Studies On Mechanisms Of Carbon Sequestration, Carbon Emissions And Soil Organic Carbon Dynamics In Different Farming Paddy Fields

This study selected three different paddy farming systems which are typical in southern regions of China,including rice-duck integrated farming system(RD), intermittent irrigation system(RW),and conventional flooded irrigation system(CK).In 2007 and 2008,dynamics of carbon sequestration by rice plants,soil carbon emissions, soil organic carbon components of three kind of paddy farming systems and their mutual relations were studied through field experiments and laboratory analysis.Mechanism of the three carbon cycle process were understood in different paddy farming systems,and the relationship between soil organic carbon components and rice yield was preliminarily affirmed during the short term scale of rice growth stage.Those studies would benefit to evaluate the impact of the changes of paddy farming systems on carbon cycling in the paddy ecosystem,and accumulate the data for soil carbon management in high-yielding rice fields in China.The main results are summarized as follows.(1)Carbon fixation by rice plant of different farming systemIn the two years of 2007 and 2008,average net carbon fixation by rice plants above ground of CK,RD and RW reached 776.6 g.m^-2,778.9 g.m^-2,705.5 g.m^-2 respectively. Net carbon fixation of RW was significantly lower by 9.2%and 9.4%than that of CK and RD.There was no significant difference between CK and RD.(2) CH₄ emissions from paddy of different farming systemsIn the two years of 2007 and 2008,the average CH₄ emission flux of RD was 13.65 mg.m^-2.h^-1,Cumulative amount of CH₄ emissions reached 34.15 g.m^-2,lower than that of CK by 19.15%and 21.4%(P＜0.01) respectively.Compared with CK,RD primarily mitigated CH₄ emission during the period of coexisting of rice and duck.In the two years of 2007 and 2008,the average CH₄ emission flux of RW was 10.03 mg.m^-2.h^-1,Cumulative amount of CH₄ emissions reached 25.49 g.m^-2,lower than that of CK by 36.9%and36.8%(P＜0.01) respectively.Compared with CK,RW could reduce CH₄ emission during the whole growth stage of rice.Factors of soil TOC,DOC,MBC,soil temperature,catalase activity,the changing amount of carbon fixation,soil pH positively correlated with CH₄ emission,while soil HA,FA and NH₄⁺ negatively correlated with CH₄ emission.The one of important reasons for mitigating CH₄ emission of RW was that RW could effectively reduce soil DOC.(3) CO₂ emission from paddy soil of different farming systemIn the two years of 2007 and 2008,the average CO₂ emission flux of RW was 159.4 mg.m^-2.h^-1,Cumulative amount of CH₄ emissions reached 385.4 g.m^-2,more than that of CK by 29.4%and 22.0%(P＜0.01) respectively,while there was no significant difference between CK and RD.Factors of soil TOC,ROC,the changing amount of carbon fixation positively correlated with CO₂ emission,while soil HA,FA,DOC,MBC and soil pH negatively correlated with CH₄ emission.(4)Dynamics of soil organic carbon components in paddy of different farming systemsIn the two years of 2007 and 2008,the average soil TOC content of CK was 11.64 g.kg^-1,and the content of soil ROC,DOC,MBC,HA,FA was 2.21 g.kg^-1,0.57 g.kg^-1,0.29 g.kg^-1,0.34 g.kg^-1,2.09 g.kg^-1 respectively,while which proportion of TOC was 22.31%,4.46%,2.38%,3.81%,19.88%respectively.In the two years of 2007 and 2008,the average soil TOC content of RD was 11.05 g.kg^-1,and the content of soil ROC,DOC,MBC,HA,FA was 2.62 g.kg^-1,0.51 g.kg^-1,0.27 g.kg^-1,0.46 g.kg^-1,2.08 g.kg^-1 respectively,while which proportion of TOC was 24.14%,4.60%,2.46%,4.18%,19.02%respectively.In the two years of 2007 and 2008,the average soil TOC content of RD was 10.79g.kg^-1,and the content of soil ROC,DOC,MBC,HA,FA was 2.58 g.kg^-1,0.44 g.kg^-1,0.25 g.kg^-1,0.40 g.kg^-1,2.13 g.kg^-1 respectively,while which proportion of TOC was 24.27%,3.99%,2.24%,3.76%,20.16%respectively.Compared with CK,RD increased soil ROC,HA and its proportion in the TOC;RW slowed down the accumulation speed of soil organic carbon,improved ROC and its proportion in the TOC,and decreased DOC and its proportion in the TOC,also increased the proportion of FA in the TOC.