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy14010100/s1, Table S1: Field operations of the wheat–maize–sunflower rotation system with and without green manure over six experimental years.

Author Contributions

Performed the experiments: N.Z., X.L., H.L., J.M. and J.C. Analyzed the data: N.Z., J.Z., X.W., L.B. and Z.W. Revised the manuscript critically for important intellectual content: Z.W. Wrote the paper: N.Z. and Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Inner Mongolia Natural Science Foundation (No. 2022QN03032), the earmarked fund for CARS-Green manure (CARS-22), and the Inner Mongolia Science and Technology Plan Project (No. 2023YFHH0011).

Data Availability Statement

The data are available in the article and the Supplementary Material.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Daily precipitation (vertical bars) and mean air temperature (solid curves) in the spring wheat (a), maize (b), and sunflower (c) seasons. The growing periods of spring wheat, maize, sunflower, and vetch are labeled.
Figure 1. Daily precipitation (vertical bars) and mean air temperature (solid curves) in the spring wheat (a), maize (b), and sunflower (c) seasons. The growing periods of spring wheat, maize, sunflower, and vetch are labeled.
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Figure 2. Graphic description of wheat–maize–sunflower rotation system with (b) and without (a) green manure planting. Vetch was sole-cropped after spring wheat harvest, while it was intercropped with maize and sunflower.
Figure 2. Graphic description of wheat–maize–sunflower rotation system with (b) and without (a) green manure planting. Vetch was sole-cropped after spring wheat harvest, while it was intercropped with maize and sunflower.
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Figure 3. The soil nutrients of (a) organic matter (SOM), (b) alkaline hydrolyzed nitrogen (N), (c) available phosphorus (P), and (d) exchangeable potassium (K) contents with and without green manure (GM) in the six experimental years (Y). The p-values were obtained after two−way ANOVA for Y and GM, where ns indicates no significant differences. Values are means ± standard errors (n = 3). The red asterisks indicate significant differences at the p < 0.05 level with and without green manure (GM).
Figure 3. The soil nutrients of (a) organic matter (SOM), (b) alkaline hydrolyzed nitrogen (N), (c) available phosphorus (P), and (d) exchangeable potassium (K) contents with and without green manure (GM) in the six experimental years (Y). The p-values were obtained after two−way ANOVA for Y and GM, where ns indicates no significant differences. Values are means ± standard errors (n = 3). The red asterisks indicate significant differences at the p < 0.05 level with and without green manure (GM).
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Figure 4. The grain yields (oven-dried) of (a) spring wheat, (b) maize, and (c) sunflower, as well as (d) the wheat equivalent yield of wheat−maize−sunflower rotation system with and without green manure (GM) in two rotation cycles (RCs). The p-values were obtained after a two−way ANOVA for RC and GM, where ns indicates no significant differences. Different lowercase letters mean significant differences with and without GM in each RC at p < 0.05 per Fisher’s least significant difference (LSD) test. Values are means ± standard errors (n = 3).
Figure 4. The grain yields (oven-dried) of (a) spring wheat, (b) maize, and (c) sunflower, as well as (d) the wheat equivalent yield of wheat−maize−sunflower rotation system with and without green manure (GM) in two rotation cycles (RCs). The p-values were obtained after a two−way ANOVA for RC and GM, where ns indicates no significant differences. Different lowercase letters mean significant differences with and without GM in each RC at p < 0.05 per Fisher’s least significant difference (LSD) test. Values are means ± standard errors (n = 3).
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Table 1. The initial soil nutrients at the experimental site.
Table 1. The initial soil nutrients at the experimental site.
Soil Depth
(cm)		SOM
(g/kg)		AN
(mg/kg)		AP
(mg/kg)		EK
(mg/kg)		TN
(g/kg)		TP
(g/kg)		TK
(g/kg)		WS (g/kg)pH
0−2013.07326.21300.800.4020.00.588.8
20−409.33611.5630.730.5617.50.528.9
SOM, soil organic matter; AN, alkaline hydrolyzed nitrogen; AP, available phosphorus; EK, exchangeable potassium; TN, total nitrogen; TP, total phosphorus; TK, total potassium, WS, total water−soluble salt.
Table 2. The yield components, aboveground biomass (AGB), and harvest index (HI) of spring wheat, maize, and sunflower with and without green manure (GM) over two cycles of wheat–maize–sunflower rotation system.
