ABSTRACT

Soil acidification induced by nitrogen (N) deposition has important effects on species diversity and ecosystem function. The potential role of phosphorous (P) limitation on these effects was overlooked and observed only after decades of atmospheric N deposition. Few studies have addressed the long-term effect of P addition on soil acidification resulting from N deposition. Changes in precipitation patterns could also have important effects on soil acidification in semi-arid regions. To investigate the effects of long-term N, P, and water addition on soil acidification, a nine-year experiment was conducted on the semi-arid steppe in north-eastern China, in which N, P and water were added in a factorial design. N addition decreased soil pH significantly, P addition decreased soil pH significantly under ambient precipitation, while there was no effect on pH when water was added. Under the ambient precipitation, N and P addition decreased soil acid buffering capacity (ABC) by 86% and 42%, respectively, and NP addition decreased ABC by 63%. Under water addition, N and NP addition decreased ABC by 74% and 58%, respectively, P addition increased ABC by 83%. Addition of water changed the way soil N and P affect soil ABC. Without additional water, there was a direct negative influence of inorganic N on soil ABC, whereas with additional water there was an indirect effect on soil pH. Without addition of water P had a positive effect on soil ABC via soil effective cation exchange capacity, whereas with addition of water, the effect of P occurred via its effects on soil organic matter. Our results suggest that long-term P addition could alleviate damage to the soil acid buffering system due to N deposition to some extent by increasing ABC. We conclude that water availability mediates the effects of N and P addition on soil ABC.