Effects Of Simulated Rainfall Change On Seasonal Variation Of Soil Respiration In Songnen Meadow Steppe And Control Mechanism

In future,precipitation change is likely to strongly affect ecosystem carbon(C)cycling,especially soil respiration(Rs),the largest C flux from the terrestrial ecosystem to the atmosphere.As resulting from the different substrate sources and biological processes,the heterotrophic(Rh)and autotrophic(Ra)components of soil respiration may respond differently to changes in precipitation.Understanding of the differential response of soil respiration and its components to precipitation will help us assess the storage and stability of soil carbon under future scenarios of climate change.We carried out an experiment with six precipitation treatments in the Songnen Grassland,including 30%and 50%increased precipitation,ambient control(0%)and 30%,50%and 70%decreased precipitation treatments.Soil respiration was measured in situ over three years in the field.At the same time,we performed an incubation experiment by adding ¹³C labeled glucose to pre-treated soils.The following results were obtained:(1)Soil moisture content(SWC)was significantly affected by precipitation treatments.Decreased precipitation treatments increased soil temperature(ST)by decreasing aboveground biomass.The soil organic carbon,total carbon and total nitrogen were highest under 30%decreased precipitation treatment.In 2018 and 2019,the aboveground biomass decreased significantly with the decrease in rainfall,and the root/shoot ratio under decreased precipitation treatments were higher than those of the increased precipitation treatments.Soil enzyme activities were significantly affected by interannual and seasonal variations,but the precipitation treatments had no significant effects on them.Bacteria,fungi,and total PLFAs were significantly affected by precipitation treatments and seasonal variation,and they decreased significantly with the decrease in rainfall.The reason could be that long-term drought leads to dormancy or physiological death of microorganisms.Precipitation treatments had no significant effects on fungi/bacteria ratio(F:B).(2)Seasonal variation of Rs followed a bell shaped curve,and depended on the seasonal variations of ST and SWC.Total Rs,Rh and Ra were significantly affected by precipitation treatments and seasonal variation.Decreased precipitation treatments significantly reduced soil respiration due to the changes in microbial biomass and activity.In addition,soil moisture,microbial and root activities were more sensitive to decreased precipitation than increased precipitation,resulting in a nonlinear and asymmetric response of Rs to precipitation change.From 2017 to 2019,all decreased precipitation treatments reduced growing season Rs flux,in which the proportion of growing season flux of Rh was greater than that of Ra.ST,SWC,aboveground biomass,microbial biomass and activity were positively correlated with Rs,while F:B was negatively correlated with Rs and Rh.(3)In the incubated experiment,we found the cumulative priming effect(PE)of soil organic matter decomposition was highest under 30%decreased precipitation treatment,mainly because higher microbial biomass and activity were found under this treatment.PE in the subsoil was higher than that in the topsoil.Because soil nutrients are limited in the subsoil,the input of exogenous organic carbon altered the stoichiometric characteristics of the substrate,which was benefit to the utilization of microorganisms and promoted the PE.Microbial carbon use efficiency(CUE)also was highest under 30%decreased precipitation treatment,and gradually decreased with the decreasing precipitation.The CUE of topsoil was higher than that of subsoil.In conclusion,precipitation change affected the seasonal variation and magnitude of R_S by regulating biological and abiotic factors.There was an asymmetric response of Rs to precipitation change and soil CO₂ emissions decreased with the decreasing precipitation.At the same time,changes in rainfall patterns may affect the soil carbon storage and stability by changing PE and CUE in the Songnen grassland in future.These findings enable us to understand the mechanism of soil organic carbon mineralization and to predict the seasonal and inter-annual variation of soil CO₂ flux in Songnen meadow steppe under future scenarios of precipitation change.