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Article

Effect of Agroforestry Systems on Soil NPK and C Improvements in Karst Graben Basin of Southwest China

by
Long Wan
1,2,
Jiaqi Yang
1,2,3,
Chenghao Zheng
1,2,
Jianbin Guo
1,
Jinxing Zhou
1,2,*,
Yuguo Han
1 and
Ansa Rebi
1,2
1
School of Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China
2
Jianshui Research Station, Beijing Forestry University, Beijing 100083, China
3
Jinan Environmental Research Academy, Jinan 250102, China
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(6), 1179; https://doi.org/10.3390/agronomy14061179
Submission received: 19 April 2024 / Revised: 18 May 2024 / Accepted: 27 May 2024 / Published: 30 May 2024
(This article belongs to the Section Agroecology Innovation: Achieving System Resilience)
Browse Figures
Versions Notes

Abstract

:
Agroforestry systems can fully exploit the ecological benefits of an ecosystem’s component plants, and improve the ecological environment, soil quality, and land use efficiency, all of which have recently attracted the attention of many scholars. Southwest China’s karst graben basins have barren soil that needs immediate improvement. The karst graben basin in southwest China was used for this study to examine the impacts of several forest–grass composite systems of Bingtang orange–alfalfa–ryegrass on soil improvement, which is located in Jianshui County, Honghe Prefecture, Yunnan Province. The experiment had four treatments, Bingtang orange–alfalfa sowing (B2), Bingtang orange–ryegrass × alfalfa mixed sowing (A), Bingtang orange–ryegrass × alfalfa intercropping (R), and Bingtang orange monocropping (CK). The results showed that different forest–grass composite patterns had noticeable effects on improving the soil’s organic carbon (SOC), total nutrients, and available nutrients, especially in the rainy season when plants grew vigorously. Forage grass intercropping under forest exerted the best effect on soil improvement in the surface layer, and the effect decreased with the increase in soil depth. Alfalfa intercropping under Bingtang orange forest had the strongest effect on improving SOC, total N, ammonia nitrogen, and nitrate nitrogen, whose content increased by 30.7%, 27.3%, 35%, and 36.3%, respectively, in the dry season and 38%, 46.7%, 48.7%, and 55.3%, in the rainy season. However, the effect of alfalfa–ryegrass intercropping under the Bingtang orange forest on soil total P, total K, and available P was better than that of the Bingtang orange–alfalfa intercropping system. The C:N ratio is more suitable in the Bingtang orange–ryegrass-alfalfa composite system. Forage grass intercropping under the Bingtang orange forest effectively improved the soil NPK and C status, and the results of this study provided a basis for the selection of forest–grass composite patterns for soil improvement.

