With the growth rate of the global population, it is expected that the global population will exceed 9 billion by 2050, which will lead to an increased demand for food (Gerland et al. 2014). Maize and soybeans are major food crops in the world and play an important role in meeting food demands for daily dietary energy requirements (Chen et al. 2017; Du et al. 2018). However, a single long term planting mode will cause black soil quality decline in Northeast of China, which is not conducive to the sustainable development of agricultural production. Therefore, it is necessary to search for various planting models to increase the yield of staple grains, oils, and protein crops from limited arable land to meet the food demand Moreover, improving the stability of soil ecosystems has become a major challenge in world agricultural production.
Intercropping systems are widely used in agricultural production. Previous research has confirmed that the main advantages of the intercropping system include the following four aspects: (i) increased grain crop yield, (ii) efficient utilization of land, water, and radiation, (iii) improved resistance of crops to unfavorable environments, and (iv) low cost of fertilizers (Yong et al. 2015; Liang and Shi 2021). Recently, studies have indicated that the maize/soybean intercropping system can significantly increase the yield of corn and soybeans, effectively inhibiting weeds, pests, and diseases (Stoltz and Nadeau 2014). Moreover, Yong et al. (2015) and Chen et al. (2017) found that soybean intercropping can improve soybean nitrogen utilization efficiency and total N accumulation in the soil. Moreover, He et al. (2013) indicated that plant P uptake also increased in maize/soybean intercropping. Although the effect of maize/soybean intercropping on nutrient uptake, land/light utilization, and crop yield has been investigated in previous studies, information on the effect of the maize/soybean intercropping system on the soil ecosystem is lacking, especially on soil quality and microbial communities.
Soil aggregates are closely related to soil quality as a basic element of soil properties. Soil aggregates are heterogeneous ensembles comprising mineral particulates and organic substances (Wang et al. 2020). Based on the formation process, aggregates can be divided into microscopic and macroscopic aggregates (Six et al. 2004). Microaggregates (<0.25 mm) are formed by the action of microorganisms, and their structural stability is significantly related to persistent organic binders (Voltolini et al. 2017). Macroaggregates (>0.25 mm) are formed by the aggregation of microaggregates under the action of organic binders (such as plant roots and fungal hyphae) (Li et al. 2019). The content and characteristics of aggregates of different sizes have significant effects on soil structure, water permeability, and microbial activity. According to previous research, soil agglomeration can enhance the absorption of minerals and increase the stability of the soil structure by increasing the tightness of the bond between organic carbon and mineral particles (Tan et al. 2017). Moreover, the presence of soil aggregates enhanced the organic carbon content of the topsoil. Rabot et al. (2018) found that soil aggregates are a key factor in enhancing soil carbon storage and can retain approximately 90% of organic carbon. Overall, aggregates are an important indicator of soil structural stability and fertility levels and are directly affected by land use and related management practices, such as the pattern of cropping rotation, fertilization, and tillage (Conant et al. 2003; Liu et al. 2018; Guo et al. 2020). Therefore, soil aggregates at various spatial scales in the maize/soybean intercropping system should be investigated, which will be meaningful for understanding the effect of planting diversity on soil quality, especially on black soil quality.
In addition, arbuscular mycorrhizal fungi (AMF), an important part of soil microorganisms, are essential for soil ecosystem functions and play an important role in soil nitrogen transformation, carbon fixation, organic matter decomposition, and nutrient cycling. Likewise, the previous study has shown that AMF also contribution to the soil aggregates formation (Bossuyt et al. 2001; Cardoso and Kuyper 2006). Zhang et al. (2019) found that AMF colonization significantly increased the content of macroaggregates (> 2mm) compared to without AMF colonization during the karst soil system (Zhang et al. 2019). Generally, AMF communities and species are affected by the planting time and cultivation systems (Duchicela et al. 2013). Song et al. (2007) indicated that the composition of the rhizosphere fungal community was significantly different between wheat/maize, wheat/fava bean, and fava bean/maize intercropping systems. Li et al. (2013) found that the abundance of microorganism communities in soybean and sugarcane monocropping systems was higher than that in the intercropping system. Similarly, the evolution of the AMF diversity and community in the rhizosphere soil and roots of maize/soybean intercropping areas was observed in our previous study (Zhang et al. 2020). However, the role of AMF in soil microecology and its contribution to soil quality in the maize/soybean intercropping system is still poorly understood and requires further investigation.
Therefore, to address this knowledge gap, we conducted a long-term experiment with black soil in northeast China to explore the influence of a maize/soybean intercropping system on soil properties and its relation to AMF composition at different soil depths under two nitrogen fertilizer treatments. The main objectives of this study were to (i) explore the distribution and stability of soil aggregates at different depths, (ii) evaluate the difference of AMF community in soil profile (0-60cm), and (iii) identify the relationship between AMF and soil properties in the maize/soybean intercropping system via structural equation modeling analysis.