| The increase of atmosphere CO2 concentration and global warming are the main features of climate change,which is the greatest challenge for maintaining the sustainable development of earth's ecological and human society system.The climate change has showed the great influence on carbon(C)cycle and productivity of terrestrial ecosystem,especially for the agriculture ecosystem.In the other word,as the biggest C pool in terrestrial ecosystem,even a minor change of soil C pool can greatly affect the CO2 concentration and C cycle in atmosphere,so soil can quickly response to climate change.Moreover,the cropland pool plays the key role in global climate change,since this pool is adjustable within a short period of time by human management.Compared to upland soil around the world,rice cultivation is a tillage process benefiting soil organic matter(SOM)accumulation,however,the effect of climate change on SOM composition and function in paddy field is still unclear.Based on the research of the effect of climate change on the productivity and process of soil-plant in the paddy ecosystem.And to deepen our understanding of the impact mechanism of climate change on soil C balance and to design a successful C sequestration strategy,this study was major on the molecular composition of root exudates and SOM in a paddy field under the simulated climate change.The study was carried out with the help of a field experiment,which was based on the rice-wheat rotation system and initiated by our group in Changshu Municipality,Jiangsu Province in 2010.And four treatments were included in the field experiment:elevated atmospheric CO2 concentration up to 500 ?mol·mol-1(CE),warming of crop canopy air by 2? over ambient(WA),and simultaneously elevated atmospheric CO2 concentration and canopy air(CW),and a control of ambient condition(CK).As a key driver in rhizosphere,root exudates have a strong influence on microbial activities and SOM stability,thus,the rice root exudates was collected at different growth stages(the tillering,heading and ripening stage)and the exudation rate was calculated.At heading stage,part of root exudates was separated for composition analysis by liquid chromatography/mass spectrometry(LC/MS).Considering the rhizosphere is the hotspot for substance and energy exchange among plant,microbes and soil,the rhizobox was designed and imbedded into the experimental plots to sample rhizospheric soil at the rice ripening stage.In addition,the soil samples were fractionated into three aggregates by wet-sieving:macroaggregates(2000-250 ?m),microaggregates(250-53?m),and clay&silt fraction(<53 ?m).The soil microbial communities were measured by phospholipid fatty acids(PLFAs),and the molecular composition of SOM was analyzed by off-line pyrolysis gas chromatography/mass spectrometry(Py-GC/MS).The main results obtained are as follows:1.The change of quantity and chemical composition of root exudates under simulated climate changeCompared to CK,CE significantly increased the root biomass at the tillering and heading stages by?30%,no change was observed at the ripening stage.However,WA and CW mainly increased the root biomass at the heading and ripening stages,no change was noticed at the tillering stage.The root exudation rate at different stages was always as follows:tillering stage(0.57-0.74 mg C·plant-1·h-1)<ripening stage(1.20-1.36 mg C·plant-1·h-1)<heading stage(6.65-9.40 mg C-plant-1·h-1).Only CE treatment significantly increased the root exudation rate at the tillering and heading stage by?30%.The composition of root exudates was analyzed at the heading stage,the results indicated that root exudates were do minated by the groups of amino acids,fatty acids,organic acids,phenolic compounds and sugars.The change of root exudates composition was only detected under WA treatment,which increased the relative abundance of amino acids but decreased that of phenolic compounds.These results suggested that elevated CO2 increased the production of root exudates without composition change,warming shifted the composition of root exudates without production alter,neither production nor composition was affected by their combination treatment due to the antagonistic effect.2.The change of soil organic carbon and microbial PLFAs contents under simulated climate changeThe climatic treatments could not change the soil water content in paddy filed,compared to CK,only CE and CW significantly decreased soil pH value.The content of dissolved organic carbon(DOC)was increased by 30%under WA treatment,no change was found under CE and CW treatment.All the climatic treatments significantly increased the soil organic carbon(SOC)and total N contents.The significant correlation between SOC content and root biomass indicated that the SOM was mainly affected by the plant-derived organic matter(OM).The content of total.PLFAs was ranged from 54.95 to 87.83 nmol·g-1,and WA increased the content of total PLFAs,bacterial and fungal PLFAs by 53%,41%,32%,respectively.These increases probably related to the changed composition of root exudates.WA increased and decreased the relative abundance of amino acids and phenolic compounds,respectively,which was beneficial to microbial growth.Specially,WA decreased the relative abundance of p-cumaric acid,then resulted in the increased fungal biomarkers and decreased bacteria/fungal ratio(B/F).CE increased the content of bacterial PLFAs by 17%and increased the B/F ratio,which was probably due to the preferential utilization of increased root exudates by bacteria.None of microbial PLFAs properties was changed obviously under CW treatment.3.The change of SOM molecular composition under simulated climate changeThe pyrolysates of soils under all treatments were dominated by alcohols,amides,fatty acids,heterocyclic