1. Introduction

A karst graben basin is a karst basin formed by Cenozoic fault activity. The landforms of karst graben basins are widely distributed in southwest China [1]. Due to the region’s abundant light and heat resources in karst graben basins, fruit trees with high production and good quality, such as Bingtang oranges, are frequently planted there. The soil in the karst graben basins is developed from limestone, which is barren and the soil quality is further degraded due to long-term unreasonable agricultural activities [2,3]. These factors significantly limit the development and growth of local fruit trees, which ultimately has a negative effect on the benefits of fruit trees for production and the financial advantages of farmers [4]. It is important to implement strategies that can raise the local land quality and production in order to lessen this negative impact [5,6].
Forages or crops are sown beneath the forest in an agroforestry system, which is a hybrid ecosystem of agriculture and forestry, throughout the growth stages of the young perennial trees [7]. In many regions of the world, agroforestry practices have a long history [8,9]. Scientists started to become interested in agroforestry in the 1980s as a way of boosting and sustaining agricultural production in marginal lands and remote tropical regions [10,11]. A forest–grass intercropping system is a widely applied agroforestry system, and various ecological and economic benefits can be obtained by intercropping woody plants and grass on the same land [12,13].
The agroforestry system can impact soil’s physical and chemical properties [14,15]. Some research has shown quantitative evidence that agroforestry systems have the potential to significantly enhance soil quality and long-term soil productivity [16,17,18]. The research has highlighted the positive effects of agroforestry on soil organic matter (SOC), and nutrient availability [19,20,21]. We examined the long-term effects of agroforestry on soil organic carbon (SOC) content [22], including the analysis of data from multiple studies and provided quantitative evidence of the significant increase in SOC in agroforestry systems compared to conventional agricultural practices [23]. Agroforestry systems contribute to C sequestration, mainly due to an increase in the above- and below-ground C inputs. The evidence for C sequestration in the soils of agroforestry systems was mainly found in tropical and subtropical regions [24].
Numerous studies have shown that an appropriate forest–grass composite system can improve soil characteristics and structure, strengthen soil microbial activity, and increase soil organic matter [25,26]. After intercropping fruit trees with grasses, nutrient elements in the soil were found to increase more than initial soils [27]. Following the combined treatment of the grass and forest, the amount of nitrate nitrogen in the soil was effectively enhanced [28]. Implementing a forest–grass treatment could boost the soil’s organic matter, microbial population, and microbial content as well as facilitate the supply of nitrogen [29]. However, the effects of forests and grass intercropping ecosystems on the soil quality were complicated. Some studies showed that the combined treatment of forest and grass had a positive effect on soil N, but a negative effect on soil P and K [30].
Alfalfa (Medicago sativa) is a high-quality perennial leguminous forage crop which can effectively increase the nitrogen content in soil, so it is widely used in forest–grass intercropping systems [31]. Alfalfa is a nitrogen-fixing plant, which lives in a symbiotic relationship with the nitrogen-fixing bacteria–the rhizobia–which lives in nodules in the plant’s roots [32]. The results indicated that intercropping alfalfa with ziziphus and walnut trees can effectively increase the nitrogen content in the soil and biomass of plants [33,34]. However, it is easy to form a soil barrier with karst soil to hinder the acquisition of phosphorus by plants because of its high calcium content and pH [35]. Therefore, it is necessary to increase the chances for the effective utilization of soil phosphorus in forest–grass systems. A large number of studies have found that leguminous forage crops and gramineous grass intercropping have different effects on soil nutrients [36], among which leguminous grass is more conducive to the decomposition and release of soil organic matter and improvement in nitrogen [37], while gramineous grass can activate soil phosphorus and potassium under the joint action of root exudates and associated microorganisms. Therefore, leguminous forage crop–gramineous grass intercropping under forests can give full play to their advantages for the soil improvement in the forest–grass composite systems.
The introduction of grass planting under the forest will also increase the competition between grass and fruit trees for soil nutrients, and there is also some competition between two types of grass, leading to an especially complicated competitive and complementary action of the forest–grass system for Bingtang orange trees [38], even though it is possible to increase the utilization of nitrogen and phosphorus under Bingtang orange (Citrus sinensis L. Osbeck) trees by planting alfalfa and ryegrass (Lolium perenne L.) in the same forest–grass system. In order to explore the best forest–grass intercropping system in the area, it is crucial to understand the impact of various Bingtang orange–grass intercropping patterns on soil improvement in karst graben basins.
A significant issue that requires immediate attention is how to plant a successful forest–grass intercropping pattern in karst graben basins while also enhancing the physical and chemical characteristics of the soil. Few studies have explored the interaction of the three types of plants such as leguminous forage, gramineous grass, and fruit forest. This study mainly aims to explore the effects of different intercropping patterns of Bingtang orange forest mixed with alfalfa and ryegrass on the improvement in soil quality. We have investigated whether the three types of plants have synergistic effects or competition effects to improve soil organic matter and the poor phosphorus nutrients of karst soil, and promote plant growth. The results of our study will hopefully prove helpful in guiding the selection of agroforestry systems in karst graben areas.

2. Materials and Methods

2.1. Site Description

The study area is located in Jianshui County, Honghe Prefecture, Yunnan Province (102°56′55″ E and 23°37′45″ N), which is a typical karst graben basin landform (Figure 1). At an altitude of 1520 m, the study area belongs to the south subtropical monsoon climate, with an annual average temperature of 19.8 °C, an annual average relative humidity of 72%, an evaporation of 2311 mm, and an annual average precipitation of 805 mm. In addition, it has distinct dry and rainy seasons, and the frost-free period is 307 d throughout the year. The orchard soil tested was red soil developed from limestone, with clay texture, uniform soil fertility, and flat terrain.

