Studies On Soil Respiration And Carbon Balance Under Different Management Practices In Cotton Field Of Oasis In Arid Region

Soil respiration as an important part of the exchange of CO2between the terrestrial ecosystemsoil and the atmosphere, plays a key role in the global carbon cycle. The different agriculturalmanagement practices strongly influence greenhouse gas exchange and the carbon cyclebetween agro-ecosystems and the atmosphere. As the most important arid region of China,Xinjiang also is the most important cotton-producing region. Since entering the21st century,agricultural production technology in Xinjiang has experienced significant changes, large-areapopularization and application of water-saving under-mulch-drip irrigation, increased cropfertilization, as well as the long-term implementation of crop stubble incorporationtechnology, which have created constant improvement in crop production but also causechanges of carbon distribution pattern and process in the arid oasis cropland ecosystems.Based on the above phenomenon, the paper from the perspective of soil respiration rate, fieldexperiment was conducted in situ to determinate the soil respiration rate of farmland duringthe growing season of cotton. To explore the effects of different agricultural managementpractices on the soil respiration of agro-ecological systems, the component changes of soilrespiration, soil organic carbon （SOC）, soil microbial biomass carbon （MBC） and soil carbonsink balance. To clear the influence mechanism of different agricultural managementpractices on the soil carbon flux and carbon balance, and to analyse the relationship betweenthe soil organic carbon content and soil carbon emissions of cotton fields under differentagricultural management practices. Further, the results obtained can provide a theoreticalbasis for reasonable agricultural management practices were arranged and the formulation oflow-carbon cultivation techniques that to improve agricultural soil carbon sequestrationability in oasis croplands of arid region. Below are the main results in this thesis.1. Effects of different agricultural management practices on the dynamic changes of soilrespiration rate were studied. The result showed that cotton soil respiration rate has asignificant seasonal characteristic under different agricultural management practices. Soilrespiration rate reached a peak in mid-July and then reduced to the minimum inmid-October after cotton harvest. The diurnal variations of soil respiration rate showed asingle peak curve, and the highest peak of full flowering stage appeared at15:00-17:00and full bolling stage appeared at15:00and the minimum value appeared at04:00.Further, there was highly significant （P<0.01） positive correlations between soilrespiration rate and soil temperature in10cm depth. The correction coefficient of soilrespiration rate in cotton field in09:00-13:00close to one under different agriculturalmanagement practices, indicating that this period of soil respiration rate is most representative of the day’s average soil respiration rate. Additionally, the average soilrespiration rate under conventional flood irrigation was smaller than that under dripirrigation under mulch, and the soil respiration rate with （NPK+OM） treatment was thehighest, and then followed by OM, CK and NPK among the fertilizer treatments. Underthe condition of interaction between fertilization and irrigation practices, the average soilrespiration rate was the strongest under drip irrigation under mulch method andNPK+OM treatment.2. The relationship between the soil respiration rate and soil temperature and soil moisturecontent under different agricultural management practices were studied. The resultshowed that the average temperature sensitivities (Q₁₀) of soil respiration underconventional flood irrigation and drip irrigation under mulch were2.43and2.03,respectively. The Q₁₀ value under conventional flood irrigation was smaller than thatunder drip irrigation under mulch. Under different agricultural management practices, theveracity is higher when compound equation be used to simulated soil respiration rate, andthe coefficient of determination （R²ï¼‰ was0.64to0.72. Therefore, considering the effectsof soil temperature and soil moisture content on soil respiration rate could improve theaccuracy of soil respiration rate in regional research.3. Effects of different agricultural management practices on the contribution of eachcomponent to soil respiration were studied. The result showed that the fertilizertreatments ratio of root respiration contribution to soil respiration was fluctuated between36.38%and58.74%under drip irrigation under mulch method, whereas between33.73%and52.03%under conventional flood irrigation method. Root respiration contributionwas the highest at the bolling stage. During the whole growth period, the root respirationcontribution under drip irrigation and flood irrigation was averagely48.05%and44.31%,respectively.4. Effects of different