(5) Relativity and sensitivity of soil organic carbon componentsThe absolute amount of ROC had no correlations with other SOC components except positively with MBC/TOC,but its effective carbon response index(ECRI) showed significant positive correlation with DOC and DOC/TOC,while negative correlation with HA.DOC and its proportion in TOC had significant positive correlation with MBC, MBC/TOC,FA,while negatively correlated with HA,HA/TOC,HA/FA.Different farming systems of rice had different influence on the relationship among SOC components.ECRI of SOC components had different roles of their absolute amount for analysis of correlations between SOC components,because ECRI represented changing amount of SOC components,could dynamically reveal the causal relationship among SOC components.Combined use of ECRI and absolute amount of SOC components in correlation analysis could comprehensively reflect the relationship among SOC components at the short-term scale.ECRI of SOC components and their effective coefficient of variation(ECV) could be used to effectively analyze the changing sensitivity of SOC components.The sensitivity of MBC,MBC/TOC, DOC/TOC,DOC,HA,FA,ROC,ROC / TOC,TOC,HA/TOC,FA/TOC,HA/FA descended from strong to weak.(6) Correlations among rice carbon fixation,soil carbon emissions and organic carbon componentsThe process of rice carbon fixation mutually interrelated to the process of soil carbon emission and the change of organic carbon components.Accumulative amount of carbon fixation by rice plants positively correlated with soil TOC,HA,HA/TOC,FA,FA/TOC which transformed rather slowly in the soil,while had no significant correlations with the active soil organic carbon which transformed faster in the soil.Contrarily,the changing amount of carbon fixation positively correlated with soil ROC,MBC,MBC/TOC,which proved that input of fresh organic carbon into the soil could cause the change of soil active organic carbon sensitively. CH₄ emission flux had significant positive correlation with the changing weight of carbon fixation,but no significant relationship with the accumulative of carbon fixation. While both of the two factors positively correlated with CO₂ emission flux.For CK,soil TOC was significantly positively correlated with CO₂ emission flux, while DOC,DOC/TOC,MBC and MBC/TOC showed significant negative correlations with it.In terms of RD,CO₂ emission flux positively correlated with soil ROC,while negatively correlated with DOC,DOC/TOC and HA.For RW,more TOC and less HA content in the soils would promote CO₂ emission.Both higher active and less active soil organic carbon components had significant correlations with CH4 emission flux for the three treatments.Among the indicators of soil organic carbon,DOC,DOC / TOC,MBC and MBC / TOC showed significant positive correlations with CH4 emission flux,while HA and HA / TOC significantly negatively correlated with it.FA and FA / TOC also showed negative correlations with CH4 emission flux for RD and RW.For CK and RD,there was a significant positive correlation between soil TOC and CH4 emission flux.In a word,the rich carbon source is the prerequisite for CH4 formation,but the amount of CH4 emissions are directly related to the amount of MBC and DOC in paddy soil.(7) Relationship of SOC components and rice grain yieldIn the terms of absolute amount of SOC components,soil TOC,DOC,DOC/TOC, MBC had significant positive correlations with rice yield,while HA/TOC,FA/TOC, HA/FA,ROC/TOC negatively correlated with the yield.In the terms of ECRI,DOC/TOC, MBC,MBC/TO,ROC,ROC/TOC,FA/TOC had significant negative correlations with rice yield,while HA,HA/TOC,TOC positively correlated with the rice yield.The results revealed higher content of soil TOC,DOC and MBC,slower changing rate of ROC,DOC and MBC,faster accumulative rate of TOC,and faster transferring rate of HA are the important conditions to gain higher rice yield.The results showed RD and RW did not decrease the rice grain yield compared with CK.