Effects Of Rest-grazing On Soil Carbon Pool And Carbon Components In Cold Season Pasture Of Alpine Meadow

Managing soil carbon pools in grasslands through rest-grazing management is an important way to respond to China’s "double carbon" target and an important measure to maintain the sustainable development of alpine grassland ecosystems on the Qinghai-Tibet Plateau.However,the resting period of cold season pastures in alpine meadows on the Qinghai-Tibet Plateau is mostly concentrated in the re-greening period,the trampling and stripping of grassland by grazing livestock after early spring soil thaw was ignored.In this experiment,we combined the degree of soil thawing and grass coverage in early spring in alpine meadows to precisely delineate the spring resting period.A study of the effects of different rest-grazing times in spring on soil carbon pools and carbon fractions to provide ideas for the management of soil carbon pools in alpine meadows.The rest-grazing test was set up in a typical alpine meadow cool-season pasture in the town of Zhuaxixiulong,Tianzhu Tibetan Autonomous County,Wuwei City,Gansu Province,in 2018.Taking the local traditional restgrazing as a control,five rest-grazing time were set: 1)the critical time of soil thawing(soil surface begins to thaw,ST1),2)late stage of soil thawing(soil thaw depth >10 cm,ST2),3)early stage of plant re-greening(plant re-greening coverage reached 30-40%,RG1),4)later stage of plant re-greening(re-green plant coverage reached 80%,RG2),5)local traditional rest-grazing(dominant plant height was approximately 5 cm,CK).After the 4th year(2021)and 5th year(2022)of restgrazing treatment,the soil carbon pool,carbon components,soil physical and chemical factors affecting soil carbon pool,soil extracellular enzyme activities,microbial community structure characteristics and carbon transforming microorganisms were analyzed during the traditional rest-grazing period(early June)and the vegetation growth boom period(late July).The influence mechanism of different rest-grazing on soil carbon pool and carbon components in alpine meadow was analyzed and discussed,the main results are as follows:(1)Bringing forward the start of rest-grazing in spring allows soils to maintain high levels of soil moisture,soil macro-aggregates water-stable soil aggregate stability index,ammonium nitrogen,available phosphorus,soil organic carbon and active organic carbon contents were kept higher in advanced rest-grazing period.Soil organic carbon pool content was higher in the critical period of soil thawing,reaching85.70-97.06 g/kg.The delayed grazing period in spring increased soil temperature,soil water seepage rate,soil compactness and soil micro-aggregate content,and maintained a high mineral-associated organic carbon,but the soil nutrient use efficiency decreased.(2)Earlier rest-grazing time increased soil N/P acquisition enzymes and soil Chydrolase,and decreased soil carbon-oxidase activity.Soil alkaline phosphatase activity increased with earlier rest-grazing time β-N-acetylaminoglucosidase showed less variation between treatments,with ST2 and RG2 treatments being lower.Soil surase activity being significantly highest in RG2);β-glucosidase activity tended to increase with the advancement of the rest-grazing time.Soil C-oxidase enzymes including polyphenol oxidase and peroxidase tended to increase gradually with delayed rest-grazing.There were seasonal and climatic differences in soil enzyme activity between months and years.(3)Changes in the timing of spring rest-grazing can significantly affect soil microbial community structure,with earlier rest-grazing times reducing the number and variety of soil microbial endemic species,as well as sample spatial heterogeneity The Shannon index,Simpson index and chao1 index of soil microorganisms were all significantly highest in the CK treatment.Except for the CK treatment,microbial diversity increased significantly(P<0.05)with earlier restgrazing in June and with later rest-grazing in July.(4)Soil carbon conversion functional microbial species,abundance and carbon metabolic pathways in June of fifth year of the rest-grazing trial showed significant differences between treatments(P<0.05).Soil C-fixation genes sdh A and acc C were significantly enriched in the CK treatment,and C-sequestration genes fhs and CCSA were more abundant in the ST1 and ST2 treatments.Soil C-degradation genes ace E and pgm were mainly enriched in the RG2 treatment;C-degradation gene K10622 was mainly enriched in the ST1 and ST2 treatments.Soil C-fixation genes were mainly enriched in the prokaryotic carbon sequestration pathway and fatty acid synthesis pathway;soil C-degradation genes were mainly enriched in glycolysis and glycogluconeogenesis,methane metabolism,TCA cycle,fatty acid degradation,nitrotoluene explanation and RNA degradation pathway.(5)Soil stable organic carbon was positively influenced by soil temperature,soil C-oxidase and C-fixation functional microbial abundance,while soil labile carbon fraction was positively influenced by soil moisture,C-hydrolytic enzymes and Cdegrading functional microbes.Structural equation modelling showed that the earlier rest-grazing time promoted the production of soil macro-aggregates,which increased the easily degradable organic carbon fraction and hydrolase activity,while microbial activity increased the mineral-bound organic carbon,which increased the soil carbon pool.Increased abundance of carbon-degrading microorganisms and hydrolytic enzyme activity reduce soil carbon pool decomposition.In summary,rest-grazing during the critical time of soil thaw increases soil water content,mac-aggregrate content,hydrolase activity and C-degradation functional gene abundance,and promoting an increase in the organic carbon pool,particulate organic carbon fraction and readily degradable organic carbon fraction.Whereas local traditional rest-grazing more increases soil temperature and mic-aggregrate,increases microbial mineralisation and makes the soil organic carbon pool is reduced.From the perspective of soil carbon pool management,this study suggests that rest-grazing during the critical period of soil thawing,which has a positive effect on soil carbon pool enhancement.