HR: 1340h
AN: B23A-0938    [Abstracts]
TI: Influence of Organic Agriculture on the Net Greenhouse Effect in the Red River Valley, Minnesota
AU: * Phillips, R L
EM: rebecca@aero.und.edu
AF: University of North Dakota, Clifford Hall 361, Grand Forks, nd 58202 United States
AB: Fluxes for the suite of biologically-produced greenhouse gases (CH$_{4}$, N$_{2}$O and CO$_{2}$) are strongly influenced by agriculture, yet the influence of organic agriculture on all three gases, which comprise the net greenhouse effect (GHE), is not clear in the context of large-scale agricultural production. Greenhouse gas mitigation potential will depend upon the net balance for all three gases [GHE balance (CO$_{2}$ equiv.)= CO$_{2}$ $_{flux}$+ 23CH$_{4}$$_{flux}$ + 296N$_{2}$O$_{flux}$]. On-farm, field-scale experiments were performed to test the hypothesis that the net GHE at the soil-atmosphere interface is reduced under organic wheat production, compared with conventional, and that effects vary inter-seasonally. Trace gas fluxes were measured at the soil-atmosphere interface for organic and conventional wheat farms in the Red River Valley, Minnesota, one of the most productive agricultural regions in the US. We utilized 40-60 ha field pairs planted with hard red spring wheat (Triticum aestivum L.). Treatment pairs were located 6km apart and consisted of fields continuously cropped for wheat/soybean/sugar beet production for over 20 yr. Ten random, permanent points were generated for each 8.1 ha sub-plot nested inside each field. Each field pair was similar with respect to crop, climate, cultivation history, tillage, rotation, soil texture, pH, macronutrients, bulk density, and water holding capacity. Differences between treatments for the last five years were soil amendments (compost or urea) and herbicide/fungicide application versus mechanical weed control. We collected gas fluxes at each of the 41 points from April (wheat emergence) until the end of July (maturity) to determine the hourly and seasonally integrated net GHE for each management practice, given similar soil/plant/climatic conditions. Moreover, we analyzed inter-seasonal variability to determine the relationship between wheat phenology and flux under field conditions for soil temperature and moisture (water-filled pore space). The net GHE for organic fields was less spatially and temporally variable than conventional, with average daily flux between 0.48 and 1.44 g CO$_{2}$ equiv. m$^{-2}$ d$^{-1}$. Average daily flux in conventional fields ranged between 0.48 and 3.12 g CO$_{2}$ equiv. m$^{-2}$ d$^{-1}$, with highest values in April and May. While soil moisture in organic fields was significantly greater than conventional, it did not interact with treatment to affect trace gas flux. Instead, the effect of organic on N$_{2}$O, CO$_{2}$ and the net GHE was strongly influenced by crop stage, an agronomically meaningful proxy integrating time and plant growth conditions. Most CH$_{4}$ flux observations were 0. Integrated fluxes for each of the 40 sites over the growing season was averaged by field pair and treatment. Although the magnitude of the treatment effect for average seasonal integrated flux varied between field pairs for CO$_{2}$ and N$_{2}$O fluxes, the overall influence of treatment on the net GHE was similar. Overall, soils under organically produced wheat emitted 200 kg CO$_{2}$ equiv. ha$^{-1}$ per season less than conventionally produced wheat. We observed 1) the net GHE for similar field sites in the Red River Valley was reduced under organic versus conventional agriculture, 2) N$_{2}$O flux in organic fields was significantly lower than conventional fields for both field pair sites, and 3) the effect of treatment on CO$_{2}$ flux was site specific.
DE: 4805 Biogeochemical cycles (1615)
DE: 1818 Evapotranspiration
DE: 1615 Biogeochemical processes (4805)
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting