Abstract: The effects of soil erosion on soil quality of sloping farmland in thick Mollisol region under different tillage methods remain unclear. Thus, three field plots with three tillage treatments of no ridge, longitudinal narrow-ridge and wide-ridge without crop planting were established across a 320-m natural slope, located at thick Mollisol region in Keshan County, Heilongjiang Province. Soil erosion rates were estimated by rare earth element (REE) tracing based on the point sampling placement method, soil erosion intensity and its spatial distribution were analyzed. A soil quality evaluation system was constructed using the minimum data set method, and the responding characteristics of soil quality to soil erosion intensity grades were analyzed on sloping farmland in thick Mollisol regions. The results showed that: 1) The average slope erosion rates of the no-ridge, longitudinal narrow-ridge, and longitudinal wide-ridge treatments were 1725.7, 2106.5, and 1898.4 t·km^-2·a^-1, respectively. Compared with the no-ridge treatment, the average slope erosion rates of longitudinal wide-ridge and narrow-ridge treatments increased by 10.0% and 22.1%, respectively. The slope section of 0-280 m was dominated by light and moderate erosion, whereas the slope section of 280-320 m was mainly dominated by soil deposition. 2) Clay content, soil organic carbon, total nitrogen, available phosphorus, and β-1,4-glucosidase were screened to construct the minimum data set for assessing soil quality. Among them, soil organic carbon and available phosphorus were the core factors affecting soil quality. The average soil quality index (SQI) values for the no-ridge, longitudinal narrow-ridge, and longitudinal wide-ridge treatments were 0.49, 0.51, and 0.50, respectively. 3) Under the three tillage methods, the SQI exhibited a significant negative correlation with soil erosion intensity grades. Soil erosion intensity directly affected SQI, and indirectly influenced SQI by altering soil physical and chemical properties. In conclusion, soil erosion was the core factor driving soil quality degradation of sloping farmland in the study area.

Key words: thick-layer Mollisol region, soil erosion-deposition, minimum data set, soil quality degradation