Table 2. The yield components, aboveground biomass (AGB), and harvest index (HI) of spring wheat, maize, and sunflower with and without green manure (GM) over two cycles of wheat–maize–sunflower rotation system.
TreatmentCycle 1 (2015–2017)Cycle 2 (2018–2020)Two-Way ANOVA
Without GMWith GMWithout GMWith GMCGC × G
Spring wheat (2015 and 2018)
Spike number (m−2)720 ± 3726 ± 5728 ± 4723 ± 5nsnsns
Grain number (spike−1)34.7 ± 1.236.8 ± 2.236.4 ± 1.932.2 ± 2.4nsnsns
1000-grain weight (g)53.9 ± 1.850.5 ± 0.950.1 ± 3.147.5 ± 0.8nsnsns
AGB (Mg ha−1)13.3 ± 0.313.2 ± 0.113.6 ± 0.413.9 ± 0.3nsnsns
HI0.51 ± 0.000.51 ± 0.010.48 ± 0.010.48 ± 0.01**nsns
Maize (2016 and 2019)
Ear number (m−2)7.40 ± 0.267.33 ± 0.047.20 ± 0.307.20 ± 0.36nsnsns
Kernel number (ear−1)654 ± 29670 ± 29634 ± 19667 ± 13nsnsns
100-kernel weight (g)35.1 ± 0.735.3 ± 0.733.4 ± 0.2 b36.6 ± 1.3 ansnsns
AGB (Mg ha−1)30.4 ± 0.933.3 ± 0.129.2 ± 0.9 b32.3 ± 1.6 ans*ns
HI0.50 ± 0.01 a0.47 ± 0.01 b0.52 ± 0.02 a0.47 ± 0.02 bns*ns
Sunflower (2017 and 2020)
Head number (m−2)3.05 ± 0.063.08 ± 0.073.00 ± 0.122.86 ± 0.02**nsns
Seed number (head−1)970 ± 931128 ± 991253 ± 1511467 ± 30nsnsns
Seed setting rate (%)88.7 ± 3.488.5 ± 183.6 ± 0.586.3 ± 2.3nsnsns
100-seed weight (g)17.7 ± 0.918.8 ± 0.823.4 ± 1.2 b26.6 ± 1.3 a*nsns
AGB (Mg ha−1)12.7 ± 0.213.2 ± 0.111.3 ± 0.511.7 ± 0.1nsnsns
HI0.29 ± 0.000.28 ± 0.000.33 ± 0.01 b0.35 ± 0.01 a****
Different lowercase letters within a row of the same rotation cycle represent significant differences at p < 0.05 per he Fisher’s least significant difference (LSD) test. Values are means ± standard errors (n = 3). C, rotation cycle; G, green manure; C × G, interaction between rotation cycles and green manure. *, p < 0.05; **, p < 0.01; ns, not significant difference. The same applies to the tables below.
Table 3. Pearson’s correlation coefficients between grain yield and yield components for spring wheat, maize, and sunflower.
Table 3. Pearson’s correlation coefficients between grain yield and yield components for spring wheat, maize, and sunflower.
Wheat Yield (2015 and 2018)Maize Yield (2016 and 2019)Sunflower Yield (2017 and 2020)
Spike number−0.17Ear number0.67 *Head number−0.68 *
Grain number−0.04Kernel number0.14Seed number0.57
1000-grain weight0.25100–kernel weight−0.35100–seed weight0.03
AGB0.53AGB0.65 *AGB0.58 *
HI0.42HI0.60 *HI−0.08
Seed setting rate0.03
Spike number, grain number, 1000–grain weight, aboveground biomass (AGB), and harvest index (HI) were tested for spring wheat. Ear number, kernel number, 100-kernel weight, AGB, and HI were analyzed for maize. Head number, seed number, 100–seed weight, AGB, HI, and seed setting rate were used for sunflower. *, p < 0.05.
Table 4. Inputs and outputs of wheat, maize, and sunflower with and without green manure (GM) across two cycles of wheat–maize–sunflower rotation system.
Table 4. Inputs and outputs of wheat, maize, and sunflower with and without green manure (GM) across two cycles of wheat–maize–sunflower rotation system.
ItemSpring WheatMaizeSunflowerPrice
Without GMWith GMWithout GMWith GMWithout GMWith GM
Average inputs
Seed (kg ha−1)375375454530305, 20, and 40 CNY kg−1 for wheat, maize, and sunflower, respectively
Green manure seed (kg ha−1)05003003030 CNY kg−1
Urea (kg ha−1)3873874854854854854 CNY kg−1
Diammonium phosphate (kg ha−1)2612612612612612613.7 CNY kg−1
Potassium sulfate (kg ha−1)1801801801801801803.3 CNY kg−1
Herbicide (bottle ha−1)151522.522.522.522.520 CNY bottle−1
Irrigation (m3 ha−1)1875187522502250112511250.8 CNY m−3
Plastic film mulching (kg ha−1)00505037.537.512 CNY kg−1
Labor (No. ha−1)000055150 CNY laborer−1
Machinery (times year−1)
Rotary tillage121212675 CNY ha−1
Soil rolling121111150 CNY ha−1
Sowing121212750 CNY ha−1
Harvest111111750 CNY ha−1
Plough tillage111111750 CNY ha−1
Total (CNY hm−2)98581293310325126501032512650
Average outputs, crop yield (Mg ha−1, 13%, 14% of wheat, maize moisture content)
Cycle 1 (2015–2017)6.86.815.215.53.73.83200, 2100, and 6000 CNY mg−1 for wheat, maize, and sunflower, respectively
Cycle 2 (2018–2020)6.66.715.115.33.74.13200, 2400, and 6400 CNY mg−1 for wheat, maize, and sunflower, respectively
Net income (CNY hm−2)
Cycle1 (2015–2017)11,902882721,59519,90011,87510,150
Cycle 2 (2018–2020)11,262850725,91524,07013,35513,590
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Zhao, N.; Zhang, J.; Li, X.; Ma, J.; Cao, J.; Liu, H.; Wang, X.; Bai, L.; Wang, Z. Limited Advantages of Green Manure Planting on Soil Nutrients and Productivity in Intensive Agriculture: A Case Study of Wheat–Maize–Sunflower Rotation in Hetao Irrigation District. Agronomy 2024, 14, 100. https://doi.org/10.3390/agronomy14010100

