Effects Of Simulated Warming And Nitrogen Addition On Soil CO₂ Emissions,Microbial Communities And Activities At Different Soil Depths In A Young Subtropical Chinese Fir Forest

As the second largest carbon(C)flux between the atmosphere and terrestrial ecosystems,soil respiration(Rs)plays vital roles in regulating ecosystem C cycling.More than 50%of soil respiration from the microbial mineralization of soil organic matter(SOM).At present,the research of soil carbon emissions mostly focus on surface soil,while soil carbon emissions and the response of microorganisms at different depths are not well knowa Although carbon concentrations decline with soil depth in profiles,the total contribution from subsurface soil layers(below 30 cm soil depth)can be 50%of the total organic C in a 1 m profile.In order to estimate and predict the impact of climate changes on ecosystem C cycle accurately,it's necessary to gain more knowledge regarding the C emissions and microorganisms in subsurface soil.To elucidate the effect of.global climate change on soil carbon oxidate(CO2)emissions,microbial biomrass,cormmunity structure and extracellular enzyme activities,we simulated soil warming(W)and nitrogen addition(N)at a young Chinese fir mesocosm The Fick's laws of diffusion was used to estimate soil CO2 flux at different depths(0-10 cm,10-20 cm,20-40 cm and 40-60 cm).Microbial biomass C and nitrogen was monitored by chloroform fumigation extraction.Microbial community composition and extracellular enzyme activities were assessed by phospholpid fatty acids(PLFAs)analysis and kinetics of enzymatic activities,respectively.Enzyme activities involve the C-acquire(?-glucosidase(BG),?-D-cellobiosidase(CBH),phenoloxidase(PHO)and peroxidase(PEO))and nitrogen-acquire(N-acetyl-?-D-glucosaminidase(NAG)).(1)Results showed that the annual average of CO2 efflux was 2.36 ?mol m-2 S-1 in control treatment,and subsurface soil contributed 10%of soil CO2 production W significantly increased surface soil CO2 fluxes(17%),improved the contribution of subsoil(20-60 cm)to soil CO2 emission(19%).But N decreased surface soil CO2 fluxes(7%),including a reduction of CO2 production in the topsoil(11%in the 0-20 cm layer),and an increase in subsoil(44%).However,the combination of nitrogen addition and soil warming(WN)did not affect surface soil CO2 fluxes,increased the contribution of CO2 production in subsoil(27%)instead.(2)W significantly increased microbial biomass carbon(MBC)at depths of 20-40 cm(29%)and(51%)in comparison to CT,whiling decreased MBC at depths of 0-10 cm and 10-20 cm by 60%and 30%,respectively.N reduced MBC by 15%at 0-10 cm layer,but did not affect MBC at others layers.WN inhibited MBC(27%)at 0-10 cm layer.(3)W and N did not affect microbial communities at 0-10 cm soil layer,but significantly modified microbial communities below 10 cm soil depth.W and WN significantly enhanced the abundances of Gram-positive bacteria below 10 cm depth in comparison to CT.However,N improved the abundances of Gram-negative bacteria below 10 cm depth.(4)W and N enhanced enzyme activities in topsoil Nevertheless,W inhibited enzyme activities in subsoil,N stimulating enzyme activities at 20-40 cm layer.In 0-10 cm and 10-20 cm layers,W and N dramatically improved BG(26%-44%),CBH(18%-66%),NAG(13%-74%),PHO(32%-69%)and PEO(18%-20%).In 20-40 cm layer,W induced a reduction in BG(28%),CBH(32%),NAG(32%)and PEO(13%).While N stimulated BG(5%)and CBH(16%),inhibited PHO(50%)and PEO(12%).In 40-60 cm layer,W significantly decreased BG(31%)and PHO(14%)as well as N decreased BG(58%),CBH(53%),PHO(59%)and PEO(32%).This finding illuminated that warming and nitrogen addition all stimulated the CO2 emission in subsoil;nitrogen addition decreased it in topsoil,while warming increased it in topsoil.In the other hand,warming stimulated extracellular enzyme activities in subsoil and microbial biomass in topsoil,but suppressed microbial biomass in topsoil and extracellular enzyme activities in subsoil.In consequence,soil enzymes may drive CO 2 production in 0-20 cm soil depth,so does microbial biomass in 20-60 cm soil depth.In summary,the capacities of carbon storage in subsoil weakens in the context of global climate changes,which is important as a source for CO2 in the global carbon cycle.The mechanisms controlling C dynamics are the different in topsoil and subsoil.Our results suggest that the validity of predictive future soil C dynamics would be improved by the inclusion of both soil organic carbon mineralization and microorganisms response to global climate changes in subsoil.