This study aimed to investigate the impact of different topographic factors (such as slope position and slope gradient) and ridge tillage methods (such as transverse ridge tillage and longitudinal ridge tillage) on soil nutrient content and fertility indexes of sloping arable land, which would provide a scientific basis for selecting the ridge tillage method and maintaining soil fertility of sloping arable land in this region.

The study was conducted on a typical long, gentle slope arable land in Hongxing Farm, Bei’an City, Heilongjiang Province. Grid-based sampling was conducted in plots with two different ridge tillage methods (transverse ridge tillage plot, 18 points; longitudinal ridge tillage plot, 11 points). Comparative analysis was carried out to assess the effects of slope position and slope gradient on soil nutrient contents and the Soil Fertility Index (SFI) under the two ridge tillage methods. Analysis of variance (ANOVA) was used to test the impact of topographic factors, ridge tillage methods, and their interactions on the differences in soil nutrient spatial distribution between the two plots. Finally, variance partitioning analysis (VPA) was used to quantify the contribution of each factor to the explanatory degrees of SFI.

The differences in soil nutrient content and fertility index between the two plots were significant. The mean values of soil organic matter, total nitrogen, total phosphorus, total potassium, effective phosphorus, and available potassium were all significantly higher in the transverse ridge tillage plot compared with the longitudinal ridge tillage plot. However, the pH value was significantly lower in the transverse plot than that in the longitudinal plot. Consequently, the SFI of the two plots was ranked as follows: transverse ridge tillage>longitudinal ridge tillage (P<0.05). Slope position and slope gradient significantly influenced the spatial distribution of soil nutrients in plots with different ridge tillage methods, resulting in significant differences in the explanatory degree of each influencing factors on SFI variation between the two plots. In the transverse ridge tillage plot, the slope gradient differences caused by the micro-terrain (explanatory degrees, 32.62%) were the main drivers of soil nutrient differentiation. In this plot, soil total and effective phosphorus decreased as the slope increasing. In the longitudinal ridge tillage plot, soil organic matter, total nitrogen, and total phosphorus were the highest at the middle slope, and soil organic matter, total nitrogen, total potassium, effective phosphorus, and available potassium initially decreased and then increased with rising slope. In this plot, the slope position (explanatory degrees, 6.81%) and slope gradient (explanatory degrees, 7.22%) influence the distribution of soil nutrients. A comprehensive analysis of the SFI across the entire slope surface revealed that ridge tillage method had the highest explanatory degrees for the spatial variation of SFI (15.46%), followed by slope position (9.54%). The combined explanatory degree of the interaction between ridge tillage method and the two topographic factors, slope gradient and slope position, was 9.49%.

Topographic factors played a key role in soil nutrient migration in sloping arable land, and the impact of each factor (slope gradient and slope position) varied significantly under different ridge tillage methods. In transverse ridge tillage, slope differences caused by internal micro-terrain helped retain soil nutrients, while in longitudinal ridge tillage, slope gradient and slope position influenced the spatial differentiation of soil nutrients. Across the entire slope surface, the contribution of ridge tillage method to the spatial variation in soil fertility was greater than that of topographic factors. Therefore, the management of black soil sloping farmland needed to consider the combined effects of ridge tillage methods and topography.

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.