CO₂Emissions And Soil Organic Carbon Contents Of Cropland Soil Under Different Tillage Practices

Quantifying cropland soil CO₂efflux and soil organic carbon (SOC) will help toprecisely evaluate the contributions of cropland soil to global warming as a carbonsource or carbon sink. In Northeast China, it is rarely reported which conservationtillage practices would be most beneficial for increasing SOC and decreasing soilCO₂efflux. Based on this background, this thesis made CO₂efflux and SOCmeasurements on a Black soil at the long-term crop rotation (continuous corn andcorn-soybean) and conservation tillage (no-tillage, NT; mouldboard plough tillage,MP; ridge-till, RT) experiment in Dehui, Jilin Province, China, and on differentongoing long-term tillage (NT; MP; RT; zone-tillage, ZT) experiments on aBrookston Argiaquoll in southwestern Ontario, Canada. Effect of different tillagesystems on the stock and distribution of SOC was studied to determine the mostappropriate tillage for these regions.In northeast China experiment field in Dehui, soil CO₂efflux rates weremeasured2-h intervals over48hours four times during the growing season of2008,with the purpose of finding the best time of day for measuring cropland soil CO₂efflux. Soil CO₂efflux rate measured between09:00and11:00h represents the dailyaverage (24hour) soil CO₂efflux rate quite well during sunny days. A procedure wasdeveloped to adjust the CO₂efflux rates measured outside (between07:00and21:00h) the optimum measurement time window (09:00to11:00h) to estimate dailyaverage soil CO₂efflux rate.Based on the optimum measuring time, black soil CO₂efflux rate was measuredduring the growing season, non-growing season and winter between2008and2010to determine seasonal effects, and determine if conservation tillage was helpful inaccumulation of SOC in the ongoing long-term tillage and crop rotation experimentin northeast China. There was no significant difference in soil CO₂efflux rate amongtillage treatments. Total winter, non-growing season and annual soil CO₂emissionswere0.29-0.45Mg C ha^-1,0.69-0.81Mg C ha^-1, and5.32-6.40Mg C ha^-1,respectively, for NT, MP and RT tillage treatments. Annual soil CO₂emission under NT was significantly higher than MP tillage for the two crop rotations, monoculturecorn and corn-soybean. The contributions of winter soil respiration to annual soilCO₂emission ranged from5.1to7.1%among tillage treatments. Winter soilrespiration clearly plays a significant role in annual soil carbon budgets. Correlationsbetween annual soil CO₂emission and SOC were significant and negative at5-10cm and10-20cm indicating that soil CO₂efflux has the greatest impact on SOC atthese two depths.In southwestern Ontario, Canada, NT produced SOC stratification relative toMP with significantly greater SOC in surface soil (0-5cm) but lower SOC insubsurface soil (10-30cm). The ZT produced SOC and total soil nitrogen levels thatwere similar to NT near the soil surface, but similar to MP in the subsurface. On anequivalent soil mass basis (4570Mg ha^-1), ZT increased SOC stock (80.6Mg C ha^-1)by11.2%and12.5%compared with NT and MP, respectively. It was concludedthat ZT has the potential to improve both SOC sequestration and selected soilphysical properties relative to NT and MP on fine-textured soils in cool, humidclimates.The RT created distinctly different soil environments relative to NT and MP,which may in turn affect soil properties. After29years of RT, an improvement wasfound in soil quality for the Brookston Argiaquoll, which was indicated by lower soilpenetration resistance and soil bulk density relative to NT and significantly higherSOC stock within the plow layer based on an equivalent soil mass basis relative toMP. We concluded that RT could be more appropriate for the long-term tillagemanagement relative with NT and MP in this region.