Environmental controls on soil pH in planted forest and its response to nitrogen deposition

Background: Soil pH is important for controlling many soil properties. The variation in soil pH can be associated with changes in climate, soil buffering system, nitrogen deposition, and plants. However, there still lacks a comprehensive study exploring the effects of all these factors on soil pH simultaneously. Here we aimed to investigate the environmental controls on the spatial variation of soil pH in planted forests across Northern China and reveal its response to different-forms of nitrogen deposition for different species of tree plantations. Methods: We sampled 1980 soil profiles from 660 planted forest plots (3 profiles in each plot) in Northern China. We used correlation analyses and structure equation models (SEM) to explore the impacts of multiple environmental factors on soil pH. Results: Climate (water balance, temperature) and soil inorganic carbon accounted for most variations of soil pH. Specifically, the concentration of hydrogen ions ([H+]) varied almost isometrically with soil inorganic carbon, which was also the major buffering system in this region. Nitrogen deposition affected both soil pH values and soil buffering system. Results from structure equation model indicated that nitrate nitrogen directly decreased soil pH, while ammonium nitrogen mostly affected soil pH indirectly through its impacts on soil inorganic carbon. The responses of soil pH to nitrogen deposition were species-specific, and conifer stands tended to have higher soil acidification rate than stands of other tree species. Conclusions: Our study provides important information for understanding mechanisms controlling the spatial pattern of soil pH in planted forests and highlights the need to develop informed policies for soil resource management under increasing threats from anthropogenic nitrogen deposition.