Quantitative assessment of soil acidification in four Chinese forests affected by nitrogen deposition

Aims Nitrogen (N) deposition increases risks of accelerating soil acidification, but its impact could vary depending on acid-neutralizing capacity (ANC) and clay mineralogy. We analyzed effects of N deposition on acidification in four forest soils that differed in parent materials. Methods We developed the method for assessing contribution of external proton input, N transformation, and plant uptake to soil acidification quantitatively in forest soils derived from loess (Qipanshan in Shenyang Liaoning), sandstone (Dinghushan in Zhaoqing Guangdong), granite, and basalt (Jianfengling in Hainan). We further estimated the acidity required for soil pH decreases using ANC (sum of cations) to predict soil pH change in forests affected by N deposition. Results N deposition contributed 30%, 45%, and 4% of the total proton generation through nitrification in the Qipanshan, Dinghushan, and Jianfengling soils, respectively. Heavy N deposition (50 kg N ha(-1) yr(-1)) also increased acidification through net cation accumulation in growing plant biomass (3.0 kmol(c) ha(-1) yr(-1)) in Dinghushan, where soil pH decreased by 1 unit in 30 years. Acidity could account for 56% of exchangeable Al accumulation (268 kmol(c) ha(-1)), but not for depletion of ANC or total base reserves in the Dinghushan soil. Proton generation associated with N deposition was smaller for the depletion of ANC or total base reserves required for soil pH decrease in Qipanshan and Jianfengling. Conclusion We found quantitatively that acidification progressed through increased nitrification and plant productivity in the forest soil due to chronic N deposition and low ANC and basic cation reserves derived from sandstone.