Changes of above- and belowground carbon input affected soil microbial biomass and community composition in two tree species plantations in subtropical China

We employed Detritus Input and Removal Experiment( DIRT) to examine the effects of carbon( C) input on soil microbial biomass and community composition in 19-year-old Chinese fir( Cunninghamia lanceolata( Lamb.) Hook) and Mytilaria laosensis forests in subtropical China. The treatments included:( 1) reciprocal litter transplant; ( 2) litter removal; ( 3) no inputs( litter removal+ root trenching); and( 4) control. Each treatment had four 2 mx2 m plots. From January to December 2012,we collected the top 5 cm soil layer samples monthly from C. lanceolata and M. laosensis plots.Chloroform fumigation-extraction was used to determine the soil microbial biomass C and nitrogen( N) contents.Phospholipid fatty acid( PLFA) analysis was used to characterize the microbial community composition in soil samples collected in June 2012. Results suggested that the C input manipulations induced differences in soil microbial biomass and community composition were dependent on tree species. C input manipulations had no significant effects on soil microbial biomass C and N under M. laosensis. However,the concentrations of microbial biomass C and N seemed to increase under C. lanceolata. PLFA analysis showed that M. laosensis litter significantly increased the contents of bacterial PLFAs,fungal PLFAs and actinomycetes,and significantly enhanced the percent abundance of gram-negative bacteria and arbuscular mycorrhizal fungi under C. lanceolata. Litter removal significantly increased the contents of gram-positive bacteria,gramnegative bacteria, arbuscular mycorrhizal fungi, actinomycetes and fungi by 22%, 29%, 44%, 25% and 52%,respectively,and also increased the fungi to bacteria ratio by 21% compared with the control in C. lanceolata soil.However,litter removal and root trenching did not change soil microbial community composition in C. lanceolata and M.laosensis soils. Seasonal patterns of microbial biomass C and N were different between C. lanceolata and M. laosensis plantations. Nutrient availability rather than soil temperature,moisture or fresh litter input,may determine seasonal fluctuation of soil microbial biomass in this study. Our results suggested that further studies are required to confirm the relative contribution of above- and belowground C inputs in different tree species plantations in subtropical China.