Human-induced nitrogen (N) deposition may have significant impact on soil microorganisms in forest ecosystems. In this study, we combined a coordinated experimental study at six forest sites in eastern China and a global-scale meta-analysis of field fertilization experiments in forests to explore the effects of N addition on soil microorganisms in forest ecosystems. The experimental study was based on the NEECF (Nutrient Enrichment Experiments in China’s Forests) network, which includes six sites and eight natural forests (from temperate to tropical) in eastern China. The meta-analysis had 1569 observations extracted from 110 papers from the literature. Measured/extracted variables included soil chemistry, microbial biomass, community structure (PLFA) and enzyme activities.
Results/Conclusions
The experimental study at the six NEECF sites showed that (1) 6-7 years of N addition increased soil available N, suppressed N-acquisition enzymes in temperate forests, and inhibited fungal PLFA abundance in most forests; and (2) had no significant effect on microbial biomass carbon (C) and N, total and bacterial PLFA abundance, and C and phosphorus (P) acquisition enzyme activities across all six forests. Moreover, the global-scale meta-analysis indicated that (3) N addition led to lower microbial biomass C but higher microbial biomass N, and inhibited fungal (but not bacterial) PLFA abundance; (4) N addition stimulated C and P acquisition enzyme activities, inhibited oxidative enzyme activities, and had enzyme-specific effect on N-acquisition enzymes; and (5) the effect size of N addition did not vary between AM (arbuscular mycorrhizal) vs. ECM (ecto mycorrhizal) forests or depend on the amount of N addition, but it correlated with the duration of N addition. Molecular analysis of soil microbial communities (DNA sequencing and functional gene) is in progress and will be presented at the meeting. Overall, combining coordinated multi-site experimental study and global-scale meta-analysis of the literature should improve our mechanistic understanding of the responses of soil microorganisms to human-induced nitrogen deposition in forest ecosystems.