Evaluation of the quality of cultivated-layer soil based on different degrees of erosion in sloping farmland with purple soil in China

The assessment of cultivated-layer soil quality (CLSQ) is important for improving crop growth capacity and is the premise for taking effective remedial measures. The cultivated-layer soil quality index (CLSQI) is a significant tool for evaluating CLSQ. This study aims to select the most effective indicators to construct a minimum data set (MDS) for soil in erosion-impacted hilly areas with purple soil in China and to analyse the association between CLSQ and the erosional environment. Taking the erosion of purple soil of sloping farmland in China as the focus of this study, different combinations of scoring methods, indicators, and data set methods were compared. In this study, the MDS is constructed by principal component analysis (PCA) in conjunction with the norm value, and the accuracy of the MDS is verified by the Nash effective coefficient. Furthermore, the nonlinear scoring (NLS) method is used to normalize the indicator data and then assess the CLSQ of soils with different degrees of erosion using the CLSQI. The results showed the following. 1) The soil indicators significantly varied under the different erosion degrees: the soil productivity showed a decreasing trend with increasing erosion, while the soil anti-erosion ability and tillage performance showed increasing trends with increasing erosion. 2) For the study area, the generated MDS included saturated hydraulic conductivity, bulk density, shear strength, penetration resistance and available phosphorus. 3) The CLSQI-NLS evaluation method was most suitable for this research topic, and combined with the MDS indicators, it could be used to accurately evaluate the CLSQ. 4) As the erosion degree intensified, the CLSQI showed an obvious decreasing trend, and the CLSQI under soil erosion of 20 cm (S-20, 0.393) was significantly lower than that under other erosion degrees, indicating that soil erosion gradually reduced the CLSQ. After the cultivated-layer soil was eroded and the subsoil was converted to cultivated-layer soil, the soil quality obviously decreased. The indexing approach used in this study provides a practical and time- and cost-effective method for the quantitative evaluation of CLSQ for erosion-prone soil in hilly areas with purple soil worldwide.