2.2. Experimental Design

The plant spacing of the Bingtang orange was 1.5 m × 3 m, and the trees were oriented in an east–west direction. The average plant height was 146.7 cm, the basal diameter was 3.7 cm, the north–south crown width was 139 cm, the east–west crown width was 132 cm, and the crown height was 3.2 m. Before planting the experimental grass, soil samples were collected through the double diagonal sampling method [39] from depths of 0–20 cm, 20–40 cm, and 40–60 cm in the experimental field. We took 16 samples each containing five sub-samples from every soil layer for the four corners and center of the sample plot. The basic physical and chemical properties of the soil samples are shown in Table 1.
The leguminous forage crops, alfalfa (WL525HQ) and gramineous grass ryegrass (MATHILDE), were selected as symbiotic grass. Three forest–grass intercropping patterns were set up in the experiment, namely, Bingtang orange–alfalfa sowing (B), Bingtang orange–ryegrass × alfalfa mixed sowing (A), and Bingtang orange–ryegrass × alfalfa intercropping (R). Meanwhile, Bingtang orange monocropping was set as the control treatment (CK) (Table 2). Each treatment was performed in four replicates, with a total of 16 plots. Each plot covered 3 m × 3 m and consisted of 6 Bingtang orange trees, and a 1 m buffer interval was set between different plots. In September 2020, the grass was sown in lines in the same direction under the Bingtang orange forest. Before sowing, the soil surface layer (0–20 cm) was ploughed, the soil depth was 4–5 cm, and the row spacing was 30 cm (Figure 2). A 1 m buffer interval was set between different treatments, and each treatment was repeated four times; there were a total of 16 quadrats, each with an area of 3 × 3 m, and six Bingtang orange plants. When the grass height reached 30–40 cm (about 60 days in the dry season and 30 days in the rainy season), the grass was mowed manually. The field management was the same as that in the local Bingtang orange orchard, and timely fertilization, sprinkler irrigation, and pesticide application were carried out according to the growth of Bingtang orange trees and grass, and the amount of fertilization, irrigation, and pesticide application for each treatment was the same. The sprinkling irrigation was used for the irrigation of the plot. In the dry season, when there is little rain or few rain in 15 days, irrigation for about 4–8 h was carried out for all the plots. In February, March, and May, 100 g of urea was applied to each tree, and in April and July, 150 g of urea was applied to each tree. Some pesticides, namely methyl thiophanate, imidacloprid, and Avermectin were applied for the prevention of pests.

2.3. Measurements

Soil samples were taken at the depth of 0–20 cm, 20–40 cm, and 40–60 cm using the five-point sampling method on 10 April 2021 (dry season) and 26 August 2021 (rainy season). We also sampled the active topsoil of 0–20 cm on 20 May 2022 (dry season) to compare the impact on the chemical properties. In the laboratory, soil organic carbon (SOC) was determined by the potassium dichromate oxidation method, total nitrogen was determined by the ammonia determination method, total P was determined by the ammonium molybdate method, total K was determined by a flame photometer, ammonium nitrogen and nitrate nitrogen were determined by the hydrazine sulfate reduction method, and rapidly available phosphorus was determined by NaHCO3 leaching-Mo-Sb colorimetry. The soil total K was not determined in 2022.

2.4. Data Processing and Analysis

Data were subjected to analysis of variance via SPSS 20.0 (SPSS Corp, Chicago, IL, USA), and the normality of the variables was assessed using the Kolmogorov–Smirnov test. A two-way analysis of variance (ANOVA) was performed to compare the selected variables of different forest–grass intercropping treatments (p < 0.05). Graphs were drawn using Origin 9.0 (Origin Lab, Northampton, MA, USA).

3. Results

3.1. Soil SOC Characteristics

Figure 3 demonstrated that SOC under various forest–grass intercropping regimes had varied temporal and spatial distribution features. With a significant difference between the two seasons (p < 0.05), the content of SOC in various treatments was ordered as follows: rainy season > dry season. In all dry and rainy seasons, the SOC content under various treatments dropped as soil depth increased, and the difference across layers was considerable. The SOC in the top layer of 0–20 cm was between 12 and 17 g·kg−1 in 2021 and increased to 15–22 g·kg−1 in 2022. In the dry and rainy seasons of 2021, the SOC was sorted as B2 > R > A > CK and the content of soil organic matter in B2 was significantly higher than that in other intercropping treatments (p < 0.05). In 2022, the SOC content of A and R increased significantly and was greater than that of B2 treatment. The SOC of all the intercropping treatments was higher than the monocropping of Bingtang orange trees. Compared with CK, the content of soil organic matter at 0–20 cm, 20–40 cm, and 40–60 cm soil depths in B2 increased by 35%, 31%, and 26% in the dry season, and by 40%, 38%, and 36% in the rainy season in 2001. In 2022, the SOC in the A treatment was the highest, which reached 20.77 g·kg−1 and increased by 58% more than the CK treatment.

