Transformation From NHx to NOy Deposition Aggravated China's Forest Soil Acidification

Elevated nitrogen (N) deposition and changes in reduced or oxidized component contribution greatly affect soil acidification. China has experienced a significant transformation of N deposition components from NHx to NOy over the past 40 years, but the effects of component transformation on soil acidification are poorly understood. Therefore, long-term monitoring data and literature on N deposition, combined with the results of isotope experiments, were used to explore the contributions of different N forms on soil acidification in China's forests. Here, all processes related to NHx and NOy, including the transformation to NH4+ and NO3- and subsequent N cycling in the soil, were considered. We found that N-induced soil acidification in 80% area of China's forests was dominated by NHx deposition, and the other areas (South China) were dominated by NOy deposition in 2010s. From 1980 to 2019, the average contribution of NHx was higher than that of NOy but the latter contribution continued to increase. Meanwhile, the results showed that soil acidification increased with the decrease of the ratio of NHx to NOy (R-NHx/NOy), which is mainly because NOy is more easily leached in the form of NO3- than that of NHx under the influence of different plant preferences and soil retention rates, resulting in a higher net proton production of NOy. Our research has powerful implications for policymaking, provides a theoretical basis for formulating different N reduction policies in different regions, and points out that the synergistic effect of R-NHx/NOy changes should be considered to alleviate soil acidification.

Automated summary

N deposition in China's forests has led to soil acidification, with NHx deposition dominating in 80% of the area and NOy deposition dominating in South China. The contribution of NOy has been increasing over the years, while the contribution of NHx has been higher but decreasing. Soil acidification is influenced by the ratio of NHx to NOy, as NOy has a higher net proton production due to leaching in the form of NO3-. This research is important for policymaking and emphasizes the need to consider the synergistic effect of R-NHx/NOy changes to alleviate soil acidification.