Wheat–maize (WM) and wheat–soybean (WS) double-cropping rotation systems are predominant in the North China Plain, with implications for national agricultural output and sustainability. As rotation systems exert legacy effects on soil health and crop productivity, the role of crop rotation in shaping the root-associated microbiome of the succeeding crops has emerged as a pivotal aspect of crop management research. Here, the effects of the preceding two cycles of WM and WS rotations on the recruitment and filtering of wheat root-associated bacterial communities across wheat developmental stages were investigated. Our results revealed that bacterial community diversity and composition were primarily influenced by compartment and developmental stage, while the preceding rotation systems had a slight but significant effect on wheat root-associated bacterial communities. The co-occurrence networks under WM were more complex in the wheat rhizosphere and rhizoplane, with the operational taxonomic units (OTUs) related to cellulolysis showing greater connectivity. The co-occurrence networks under WS were simple but stable in the rhizosphere and complex in the rhizoplane and endosphere, with the OTUs related to ureolysis and nitrogen fixation showing greater connectivity. While both stochastic and deterministic processes contributed to the assembly of wheat root-associated bacterial communities, the contributions of deterministic processes under WS were 19.4–38.5% higher than those under the WM rotation across the root-associated compartments, indicating the substantial impact of a soybean legacy effect on wheat root selection of microbes. Plant growth-promoting rhizobacteria with the potential to fix nitrogen, produce indole-3-acetic acid, and inhibit diseases such as Betaproteobacteriales, Azospirillales and Dyella sp., were identified within the OTUs that were consistently enriched across all the wheat root-associated compartments and developmental stages, which were also important predictors of wheat yield. This study elucidates the role of crop rotation in modulating the dynamics of crop root-associated bacterial communities, and underscores the potential of targeted microbiome manipulation for optimizing wheat production and enhancing soil health.
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This study aims to investigate the impact of residual nitrogen from the wheat season on the soil nitrate content and distribution during the soybean growth season in a winter wheat-summer soybean rotation system. It aims to clarify the dynamic relationship between soil nitrate content and the development of root nodules and root system architecture in soybean and reveal the pathways through which residual nitrogen from wheat season and soil nitrogen dynamics during the soybean season affect soybean yield.
According to the residual nitrogen levels of wheat stubble soil in double cropping region of the Huang-Huai-Hai Plain, five residual nitrogen levels were set: 5.25 mg·kg-1 (N5), 10.00 mg·kg-1 (N10), 20.00 mg·kg-1 (N20), 40.00 mg·kg-1 (N40), and 60.00 mg·kg-1 (N60) in 2021 and 2022. The soil nitrate content in the root layer (0-40 cm) was measured before soybean sowing, at the six-leaf stage, the flowering stage, and the early pod-filling stage. The above-ground and below-ground biomass, root nodule dry weight, and root traits at the six-leaf stage and the early pod-filling stage, as well as the yield at the harvest stage were analyzed.
The residual nitrogen increased the soil nitrate content during the soybean growth season, and it rapidly leached with rainfall. Differences in soil nitrate content among treatments persisted until the early pod-filling stage and the six-leaf stage in 2021 and 2022, respectively. The soil nitrate content at the six-leaf stage was 23.44-24.42, 24.98-28.07, 16.99-28.21, 23.81-45.34, 33.37-53.78 mg·kg-1 in 2021, and 7.63-7.84, 8.02-8.86, 8.32-8.71, 9.43-10.01, 15.40-17.92 mg·kg-1 in 2022. The soil nitrate content in the root layer at the six-leaf stage was significantly correlated with soybean yield, when it fell within the range of 17.83-40.33 mg·kg-1, the yield increased with its increase; when it was 7.63-17.83 or 40.33-53.78 mg·kg-1, the yield decreased with its increase; the yield reached its maximum at 7.63 or 40.33 mg·kg-1. At the six-leaf stage, an increase in soil nitrate content resulted in higher above-ground biomass, root area, and root width, while it reduced below-ground biomass, root nodule dry weight, lateral root number, and root tip number. At the early pod-filling stage, above-ground biomass, root area, taproot root length, and root tip number showed a decreasing trend after an initial increase and subsequent decrease with the soil nitrate content at the six-leaf stage. Among the root nodules and root traits significantly affected by soil nitrate content at the six-leaf stage, root nodule dry weight, root area, root width, and root tip number were the main factors influencing soybean yield.
Residual nitrogen from the wheat season affects the soil nitrate content in the root layer during the soybean growth season both temporally and spatially. It regulates the nitrate content in soybean root layer at six-leaf stage, and thus impact above-ground biomass and yield by influencing the development of soybean root nodules and root systems. We recommend to apply nitrogen with appropriate amount and period based on residual nitrogen levels in order to control the nitrate content in the root layer of soybean during the seedling stage at around 7.63 or 40.33 mg·kg-1. This will enhance nitrogen fertilizer utilization efficiency and achieve higher yields.
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