3.2. Characteristics of Total N, Total P, and Total K in Soil

Rainy season > dry season was used to order the total N, P, and K contents of the soil during various times (Figure 4, Figure 5 and Figure 6). Under various treatments, there were significant changes in the vertical distribution of soil total N, total P, and total K (p < 0.05), and they all displayed a declining tendency as the soil depth increased during dry and rainy seasons. Under various forest–grass intercropping patterns, clear changes in soil total N, total P, and total K were observed. In the dry and rainy seasons of 2021 and 2022, soil total N, total P, and total K in different forest–grass intercropping patterns were significantly higher than those in monocropping Bingtang orange forest in different soil layers (p < 0.05).
Soil total N in B2 treatment was significantly higher than those in other intercropping treatments (Figure 4), but there was no significant difference between R and A treatments (p > 0.05). The soil total N for B2 treatment in the dry and rainy season in 2021 were 1.29 g·kg−1 and 1.51 g·kg−1 in the soil layer of 0–20 cm, respectively. However, the soil total N for CK treatment in the topsoil was only 1.04 and 1.07 g·kg−1 in the dry and rainy seasons. Compared with CK, B2 treatment reached the highest increase in soil total N, which increased by 34%, 25%, and 23% in the dry season and 57%, 42%, and 41% in the rainy season at 0~20 cm, 20~40 cm, and 40~60 cm soil depths. In 2022, the soil total N in the topsoil increased from 0.55 g·kg−1 for CK to 1.37 g·kg−1 for CK.
The order of soil total P in different intercropping treatments was A > R > B2 > CK (Figure 5). Soil total P in the topsoil for A and R treatments were 0.82 and 0.77 g·kg−1 in the dry season and 1.00 and 0.90 g·kg−1 in the rainy season in 2021, which were significantly higher than those in other treatments (p < 0.05). Compared with CK, the soil total P for A treatment at 0–20 cm, 20–40 cm, and 40–60 cm soil depths increased by 39%, 36%, and 33% in the dry season and 46%, 39%, and 38% in the rainy season, respectively. In 2022, total P in the 0~20 cm soil increased by 0.65 g·kg−1 of CK treatment to 0.73 g·kg−1 and 1.37 g·kg−1 for the A and R treatments.
Soil total K in different intercropping treatments was sorted as R > A > B2 > CK (Figure 6). The soil total K in R and A were between 4.5 and 6 g·kg−1, which was significantly higher than that in the CK treatments at about 4 g·kg−1. Compared with CK, the R treatment led to an increase in total K at different depths of 0~20 cm, 20~40 cm, and 40~60 cm, which increased by 16%, 18%, and 22% in the dry season and 40%, 36%, and 32% in the rainy season.
Soil microorganisms are generally limited by the availability of carbon (C) and nitrogen (N) in the topsoil of 0~20 cm. The characteristics of C:N in the soil of 0~20 cm were evaluated in this study. We found that C:N in the dry season and rainy season were between 12 and 14 in 2021 (Table 3). But in 2022, C:N increased greatly in A and R treatments, mainly because the higher soil organic C through the intercropping with the ryegrass. C:N in the A treatments increased to more than 27. However, because of the increase in N in the B2 treatment, the C:N decreased in the B2 treatment.

