Belowground Carbon Processes Feedback To Climate Change Along An Elevation Gradient In Subtropical Forests

Tropical and subtropical forests exert a large influence on the terrestrial carbon(C)balance,but the effects of future increases in temperature on tropical forest C cycling are uncertain.Currently,there is no consensus on how a changing climate will impact C inputs to,C cycling within and C loss from tropical ecosystems,with effects on belowground C processes being particularly understudied.Therefore,it's crucial to understand how belowground C processes in response to climate change in tropical and subtropical forests.We quantified soil-surface CO₂ efflux(‘soil respiration';F_S),aboveground litterfall a 4.0°C MAT gradient in subtropical forests in Wuyi mountain China to examine the temperature dependency of belowground C fluxes in tropical montane wet forest,and the mechanisms of soil C cycling in response to rising temperatures by doing soil cores transplant,mycorrhizal exclusion and litterfall manipulation experiments in subtropical forests.There are several important findings as follows:(1)In subtropical forests,soil microbial respiration in high-altitude is sensitive to simulated climate change,while there is no significant change at low altitudes.Divergent response of soil microbial respiration to climate change across elevation gradient can be partly explained by the difference in soil moisture content.Simulated climate warming increased the sensitivity of soil microbial respiration at high altitudes but decreased the sensitivity of soil microbial respiration at medium and low altitudes.The adaptation of soil fungal communities at different altitudes to climate change and soil nutrients availability are the main factors that driving soil microbial respiration in subtropical montane forests.(2)Soil fungal communities were significantly changed by mycorrhizal exclusion and rising temperatures in subtropical montane forests.The risk of soil C and nitrogen(N)loss was significantly increased after mycorrhizal isolation at different elevations,mainly because the relative abundance of saprophytes was significantly increased after mycorrhizal isolation,and its open N utilization strategy aggravated the decomposition of soil organic matter.After transplanting high-altitude soil to low-altitude,soil respiration in the treatment without isolated mycorrhiza increased significantly,even greater than that in the new host soil,which mainly caused by the nutrients competition between the original saprophytes in the high-altitude soil and the newly invaded ectomycorrhizal fungi.The effects of climate change and mycorrhizal fungi on soil N availability play a decisive role in soil C cycle.In the future,due to climate change and upward migration of vegetation at different altitudes and elevations,new mycorrhizal invasion may,at least in the short-term,greatly increase soil C and N loss.(3)Priming effects(PE)induced by high-quality litter might be more sensitive to climate change compared with low-quality litter,as indicated by the PE caused by high-quality litter at warm site were 140%greater than low-quality litter while no significant difference were observed at cool site.The divergent PEs induced by the high-and low-quality litters are mainly regulated by the microbial metabolic efficiency and the investment of litter-derived energy for microbial P-mining rather than N-mining in subtropical forests.Although both high-and low-quality litters could induce positive PEs in high and low altitudes,the new added C could compensate the soil C loss caused by positive PE,resulting in no significantly impact on soil C storage.(4)Soil respiration in low altitude are more sensitive to changes in quantity of litter than at high altitudes,because litter removal significantly reduces soil respiration at low altitudes but has no signifiant effect on high altitudes.Increasing litters results in positive PE at high altitude and negative PE at low altitude.We predicat that a future increase in litterfall of 30%with an increase in atmospheric CO₂ concentrations of 150 ppm could release about 0.04 t C ha^-1 yr^-1 from high-altitude forest soil,but could accumute about0.28 t C ha^-1 yr^-1 in low-altitude soil.Overall,in subtropical forests,the response of soil respiration to climate change is mainly driven by microbial acclimation across the elevation gradient.Therefore,climate warming might not increase the CO₂ released by microbial respiration as we expected.The rapid adaptation of microorganisms to climate change can mitigate the impact of climate change on the carbon cycle process of forest ecosystems.We speculate that in the future climate warming,the input of litters increased by the increase of atmospheric CO₂concentration may partially offset the effect of soil C decomposition caused by warming,so climate warming would have little effect on soil carbon storage.Consequently,we suggest that,belowground C processes from tropical forest ecosystem could be used to predict the responses of subtropical forest ecosystems to future climate change.