Effects of nitrogen addition on soil inorganic N content and soil N mineralization of a cold-temperate coniferous forest in Great Xing'an Mountains

Nitrogen (N) plays an important role in carbon and nutrient cycling in forest ecosystems. Previous studies have shown that increased atmospheric N deposition has led to changes in forest soil N transfomations and soil N availability. However, knowledge gap exists about the impacts of N deposition on soil N dynamics of forest, especially for the boreal forests in China. In order to explore such impacts, a low-dose (Control, 0kgNhm−2years−1, Low N—10kgNhm−2years−1and High N, 40kgNhm−2years−1) and multiple forms (NH4Cl, KNO3 and NH4NO3) of simulated atmospheric N deposition experiment was conducted from 2010–2012 in a cold-temperate coniferous forest in the Great Xing'an Mountains. This paper reports inorganic N (NH4+-N and NO3−-N) content, net ammonification rate, net nitrification rate and net N mineralization rate in the 0–10cm mineral soil during the growing season from May to September in 2012, the third year after N addition. Results that during the growing season, soil inorganic N was dominated by NH4+-N, showing a significant seasonal variation. Soil NH4+-N and NO3−-N responded differently to exogenous N addition. Soil NH4+-N content was more affected by the forms of N addition, while soil NO3−-N content was more affected by the doses of N addition. Relative to control, NH4+-N content of 0–10cm mineral soil increased greatly with NH4+-N addition, which was completely different from the results observed in the first growing season of N addition in that no obvious effects of N addition on NH4+-N content of 0–10cm mineral soil was observed. In agreement with results observed in the growing season, NO3−-N content of 0–10cm mineral soil still showed a trend of enrichment with low dose N addition. For the N-limited forest ecosystem, soil net ammonification with a significant seasonal variation contributed most to soil net N mineralization, and soil net ammonification increased under NO3−-N addition, especially in the case of low dose N addition. Similar to soil net ammonification, soil net nitrification rates increased more under low-dose N addition compared to high-dose N addition. The possible explanation for our results is that a shift of 0–10cm mineral soil NH4+-N in response to N addition over time could be attributed to continue NH4+-N addition-induced increase of litterfall decomposition and decrease of microbial N immobilization and autotrophic nitrification due to a reduction in soil pH. Continual NO3−-N addition might decrease the number and activity of autotrophic nitrifying bacteria, showing a positive effect on net ammonification in the 0–10cm mineral soil. Compared with low doses of N addition, the effect of high doses of NH4+-N addition on soil acidification could be higher and high doses of NO3−-N addition might suppress the activity of nitrifying bacteria, increase the risk of leaching and denitrification, and stimulate microbial N immobilization in the N-poor soil. Our results, to some extent, suggest that increasing atmospheric N deposition will promote the accumulation of 0–10cm mineral soil NH4+-N and promote soil net N mineralization of the cold-temperate coniferous forest in Great Xing'an Mountains.