Effects Of Changes In Carbon Input On Soil Respiration Of Three Forest Types In The Subtropical-Warm Temperate Transition Zone

As the second largest carbon(C)flux in terrestrial ecosystems,soil respiration is extremely crucial in regulating global C balance and climate change.Soil respiration is regulated by multiple abiotic and biotic factors,including soil temperature,moisture,microbial community,and vegetation types.Changes in plant species diversity and forest primary productivity under global change scenarios will alter the quality and quantity of above-and blow-ground C input.However,there is no clear mechanistic understanding of how soil respiration responds to changes in C input,which greatly limits the accuracy of model simulation and prediction of soil respiration dynamics under climate change.Soil respiration in forest ecosystems is an important part of the terrestrial C cycling.Vegetation types and soil physicochemical properties vary with different forest types,which will affect litter decomposition by microorganisms and C release.Given the differences in the quality and quantity of litter under different forest types,a coniferous forest,a broadleaf forest,and a coniferous-broadleaf mixed forest were chosen to add,remove litter and exclude roots(DIRT experiment)for the 4 years(2016-2019),and explored the response of soil respiration to changes in C input.Across all the forest types,litter addition stimulated soil respiration by 11.6%,but litter removal and root exclusion inhibited soil respiration by 9.8% and 10.7%,respectively.When analyzed by each forest type,litter addition increased soil respiration in the coniferous and coniferous-broadleaf mixed forests by 17.9% and 13.3%,but did not affect soil respiration in the broadleaf forest.Litter removal reduced soil respiration by 12.4%,10.9% and 6.2% in the coniferous,broadleaf,and coniferous-broadleaf mixed forests,respectively.Root exclusion reduced soil respiration by 10.3%,9.5% and 12.4% in the coniferous,broadleaf,and coniferous-broadleaf mixed forests,respectively.The response of soil respiration to litter addition was greater than litter removal in the coniferous and coniferous-broadleaf mixed forests.The negative effect of soil respiration on the litter removal was larger than the no effect of litter addition in the broadleaf forest.This showed that the same degree of increase or decrease litter has an asymmetric effect on soil C release,which may lead to uncertainty in soil C storage budget.In addition,litter removal and root exclusion had an interacted effect on soil respiration.In the coniferous forest,litter removal decreased soil respiration by 20.0% without root exclusion but showed no response with root exclusion,and root exclusion reduced soil respiration by 20.2% without litter removal,but had no effect with litter removal.By contrast,soil respiration under litter removal was unaffected in the without root exclusion plot,but was significantly decreased by 14.4% in the root exclusion plot.Root exclusion had neutral effect on soil respiration without litter removal but marginally reduced soil respiration by 14.8% with litter removal in the broadleaf forest.The interaction showed that,in the coniferous forest,litter removal inhibited the negative effects of root exclusion on soil respiration,and root exclusion also weakened the negative effects of litter removal.The soil organic C content and microbial biomass in coniferous forests were highest.The lack of above-and below-ground C sources stimulated the mineralization of microbes on the soil C pool,resulting in additional C release,but in the broadleaf forest,litter removal and root exclusion mutually exacerbate the negative effects on soil respiration.This was because the soil organic C content and microbial biomass are low,and the lack of two C sources aggravates the decrease in the microbial community(the ratio of fungi to bacteria),resulting in a decrease in the dominance of fungi,and further inhibiting the soil C release.Across all the forest types,microbial biomass C dominated the changes in soil respiration.When analyzed by each forest type,soil respiration was regulated by microbial biomass C and soil temperature in the coniferous forest.Soil respiration depended on the ratio of fungi to bacteria in the broadleaf forest,but in the coniferous-broadleaf mixed forest,microbial biomass C explained the majority variations of soil respiration.The observations indicate that soil microorganisms could adapt to different forest ecosystems and make corresponding feedback mechanisms to regulate soil C release.By quantifying the effect of plant litter alterations under three forest types on soil respiration,this study explored the response mechanism of the ecosystem C cycling to changes in C input.Moreover,the lack of above-and blow-ground C source could stimulate and weaken the potential of soil C mineralization in different forest types,which will be overestimate or underestimate the effect of changes in C input on soil C storage and release.These findings prove that evaluating the impact of changes in C input on soil respiration at regional and global scale requires consideration of different forest types,which will help more accurately simulate and predict the response of terrestrial C cycling.