Quantification of the effect of soil erosion factors on soil nutrients at a small watershed in the Loess Plateau, Northwest China

Purpose This research aimed to quantitatively assess the effect of soil erosion factors on soil nutrients under similar rainfall conditions at a small watershed in the Loess Plateau, China, with emphasis on quantification of each factor's contribution to soil nutrients. Materials and methods Forty-five soil samples were collected and the soil texture and nutrients were analyzed. The integrated fertility index (IFI) was quantified with laboratory-measured available phosphorus, available potassium, total nitrogen, and soil organic matter based on principal component analysis and the soil erosion factors were computed. The feature importance of soil erosion factors that affect soil nutrients was estimated based on the gradient-boosting decision tree (GBDT) method. Results and discussion The GBDT optimized model revealed that the feature importance of soil erosion factors soil erodibility (K), slope length (L), slope steepness (S), vegetation cover (C), and mechanical erosion control (P) was 0.41, 0.11, 0.03, 0.39, and 0.06, respectively. The K and C factors occupied 80% of the importance and were the two predominant soil erosion factors affecting the soil comprehensive nutrients. The Pearson correlation coefficient between K, L, S, C, or P, and IFI was - 0.387, - 0.23, - 0.25, - 0.479, and - 0.532, respectively. The different order of factors between the Pearson correlation and feature importance may indicate that linear relation cannot be used to indicate the significance of each factor. Conclusions The soil erodibility and the vegetation cover could basically determine the quality of soil nutrients in the small watershed under similar rainfall conditions. The soil property itself and associated effects on the amount and rate of runoff under a certain rainfall erosivity had a dominant impact on the soil nutrients. Factor C greatly contributed to runoff resistance and protection of nutrients from erosion. The L, S, and P factors had weak effects on the IFI because of the generally low rainfall intensity, short rainfall duration, and other factors that may counteract the effects in the area.