Shrub Biomass Allocation In Southwest China And Effects Of Warming And Nitrogen Addition On The Soil Organic Carbon Fraction

With increasing global atmospheric nitrogen deposition,the carbon sequestration of terrestrial ecosystems plays an important role in confined global warming within 1.5 °C.Biomass accumulation and allocation are important to quantify the ecosystem structure and carbon stocks and fluxes within the global carbon cycle.It’s prerequisite for quantify carbon dynamics and sequestration to accurate estimate biomass reserves,specifically its component and spatial distribution pattern.Soil carbon pool is the largest carbon pool in terrestrial ecosystems,the small changes in the soil carbon pool will trigger larger fluctuation in the whole carbon cycling of the terrestrial ecosystems.Therefore,the study of soil carbon fraction is particularly important under global atmospheric nitrogen deposition and climate warming.Shrub ecosystems play important roles in global carbon cycle.Alpine scrub ecosystem of mountainous shrubs in Southwest China is the second vegetation type in the eastern QinghaiTibet plateau.The research of carbon cycle in shrubland,especially in alpine scrubland ecosystem was extremely rare compared with forests and grassland.Therefore,we investigated the biomass allocation pattern of mountainous shrubs in Southwest China to accurate calculation the vegetation carbon stock,and the effects of shortterm(two years)warming(open top chamber,OTC)and seven years nitrogen addition(N0,0;N20,20;N50,50;N100,100 kg N/ha/year)on the soil organic carbon fraction of Sibiraea angustata,which is the dominant species in the native alpine scrubland in the eastern QinghaiTibet plateau.This study provides a theoretical basis and better understand of the potential change of carbon cycle in the alpine scrubland ecosystem.The results were as follows:(1)The above-ground biomass(AGB),total biomass(TB)of the shrub community,and TB of shrub layer in mountainous shrub in Southwest China significant decreased with increasing altitude.However,the biomass of herb layer increased with increasing altitudes(P<0.001).The root/shoot(R/S)ratio of the shrub community and its component synusiae increased gradually with increasing altitudes(P<0.001).The R/S of shrub layer showed a quadratic relationship(P<0.001),and the R/S of herb layer and shrub community increased significantly with the increasing latitude(P<0.001).The biomass of shrub layer and the TB of shrub community were significantly increased(P<0.001),but the biomass of herb layer and the R/S of shrub community were significantly decreased(P<0.05)with increasing longitude.The allometric relationship between AGB and BGB was found for the shrub and herb layer,but an isometric relationship was found for the shrub community level because of the offset effects of the two layers.These findings indicate that mountainous shrub biomass accumulation and allocation are strongly related to altitude,longitude,latitude,and are significantly affected by mean annual precipitation(MAP)and mean annual temperature(MAT)in Southwest China.(2)Simulated warming after three years significantly increased the AGB of herb layer and shrub community in S.angustata native alpine scrubland by 57.8% and 7.2%,respectively,as well as significantly increased the BGB of shrub and herb layer,and shrub community by 42.5%,105.6%,and 45.6%,respectively.Simultaneously,the simulated warming significantly increased the TB of shrub and herb layer,and shrub community by 25.6%,85.7%,and 28.4%,respectively,and significantly increased the R/S of those by 33.2%,30.4%,and 36.0%,respectively.The results indicated that plants in alpine scrubland could adapt to future climate warming by modulating their biomass allocation patterns on the eastern Qinghai-Tibet Plateau.In the second year of simulated warming,OTC increased the temperature of the 5 cm soil layer in plant and remove-plant treatment by 0.51℃ and 0.56℃,and have no significant effects on soil moisture content and p H value.Simulated warming significantly decreased 0-30 cm soil organic carbon by 16.89%.It’s due to labile carbon fractions(i.e.,microbial biomass carbon(MBC),particulate organic carbon(POC),light fraction organic carbon(LFOC),macroaggregate(250～2000μm))and the recalcitrant carbon fractions(heavy fraction organic carbon(HFOC))decreased with warming in 0～30cm soil layer by 14.02%,16.67%,19.96%,8.67%,and 12.39%,respectively.The results of soil aggregate respiration showed that the carbon mineralization of microaggregates and non-aggregates increased by 15.08% and 10.98%with simulated warming,respectively.However,the carbon mineralization of macroaggregates decreased by 24.60%,and the carbon mineralization of soil decreased by 22.61% with simulated warming.The decreased of soil carbon mineralization was mainly due to the higher mass of macroaggregates and its higher contribution to soil respiration,and carbon mineralization of macroaggregates significantly reduced with warming.The soil carbon pool of 0～100cm showed no significantly difference between control and warming treatment mainly due to simulated warming increased 30-100 cm soil organic carbon.The results supported that short-term warming could not affect the global soil carbon loss,and supported the soil carbon sink under global change.(3)Nitrogen addition significantly increased the soil moisture in 0～10cm,and have no significant effects on soil moisture content and p H value in 0～100cm soil layer.The soil organic carbon(10.37%、28.2%),labile carbon(11.36%、23.42%),and recalcitrant carbon pool(11.07%,48%)of 0～100cm significant increased with middle(50kg N/ha/year)and high(100kg N/ha/year)nitrogen addition.The increased labile carbon pool mainly due to the increasing of the organic carbon content of macroaggregate(high proportion,large contribution)with middle and high nitrogen addition by 17.35% and 21.31,respectively;and the increase of organic carbon content LFOC,POC and microaggregates.The increasing of recalcitrant carbon pool was mainly due to the increase of HFOC content by 11.56% and 27.74%,respectively,and the increase of organic carbon content in <53μm aggregates by middle and high nitrogen addition.WSOC content was increased with nitrogen addition and accelerated soil carbon loss,but did not affect the soil carbon pool due to the content and proportion of WSOC were rare.High nitrogen significant increased soil carbon mineralization by 11.41%,related to the significant increase mineralization and contribution to soil respiration of macroaggregates and nonaggregates under simulated warming.To conclude,these results suggest that shrub biomass allocation is strongly affected by the altitude,longitude,latitude,MAT and MAP.Shrub communities adapted to the heterogeneous environment by different biomass allocation pattern of its synusia.It supported the isometric relationship of AGB and BGB partitioning at the community level in mountainous shrub biomes.Alpine scrubland could adapt to future climate warming by modulating their biomass especially BGB allocation patterns on the eastern Qinghai-Tibet Plateau.Surface soil organic carbon fractions are sensitive to simulated warming and nitrogen addition.Short-term warming could not affect the soil carbon pool of 0～100cm,moreover,nitrogen addition significantly increased soil carbon pool of 0～100cm,which clarified the soil carbon sink under global change.