Soil Organic Carbon Biochemistry In Alpine Meadows As Affected By Hydrological Change And Shrubbification

The Qinghai-Tibetan Planteau is important ecological security barrier in China and even the Northern Hemisphere.The plateau is facing a series of ecological degradation risks over the past half century,including hydrological change and shrubification.However,how it feedbacks to the biochemical nature of soil organic carbon（SOC）in alpine meadows remains uncertain,even though it is very important for forecasting the fate of SOC in a scenario of the changing climate.In this study,we compared biochemical characteristics of SOC between adjacent wetland and meadow soils,and investigated the changes in SOC mineralization,as affected by hydrological change（anaerobic versus aerobic conditions）.We also compared soil stoichiometry,biochemical characteristics of SOC between shrubs and meadow soils in alpine meadows,and then investigated differences in SOC mineralization and its temperature sensitivity between grassy and shrubby patches.1.Wetlands contain more SOC at three depths than meadows.The biochemical composition of SOC differed sharply between adjacent wetlands and meadows.The non-cellulosic sugar content in SOC was similar in wetland and meadow soils,but the mass ratio of hexoses（galactose plus mannose or rhamnose plus fucose）to pentoses（arabinose plus xylose）was three-fold greater in meadows than wetlands.Furthermore,there was three times more accumulation of microbial residues（sum of glucosamine,mannosamine,galactosamine,and muramic acid）in SOC in the meadow than wetland soils.In addition,meadow soils had more microbial biomass and higher activities ofβ–glucosidase,phenol oxidase,and urease per unit of SOC than wetland soils.This suggests slower transformation of plant litter by microorganisms,and thus less accumulation of microbial byproducts in the SOC of wetlands than meadows.The effects of water regime on SOC decomposition were dependent on the degree of SOC degradation.Anaerobic conditions decreased the mineralization of less degraded SOC in wetland soils but increased the mineralization of more degraded SOC in meadow soils compared with aerobic conditions.2.The SOC content and soil stoichiometric C/P and N/P ratios in shrubby patches（Caragana brevifolia,Potentilla fruticosa,Spiraea alpina and Salix oritrepha）markedly increased with increasing plant size compared with herbaceous areas,indicating the increased P limitation to microorganisms relative to C and N with SOC accumulation under shrubs.The mass ratio of galactose plus mannose to arabinose plus xylose in the non-cellulosic carbohydrate pool of plant input was higher under shrubs than under grasses but was generally similar in soils between herbaceous and shrubby patches.Furthermore,the accumulation of microbial residues in SOC was higher under herbaceous than under shrubby.This suggests that shrubs decreased accumulation of microbial necromass in SOC.Stepwise regression revealed that lower total CO₂ efflux per unit mass of SOC at either 15℃ or 25℃ from shrubby than from herbaceous patches over the 76-day incubation was mainly associated with the increased soil C/P or N/P ratio under shrubs.Shrubs decreased temperature sensitivity of SOC decomposition compared with grasses only at 20–40 cm soil depth,where microbial-synthesized substances had greater dominance over organic matter than at0–20 cm soil depth and were less abundant under shrubs relative to grasses.In summary,the biochemical composition of SOC differed sharply under adjacent wetlands relative to meadows because of the strongly decreased microbial action on the plant residues due to O₂ limitation in wetlands.The effects of hydrological conditions on SOC decomposition were dependent on the degree of SOC degradation.In addition,increasing P limitation to microorganisms by shrubification resulted in the decreased SOC decomposability and microbial-synthesized substances determined the temperature sensitivity of SOC decomposition.We conclude that both wetland drainage and grassland waterlogging increase SOC mineralization（measured as CO₂ evolution）.Shrubification mitigates CO₂ emissions in grasslands by decreasing SOC mineralization and its temperature sensitivity in the context of global warming.