Effects of six years of simulated N deposition on gross soil N transformation rates in an old-growth temperate forest

Elevated atmospheric nitrogen (N) deposition has been detected in many regions of China, but its effects on soil N transformation in temperate forest ecosystems are not well known. We therefore simulated N deposition with four levels of N addition rate (N0, N30, N60, and N120) for 6 years in an old-growth temperate forest in Xiaoxing'an Mountains in Northeastern China. We measured gross N transformation rates in the laboratory using N-15 tracing technology to explore the effects of N deposition on soil gross N transformations taking advantage of N deposition soils. No significant differences in gross soil N transformation rates were observed after 6 years of N deposition with various levels of N addition rate. For all N deposition soils, the gross NH4 (+) immobilization rates were consistently lower than the gross N mineralization rates, leading to net N mineralization. Nitrate (NO3 (-)) was primarily produced via oxidation of NH4 (+) (i.e., autotrophic nitrification), whereas oxidation of organic N (i.e., heterotrophic nitrification) was negligible. Differences between the quantity of ammonia-oxidizing bacteria and ammonia-oxidizing archaea were not significant for any treatment, which likely explains the lack of a significant effect on gross nitrification rates. Gross nitrification rates were much higher than the total NO3 (-) consumption rates, resulting in a build-up of NO3 (-), which highlights the high risk of N losses via NO3 (-) leaching or gaseous N emissions from soils. This response is opposite that of typical N-limited temperate forests suffering from N deposition, suggesting that the investigated old-growth temperate forest ecosystem is likely to approach N saturation.