Differential impacts of nitrogen compounds on soil acid phosphatase activity in a meadow steppe

Background Nitrogen (N) addition profoundly alters soil phosphatase activities and induces widespread microbial phosphorus (P) limitation in grassland ecosystems. However, the long-term effects of different rates and chemical forms of N addition on soil acid phosphatase (ACP) activity and P availability remain unclear. Here, based on a 10-year N addition experiment, we investigated the effects of a range of N addition rates (0-50 g N m(-2) yr(-1)) applied as either (NH4)(2)SO4 or urea on ACP activity in a northern China meadow steppe, with and without mowing management. Results We found that ACP activity remained unaffected by N addition until certain thresholds were reached (< 10 g N m(-2) yr(-1) for [(NH4)(2)SO4] and < 20 g N m(-2) yr(-1) for urea), beyond which it declined sharply. (NH4)(2)SO4 exerted a stronger inhibitory effect on ACP activity compared to urea, with the former primarily acting through soil acidification and the latter through a decrease in bacterial diversity. In contrast, mowing regimes did not significantly alter ACP activity under either N form. Conclusions Our findings underscore the necessity of accounting for both N addition rates and N forms when assessing atmospheric N deposition impacts on soil phosphatase activity. These results inform the development of management strategies to mitigate declines in soil enzyme activity and enhance the long-term sustainability of grassland ecosystems.