Research advances on the mechanisms of soybean and maize influence nitrogen supply in subsequent crops

Objectives We reviewed the effects of soybean and maize production on the soil’s physical properties and nitrogen supply capacity to subsequent crops from crop root morphology and residual nutrient availability. We aim to examine how soil texture could increase the rotational system’s productivity and sustain soil fertility.

Major advances  Soybean roots are reticular in shape with many branches and lateral roots. New roots rapidly replace the old roots during the whole growing period. This stabilizes the soil texture and shows a multistage complex form rich in internal aggregate pores, conducive to the root development of the subsequent crops. After soybean cultivation, the ensuing stable soil aggregate structure lays a good foundation for soil N cycling, such as increasing the total soil organic nitrogen mineralization and transformation and enhancing the potential of soil N supply to the subsequent crops. Maize requires a high N fertilizer application, leading to a corresponding high residual N in the soil. The residual N is mainly in the form of NO₃^–-N and microbial nitrogen. NO₃^–-N is unstable and easily lost in soils, reducing its availability for use by subsequent crops. Soybean straw has a low C/N ratio and is easily utilized by soil microorganisms after returning to the field, accelerating the soil N cycle and N use by subsequent crops. Soybean roots have many dead root nodules, sediments formed by roots during the growth process, high levels of root exudates such as glycine and serine, and a low C/N ratio of sediments. This further increases their mineralization and transformation into an important N source for later crops. Therefore, the N supply capacity of soils after soybean cultivation is better than maize.

Expectations  A beneficial crop rotation should consider proportion of key factors that can improve the efficient utilization of residual soybean nitrogen (N), N management for minimal loss after maize production, and the mechanism of root-soil microbial interaction during N transfer and transformation. Since straw decomposition can accelerate soil organic carbon release, reducing greenhouse gas emissions such as annual CO₂ and improving soil carbon sink are worthy of future research.