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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. [ABSTRACT FROM AUTHOR]

Purpose: Ammonium and nitrate are the main components of anthropogenic nitrogen (N) from atmosphere, while their ratio varies worldwide. However, it remains unclear whether forest soil ecosystem changes differ when various ratios of mixed N are added. Methods: Ammonium and nitrate were mixed in different ratios (3:7, 4:6, 5:5, 6:4, and 7:3) and forest soils were fertilized for four years. Then, the soil pH, microbial biomass, enzymatic activities, and organic carbon (SOC) were determined. The potential mechanism was analyzed using structural equation modeling. Results: Ammonium addition induced a greater fungal biomass decrease than nitrate addition (-0.618 vs. -0.329). The fungal biomass decrease further led to a decline in degradation enzymes, which resulted in SOC accumulation. Phosphatase activity increased and correlated with C-degrading enzymatic activities after N addition, indicating that phosphorous may become the limiting factor that controls degradation. Both ammonium and nitrate addition caused soil acidification (P < 0.05), but the acidification did not affect the enzymatic activities (P > 0.05). Moreover, when the added mixed N component ratios were closer to the ambient N component ratios, fewer changes in soil microbial biomass, enzymatic activities, and SOC were observed. This can be explained by the home-field advantage, because soil microbes may have adapted more readily to the ambient N components of the sample site and display fewer responses when the added N is nearly similar to the ambient N types. Conclusion: When designing N addition field experiments, the type of N compounds should be considered, besides the amount and duration. [ABSTRACT FROM AUTHOR]