Changes in soil chemical properties following afforestation of cropland with Robinia pseudoacacia in the southeastern Loess Plateau of China

Abstract

Over the past several decades, a vegetation restoration project has been conducted in the southeastern Loess Plateau of China by converting cropland into forest plantations to combat soil erosion. However, the effects of land use change on soil fertility are poorly documented. Here, we assessed soil chemical properties in areas of the southeastern Loess Plateau following afforestation of cropland with Robinia pseudoacacia plantations that had been established for 16 years and 49 years, a 49-year-old naturally restored forest, and adjacent cropland. We found that the effect of afforestation on soil organic carbon and total nitrogen mainly occurred at shallow depths. Soil organic carbon and total nitrogen concentrations in the topsoil of both 49-year-old forests were significantly higher than those in cropland. However, the highest total phosphorus concentrations were observed in the topsoil of cropland due to the application of fertilizer. The nitrogen-fixing effect of leguminous Robinia pseudoacacia was not obvious through comparisons of soil total nitrogen within 49-year-old naturally restored and planted forests. Our results indicate that both natural restoration and planting can increase soil organic carbon stocks, but that the increase after 16 years is slight. In this experiment, we found that soil organic carbon concentrations were negatively correlated with soil pH for all four land use patterns, and that the strength of the correlation increased with time since restoration. In contrast to cropland and naturally restored forest, plantations of both ages showed soil acidification across their entire profiles. This may be due to the root exudate and rhizosphere environment created by R. pseudoacacia. These results provide important insights about the impacts of ecological restoration on soil chemical properties in the Loess Plateau and elsewhere.