Variation characteristics of microorganisms at different soil depths of typical forests in southwest China.

Microbial biomass and community structure play a significant role in soil carbon cycling. There is a large amount of organic carbon in the subsoil, but most studies on soil microbial community have focused on the surface soil. The changes and influencing mechanisms of microbial community in subsoil are unclear. We analyzed soil microbial biomass and community structure at different soil depths (0-20, 20-40, 40-60, 60-80, and 80-100 cm) in three typical forests in southwest China, Xishuangbanna tropical rain forest, Ailao Mountain subtropical broad-leaved forest, and Lijiang temperate coniferous forest, by using phospholipid fatty acid technology, to explore their variation characteristics and influencing factors in different forests and soil depths. The results showed that contents of soil organic carbon and total nitrogen decreased gradually, microbial biomass declined significantly. The ratio of Gram-positive bacteria to Gram-negative bacteria (G+:G-) reduced gradually, while the ratio of fungi to bacteria (F:B) increased with the increasing soil depth. Microbial community turned from G--dominated which adapted to eutrophic environment into G+-dominated which adapted to oligotrophic environment. The three forest types differed little in soil microbial biomass, but different significantly in microbial community structure. Ailao Mountain subtropical broad-leaved forest and Lijiang temperate coniferous forest had much higher F:B at 0-20 cm than Xishuangbanna tropical rain forest, while significantly higher G+:G- at 0-100 cm in Xishuangbanna tropical rain forest was observed. Results of the redundancy analysis showed that the contents of soil organic carbon and total nitrogen were the main factors determining microbial biomass, with combined explanation of 78.3%. Results of the stepwise regression analysis showed that C:N was the most important driving factor on F:B and G+:G-. The change in microbial community structure and the decrease in biomass along soil profile might strongly affect the dynamics of soil organic carbon in southwest China forests.

N and P combined addition accelerates the release of litter C, N, and most metal nutrients in a N-rich subtropical forest.

Imbalanced nitrogen (N) and phosphorus (P) depositions are profoundly shifting terrestrial ecosystem biogeochemical processes. However, how P addition and its interaction with N addition influence the release of litter carbon (C), N, P, and especially metal nutrients in subtropical forests remains unclear. Herein, a two-year field litterbag experiment was conducted in a natural subtropical evergreen broadleaved forest of southwestern China using a factorial design with three levels of N addition (0, 10, and 20 g N m-2 y-1) and P addition (0, 5, 15 g P m-2 y-1). During two years of decomposition, N- and P-only addition treatments decreased the accumulated mass loss and release rates of litter C, N, P, K, Na, and Mn (p < 0.05); N and P coaddition treatments increased the accumulated mass loss and release rates of litter C, N, K, Na, Mn, and Cu (p < 0.05) and decreased the accumulated release rates of litter P and Mg (p < 0.05); the C/P and N/P ratios of the residual litter increased under the N-only addition treatments (p < 0.05) and decreased under the P-only addition and N and P coaddition treatments (p < 0.05). Overall, the results suggest that combined N and P supply can increase biological activities and thus accelerate the release of litter C, N, and most metal nutrients, as expected within the framework of ecological stoichiometry and growth rate hypothesis. Our study also highlights that the effect of N addition on litter C and nutrients release depends on P availability.

The amounts and ratio of nitrogen and phosphorus addition drive the rate of litter decomposition in a subtropical forest.

Nitrogen (N) and phosphorus (P) control biogeochemical cycling in terrestrial ecosystems. However, N and P addition effects on litter decomposition, especially biological pathways in subtropical forests, remain unclear. Here, a two-year field litterbag experiment was employed in a subtropical forest in southwestern China to examine N and P addition effects on litter biological decomposition with nine treatments: low and high N- and P-only addition (LN, HN, LP, and HP), NP coaddition (LNLP, LNHP, HNLP, and HNHP), and a control (CK). The results showed that the decomposition coefficient (k) was higher in NP coaddition treatments (P < 0.05), and lower in N- and P-only addition treatments than in CK (P < 0.05). The highest k was observed with LNLP (P < 0.05). The N- and P-only addition treatments decreased the losses of litter mass, lignin, cellulose, and condensed tannins, litter microbial biomass carbon (MBC), litter cellulase, and soil pH (P < 0.05). The NP coaddition treatments increased the losses of litter mass, lignin, and cellulose, MBC concentration, litter invertase, urease, cellulase, and catalase activities, soil arthropod diversity (S) in litterbags, and soil pH (P < 0.05). Litter acid phosphatase activity and N:P ratio were lower in N-only addition treatments but higher in P-only addition and NP coaddition treatments than in CK (P < 0.05). Structural equation model showed that litter MBC, S, cellulase, acid phosphatase, and polyphenol oxidase contributed to the loss of litter mass (P < 0.05). The litter N:P ratio was negatively logarithmically correlated with mass loss (P < 0.01). In conclusion, the negative effect of N addition on litter decomposition was reversed when P was added by increasing decomposed litter soil arthropod diversity, MBC concentration, and invertase and cellulase activities. Finally, the results highlighted the important role of the N:P ratio in litter decomposition.