agricultural management practices on the soil carbon emissions,organic carbon content and carbon balance were studied. The result showed that soilcarbon emissions were smaller under conventional flood irrigation than those under dripirrigation under mulch, highest under NPK+OM fertilization （followed by OM, CK, andNPK）, and larger with stubble incorporation relative to removal. Soil organic carboncontent of the cotton field surface was highest, with lower distribution trends as soil layerdepth increased under different management practices. Soil organic carbon content in the0-40cm soil layer was larger under conventional flood irrigation than under dripirrigation under mulch, highest under NPK+OM treatment （followed by OM, NPK, andCK）, and larger under stubble incorporation relative to removal. Cotton field soil carbon emissions were best correlated with organic carbon content in the20-40cm soil layerduring the growth season. Soil carbon sink intensity was larger under drip irrigation undermulch than under conventional flood irrigation, highest under NPK treatment （followedby NPK+OM, OM, and CK）, but had no significant difference between the stubbletreatments. Under combined treatments, soil carbon sink intensity was strongest underdrip irrigation, stubble incorporation, and NPK and NPK+OM fertilization.5. Effects of different agricultural management practices on the soil microbial biomasscarbon were studied. The result showed that the soil microbial biomass carbon of surfacesoil was highest, with lower distribution trends as soil layer depth increased of the oasiscotton field in arid region. Soil microbial biomass carbon content and soil microbialquotient （qMB） were smaller under conventional flood irrigation than those under dripirrigation under mulch among the irrigation treatments, smaller with stubble removalrelative to incorporation among the stubble treatments, and highest under NPK+OMfertilization （followed by OM, CK, and NPK） among the fertilization treatments. Therewas highly significant （P<0.01） positive correlations between soil microbial biomasscarbon and total soil organic carbon （TOC）. Under the different agricultural managementpractices, the soil microbial metabolic quotient （qCO2） were smaller under under-mulchdrip irrigation than under conventional flood irrigation among the irrigation treatments,smaller with stubble incorporation relative to removal among the stubble treatments, andlowest under NPK+OM fertilization （followed by OM, NPK, and CK） among thefertilization treatments. Under the condition of interaction between different agriculturalmanagement practices, qCO2was the lowest under under-mulch drip irrigation methodand NPK+OM treatment with stubble incorporation. Therefore, in arid region, cottonproduction using cropland management practices, such as water saving technology of dripirrigation under mulch, combined application of NPK fertilizer and organic manure andstubble incorporation, could not only maintain good soil properties and potential forsustainable use, but also promote to the accumulation of soil organic carbon and reducecarbon discharge.6. Effects of different agricultural management practices on the cotton photosyntheticmaterial production and the relationship with soil respiration rate were studied. The resultshowed that under the different agricultural management practices, the canopy apparentphotosynthesis rate （CAP）, total soil respiration （Rt） of cotton field, root respiration （Rr）,net primary productivity （NPP）, and lint yield of cotton under conventional floodirrigation were smaller than that under drip irrigation under mulch among the irrigationtreatments, and the CAP, Rt, Rr, Rr/Rt, NPP and lint yield of cotton under stubble removal were smaller than that under incorporation among the stubble treatments, and the CAP, Rr,Rr/Rt, NPP and lint yield of cotton with （NPK+OM） treatment were the highest, and thenfollowed by NPK, OM and CK among the fertilizer treatments, but the Rt with（NPK+OM） treatment were the highest, and then followed by OM and NPK/CK. Weconduct to fit the relationship between the cotton canopy apparent photosynthesis rate andthe total soil respiration rate and the root respiration rate, clearly, the fitting degree ofquadratic equation was better than that of the linear equation. Besides, the fitting degreeof the two equations of the cotton canopy apparent photosynthesis rate with the total soilrespiration rate and the root respiration rate in full flowering stage was better than that infull bolling stage. Under the same growth stage, the fitting degree of the two equations ofthe cotton canopy apparent photosynthesis rate with the root respiration rate was betterthan the fitting degree with the total soil respiration rate. Additionally, the fitting degreeof the net primary productivity with the cotton canopy apparent photosynthesis rate waslarger than that with the total soil respiration rate, the fitting degree in full flowering stagewas better than that in full bolling stage.