Response of alpine soils to nitrogen addition on the Tibetan Plateau: A meta-analysis

- Nitrogen addition decreased soil pH.
- Nitrogen addition increased soil fertility.
- Effects of nitrogen addition varied with vegetation types and nitrogen types.
- Effects of nitrogen addition varied with mean annual temperature and precipitation.

A meta-analysis approach was used to identify general tendency of alpine soil responses to nitrogen fertilizer on the Tibetan Plateau. Nitrogen addition increased ammonium nitrogen (NH4+) and nitrate nitrogen (NO3−), but decreased pH. Effects of nitrogen addition on soil organic carbon (SOC), total nitrogen (TN), C:N ratio and pH differed among forests, alpine meadows and alpine steppes. Effects of nitrogen addition on NH4+, NO3−, soil microbial biomass carbon (MBC) and nitrogen (MBN) differed between alpine meadows and forests. Effects of NH4NO3 addition on SOC, TN, C:N ratio, NH4+, NO3−, MBC and pH differed with those of urea addition. The effect of nitrogen addition on pH was negatively correlated with mean annual temperature and mean annual precipitation. Soil conditions in control plots were positively correlated with the effect of nitrogen addition on pH, but negatively correlated with the effect of nitrogen addition on TN. Nitrogen addition rate was correlated with the effect of nitrogen fertilizer on NO3− and pH. Nitrogen addition duration was positively correlated with the effect of nitrogen fertilizer on pH. Therefore, effects of nitrogen fertilizers on alpine soils varied with nitrogen fertilizer types; climatic warming and precipitation change regulated effects of nitrogen fertilizer on alpine soils; and response of alpine soils to nitrogen addition may depend on initial soil conditions, rate and duration of nitrogen addition on the Tibetan Plateau.