Effects Of Simulated Warming And Precipitation Changes On Key Carbon Processes In Alpine Grassland Ecosystems

Grasslands are important and widely distributed terrestrial ecosystems,of which alpine and polar tundra ecosystems distributed at high altitudes and high latitudes are extremely sensitive to climate change.The alpine grasslands,as representatives of the alpine ecosystems of the Tibetan Plateau,cover about 61% of the Tibetan Plateau.It is an important ecological barrier in China.Over the past decades,changes in temperature and precipitation patterns due to human activities have significantly affected the structure and function of alpine grassland ecosystems on the Tibetan Plateau.This in turn affects the carbon cycling processes of grassland ecosystems and their feedback effects on climate change.Therefore,studies on how alpine grassland ecosystem carbon cycle processes respond to long-term climate change can help to accurately understand the processes of alpine grassland carbon cycle changes under climate change and its future development trends.This study was conducted on an alpine grassland ecosystem in the northeastern Tibetan Plateau,relying on a large-scale “warming and precipitation” control experiment platform established at the Qinghai Haibei Alpine Grassland Ecosystem National Field Scientific Observation and Research Station(two levels of temperature,control and 2°C warming,and three levels of precipitation,control,50% reduction of precipitation and 50% increase of precipitation).Based on 12 consecutive years(2011-2022)of warming and precipitation change treatments,combined with indoor incubation and field monitoring analyses.The effects of long-term warming and precipitation changes on the biodegradability of soil dissolved organic carbon(DOC)and its temperature sensitivity were quantified.The response of soil respiration to longterm warming and precipitation alterations was analyzed.The response of ecosystem carbon fluxes to long-term warming and precipitation changes was revealed.The main findings and conclusions are as follows:(1)Warming increased the DOC concentration of soil surface pore water.The effect of warming on soil pore water DOC concentration was dependent on precipitation changes,and the effect of warming on soil pore water DOC concentration under increased precipitation was limited.Temperature increase significantly increases the DOC content of the soil profile(promoting the accumulation of small molecule organic carbon).Moreover,long-term warming exacerbates soil microbial N limitation and increases the degree of DOC biodegradability in the soil.Both reduced and increased precipitation treatments increase the accumulation and biodegradability of DOC in deep soils.Warming and precipitation changes(reduced and increased precipitation)significantly reduced the temperature sensitivity of soil DOC biodegradation.(2)Warming and increasing precipitation significantly increased soil respiration,while decreasing precipitation significantly inhibited soil respiration.The warming significantly increased autotrophic respiration,increased precipitation significantly promoted heterotrophic respiration,and decreased precipitation significantly inhibited autotrophic respiration.This indicates that the response of soil respiration to increased temperature and reduced precipitation is dominated by autotrophic respiration,while the response to increased precipitation is dominated by heterotrophic respiration.In addition,the response of soil respiration to warming varied in a “significant boost-no significant change-significant boost” pattern over three broad phases(2011-2016,2017-2020,2021-2022).For heterotrophic respiration,warming has no significant effect in the first and second phases,but it even weakens heterotrophic respiration in the third phase.For autotrophic respiration,warming had a positive effect at all three phases,but the positive effect was strong-weak-strong.In addition,it was found that warming reduced the temperature sensitivity of soil respiration,and thermal adaptation occurred in all three phases.These results support projections of a long-term,self-reinforcing carbon feedback from alpine grassland ecosystem to the climate system as the world warms.(3)On an interannual scale at different phases,warming and increased precipitation increased the amplitude and peak width of annual soil respiration during phases I and III,and therefore both warming and increased precipitation contributed to cumulative soil respiration carbon emissions.In Phase II,although the warming treatment significantly increased the minimum soil respiration rate for the annual soil respiration process,it did not affect cumulative soil respiration carbon emissions.In addition,although the water reduction treatment significantly increased the minimum soil respiration rate in all three stages,the reduction in precipitation suppressed the cumulative carbon emissions from soil respiration.(4)Increasing precipitation significantly increases net ecosystem carbon uptake,but decreasing precipitation significantly inhibits net ecosystem carbon uptake.In addition,the effect of warming on net ecosystem carbon exchange was dependent on precipitation.Increasing warming increases carbon uptake when there is sufficient moisture and significantly inhibits carbon uptake when there is insufficient moisture.There are no significant interannual dynamics of decreased precipitation on net ecosystem carbon uptake,while the positive effect of increased precipitation on net ecosystem productivity diminishes to stabilization as the experimental year progresses.These results suggest that future warming and humidification will significantly increase carbon sequestration in alpine grassland ecosystems,but that this positive effect will diminish as ecosystems adapt to climate change.In summary,this study analyses the response of key processes in the carbon cycle of alpine grassland ecosystems on the Tibetan Plateau to long-term warming and precipitation changes(reduced and increased precipitation).The main findings are that warming facilitates the accumulation of dissolved organic carbon in alpine grassland soils.The contribution of warming to soil respiration is mainly from autotrophic respiration and alternates between significant increases and no significant changes in soil respiration,suggesting that alpine grassland ecosystems are a long-term,selfreinforcing carbon feedback to the climate system as the world warms.Future warming and humidification will significantly increase carbon sequestration in alpine grassland ecosystems,but this positive effect will diminish as ecosystems adapt to climate change.The above results help to improve the understanding of the response of the alpine grassland carbon cycle to climate change and provide important data to support model simulations and predictions of the response and feedback of the alpine grassland carbon cycle to climate change on the Tibetan Plateau.