| The world plantation area is increasing in recent years and China has become the world’s largest plantation owner. It is of great significance to study the soil respiration (Rs) in forest plantations under different tree species for accurately evaluating the carbon sequestration potential and the ability of mitigating future climate change. Future climate change is expected to alter the primary productivity of forest ecosystems, and thus potentially change the quality and quantity of aboveground litter inputs which could affect soil carbon emission and sequestration. Tree species may have different soil microbial biomass and community composition due to the differences in quality and quantity of aboveground litter inputs and decomposition rate, thus influence the carbon and nitrogen cycling. But studies on the effect of tree species litter on soil microbial biomass and community composition are still limited. A litter manipulation experiment was carried out in 19-year-old Mytilaria laosensis and Cunninghamia lanceolata plantations replanted in C. lanceolata woodland in subtropical China between January 2012 and February 2013. Three treatments including litter exclusion, reciprocal litter transplant and control were conducted within 2x2m2 plots in randomized block design. Soil respiration rate (Rs) and microbial biomass carbon and nitrogen were measured monthly and soil microbial community composition were also investigated by a one time-point measured soil PLFA profiles. The objective of this study were to (1) compare the Rs under the two tree species and assess the relative contribution of litter Rs of different tree species; (2) Investigate the impact of tree species and litter quality on soil microbial biomass and community composition and its significance to maintain soil fertility. The results showed that:(1) Tree species significantly affected soil Rs. The annual Rs flux and mean Rs rates were 1068 g C·m-2 a-1 and 2.95 μmol CO2·m-2 s-1 under M.laosensis and 823 g C·m-2 a-1 and 2.37μmol CO2 ·m-2 s-1 under C.lanceolata. litter exclusion and transplant significantly decreased the annual fluxes under M.laosensis plots by 29.8% and 14.2%, respectively. Litter exclusion caused a slight decline in annual flux but litter transplant markedly increased the annual flux by 37.8% in C.lanceolata plots. Using the decay constants of the two litter species in a reciprocal litter transplant decomposition experiment, priming effects were calculated as the amount of litter-derived respiration minus decomposed carbon from litter. Results showed that M. laosensis litter aroused higher priming effect than C.lanceolata litter, suggesting that the quantity and quality of litter input caused by different tree species could result in different below-ground responses. The temperature sensitivity (Q10) of Rs decreased to 1.86 and 2.05 in the litter exclusion and transplant plots compared to 2.14 in the control plots under M.laosensis plantations. However, the Q10 in the litter exclusion, litter transplanting and control plots were 1.40,1.52 and 1.52 in C.lanceolata plantations, respectively, which are lower than other studies in subtropical China.(2) Coreelations showed that Rs were exponentially correlated with the soil temperature and negatively correlated with the soil moisture under both M. laosensis and C.lanceolata plots. Non-liner regressions of soil temperature and soil moisture together accounted for 68.9-77.0% and 36.2-63.8% of monthly variation in soil respiration in M.laosensis and C.lanceolata plots, respectively, indicating that other factors were responsible for interpreting variations in Rs.. Linear regression analysis showed that Rs was significantly correlated with annual litter biomass, fine root biomass, soil organic carbon storage (SOC), light fraction organic carbon storage (LFOC), microbial biomass carbon(MBC) concentration and dissolved organic carbon(DOC) concentration (P<0.05), which were significantly different between species. Stepwise regression analysis showed that litter biomass and MBC concentration was the most important affecting factors of Rs. Our results suggest that tree species induced difference of Rs may be attributed to the changes in the quantity and quality of litter and the microbial biomass and we concluded that the effects of litter manipulation on Rs were dependent on tree species, the contributions of litter to Rs. can be attributed to the quantity and quality of litter input, the changes in soil labile carbon and soil microbial biomass in subtropical forest plantations.(3) Litter manipulation induced changes in soil microbial biomass and community composition were dependent on tree species. Litter manipulation had no significant effects on soil microbial biomass C and N under M. laosensis plantation. 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, gram-negative 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 revealed that further studies are required to confirm the relative contribution of above-and belowground C inputs in different tree species plantations in subtropical China. |
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