关键词: 小麦-大豆轮作, 土壤硝态氮, 土壤残留氮, 根瘤, 根系构型, 大豆产量

Abstract:

【Objective】 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.【Method】 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.【Result】 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.【Conclusion】 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.

Key words: wheat-soybean rotation system, soil nitrate, soil residual nitrogen, root nodule, root system architecture, soybean yield