Interaction between long gently sloping topography and contour-ridge on the spatial partitioning of soil organic carbon in a black soil

The contour ridge tillage had a certain effect on reducing soil organic carbon (SOC) loss in sloping cropland, but the unique long and slow topography would interact with contour ridge cropping on the spatial differentiation of SOC in sloping cropland of the black area. However, the risk of SOC loss caused by this interaction has not attracted enough attention yet. In this study, Hongxing Farm of Beian Branch, Heihe City, Heilongjiang Province, a typical black soil area, was taken as the study area, and the two directions of the contour ridge cropping and longitudinal waterline direction were divided according to the study purpose under the contour ridge cropping measure, and a total of 75 sample sizes were taken in the two directions. The geographic detector model as well as one-way ANOVA and Pearson's correlation analysis were conducted to explore the spatial differentiation of SOC and its interactive effects. In the contour ridge tillage direction, SOC content showed a trend of gradual increase from the top position of the slope to the waterline in the ridge furrow; and it showed a trend of increase and then decrease from the top position of the slope to the waterline in the ridge platform. Along the longitudinal waterline direction, the SOC content showed a trend of increasing from lower to upper position in the ridges, and it showed a trend of increasing and then decreasing in the ridge platform. Soil organic carbon content was significantly greater in ridges than in furrows (P < 0.05). The soil organic carbon content of the upper was still significantly different from the lower due to the waterline created by the broken ridge (P < 0.05). Spatial differentiation of soil organic carbon resulted in greater spatial variability of SOC along the direction of contour ridge cropping and less spatial variability along the direction of the longitudinal waterline. Pearson's correlation analysis showed that organic carbon was significantly negatively correlated with the erodibility factor (correlation coefficient of -0.228 and -0.238, P < 0.05 for furrow and platform, respectively), and that β-glucosidase (BG) and microbial biomass carbon (MBC), which are related to carbon cycling, were highly significantly positively correlated with the erodibility factor in furrow (correlation coefficient of 0.398 and 0.676, P < 0.01). Geographic detector analysis showed that, among the single factors, the longitudinal waterline had the greatest effect on soil organic carbon differentiation, and its explanatory rate reached more than 61% and 52% for ridges and ridgetops, respectively; among the two factors, the interaction between longitudinal waterline and other factors had the greatest effect, and all of them strengthened the explanatory power of soil organic carbon, especially the interaction between longitudinal waterline and topography was the most obvious, and the explanatory rate in ridges and ridgetops both reached more than 90%. The spatial differentiation of SOC in sloping cropland in the black soil area was mainly affected by the interaction between the longitudinal waterline and topography on the downslope. Although the contour ridge tillage was able to intercept runoff, the broken ridges generated by the influence of the topography on the long and gentle slopes will exacerbate the loss of SOC induced by soil erosion. Therefore, it is suggested to consider the joint influence of contour ridges and topography at the same time in the management of black soil slope cropland, and to pay enough attention to the potential risk of contour ridges, so as to achieve the optimized effect for erosion prevention.