Effects Of Wetland Degradation On Soil Aggregate Organic Carbon And Its Fraction Organic Carbon In Zoige,China

Zoige wetland is the most typical alpine wetland in the world,and its huge carbon reserves play an important role in the global carbon cycle.However,during the past several decades,the alpine wetlands have degraded in different degrees due to the influence of natural and human factors.It is quite significant to correctly assess carbon sequestration potential of alpine wetland soils that effects of wetland degradation on soil aggregate organic carbon and its fraction organic carbon is investigated.There are four types of wetlands which are selected in the Zoige Natural Wetland Reserve,including Pristine marsh(RPM),Light degradation marsh(LDM),Moderate degradation marsh(MDM)and Heavy degradation marsh(HDM).The paper studied the dynamical characteristics of soil aggregate organic carbon and its physical component organic carbon,chemical bound organic carbon and easily oxidized organic carbon(EOC)in different degraded wetlands by field investigation and laboratory analysis,mathematical statistics,and infrared spectroscopy.The main conclusions are as follows:(1)The degradation of wetlands changed soil aggregate compositions and the reduction of aggregate organic carbon contents.Compared with the RPM,the contents of soil macro-aggregates and silt-clay aggregates in degraded wetlands were significantly decrease by 45.1-53.2,42.1-51.0%,respectively,but the contents of micro-aggregates significantly increased by 40.2-87.2%.The aggregate organic carbon contents in the MDM and HDM significantly decrease by 39.1-84.8,42.7-82.6%,respectively in comparison to PRM,while it in the LDM has no significant change in comparison to PRM.When marsh wetland was degenerated into meadow and degraded meadow,the contribution rate of soil macro-aggregate organic carbon decreased by 35.9-58.0% significantly,however the contribution rate of micro-aggregate organic carbon increased by 28.9-51.2% significantly.The wetland degradation also changed functional groups composition of soil organic matters in aggregates.Active organic carbons such as alcohol phenolic carbons,polysaccharide carbons increased,while aliphatic carbons and aromatic carbons decreased,thus reduced the aromatization of soil organic matters in aggregates and weakened the stability of soil aggregate organic carbon.(2)The degradation of alpine marshes led to the reduction of the physical fractions of aggregate organic carbon contents,including fine particle organic carbon(fPOC),internal particle organic carbon(iPOC),silty+clay organic carbon(s+c_C).Compared with theRPM,the contents of the fPOC,iPOC and s+c_C in degraded wetland soils respectively decreased by 4.0-86.0,1.2-88.7,34.5-50.0%.The contribution rate of soil physical fraction carbon of each graded aggregate was shown that the more serious the degradation of wetlands,the lower the contribution rate of the fPOC and iPOC,while the contribution rate of the s+c_C was opposite.(3)The contents of Ca bound soil organic organic carbon(Ca-SOC)and Fe(Al)bound soil organic organic carbon(Fe(Al)-SOC)decreased with the degradation of wetlands,but their proportions to soil organic carbon increased with the degradation of wetlands.The degradation of wetlands also caused the increase of the Ca-SOC content in the macro-aggregates,while those of chemical bound organic carbon in the other aggregates decreased with wetland degradation.In the process of wetland degradation,The contribution rate of organic carbon in chemical binding state of macro-aggregates decreased significantly,but that of organic carbon in chemical binding state of micro-aggregates increased significantly.(4)The degradation of wetland decreased soil EOC content and oxidative stability of organic carbon,and those of aggregates.The content of soil EOC and oxidative stability of organic carbon respectively decreased by 36.4-69.3 and 7.9-77.4% in degraded wetland soils,compared with RPM.They decreased by 13.7-73.3% and 23.0-73.8% in aggregates in comparison to RPM,respectively.(5)The EOC and aggregate organic carbon(>0.25 mm size fraction)could be used as sensitive indicators to reflect changes in soil carbon storage under the condition of wetland degradation.The EOC is the positive factor affecting soil total organic carbon(TOC)for RPM and LDM.The influence of the EOC on TOC was weakened,while the effect of aggregate organic carbon on TOC was enhanced with wetland degradation.Here,aggregate organic carbon became the positive factor affecting TOC of MDM and HDM.During the succession process of the RPM to the LDM,MDM and HDM,the influence factors of >0.25 mm aggregate organic carbon changed from the EOC to the physical component carbon,while those of<0.25 mm aggregate organic carbon changed from the physical component carbon to chemical bound organic carbon.As a result,the major mechanism of carbon sequestration was transformed from physical mechanism into physiochemical mechanism.