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

Abstract

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 15N 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 15N 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.