RESULTS
Across all the studies, the mean effect size of N addition on soil total microbial biomass was -7.69 % (P < 0.01). However, the responses of total microbial biomass to various N types were different. NH4+-N addition led to a rather significant microbial biomass decline than other N types (-18.9 %,P < 0.01). In all types of NH4+-N fertilizers, (NH4)2SO4 induced the most aggravated microbial biomass decline (-21.3 %, P< 0.01). NO3-N addition also significantly inhibited total microbial biomass (-7.26 %, P< 0.05) but significantly less than NH4+-N (ANOVA, P < 0.05). In all types of NO3-N fertilizers, KNO3 addition induced the most serious microbial biomass decline (-21.8 %, P < 0.01). Contrarily, total microbial biomass increased when Ca(NO3)2 was fertilized (+3.66 %). NH4NO3 addition significantly decreased microbial biomass as well (-7.51 %, P < 0.01), but also less than single NH4+-N (ANOVA,P < 0.05). CO(NH2)2addition induced the smallest microbial biomass decrease (-6.99 %,P < 0.05) among all N types (Fig. 1 ).
Further analysis revealed that different soil microbial characteristics displayed various responses to these N fertilizers. Soil fungal biomass significantly decreased after N addition (-14.2 %, P < 0.01). Among all types of N fertilizers, NH4+-N induced to the greatest fungal biomass (-47.6 %, P < 0.01) than other N types (ANOVA,P < 0.05), especially for (NH4)2SO4 (-54.5 %,P < 0.01, Fig. 2A , Supplementary OnlineMaterial 6 ). Similarly, soil bacterial biomass significantly decreased after N addition (-11.3 %, P < 0.05). For all types of N fertilizers, bacterial biomass decreased the most after NH4+-N addition (-30.2 %, P< 0.01) than others (ANOVA, P < 0.05), while CO(NH2)2 addition did not significantly affect them (P = 0.071, Fig. 2B , Supplementary Online Material 6 ). Because of the less decreases of bacterial biomass than fungal biomass, F/B ratio significantly declined for after N addition (-7.70 %, P < 0.01). F/B ratio declined the most after NH4+-N addition (-27.9 %,P < 0.01). CO(NH2)2addition also induced to significant F/B ratio decline (-15.6 %,P < 0.05), but less than NH4+-N (ANOVA, P < 0.05). However, F/B ratio did not significantly change by NH4NO3 addition (P = 0.4288,Fig. 2C , Supplementary Online Material 6 ).
The biomass of both actinomycete and saprophytic fungi did not significantly change after N addition (P = 0.403 and 0.973). The changes were also not significantly different among various N types, although actinomycete biomass significantly increased after NH4+-N addition (-26.3 %, P< 0.05, Fig. 2D, E , Supplementary Online Material 6 ). AM fungal biomass decreased the most (-27.1 %, P< 0.01) among all microbial characteristics. Furthermore, NH4+-N addition induced to the most serious decline (-37.7 %, P < 0.05), while CO(NH2)2 led to more significantly biomass decline compared with NH4NO3(-36.9 vs . -20.9 %, ANOVA, P < 0.05,Fig. 2F , Supplementary Online Material 6 ). Both G+ and G bacterial biomass did not statistically significant change after N addition (P = 0.8088 and 0.2235, Fig. 2G, H , Supplementary Online Material 6 ). However, G+/ G ratio showed significant increase after CO(NH2)2addition (+14.4 %, P < 0.01), but was insignificant affected by NH4NO3 (Fig. 2I ,Supplementary Online Material 6 ).
There were significant differences in soil microbial characteristics changes among ecosystem types. In general, N addition induced significant total microbial biomass decrease in forest tundra (-11.5 %,P < 0.01), grassland (-10.9 %, P < 0.01), forest (-6.90 %, P < 0.01), and shrub (-4.77 %, P < 0.05), but increase in wetland (+12.9 %,P < 0.05). In all types of ecosystems, microbial characteristics decreased the most after NH4+-N addition. Compared with broadleaved forest (-2.70 %), in forest ecosystem, soil microbial characteristics in coniferous forest decreased more seriously (-3.51 %) after N addition for all N types, especially after NH4NO3 addition (-2.70 vs . -7.44 %, ANOVA, P < 0.05, Table 1 ,Supplementary Online Material 7 ).
The results of the regression analyses showed that soil microbial responses to N addition were significantly correlated with the N addition rate and application duration (Supplementary Online Material 4 ). Among all the types of N fertilizers, NH4+-N addition revealed the most serious total soil microbial biomass decline and CO(NH2)2 addition induced to the smallest decline (Fig. 3 ). Various microbial characteristics revealed similar trends (Supplementary Online Material 4 ).