A better understanding of soil microbial ecology is critical to gaining an understanding of terrestrial carbon (C) cycle-climate change feedbacks. However, current knowledge limits our ability to predict microbial community dynamics in the face of multiple global change drivers and their implications for respiratory loss of soil carbon. Whether microorganisms will acclimate to climate warming and ameliorate predicted respiratory C losses is still debated. It also remains unclear how precipitation, another important climate change driver, will interact with warming to affect microorganisms and their regulation of respiratory C loss. We explore the dynamics of microorganisms and their contributions to respiratory C loss using a 4-year (2006-2009) field experiment in a semi-arid grassland with increased temperature and precipitation in a full factorial design. We found no response of mass-specific (per unit microbial biomass C) heterotrophic respiration to warming, suggesting that respiratory C loss is directly from microbial growth rather than total physiological respiratory responses to warming. Increased precipitation did stimulate both microbial biomass and mass-specific respiration, both of which make large contributions to respiratory loss of soil carbon. Taken together, these results suggest that, in semi-arid grasslands, soil moisture and related substrate availability may inhibit physiological respiratory responses to warming (where soil moisture was significantly lower), while they are not inhibited under elevated precipitation. Although we found no total physiological response to warming, warming increased bacterial C utilization (measured by BIOLOG EcoPlates) and increased bacterial oxidation of carbohydrates and phenols. Non-metric multidimensional scaling analysis as well as ANOVA testing showed that warming or increased precipitation did not change microbial community structure, which could suggest that microbial communities in semi-arid grasslands are already adapted to fluctuating climatic conditions. In summary, our results support the idea that microbial responses to climate change are multifaceted and, even with no large shifts in community structure, microbial mediation of soil carbon loss could still occur under future climate scenarios.

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Net N mineralization rate (NMR), net N consumption rate (NCR), microbial biomass carbon (MBC) and nitrogen (MBN), potentially mineralizable N (PMN) and mineral N (N-NH 4 + and N-NO + and N-NO + 3 -) were measured in paddy soil at -) were measured in paddy soil at -five growth stages of rice to determine the effect of long-term fertilization in subtropical China. The studied long--term treatments included CK (no fertilization), N, NP, NPK and NPK + OM (NPK plus organic manure). The NPK + OM treatment gave the highest values of the measured variables among all treatments. There was no significant difference in other treatments except for mineral N and PMN at early growth stages. All these variables were generally highest at transplanting stage as two thirds of fertilization was applied as basal fertilizers and the rice uptake was low. Then they decreased or leveled off with the rice growth stages except for MN in all treatments. Stepwise regression revealed that NMR was significantly correlated with MBC and N-NH 4 + ( + ( + R 2 = 0.954, P < 0.01) at all rice growth stages. So, mineral plus manure fertilizer application and more mineral fertilizer as topdressing were recommended in subtropical paddy soil.

To gain insight into microbial responses to topography, annual burning, and N addition, a field experiment was conducted from April 2005 to December 2009 in a semiarid grassland of northern China. Soil physicochemical properties, microbial biomass, and microbial community composition were measured in 2006 and 2008. A larger ratio of fungi/bacteria was observed in the upper slope than in the lower slope. Interannual climate fluctuation could have modified the effects of topography on microbial biomass and composition. Burning effects on microbial biomass and composition also depended on year, which could be attributed to low fire severity resulting from decreasing fuel load over time or microbial resilience. Nitrogen addition exerted a much stronger influence on microbial biomass in 2008 compared with 2006 and reshaped microbial communities through decreasing the relative proportion of fungal groups [arbuscular mycorrhizal fungi (AMF) and nonmycorrhizal fungi] in 2008. Overall, these results highlight dynamic responses of soil microbial communities to both the intrinsic features (topography) and exogenous disturbances (fire or N deposition) of the semiarid grassland.