Responses of soil respiration and its temperature sensitivity to nitrogen addition: A meta-analysis in China

Annual atmospheric nitrogen (N) deposition in China increased approximately 8 kg N ha(-1) from the 1980s to the 2000s, which may have greatly affected soil respiration (R-s) and its temperature sensitivity (Q(10)). Although numerous individual studies have been conducted, the responses of R-s to simulated N deposition remain controversial. To reconcile the conflicting results of R-s and expand our knowledge about the response of Q(10) to N addition, a dataset with 333 independent observations in China was compiled, and a meta-analysis was performed. Our results showed that N addition increased R-s by 7.1% (P < 0.05) across all biomes. The positive response degree of R-s in croplands (27.0%, P < 0.05) was significantly greater than those in the grassland and forest biomes, which indicated that R-s in anthropogenic ecosystems might be more sensitive to N enrichment. In contrast to low and medium N levels (<= 20 g m(-2) yr(-1)), high N addition levels (> 20 g m(-2) yr(-1)) inhibited R-s due to a decrease in plant fine root biomass (RB). Compared with ammonium nitrate, urea is more likely to stimulate the release of soil C. Regression models showed that the R-s response ratio (RR) was positively correlated with pH and the RRs of microbial biomass carbon (MBC) and RB. Increases in both plant roots and microbial biomass induced by N addition directly promoted R-s, and N enrichment likely increases R-s in soil with a high pH. The temperature sensitivity parameter, Q(10), decreased by 3.7% (P < 0.05) across all biomes and showed an overall negative response for different biomes (except wetlands and deserts), N addition levels and N types. The Q(10) response ratio was positively correlated with the RR of pH. Soil acidification induced by proton release during N fertilizer transformation might accelerate the decomposition of recalcitrant organic matter and further decrease the Q(10) value. Our study provides valuable information to Chinese environmental policy- and decision-makers in their attempts to evaluate the effects of N deposition on terrestrial ecosystem C cycle.