nitrogen compounds,phenolic acids,and phenols.After normalization,compared to CK,CE increased the content of alcohols,amides,fatty acids,heterocyclic nitrogen compounds,phenolic acids,and phenols by 26?109%,however,WA and CW decreased and maintained these six main compounds,respectively.Common molecules is a type of molecules which cannot be decomposed or transformed after treatment and represents the relative stable compounds,the main groups of common molecules were aromatic(heterocyclic nitrogen compounds,phenolic acids,and phenols)and hydrophobic lipids(alcohols,amides,and fatty acids).CE and WA increased and decreased the content of common molecules,respectively,and it's maintained under CW treatment due to the antagonistic effect.These changes of SOM composition were resulted from the altered root exudates.The increased root exudates could be preferentially utilized by bacteria with protecting the SOM from decomposition under CE treatment,benefiting the accumulation of the stable compounds(aromatic and lipidic compounds).The decreased relative abundance of phenolic compounds such as p-coumaric acid in root exudates lead to increased fungal activity,consequently,more stable compounds of SOM were decomposed under WA treatment.These results indicated that CE enhanced the molecular stability of SOM,however,which was decreased under WA treatment.4.The change of molecular composition of organic matter(OM)in aggregates under simulated climate changeCompared to CK,the mass proportion of macroaggregates increased significantly under CE,WA and CW treatments by 32%,55%and 109%,respectively,however,the proportion of microaggregates decreased notably under CE,WA and CW treatments by 30%,14%and 54%,respectively.And the root biomass was significant positively and negatively correlated with the mass proportion of macro-and microaggregates,respectively.These results indicated that climate change could influence the aggregate distribution by enhanced root growth.The content of microbial PLFAs was reduced with the decreasing aggregate size.Compared to CK,WA significantly increased the total PLFAs,bacterial and fungal PLFAs across the aggregates,CE increased the total PLFAs in clay&silt fraction and bacterial PLFAs in microaggregates,and CW only increased bacterial PLFAs in microaggregates.As to the OM composition in aggregates,the relative abundance of lipids was increased gradually with decreasing aggregate size,however,that of phenolic compounds was reduced gradually,the relative abundance of phenolic compounds was 30%,20%,and 10%in macro-,microaggregates,and clay&silt fraction,respectively.After normalization,the content of hydrophobic lipids and aromatic compounds was increased in macro-and microaggregates under CE treatment compared to CK,however,WA decreased them in macro-and microaggregates.Due to the antagonistic effect,the increase of these compounds under CW treatment was smaller than CE treatment.Furthermore,the increase or decrease extent of the lipids and aromatic compounds was diminished with decreasing aggregate size,and the change in silt&clay fraction was hardly found under all climatic treatments.These results indicated that the influence of climate change on molecular composition of OM in aggregates was reduced with decreasing aggregate size.Similarly,the correlation between OM composition and microbial PLFAs properties was more apparent in larger aggregate fraction.5.Dynamics in molecular composition of SOM regulated by the mass proportion of aggregatesCompared to CK,CE significantly increased the content of common molecules in bulk soil,which was derived from the increased common molecules distributed in macroaggregates and clay&silt fraction with the unaltered common molecules in microaggregates.The increase extent of common molecules distributed in macroaggregates was smaller than the decrease extent of ones distributed in microaggregates under WA treatment,and the common molecules distributed in clay&silt was unaltered.As a result,WA decreased the content of common molecules in bulk soil.CW didn't change the content of common molecules in bulk soil,which was attributed to an increase common molecule distributed in macroaggregates followed by a decrease in microaggregates with the similar extent,and an insignificant change in clay&silt fraction.Moreover,relative to control,a significant correlation was found between the differences in mass proportion of aggregates and the differences in common molecules distributed in aggregates(R2=0.72,P<0.0001).These results suggested that the content of common molecules in bulk soil were determined by the ones distributed in aggregates,which was controlled by soil aggregate composition.Therefore,the response of SOM molecular composition to climate change was regulated by the mass proportion of aggregates.In summary,elevated CO2 increased the production of root exudates,which was preferentially utilized by bacteria,and then limited the decomposition of SOM benefiting the accumulation of hydrophobic lipids and aromatic compounds in soil.The decreased relative abundance of phenolic compounds,especially p-coumaric acid,in root exudates under air warming was attributed to the fungal dominance accelerating the decomposition of these persistent compounds.Therefore,these results suggested that elevated-CO2 and air warming exhibited a different mechanism to influence the molecular composition of SOM with the mediation of root exudates.The influence of elevated CO2 and air warming on molecular composition of OM in aggregates was faded with decreasing aggregate size.Moreover,climate change could affect the dynamics of SOM molecular composition by regulating the mass proportion of aggregates through root growth. |
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