Reviews and syntheses: Soil resources and climate jointly drive variations in microbial biomass carbon and nitrogen in China's forest ecosystems

Microbial metabolism plays a key role in regulating the biogeochemical cycle of forest ecosystems, but the mechanisms driving microbial growth are not well understood. Here, we synthesized 689 measurements on soil microbial biomass carbon (C-mic) and nitrogen (N-mic) and related parameters from 207 independent studies published up to November 2014 across China's forest ecosystems. Our objectives were to (1) examine patterns in C-mic, N-mic, and microbial quotient (i.e., C-mic / C-soil and N-mic / N-soil rates) by climate zones and management regimes for these forests; and (2) identify the factors driving the variability in the C-mic, N-mic, and microbial quotient. There was a large variability in C-mic (390.2 mg kg(-1)), N-mic (60.1 mg kg(-1)), C-mic : N-mic ratio (8.25), C-mic / C-soil rate (1.92 %), and N-mic / N-soil rate (3.43 %) across China's forests. The natural forests had significantly greater C-mic (514.1 mg kg(-1) vs. 281.8 mg kg(-1)) and N-mic (82.6 mg kg(-1) vs. 39.0 mg kg(-1)) than the planted forests, but had less C-mic : N-mic ratio (7.3 vs. 9.2) and C-mic / C-soil rate (1.7% vs. 2.1 %). Soil resources and climate together explained 24.4-40.7% of these variations. The C-mic : N-mic ratio declined slightly with C-soil : N-soil ratio, and changed with latitude, mean annual temperature and precipitation, suggesting a plasticity of microbial carbon-nitrogen stoichiometry. The C-mic / C-soil rate decreased with C-soil : N-soil ratio, whereas the N-mic / N-soil rate increased with C-soil : N-soil ratio; the former was influenced more by soil resources than by climate, whereas the latter was influenced more by climate. These results suggest that soil microbial assimilation of carbon and nitrogen are jointly driven by soil resources and climate, but may be regulated by different mechanisms.