AMA Style

Zhao N, Zhang J, Li X, Ma J, Cao J, Liu H, Wang X, Bai L, Wang Z. Limited Advantages of Green Manure Planting on Soil Nutrients and Productivity in Intensive Agriculture: A Case Study of Wheat–Maize–Sunflower Rotation in Hetao Irrigation District. Agronomy. 2024; 14(1):100. https://doi.org/10.3390/agronomy14010100

Chicago/Turabian Style

Zhao, Na, Jun Zhang, Xiaohong Li, Jun Ma, Jufeng Cao, Hanjiang Liu, Xiquan Wang, Lanfang Bai, and Zhigang Wang. 2024. "Limited Advantages of Green Manure Planting on Soil Nutrients and Productivity in Intensive Agriculture: A Case Study of Wheat–Maize–Sunflower Rotation in Hetao Irrigation District" Agronomy 14, no. 1: 100. https://doi.org/10.3390/agronomy14010100

APA Style

Zhao, N., Zhang, J., Li, X., Ma, J., Cao, J., Liu, H., Wang, X., Bai, L., & Wang, Z. (2024). Limited Advantages of Green Manure Planting on Soil Nutrients and Productivity in Intensive Agriculture: A Case Study of Wheat–Maize–Sunflower Rotation in Hetao Irrigation District. Agronomy, 14(1), 100. https://doi.org/10.3390/agronomy14010100

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