Response of erosion reduction effect of typical soil and water conservation measures in cropland to rainfall and slope gradient changes and their applicable range in the Chinese Mollisols Region, Northeast China

Mollisols are rich in organic matter, which makes them suitable for cultivation and for enhancing global food security. Mollisols are experiencing severe soil erosion due to overfarming and a lack of maintenance. Thus, suitable soil and water conservation measures (SWCMs) are needed to protect Mollisols. However, how SWCMs respond to changes in slope gradient and rainfall and their effective application area remain ambiguous. Using a long-term field observation dataset, meta-analysis, and statistical test methods, we reveal the typical erosion reduction effect of SWCMs and their responses to changes in slope gradient and rainfall. Next, we calculated a coupling factor, P·S, by slope gradient and rainfall in flood season to determine the effective SWCMs application threshold. Compared with bare land, no-tillage (NT), contour ridge tillage (CT), ridge hedgerows (RH), and terraced fields (TF) had an average erosion reduction coefficient of 91.5%. There was a significant exponential increase in the correlation between P·S and the soil erosion amount in plots with typical measures. According to this correlation and soil loss tolerance (200 t/km2), the upper limits of P·S (NT: 564 mm; CT: 885 mm; RH: 1135 mm) were determined and utilized to determine the effective application areas (NT: 311.40 103 km2; CT: 320.86 103 km2; RH: 323.72 103 km2) at the plot scale. In wet years, the applicable area of SWCMs declined toward the low-elevation foothills and flat terrain. All the results are applicable when slope length within 20 m. These results provide a basis for the precise allocation of SWCMs in Mollisols area and promote the scientific utilization of Mollisols resources.