Response of carbon exchange between shrub and grass patches to increased seasonal precipitation:a comparative analysis based on aboveground net primary productivity and leaf area index standardization
Aims With the intensification of climate change and human activities,the phenomenon of shrub encroachment in arid and semi-arid grasslands is widespread,significantly impacting the carbon sequestration function of grassland ecosystems.Water availability is the primary limiting factor in the semiarid grassland of Nei Mongol,and future changes in precipitation patterns have important implications for carbon exchange in grassland ecosystems.However,there is limited research on the effects of precipitation changes on shrub-encroached grassland ecosystems,particularly on the carbon exchange processes within heterogenous patches.The underlying mechanisms remain unclear.Methods In this study,we conducted a seasonal precipitation manipulation experiment by increasing snowfall in winter and rainfall in summer in shrub-encroached grassland dominated by Caragana microphylla in Nei Mongol.Carbon exchange parameters,such as net ecosystem carbon exchange(NEE),gross ecosystem productivity(GEP),and ecosystem respiration(ER)of shrub patches and grass patches,were measured and compared using standardized parameters based on aboveground net primary productivity(ANPP)and leaf area index(LAI).The study investigated the impact of increased seasonal precipitation on carbon exchange in shrub-encroached grassland and the differential responses of heterogeneous patches.Important findings 1)Increased summer rainfall significantly enhanced|NEE|,GEP and ER of the grass patches,while increased winter snowfall significantly reduced|NEE|ANPP,GEPANPP and ERANPP of the grass patches.Increased summer rainfall significantly enhanced the GEP and ER of the shrub patches,while the effect on|NEE|was not significant.Additionally,increased winter snowfall had a positive impact on carbon exchange processes in the shrub patches.Overall,|NEE|,GEP and ER of the shrub patches were significantly higher than those of the grass patches.In comparison to a wet year(2021),the carbon exchange in a dry year(2020)was more sensitive to increased precipitation.2)Carbon exchange in the shrub patches(|NEE|,GEP and ER)was positively correlated with soil water content and leaf biomass.Increased summer rainfall mainly promoted carbon exchange by enhancing deep soil water content(40-80 cm)and lowering soil temperature.Carbon exchange in the grass patches was positively correlated with shallow soil water content(0-20 cm)and ANPP,and negatively correlated with soil temperature and root-to-shoot ratio.Increased summer rainfall primarily enhanced carbon exchange in grass patches by raising shallow soil water content and lowering soil temperature,while increased winter snowfall hindered carbon exchange by increasing deep soil water content and stimulating belowground biomass.3)Standardized carbon exchange parameters based on ANPP better revealed the differential responses of the shrub patches and grass patches to changes in precipitation.These research findings provide an important scientific basis for accurately assessing the carbon sink function and carbon sequestration potential of arid and semi-arid grasslands ecosystems under climate change.
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