3.3. Characteristics of Available Nutrients in the Soil

The content of soil-available nutrients in various forest–grass intercropping treatments was higher in the rainy season than it was in the dry season, and the distribution law of total nutrients and soil-available nutrients in various forest–grass composite patterns was consistent over time (Figure 7, Figure 8 and Figure 9). Under various forest–grass composite patterns, the amount of soil nutrients that were readily available to plants exhibited a decreasing tendency as soil depth increased. The maximum levels of nitrate and ammonium nitrogen in B2 were found in various treatments of forest–grass compounds.
The ammonium nitrogen content in B2 at a 0–20 cm soil depth in the dry and rainy seasons was 113.53 mg·kg−1 and 142.68 mg·kg−1, respectively, and the nitrate nitrogen content was 23.49 mg·kg−1 and 28.28 mg·kg−1, respectively. The content of ammonium nitrogen and nitrate nitrogen in different intercropping treatments was sorted as B2 > (R and A) > CK in 2021. The R and A had no statistically significant difference (p > 0.05). Compared with CK, soil ammonium nitrogen content for B2 treatment at 0~20 cm, 20~40 cm, and 40~60 cm soil depths increased by 40%, 33%, and 32% in the dry season and 58%, 43%, and 45% in the rainy season, respectively. Soil nitrate-nitrogen content increased in the intercropping treatments, which increased from 15 to 20 mg·kg−1 for CK treatment to 25~30 mg·kg−1 for the B2 treatment in the topsoil. In 2022, soil ammonium nitrogen and nitrate nitrogen in the 0~20 cm soil were highest in the R treatment, which exceeded the B2 treatment because of the compound effect of the alfalfa and ryegrass intercropping.
The content of soil rapidly available phosphorus in different forest–grass composite patterns was different, sorted as A > R > B2 > CK. Soil available phosphorus in A and R treatments was significantly higher than that in other treatments. Compared with CK, the A treatment led to the greatest increase in the soil available P, which increased by 47%, 36%, and 35% in the dry season and increased by 56%, 38%, and 37% in the rainy season at 0–20 cm, 20–40 cm, and 40–60 cm soil depths, respectively. In 2022, soil-available phosphorus was also higher in the A and R treatments.

4. Discussion

The ecological advantages of the plants that make up an ecosystem can be fully exploited by agroforestry systems, which can also enhance the ecological environment, soil quality, and efficiency of land use—all factors that have recently caught the interest of many academics. The barren soil in the karst graben basins of southwest China needs quick treatment.

4.1. Influence of Forest–Grass Composite Patterns on SOC

This study demonstrated how Bingtang orange–alfalfa–ryegrass agroforestry methods can improve the SOC content. Previous studies have demonstrated that intercropping fruit trees in agroforestry systems increase carbon sequestration [40]. Compared with monocropping, the fruit tree–grass intercropping pattern in this study increased the aboveground and underground biomass of vegetation in the area between fruit tree–grass, creating conditions for the input of SOC, so the SOC under the three forest–grass intercropping patterns was higher than that in CK treatment. SOC increasing by a fruit tree–grass intercropping system contributes to the C sequestration increasing in the soil of agroforestry system, which is conducive to the carbon neutrality target of southwest China [41]. The litter and sediment of plant roots are important sources of SOC [42], so SOC matches the distribution of plant roots and decreases with the increase in soil depth [43]. Because there are better hydrothermal conditions in the rainy season than those in the dry season, the plants in the forest–grass system show relatively vigorous physiological activities, which is beneficial to the input of soil carbon by plants and the storage and accumulation of organic carbon in the rainy season [44]. As a result, the SOC of the forest–grass intercropping system is significantly higher during the rainy season than it is during the dry season [45]. As the fastest growing of the three forest–grass intercropping treatments in the first year, alfalfa had better-established roots and robust physiological activities, which allowed B2 to achieve the maximum SOC. However, as ryegrass grows, its larger root system can provide significantly more carbon to the system [46]. In the second year, SOC in the intercropped alfalfa, ryegrass, and Bingtang orange treatments (A and R) showed higher levels than the B2 treatments. The different intercropping patterns can change the structure of SOC [47]. A study has also shown that ryegrass intercropping in an agroforestry system boosted carbonyl carbon and decreased alkyl, O-alkyl, and aromatic carbon, which can also lead to a significant rise in the soil’s carbon content [48]. Alfalfa, ryegrass, and Bingtang orange intercropping can significantly boost the microbial community’s functional diversity to increase SOC, which is another factor contributing to the SOC increase.

