Effects Of Global Change Driving Factors On Ecosystem Carbon Stable Isotope Composition And Soil Carbon Pool In A Semi-arid Grassland Of Northern China

Since the Industry Revolution,resulted from intensified anthropogenic activities,atmosphericgreenhouse gas concentration has been increasing rapidly,which leads to climate warming.Global precipitation regimes have been changing with intensifying global hydrological cycles as a result of climate warming.In addition,increasing artificial fertilization and fossil fuel combustion has been exacerbating atmospheric nitrogen?N?deposition.Global change?e.g.,rising CO₂,climate warming,changing precipitation regimes,and atmospheric N deposition?has exerted profound influences on terrestrial ecosystem carbon?C?cycle.As an important component of ecosystem C cycle,soil organic carbon?SOC?dynamics under global change are crucial for evaluating C budgets and cycle at regional or global scales.As part of a field manipulative experiment,this study was conducted to examine the effects of four global change driving factors?elevated CO₂,climate warming,increased precipitation,and N addition?on SOC in a semi-arid grassland in Duolun County,Inner Mongolia,China.The?¹³C of plant-soil system,fraction of new carbon?FNC?,mean residence time?MRT?,and C fractions?i.e.labile pool?LP?,recalcitrant pool?RP?,light fraction?LF?,and heavy fraction C?HF??in SOC were evaluated.Across the 2 years from 2014 to 2015,elevated CO₂ decreased shoot?¹³C by 1.8‰and 1.4‰?absolute change?at community and C₃ functional group levels,respectively.At the species level,elevated CO₂ declined shoot?¹³C of Agropyron cristatum,Astragalus scaberrimus,Potentilla tanacetifolia,Leymus chinensis,Stipa krylovii,and Cleistogenes squarrosa by 1.1‰,1.4‰,1.3‰,0.8‰,0.9‰,and 1.3‰,respectively.Night warming had no effects on shoot?¹³C at community,functional groups,and species levels.Increased precipitation significantly suppressed shoot?¹³C of C₃ functional group by 0.8‰,whereas had no effects on that at community or C₄ functional group level.However,increased precipitation enhanced shoot?¹³C of Cleistogenes squarrosa by 1.2‰.Nitrogen addition stimulated shoot?¹³C by 1.2‰and 1.3‰at community and C₄ functional group levels,respectively.However,N addition did not influence shoot?¹³C of C₃ functional group and any species.No interactive effects of elevated CO₂,night warming,increased precipitation,and N addition on community?¹³C were found across the 2 years?2014-2015?.Elevated CO₂,increased precipitation,and N addition had interactive effects on shoot?¹³C of C₃ functional group.Elevated CO₂ reduced root?¹³C at 0-10 cm depth by 0.9‰in 2015.Increased precipitation suppressed root?¹³C at 10-20 cm depth by 0.75‰.However,neither night warming nor N addition influenced root?¹³C.Elevated CO₂,night warming,and increased precipitation interactively affected root?¹³C at the depth of 10-20 cm.Across the 3 years?2015-2017?,soil?¹³C was not influenced by all the 4 treatments.However,N addition increased soil?¹³C by 0.2‰at the deapth of 0-10 cm in 2016.Across the 3 years from 2015 to 2017,soil FNC_shoot?fraction of new C from shoot?did notrespond to night warming,increased precipitation,or N addition.However,night warming elevated soil FNC_shoothoot by 7.3%?absolute change?in 2015.Night warming stimulated soil FNC_root?fraction of new C from root?by 5.4%across the 3 years?2015-2017?.In addition,increased precipitation enhanced soil FNC_rootoot by15.0%in 2017.Nitrogen addition suppressed soil FNC_rootoot by 7.7%in 2015.Across the 3 years?2015-2017?,night warming lengthened soil MRT_root(mean residence time of soil calculated based on root?¹³C)by 36.8yr.Increased precipitation shortened soil MRT_shoot(mean residence time of soil calculated based on shoot?¹³C)by 109 yr,but extended soil MRT_rootoot by 26.4 yr.Nitrogen addition reduced soil MRT_shoothoot by 108 yr.In 2016,elevated CO₂ had no effect on LF at the depth of 0-10 cm,but decreased HF by 0.1 mgg^-1,and thus led to 16.0%?absolute change?reduction of LF:HF.In addition,elevated CO₂ increased LF at the depth of 10-20 cm by 0.8 mg g^-1.Increased precipitation declined HF by 0.1 mg g^-1,and decreased LF:HF by 12.3%.Increased precipitation did not affect LP,whereas suppressed LP1 by 0.5 mg g^-1.Neither night warming nor N addition had effects on soil C fractions.Increased LF under the elevated CO₂ treatment can enhance soil C sequestration,whereas thedecreased HF indicate that increasing CO₂ may stimulate the decomposition and turnover of SOC,and consequently offset the accumulated C induced by elevated CO₂.In addition,although increased precipitation stimulated soil FNC,the declined HF and LP1 may have adverse effects on soil C sequestration.The decreased FNC due to reduced species richness,and the suppressed MRT under the N addition treatment may also have negative effects on soil C sequestration.