Microbial adaptation to long-term N supply prevents large responses in N dynamics and N losses of a subtropical forest

Atmospherically-deposited nitrogen (N) can stimulate complex soil N metabolisms and accumulations over time. Whether long-term (decadal) N deposition effects on soil N transformations and functional microbes differ from the short-term (annual) effects has rarely been assessed. Here we conducted a laboratory 15N tracing study with soil samples from a short-term (one year) N addition site and a long-term (12 years) site in a subtropical forest. The effects of simulated N deposition on soil N2O emissions, N transformation rates and microbial nitrifying and denitrifying genes were determined. Our results showed that: (1) long-term N addition did not change soil N2O fluxes significantly in comparison to the short-term N addition. Denitrification, heterotrophic nitrification and autotrophic nitrification contributed 53%, 28% and 18% to total N2O emissions, respectively. (2) Autotrophic nitrification was the dominant N transformation process, except for the high-N treatment at the long-term site. The magnitude of soil N transformation rates was significantly different among N addition treatments but not between short- and long-term N addition sites. However, long-term N addition changed the responses of specific N transformation rates to N addition markedly, especially for the rates of nitrification, organic N mineralization to NH4+, NO3− immobilization and dissimilatory NO3− reduction to NH4+ (DNRA). (3) Responses of ammonia oxidizing archaea and bacteria (AOA and AOB) were more variable than those of denitrifying N2O-producers (nirK) and denitrifying N2O-reducers (nosZ), particularly at the long-term site. (4) The close correlations among N2O flux, functional genes and soil properties observed at the short-term site were weakened at the long-term site, posing a decreased risk for N losses in the acid subtropical forest soil. There is evidence for an adaptation of functional microbial communities to the prevailing soil conditions and in response to long-term natural and anthropogenic N depositions. [Display omitted] •Denitrification was the dominant N2O production pathway in the studied forest soil.•The magnitude of N transformation rates was similar at the short- and long-term sites.•Denitrifiers were stimulated by low- and moderate-N addition at both sites.•Long-term N addition induced larger changes in the response of nitrifiers.