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

Abstract

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 (Rs) and its temperature sensitivity (Q10). Although numerous individual studies have been conducted, the responses of Rs to simulated N deposition remain controversial. To reconcile the conflicting results of Rs and expand our knowledge about the response of Q10 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 Rs by 7.1% (P < 0.05) across all biomes. The positive response degree of Rs in croplands (27.0%, P < 0.05) was significantly greater than those in the grassland and forest biomes, which indicated that Rs 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 Rs 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 Rs 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 Rs, and N enrichment likely increases Rs in soil with a high pH. The temperature sensitivity parameter, Q10, 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 Q10 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 Q10 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.