4.2. Influence of Forest-Grass Composite Patterns on Total Nutrients in Soil

N, P, and K are essential mineral nutrients for plants and important participants in various physiological and metabolic activities in the growth of plants [49]; therefore, their content is an important index to evaluate the quality of soil nutrients. Alfalfa is a type of perennial leguminous nitrogen-fixing crop. Symbiotic nitrogen-fixing rhizobia in the forest–grass system fixes nitrogen from the air, raising the total amount of nitrogen in the soil [50]. Therefore, the three forest–grass intercropping patterns had higher soil nitrogen contents than the CK treatment, and B2 had the highest total nitrogen level in the soil. This was particularly evident during the rainy season when alfalfa flourished vigorously. However, because ryegrass absorbed soil nitrogen in the forest–grass system where alfalfa and ryegrass coexisted, the soil N concentration in treatments A and R was lower than that in treatment B. A study has identified that rhizobia’s presence allows grass and legumes to coexist without competing for N [51], and the intercropping of gramineous pasture ryegrass and leguminous forage alfalfa increased soil nitrogen can lead to an increase in nitrogen contents in the shoots and roots [52].
The shallow layer of the fruit tree–grass system comprised more root systems and contained a larger soil carbon content, which was more favorable for the development of soil aggregates, even if alfalfa and ryegrass would partially consume P and K in the soil to meet their growth demands [53]. The interception and storage of fertilizers applied to soil might be made easier by the adsorption properties of soil aggregates, lowering the leaching loss of P and K fertilizers from the forest–grass system, which may be the main reason the total P and total K in forest–grass intercropping systems was higher than that in CK treatment. Many reasons may lead to the variation in soil levels of P and K. It was concluded that the mobilization of P in the rhizosphere may be increased by organic anion exudation and the acid phosphatase activity of tree roots [54]. Because ryegrass absorbed soil P and K less efficiently than alfalfa, soil P and K levels are likely greater in the alfalfa and ryegrass intercropping agroforestry system. According to a prior study, ryegrass’s relative competitiveness in intercropping was unaffected by the P deficit [55]. The agroforestry patterns in our study had excellent promotional benefits because intercropping with alfalfa and ryegrass in agroforestry systems could mitigate the negative impacts of excessive nitrogen fertilizer application on the environment [56]. Soil K plays a particularly crucial role in many physiological processes vital to the growth and stress resistance of plants. One study found that available soil K level was high in the agroforestry system [57]. However, in our study, the soil K content in all treatments was less than 6 g·kg−1. This content was low and also limited the plants’ growth. Average soil reserves of K are generally large, but most of it is not plant-available. Therefore, there is a need to study the agroforestry impacts on soluble K variation in the soil in the near future. C:N stoichiometry concerns the balance of the multiple chemical elements in ecological interactions [58]. The suitable C:N range for terrestrial microbial communities is 20–25. Because soil C sharply increased after the second year of the agroforestry system, in our study, the soil C:N ratio significantly increased from 12~14 to 19~27 in the Bingtang orange–alfalfa–ryegrass ecosystem, being more suitable for the soil microbial communities. However, the soil C:N ratio of Bingtang orange–alfalfa was lower than other treatments. Researchers have found that during early establishment, alfalfa predominated in all ryegrass–alfalfa combinations, and the ryegrass companion treatments produced outstanding fodder yields in the next year [59]. The growth of ryegrass–alfalfa combinations has an impact on the soil’s N, P, and K contents, according to our results for 2022.

4.3. Influence of Forest-Grass Composite Patterns on Rapidly Available Nutrients in the Soil

The rapidly available nutrients in the soil are nutrients that can be directly used by plants, which are very important for the growth and development of plants [60]. Previous studies have found that intercropping improves soil nutrient availability [61]. In this study, the content of soil rapidly available nutrients in different forest–grass systems was analyzed to evaluate the improvement effect of different forest–grass intercropping patterns on soil. In the forest–grass intercropping system, alfalfa increased the content of organic nitrogen in soil through the nitrogen fixation effect of rhizobia, and meanwhile, the interaction between roots and microorganisms increased the decomposition and mineralization rate of organic nitrogen and improved the content and availability of ammonia nitrogen and nitrate nitrogen in soil [62]. As a result, the three forest–grass intercropping patterns had higher soil nitrate and ammonia nitrogen contents than the CK treatment, particularly in the B2 treatment, which had the highest alfalfa planting density in the first year of our study. So, in the second year, we discovered that the A and R treatments that intercropped with ryegrass had higher levels of nitrate nitrogen and ammonia nitrogen. Ryegrass is a member of the Gramineae family and can activate insoluble inorganic phosphorus in the soil as well as carboxylic acids, protons, and enzymes that cannot be used by plants. This increases the amount of quickly available phosphorus in the soil and encourages the uptake of phosphorus by symbiotic plants [63]. However, it was found that the long-term continuous growth of alfalfa can reduce soil available phosphorus [64]. Therefore, the content of rapidly available phosphorus in soil under treatments A and R was higher than that under treatments B2 and CK. Microbial and enzyme activities played an important role in the mineralization of soil organic P [65]. Meanwhile, soil organic nitrogen and available phosphorus interact with each other in the agroforestry ecosystem [66]. Researchers have discovered that the highest NH4-N dose strongly decreased the available P content, as well as increasing the root phosphatase activity which affected plant phosphorus uptake [67]. Because of the high ammonia nitrogen and nitrate nitrogen in the B2 treatment, therefore, the available P was lower in B2 than the A and R treatments.

