The Effect Of Alternating Drying And Wetting On The Decomposition Of Soil Organic Carbon

Global climate change is expected to increase the frequency of drought and heavy precipitation,which could create more frequent drying-rewetting cycles?DWC?in the soils.The rewetting of dry soil will result in a large carbon dioxide pulse,which is called the Birch effect.Although a large number of studies have been conducted to quantify the eff ect of DWC on cumulative soil respiration?CSR?,their results are often contradictory,the effect of DWC on decomposition of different soil organic carbon?SOC?pools is still unclear,and the subsequent legacy effect on SOC decomposition is not well understood.Thus,we use meta analysis and laboratory incubation to investigate the effects of DWC on soil respiration.Our meta-analysis included 524 observations from studies across diff erent ecosystems and soil types,we investigated the responses of CSR to DWC.Our laboratory incubation experiment used three long-term experimental soils representing different SOC pools?fallow for 15 years,bare for 15 years,and bare for 23 years plus extra laboratory incubation for 815 days represent labile,relatively stable,and stable SOC,respectively?,we conducted a 128-d laboratory incubation experiment to investigate how DWC affects decomposition of different SOC pools.The incubation experiment included nine 10-d DWC of 1)three constant moisture treatments at 20%,60%,and 100%water-holding capacity?WHC?,respectively,2)a DWC treatment with 10-d drying to 40%WHC and immediately rewetting to 80%WHC,and 3)a DWC treatment with 10-d drying to 20%WHC and immediately rewetting to 100%WHC,and then a 28-d extended incubation period under same constant moisture.Our results indicated that DWC led to 72%stimulation of CSR compared with lower constant-moisture control?LC?,25%inhibition compared with upper constant-moisture control?UC?,but minimal diff erence with mean constant-moisture control?MC?,respectively.The meta-regression analysis showed that the DWC eff ect on CSR depended on mean annual precipitation,soil properties?clay content,C/N ratio,TN,pH?,and the duration of drying in the DWC treatment.In addition,the eff ect size of CSR relative to UC was positively correlated with the number of DWC,suggesting that the slower-cycling soil carbon may be more vulnerable to DWC than the faster-cycling soil carbon and the“physical mechanism”may play a role in substrate release and CO₂ emission after multiple DWC.During the DWC period,cumulative C mineralization increased linearly with increasing constant moisture content.Compared to the mean constant moisture treatment?60%WHC?,DWC did not significantly alter cumulative C mineralization in most of the cycles in fallow and bare soils,but increased it by 14%in bare+incubation soil in 100-20%WHC treatment.This result indicates that the stable SOC may be more vulnerable to DWC than the labile SOC.In addition,during the28-d extended period,DWC significantly increased C mineralization by 18%in fallow soil and decreased it by 73%in bare+incubation soil in 100-20%WHC treatment.This legacy effect induced by DWC was attributed to the amount of microbial biomass and remaining substrates.Overall,compare to the mean constant moisture control,DWC did not significantly affect soil respiration of soil labile carbon.However,different soil carbon pools respond differently to DWC,showing that stable SOC is more vulnerable.Although DWC stimulated CO₂ emission from the stable SOC during the DWC period,the released CO₂ emission from stable SOC after DWC suggesting that soil organic carbon response limitedly to global climate change.In summary,our study clearly elaborated the effects of DWC on soil carbon decomposition,and emphasized the importance of distinguishing SOC pool components in studying the effects of DWC on soil respiration.Future research is needed to explore the internal mechanisms of different SOC pools response differently to DWC and to investigate the DWC eff ects on soil respiration by in situ experiments and long-term studies.