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 ).