Effects of slope morphology and position on soil nutrients after deforestation in the hilly loess region of China

Abstract

Deforestation can lead to severe losses of soil nutrients. However, the impacts of the slope morphology and position on the spatial distributions of soil nutrients after deforestation and their relationships are unclear, thereby limiting our understanding of the mechanism of soil degradation. In this study, we aimed to identify the relationships between the changes in soil nutrients and the slope morphology (i.e., slopes with different widths or lengths) after deforestation. Soil samples were collected from forestland slopes with different morphologies after deforestation for 28 years in the hilly area of the Loess Plateau, China. We analyzed the effects of the length, width, and position of the slope on the pH, soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), nitrate-N (NO3−), ammonium-N (NH4+), available phosphorus (AP), and available potassium (AK) distributions after deforestation. The spatial autocorrelation analysis indicated that the Moran's I index value for soil nutrients was larger on deforested slope than that of forest land, thereby suggesting that deforestation increased the spatial heterogeneity of the soil nutrients. After deforestation, the spatial autocorrelation distances increased for the soil pH and TP, whereas they decreased for SOC, TN, NH4+, NO3−, and AK. The TN, NH4+, and NO3− contents were significantly higher on the narrow slope than the wide slope. Among the three slope lengths, the soil nutrient losses were greater on the longer slope because of the more severe erosion of the slope after deforestation. The slope position significantly affected the distributions of the soil nutrients. In the vertical direction, the soil nutrient contents decreased continuously in the 0–10, 10–20, and 20–40 cm soil layers, but then increased in the 40–60 cm soil layers, mainly due to the eluviation of nutrients to deep soil layers. These results demonstrate that the slope morphology and position should not be overlooked when predicting the responses of soil nutrients to deforestation in the hilly loess region of China.