[Effect of nitrogen additions on soil pH, phosphorus contents and phosphatase activities in grassland]

Phosphorus is a key nutrient for all plant species and a limiting factor for grassland ecosystem function. In recent years, in response to the rapid increase of global nitrogen deposition, soil phosphorus contents and phosphatase activities changed to varying degrees in grassland ecosystems. We conducted a meta-analysis to examine the responses of soil pH, total phosphorus (TP), available phosphorus (AP), as well as activities of alkaline phosphatase (AlP) and acid phosphatase (AcP) in soils to nitrogen addition amount, nitrogen type, experimental duration, and sampling depth. The correlation between soil pH and phosphatase response ratio was investigated. The results showed that nitrogen addition significantly reduced soil pH, TP and AlP activity, while significantly increased AcP activity, but had no significant effect on AP. Soil pH and AlP activity significantly decreased under nitrogen addition >5 g·m^-2·a^-1, and AcP activity significantly increased under high nitrogen addition (>10 g·m^-2·a^-1). The contents of TP and AP significantly decreased when nitrogen addition was 5-10 g·m^-2·a^-1. NH₄NO₃ treatment significantly reduced soil TP and increased AcP activity, while urea treatment significantly reduced soil pH and AlP activity. Across all nitrogen addition amounts, when the experiment duration was 3 to 10 years, soil TP content and AlP activity were significantly reduced. Soil pH was significantly reduced after three years nitrogen addition, and AcP activitiy was significantly increased after 10 years nitrogen addition. In the 0-10 cm soil layer, the TP content and AlP activity significantly decreased, while the AP content significantly increased. In >10 cm soil layer, the AP content was significantly decreased. The significant negative correlation between soil pH and AcP activity indicated that change in soil pH caused by nitrogen addition may be an important factor for the variation of soil phosphatase activity.