The impacts of nitrogen addition on the active carbon pools in forest soils along an urban–rural gradient

Abstract

The increased nitrogen (N) deposition accompanying urbanization alters the dynamics of soil carbon (C) in urban and peri-urban forests. However, our understanding of the magnitude and mechanism of this impact is still minimal. In this study, experimental plots were established in urban, suburban, and rural forests to investigate the effects of three levels of N addition: no addition (0), low addition (50 kg N·ha−1·yr−1), and high addition (100 kg N·ha−1·yr−1). The responses of the active components in the soil C pool, including dissolved organic C (DOC) and microbial biomass C (MBC), were monitored after simulated N deposition treatments in forests along an urban–rural gradient. The results showed that the concentrations of DOC and MBC in forest soils along the urban–rural gradient exhibited significant seasonal and site variation. Soil DOC and MBC concentrations in the rural forest were significantly affected by N addition, increasing by 13.39 % and 45.90 % under the low N treatment, respectively, and MBC concentration increased by 28.40 % under the high N treatment. On the contrary, soil DOC and MBC concentrations in urban and suburban forests did not change significantly. Among studied influencing factors, the total dissolved N provided the crucial nutrient sources for microbial growth and activity and thus became the primary factor affecting the active components in the soil C pool. The insensitivity of urban and suburban forests' active soil C pools to added N indicates that the N fertilization effect brought by urban expansion may not have a lasting impact on soil C pools of rural forests.