Effect of soil sealing on the microbial biomass, N transformation and related enzyme activities at various depths of soils in urban area of Beijing, China

Sealing of soils prevents the exchange of gas, water and nutrient between soil and other environmental compartments, and affects urban N flux, thereby resulting in certain negative impacts on soil functioning and urban environment. However, little information is available on the biogeochemical cycling and biological activities after sealing of soils in urban areas. The aim of this study was to assess the effects of soil sealing on N transformation and associated microbial properties. Soil samples were collected at four depths (0-10, 10-20, 20-30, 30-40 cm) from three land cover types: impervious land (sealing of soil), forest (vegetation) and bare land (no sealing and no vegetation) in Beijing, China. Soil inorganic N, net potential N-mineralization and nitrification rates, physicochemical properties, microbial biomass and activities of urease, protease and nitrate reductase were analyzed. Sealing of soils affected soil physicochemical properties, nutrient availability and microbial characterizations, which resulted in the reduction of soil net potential mineralization and nitrification. Soil pH and C/N ratio were increased while soil organic matter and total N were decreased after sealing of soils, especially in the 0-10 cm layer. Impervious sites had the lowest soil microbial biomass C (10.52-26.26 mg kg(-1)) and N (10.52-26.26 mg kg(-1)) among the three types of soils. In addition, microbial biomass increased along the depth gradient. Soil enzyme activities were also significantly affected by sealing of soils. Canonical correlation analysis suggested that reduction of soil net potential mineralization and nitrification in impervious sites was mainly caused by the variation of soil total N, soil organic matter and soil microbial biomass. Sealing of soils remarkably influenced the soil N transformation process and soil microbial activities in urban areas. Thus, our results suggested that reducing the negative effects of soil sealing on paved and unpaved areas could be useful measures to improve urban soil functions, and hence facilitate the remediation of the adverse impacts caused by urban N deposition.