Differences in Characteristics of Sample Sites Explain Variable Responses of Soil Microbial Biomass to Nitrogen Addition: A Meta-Analysis

Many projects have demonstrated that excessive nitrogen (N) addition changed soil microbial biomass; however, the extent varies at different sites around the world. Here, meta-analysis was performed to evaluate the effects of sample site characteristics on the responses of soil microbial biomass to N addition based on 1,642 data from 181 field experiments. Our results showed that soil microbes were more sensitive to N addition in sample sites with mild environmental and soil conditions, such as medium mean annual temperature (1–15 °C), mean annual precipitation (350–600 mm), humidity (30–35), soil carbon-to-nitrogen (C:N) ratio (9–20), soil pH (5.0–7.5), or lower ambient N deposition rate (< 25 kg N ha −1 y −1 ). Under extreme conditions, however, the soil microbial biomass showed less or even insignificant responses to N addition. Contrarily, the ambient soil microbial characteristics, such as total microbial biomass, the ratios of fungi-to-bacteria (F:B), and gram-positive to gram-negative bacteria, were linearly correlated with the microbial responses ( P < 0.05). Furthermore, these microbial characteristics are comprehensively structured by the environmental conditions and soil properties. For soil microbiota, fungal biomass decreased the most after N addition, especially arbuscular mycorrhizal fungi. Saprophytic fungal biomass changed insignificantly and actinomycete biomass even increased. These results indicate that when accurately predicting the effect of N deposition on soil microbial biomass, the characteristics of sample sites should be synthetically considered, especially soil F:B and C:N. Furthermore, more field experiments in Southern hemisphere countries should be executed in the future, as less data are available in these regions.