Effects of simulated atmospheric nitrogen deposition on inorganic nitrogen content and acidification in a cold-temperate coniferous forest soil

Abstract The effects of increasing nitrogen (N) deposition on soil N dynamics and soil acidification in boreal forests are not well understood. Previous studies have shown that changes in N availability could, in turn, affect the magnitude and direction of soil net N mineralization and nitrification rates as well as soil acidification. To better understand the responses of soil inorganic N content and acidification to increased atmospheric N deposition, a cold-temperate coniferous forest in the Daxing’anling region was selected for a low level N addition experiment with different forms of N. It was hypothesized that the soil inorganic N dynamics influenced by plant uptake and soil transformation, immobilization and loss in these N limited boreal forest soils would be sensitive to N addition, even under low level input. It was also hypothesized that the combined effects of NH 4 + – N and NO 3 - – N addition on soil acidification would be different from their individual effects. During the growing season (May to September) of 2010, soil samples of the organic (O ae horizons combined) and mineral (0–30 cm) layers of each plot were collected to measure inorganic N ( NH 4 + – N and NO 3 - – N ) contents and soil pH. The results showed that most of the NH 4 + – N and NO 3 - – N in the cold-temperate coniferous forest soil was accumulated in the organic layer, and decreased with depth. Contrary to expectations, N addition had no effect on the NH 4 + – N content of the organic or mineral soil, while low N addition significantly increased the soil NO 3 - – N content in the mineral horizons. Moreover, the addition of nitrate fertilizers such as KNO 3 and NH 4 NO 3 had a strong effect on soil NO 3 - – N content in the top 10 cm of the mineral layer. Both soil NO 3 - – N and NH 4 + – N contents showed obvious seasonal variations, which were consistent with the seasonal pattern of the soil pH values. This was that low and high N addition tended to lead to accumulated soil NO 3 - – N in the peak of the growing season. High N and NH 4 NO 3 addition significantly reduced soil pH in the mineral horizons, and the effects of NH 4 + – N and NO 3 - – N addition on soil acidification were additive, which agreed with the second hypothesis. These results suggest the responses of the cold-temperate coniferous forest to chronic N deposition are more sensitive than other N rich forests. Soil NO 3 - – N and NH 4 + – N dynamics inconsistently respond to N addition in different soil horizons, months, added N levels and N types. The difference in early response to N input between NH 4 + – N and NO 3 - – N could be related to soil N status and differences in the content and properties of N ions. Environmental variables including soil temperature and moisture have the greatest impact on the N dynamics. Soil acidification caused by N addition affects the overall performance of soil inorganic N dynamics including plant uptake and soil N transformation and loss. It should be noted that soil pH in N limited boreal forest soils may be more vulnerable to N deposition, and its critical load may be 30–50 kg N ha −1  year −1 . Long-term studies are needed to reveal the responses of N dynamics and soil pH to increased N deposition and to clarify the mechanisms responsible for soil acidification.