Diverging patterns at urban-rural forest gradients: soil organic carbon stability responses to nitrogen addition

Numerous studies have explored the impacts of nitrogen (N) deposition on soil organic carbon (SOC) dynamics. However, limited research has investigated the modulatory role of N deposition in urban to rural forests and the underlying microbial mechanisms. We carried out a 5-year field study to explore the links between microbial properties (microbial biomass carbon (MBC), microbial diversity, community composition and functions) and the different SOC fractions (particulate organic carbon, POC and mineral-associated organic carbon, MAOC) submitted to three levels of N addition rates (0, 50, and 100 kg N ha-1 yr-1) in urban–rural gradient forests in eastern China.

We discovered that N addition raised the soil ammonium nitrogen concentration in urban and suburban forests. However, it had no effect on soil acidification or POC or SOC accumulation，and in urban forest the stability was due to the 105 % to 110 % increase in the mineral-associated organic carbon (MAOC) through enhancing peroxidase activity and microbial biomass carbon. On the contrary, high nitrogen input significantly reduced SOC stability in the suburban and rural forest stands. High nitrogen input contributed to the loss of MAOC (-33.6 %) in the suburban forest stand due to the enhancement of microbial biomass nitrogen. High nitrogen addition also decreased the ratio of MAOC to SOC in the rural forest stand by 29.8 % through indirect pathways mediated by the soil Ca²⁺ concentration and polyphenol oxidase activity. We concluded that SOC in the urban forest was stable when subjected to increased nitrogen deposition, primarily due to the enhancement of MAOC driven by microbial function. This finding has contributed to a better understanding  in predicting forest carbon cycling under conditions of global climate change and urban expansion.