The Effect Of Simulated N Deposition On Gross Soil N Transformation Rates In A Typical Mixed Broadleaved-Korean Pine Forest

Nitrogen (N), as one of the main limited factors in temperate forest ecosystem, generally regulates the productivity of ecosystem. However, with the increase of human activities, large amount of N were input to forest ecosystem in wet and dry N deposition forms, changing the pattern of N cycling in ecosystem extremely. And the forest soil, as an important carrier of N storage and transformation progresses, would also response to high-leveled N deposition, including the change of N transformation rates and the modification of N2O emission rates and sources.The effect of nitrogen (N) deposition on soil gross N transformations were studied on a typical mixed broadleaved-Korean pine forest soil in Xiaoxing’an Mountains in northeastern China, which experienced 6 years of simulated N deposition. The characteristics of gross N transformations in this forest soils were not significantly affected by the repeated N fertilizations that simulated the N depositions. Nitrate (NO3-) was primarily produced via oxidation of NH4+(i.e., autotrophic nitrification), whereas oxidation of organic N (i.e., heterotrophic nitrification) was negligible. The differences in ammonia-oxidizing bacteria (AOB) or ammonia-oxidizing archaea (AOA) abundances were not significantly different among the N fertilization treatments (N0, N30, N60, N120), which was possibly the main mechanism for the non-significant effect on the gross nitrification rates. For all studied soils, the gross NH4+immobilization rates were consistently lower than gross N mineralization rates, leading to net NH4+ mineralization. Furthermore, gross nitrification rates were much higher than total NO3- consumption rates, which resulted in a built-up of NO3-, highlighting the high risk of N losses via NO3- leaching or gaseous N emissions from the studied soils. The effect of N fertilizations on the emissions of N2O was not significant except the N30 treatment, the emission rate of which was higher than that in the NO treatment (p< 0.05).51.88-84.86%of N2O emissions were from the denitrification process, which was always higher compared with fractions from nitrification, suggesting that denitrification is the relatively main process responsible for N2O emissions. The ratios of N2O-N emission from total nitrification (equal to autotrophic nitrification) observed in the present investigation were from 0.01%to 0.10%, and the ratios of N2O-N emission from denitrification were from 14.65%to 25.75%. The patterns of the ratios from nitrification and denitrification were similar:NO> N120> N30> N60, which was possibly related to soil pH-organic matter interactions.