Nitrogen deposition accelerates soil acidification in four Chinese forests

Abstract Aims Increasing nitrogen (N) deposition is hypothesized to cause a decline in soil pH in China. We tested to what extent N deposition is contributing to acidification of forest soils. Methods We quantified external proton (H+) input and net proton generation from N transformation and cation uptake by plants and acid-neutralizing capacity (ANC; sum of cations) in soil. This allowed us to estimate the required time for 1 unit drop of soil pH based on soil pH-ANC relationship in Shenyang, Dinghushan, and Jianfengling forests. Results The N deposition contributed to 30%, 45%, and 4% of total proton generation through nitrification in the Shenyang, Dinghushan, and Jianfengling forest soils, respectively. Heavy N deposition (50 kg N ha-1 yr-1) also increased proton generation associated with net cation accumulation in growing plant biomass (2.2 kmolc ha−1 yr−1) in the Guangzhou forest, where soil pH decreased by 1 unit in 30 years. Depletion of 1700 kmolc ha−1 ANC and 977 kmolc ha−1 total base reserve is required for 1 unit drop of soil pH, but the cumulative acid inputs for 30 years (120 kmolc ha−1) were much lower to meet these requirements. Acidity can account for 45% of exchangeable Al accumulation (268 kmolc ha−1) in the surface soil, but the remaining portion might be attributed to the heterogeneity of proton generation and consumption in the soil profile. Conclusion We found quantitatively that heavy N deposition accelerates soil acidification through increased nitrification and plant productivity.