5. Conclusions

In the karst graben basin, the experiment of Bingtang orange intercropping with alfalfa and ryegrass in the agroforestry system revealed that R treatment will improved NPK and C more for a long term intercropping, and more suitable for a farmer management. The SOC content in the Bingtang orange–grass intercropping system was higher in all treatments except for the Bingtang orange monocropping forest. Due to the nitrogen fixation effect of symbiotic rhizobia in alfalfa, the soil total N, ammonia nitrogen, and nitrate nitrogen in the intercropping system were higher than those in the Bingtang orange monocropping treatment, and the Bingtang orange–alfalfa treatment had the best effect on improving soil nitrogen. However, the content of nitrogen in soil decreased due to the competition among grass species when intercropping with ryegrass. The content of total P, total K, and rapidly available phosphorus in soil could be significantly increased by adding ryegrass to the intercropping system than the Bingtang orange–alfalfa intercropping system. The results of this study provide a scientific reference for the management of Bingtang orange orchards, but the better physiological characteristics and economic benefits of Bingtang orange are the ultimate aims for the response to the improvement in soil quality. Therefore, both the ecological benefits and economic benefits of the whole intercropping system should be incorporated into the scope of future research.

Author Contributions

Conceptualization, L.W. and J.Z.; methodology, L.W.; software, J.Y.; validation, J.G.; formal analysis, L.W.; investigation, J.Y. and C.Z.; resources, J.Y.; data curation, J.Y.; writing—original draft preparation, L.W.; writing—review and editing, L.W., J.Z., Y.H. and A.R.; visualization, J.Y. and C.Z.; supervision, J.Z. and J.G.; project administration, L.W.; All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key Research and Development Program of China (2022YFF1302901), the Key Research and Development Program of Yunnan Province, China (2019BC001-03), the National Natural Science Foundation of China (342207065/31700640), and the Key Research and Development Program of the Ningxia Hui Autonomous Region (2021BEG02005).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ethical restrictions.

Acknowledgments

We are grateful for the support from Mei Zhang, Fawan Liu, Yuxiong Ding, Guiying Xiao, and Liying Wang in the Forestry and Grassland Bureau of Jianshui County, Yunan Province, China.

