Soil microorganisms play fundamental roles in nutrient cycling within forests ecosystems. With elevated anthropogenic nitrogen (N) deposition at a global scale, however, little is known on how long-term N deposition affects soil microbial community in tropical forest ecosystems as well as in ecosystems with different land-use history. To address this question, we expanded on a long-term N deposition experiment in three tropical forests that vary in land-use history (primary, secondary, and planted forests) in Southern China, with variable N additions as NH4NO3 of 0, 50, 100 and 150 kg N ha-1 yr-1 (above the background), respectively. Soil bacterial and fungal communities were studied using 16s rRNA genes pyrosequencing and the internal transcribed spacer (ITS) sequencing, respectively.
(1) Fungi: Basidiomycota and Ascomycota were the dominant communities as the phylum level in all three forests (60-80% as relative abundance). The primary forest was the most diverse in terms of operational taxonomic units (OTUs). Non-metric multidimensional scaling analysis showed that the primary forest had significantly different community structures from that of the secondary and planted forests. However, a decade of N additions had no significant effects on fungal α-diversity (such as Shannon, chao1, observed species, and PD-whole-tree) for all three forests, although the dominant communities had different responses to long-term N additions, with an increasing abundance for Ascomycota in the primary forest and for Basidiomycotain the pine forest, but no changes in the secondary forest.
(2)Bacteria: Proteobacteria, Acidobacteria, and Actinobacteria were the dominant communities as the phylum level in the studied forests (70-80% as relative abundance).The total α-diversity were comparable among these forests, but non-metric multidimensional scaling analysis showed that the bacterial community structures were different between the primary and the secondary/planted forests. Similar to soil fungi, a decade of N additions had no significant effects on total α-diversity. However, the bacterial community structures were more sensitive to high N inputs in the primary forest than the other two, which generally showed no response to N inputs. Elevated N additions increased the abundances of Proteobacteria (mainly as Alphaproteobacteria and Gammaproteobacteria) and Actinobacteria, but decreased the abundance in Acidobacteria in the primary forest.
These findings suggested that long-term N deposition could alter soil microbial community structures rather than total biodiversity, and the primary forest was more sensitive to high N inputs than the secondary and planted forests. Soil acidification played an important role on changing microbial community structures in the primary forest.