Jasrotia,P., K. Kahmark, Sven Bohm, S.K. Hamilton, and G.P. Robertson
Presented at the GLBRC Sustainability Retreat (2010-02-10 to 2010-02-12 )
Globally, agriculture is responsible for 20% of the greenhouse gas emissions. In the United States, the national average from agriculture is 8%.Most of the global emissions come from the combustion of fossil fuels releasing carbon dioxide(CO2), a greenhouse gas, to the atmosphere. Agricultural emissions come from other greenhouse gases, namely methane (CH4) and nitrous oxide (N2O) in addition to CO2. While CH4 and N2O emissions are far less in quantity in the atmosphere, they have a much more potent impact on the climate. In present study, we investigated the influence of land conversion for biofuel production on N2O and CO2 emissions. Seven sites of different management histories were selected within a range of 10 miles from W.K. Kellogg Biological Station (KBS). Four sites (M1, M2, M3 & M4) represented conversions from monocultural grasslands, characterized by the dominant presence of Smooth brome (Bromus inermis Leyss), and not cultivated in the last 20 years following the criteria of Conservation Reserve Program (CRP) of the United States Department of Agriculture (USDA) (www.fsa.usda.gov/FSA/). The other three sites (L1, L2 & L3) located inside of the Lux Arbor Reserve, were cultivated in the last 10 years as corn-soybean rotation that represented a traditional agriculture management in this region. In order to study the effect of land use change on greenhouse gas fluxes (N20 and CO2), all the grasslands sites and corn sites were converted in soybean cultivation while one of the grassland sites was kept undisturbed as reference site (M4). Gas fluxes were measured from May to November 2009 on biweekly basis using in-situ closed-cover flux chambers. Results indicated large temporal variability of N2O and CO2 fluxes within and between individual days for different sites. Cumulative N20 emissions from CRP to soybean conversion ranged from 0.86 to 5234.45 g N-N2O ha-1 compared to 0.55 to 740.56 g N-N2O ha-1 from corn to soybean conversion. At the end of the season, site M1 have maximum cumulative N20 fluxes (5234.45 g N-N2O ha-1) followed by sites M3 (3865.40 g N-N2O ha-1) and M2 (1717.72 g N-N2O ha-1). Amongst Lux Arbor sites, site L1 have maximum cumulative N20 emissions (740.56 g N-N2O ha-1) followed by L2 (706.98 g N-N2O ha-1 ) and L3 (635.93g N-N2O ha-1 ). N20 fluxes from site L1 were statistically at par with reference site (M4- 739.89 g N-N2O ha-1). Similar trends were observed in CO2 emissions. Cumulative CO2 emissions at the end of season were maximum in site M1 (22443.22 t CO2 ha-1) and minimum in site L2 (6692.12 t CO2 ha-1). Simple linear regression analysis indicated significant positive correlation (R2=60.39%; p=0.001) of daily N20 fluxes with soil nitrate nitrogen content. Significant positive correlation (R2=75.78%; p=0.01) was found between daily CO2 fluxes with carbon and nitrogen (C/N) ratios of different sites. In depth understanding of these interactions at landscape scales is important to analyze and predict the environmental effects of landuse change for biofuel cultivation. A comprehensive analysis will help us to understand the implications, as well as in devising mitigation techniques for vibrant biofuel economy.
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