Nitrogen deposition is linked to changes in mineral-associated organic carbon in forest soils in the United Kingdom

Abstract Aims Nitrogen (N) is an essential element for soil microbes, and its addition to soil can have variable effects on decomposition of soil organic carbon (SOC). We sought to quantify how N addition affects soil microbial decomposition and SOC stability by examining chemically free particulate organic C (POC) and chemically bound mineral-associated organic C (MAOC). Methods Working along a naturally occurring inorganic N deposition gradient (from 6–32 kg ha −1 yr −1 ) across 26 UK forest sites, we examined correlations among POC, MAOC, and inorganic N pools, comparing the responses of organo-mineral vs. mineral soils, and broadleaf vs. conifer forests. Results POC stocks and total microbial biomass C were not affected by N deposition in broadleaved forest soils, yet MAOC stocks increased. In conifer forest soils, MAOC decreased and POC and the microbial biomass pools were unchanged. POC and MAOC stocks were significantly greater in mineral soils than organo-mineral soils, while most inorganic N measurements did not differ by soil type. Climatic factors were found to have a weak effect on soil C pools, but POC, MAOC, DOC, and microbial biomass C all decreased with increasing stand basal area. Conclusions Our results show that the effects of inorganic N addition on soil C and N cycling can depend upon local plant and soil types. Alongside N deposition, forest attributes such as stand age and forest type have a strong effect on the microbial and geochemical factors that ultimately control POC and MAOC stability.