Warming suppresses microbial biomass but enhances N recycling

Abstract

Abstract Temperate grasslands are semi-arid ecosystems where plant growth and productivity are constrained not only by water, but also by nitrogen (N). However, rising temperatures may cause a progressive decrease in the availability of water and N to plants in temperate grasslands, negatively affecting long-term C fixation. To predict whether declining N availability would occur under a future warmer climate, we quantified the warming effects on major processes and pools of N in temperate grasslands via a meta-analysis. Although microbial biomass (−6.4%) generally decreased in warming experiments, net N mineralization increased, suggesting increased N mineralization-immobilization turnover as a result of a “portfolio effect”. Contrary to our expectations, experimentally increasing temperature to temperate grasslands increased aboveground biomass (+8.3%), but did not increase N storage in vegetation. This occurred mainly because the vegetation shifted to species with lower N demands, e.g., C4 plants, reducing the role of N as a limiting factor in productivity. By contrast, N storage in the microbial biomass declined by 7.7%. Despite the large decline in soil total N (−3.8%), soil available N did not decrease with warming. Soil dissolved organic N and nitrate even showed a positive response, increasing by an average of 27.3% and 60.2%, respectively. Overall, our work indicates that under a future warmer climate, soil N availability might not decrease in temperate grasslands, even in the case of microbial decline. In the short term, warming may affect soil N availability mainly by its effect on N mineralization-immobilization turnover, rather than plant N uptake.