Effects Of Spring And Autumn Warming On Soil Respiration In An Old-field Grassland

The projection of ecosystem carbon(C)cycling responses to climate warming is one of the major challenges for terrestrial C-climate change feedback.IPCC(The Intergovernmental Panel on Climate Change)has reported(Global Warming of 1.5 °C)that observed mean land surface air temperature has risen by 1.53 °C relative to 1850-1900.Historical meteorological records and model projections reveal that the magnitudes of temperature increase on the Earth's land surface show substantial variability at seasonal scales.A strong seasonal warming,especially in spring and autumn,has been observed over the Northern Hemisphere.Spring warming can positively affect net ecosystem productivity due to advanced plant phenology and the relatively higher sensitivity of plant photosynthesis than respiration.However,a large discrepancy exists between model simulations for the effects of autumn warming on terrestrial C sink,leading to a great uncertainty for the projections of annual effects of seasonal warming on terrestrial C cycling.In addition,the simulations and projections of spring and autumn warming on terrestrial C sink are based on the assumption that the effects of spring plus autumn warming are calculated as the sum effects of spring warming and autumn warming.However,it remains elusive whether spring and autumn warming interact with each other to affect ecosystem C cycling.Therefore,to better understand and make comprehensively assessment on the effect of spring and autumn warming on ecosystem C cycling,direct evidence from field experiment is urgently needed.A manipulative experiment including four treatments(control,spring warming,autumn warming,spring plus autumn warming)was conducted since 2012 in an old-field grassland at the Global Change Ecology Farm of Henan University,Kaifeng,Henan,China.This work explored the response of soil carbon release to asymmetrically seasonal warming via assessing the effects of spring and autumn warming on soil temperature and moisture,plant leaf photosynthesis,aboveground net primary productivity,plant community structure and composition,microbial biomass carbon,soil respiration and heterotrophic respiration.With a 7-year(2012-2018)manipulation experiment,this study found that:1)Spring warming and autumn warming increased soil temperature by 1.12 ? and 1.20 ? in spring and autumn,respectively.In addition,spring warming decreased spring soil moisture by 0.53% V/V(absolute changes)and autumn warming reduced autumn soil moisture by 0.80% V/V.Spring and autumn warming decreased summer photosynthesis of Cyperus rotundus by 10.4% and 11.4%,respectively.Spring warming elevated summer photosynthesis of Cynodon dactylon by 12.2% without autumn warming but decreased it by 5.5% with autumn warming,and autumn warming increased it by 10.7% without spring warming but reduced it by 6.7% with spring warming.2)Spring warming had no effect on forb biomass whereas autumn warming decreased it by 31.8%.By contrast,spring warming stimulated grass biomass by 14.6% without autumn warming but decreased it by 28.6% with autumn warming,and autumn warming increased grass biomass by 45.7% without spring warming but reduced it by 9.3% with spring warming.Spring warming had no significant effect on forb or grass cover in summer.Autumn warming decreased the forb cover in summer by 8.02%(absolute changes)but did not affect grass cover.No interaction between spring and autumn warming on forb or grass cover was observed.3)Spring and autumn warming reduced soil respiration by 8.3% and 9.8%,respectively.In addition,spring warming decreased soil respiration by 1.0% and 15.9% without and with autumn warming,respectively.Autumn warming suppressed soil respiration by 2.5% and 17.2% without and with spring warming,respectively.Moreover,spring warming increased heterotrophic respiration by 10.2% but decreased it by 5.5% without and with SW,and AW reduced heterotrophic respiration by 2.4% and 16.3% without and with SW,respectively.The synergistically negative impacts of spring and autumn warming on soil respiration happened in spring,summer,and autumn,and the decrease in summer soil respiration dominated the annual changes.4)The decreased soil respiration could be primarily attributed to the reductions in plant photosynthesis and growth,and shifts in plant community structure triggered the legacy response of soil respiration.The nonadditive effects of spring and autumn warming on soil respiration suggest the asymmetrically seasonal warming may diminish soil C loss and its feedback to climate warming.In addition,the legacy response of soil respiration imply that asymmetrically seasonal warming could exert impact on soil C release not only at intra-annual but also inter-annual timescales,hence legacy effects should be taken into consideration for model simulations.These findings highlight the urgent need of incorporating the above mechanisms on terrestrial C cycling in an asymmetrically seasonal warming world.