Enhanced deposition of nitrate alters microbial cycling of N in a subtropical forest soil

In China, atmospheric deposition of NO3 (-) is increasing rapidly. However, information on how microbial N cycling in forest soils may respond to increasing deposition of NO3 (-) is currently lacking. Determination of process- and pool-specific N transformation rates can provide additional insights into the controls on the production and consumption of inorganic N, and microbial function. Here, we present results from a laboratory N-15 tracing study with a soil (0-10 cm) from a subtropical forest receiving fertilization for more than 2.5 years at a rate of 0, 40, and 120 kg NO3 (-)-N ha(-1) year(-1). The process- and pool-specific N transformation rates were quantified with a N-15 tracing model. The directions of changes in microbial mineralization of labile and recalcitrant organic N were opposite under increased NO3 (-) additions. Microbial mineralization of labile organic N first decreased, then increased, while microbial mineralization of recalcitrant organic N showed the opposite in response to increasing NO3 (-) additions. Ammonium immobilization into labile and recalcitrant organic N was not changed by increased NO3 (-) additions. Nitrate additions did not affect heterotrophic and gross nitrification, but stimulated autotrophic nitrification. Nitrate immobilization decreased under increased NO3 (-) additions, with a greater reduction under low NO3 (-) addition treatment compared to high NO3 (-) addition treatment. Overall, our results reflect a contrasting change in microbial mineralization of liable and recalcitrant organic N under increased NO3 (-) additions, and a contrasting effect of low and high NO3 (-) additions on microbial mineralization of liable and recalcitrant organic N. It also has implications for our understanding of NO3 (-) deposition-induced nonlinear changes in net production and loss of NO3 (-) in subtropical/tropical forest soils.