Conflicts of Interest

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

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Figure 1. Schematic diagram of the geographical location of the study area.
Figure 1. Schematic diagram of the geographical location of the study area.
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Figure 2. Experimental design of an agroforestry system.
Figure 2. Experimental design of an agroforestry system.
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Figure 3. Content of SOC under different forest–grass composite patterns. The different treatments were (B2) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping. Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
Figure 3. Content of SOC under different forest–grass composite patterns. The different treatments were (B2) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping. Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
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Figure 4. Soil total N under different forest–grass composite patterns. The different treatments were (B2) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping. Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
Figure 4. Soil total N under different forest–grass composite patterns. The different treatments were (B2) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping. Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
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Figure 5. Soil total P under different forest–grass composite patterns. The different treatments were (B2) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping. Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
Figure 5. Soil total P under different forest–grass composite patterns. The different treatments were (B2) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping. Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
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Figure 6. Soil total K content under different forest–grass composite patterns. (The different treatments were (B) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping). Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
Figure 6. Soil total K content under different forest–grass composite patterns. (The different treatments were (B) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping). Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
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Figure 7. Soil ammonium nitrogen content under different forest–grass composite patterns. The different treatments were (B2) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping. Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
Figure 7. Soil ammonium nitrogen content under different forest–grass composite patterns. The different treatments were (B2) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping. Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
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Figure 8. Soil nitrate-nitrogen content under different forest–grass composite patterns. The different treatments were (B2) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping. Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
Figure 8. Soil nitrate-nitrogen content under different forest–grass composite patterns. The different treatments were (B2) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping. Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
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Figure 9. Soil available phosphorus content under different forest–grass composite patterns. The different treatments were (B) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping. Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
Figure 9. Soil available phosphorus content under different forest–grass composite patterns. The different treatments were (B) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping. Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the different soil layers of the same treatments. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the same soil layer of different treatments.
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Table 1. The basic physical and chemical properties of the soil before the implementation of orchard.
Table 1. The basic physical and chemical properties of the soil before the implementation of orchard.
Soil DepthMoisture Content %pHSoil Bulk Density
g·m−3		Soil Organic Carbon
g·kg−1		Total N
g·kg−1		Total P
g·kg−1		Total K
g·kg−1		Ammonium Nitrogen
mg·kg−1		Nitrate Nitrogen
mg·kg−1		Rapidly Available Phosphorus
mg·kg−1
0–20 cm28.04 ± 0.855.22 ± 0.161.14 ± 0.0513.54 ± 0.511.04 ± 0.120.66 ± 0.023.94 ± 0.2570.97 ± 3.215.94 ± 0.337.12 ± 1.02
20–40 cm31.88 ± 1.245.70 ± 0.121.23 ± 0.037.47 ± 0.430.75 ± 0.050.50 ± 0.033.55 ± 0.3461.79 ± 1.653.28 ± 0.403.63 ± 0.68
40–60 cm36.85 ± 1.025.90 ± 0.181.27 ± 0.024.68 ± 0.450.47 ± 0.040.37 ± 0.023.39 ± 0.2153.34 ± 1.890.85 ± 0.301.40 ± 0.32
Table 2. Experimental treatment code and sowing schemes.
Table 2. Experimental treatment code and sowing schemes.
Planting PatternTreatment Forest–Grass Spacing (cm)Time of SowingSeeding Rate (kg·m−2)
Bingtang orange–alfalfa single sowingB260September, 20200.0018
Bingtang orange–ryegrass × alfalfa mixed sowingA60September, 20200.0018
Bingtang orange–ryegrass × alfalfa intercroppingR60September, 20200.0018
Bingtang orange pure forestCK---
Table 3. Ratios of C:N of the soil for different treatments. The different treatments were (B) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping.
Table 3. Ratios of C:N of the soil for different treatments. The different treatments were (B) Bingtang orange–alfalfa sowing, (A) Bingtang orange–ryegrass × alfalfa mixed sowing, (R) Bingtang orange–ryegrass × alfalfa intercropping and (CK) Bingtang orange monocropping.
BCKAR
the dry season in 200113.18 ± 0.68 Aa12.96 ± 0.55 Cab11.84 ± 0.71 Bb11.76 ± 0.41 Bb
rainy season in 200112.88 ± 0.97 Aab13.85 ± 0.17 Ba12.44 ± 0.48 Bb12.23 ± 0.12 Bb
dry season 202210.40 ± 0.54 Bd22.66 ± 0.75 Ab27.33 ± 0.84 Aa19.24 ± 0.36 Ac
Note: Columns followed by the different uppercase letters indicate statistically significant differences at p < 0.05 for the same treatments of different years. Columns followed by the same lowercase letter indicate statistically significant differences at p < 0.05 for the different treatments of the same year.
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Wan, L.; Yang, J.; Zheng, C.; Guo, J.; Zhou, J.; Han, Y.; Rebi, A. Effect of Agroforestry Systems on Soil NPK and C Improvements in Karst Graben Basin of Southwest China. Agronomy 2024, 14, 1179. https://doi.org/10.3390/agronomy14061179

AMA Style

Wan L, Yang J, Zheng C, Guo J, Zhou J, Han Y, Rebi A. Effect of Agroforestry Systems on Soil NPK and C Improvements in Karst Graben Basin of Southwest China. Agronomy. 2024; 14(6):1179. https://doi.org/10.3390/agronomy14061179

Chicago/Turabian Style

Wan, Long, Jiaqi Yang, Chenghao Zheng, Jianbin Guo, Jinxing Zhou, Yuguo Han, and Ansa Rebi. 2024. "Effect of Agroforestry Systems on Soil NPK and C Improvements in Karst Graben Basin of Southwest China" Agronomy 14, no. 6: 1179. https://doi.org/10.3390/agronomy14061179

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