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

Abstract. 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.