Changes in soil nutrient and enzyme activities under different vegetations in the Loess Plateau area, Northwest China

This study examined the changes in soil properties and soil quality 30 years after cultivated farmland was restored back to forest land in Loess Plateau, China. Specifically, organic matter, total nitrogen (N) and phosphorus (P), available N, P and Potassium (K) contents in soils were tested and analyzed. In addition, enzyme activities of α-amylase, saccharase, polyphenol oxidase, cellulase, urease, catalase and alkaline phosphatase were also investigated. The study area has mostly been restored, in the past 30 years, back to grassland in some parts, and the other parts of forest lands of black locust, korshinsk peashrub, Chinese pine, mixed trees of Chinese pine and amorpha, and mixed trees of black locust and amorpha. Soil properties on a sloped farmland, located in the study area but has not had a chance to be restored, were tested and the result was treated as the soil properties of non-restoration. Soil properties of an 80-year old forest land of Chinese arborvitae were tested and the result was treated as that of a climax community. The study showed that after 30 years of restoration, nutrients content in the soil of mixed forest of black locust and amorpha increased significantly. However, nutrients content in the soil of mixed forest of Chinese pine and amorpha decreased. As to soil enzyme activities, saccharase, cellulase, urease, catalase and alkaline phosphatase increased while polyphenol oxidase activity decreased compared to non-restoration and climax community soils. The study also found that the organic matter content was relatively low in the restored soils, compared with not-restored land. This may be caused by the high enzyme activity per unit of organic carbon in the soils of the study area which tends to help decompose and therefore decrease the organic matter in soil.