Saturday, 15 July 2006

Soil Carbon Dioxide and Nitrous Oxide Efflux in Agricultural Watersheds with Agroforestry and Grass Contour Buffer Strips.

Peter Motavalli1, Neal Bailey2, Ranjith P. Udawatta3, and Kelly Nelson2. (1) Univ of Missouri-Columbia, Dept of Soil, Environmental, and Atmospheric Sciences 333 Natural Resources Building, Columbia, MO 65211, (2) Univ of Missouri, Division of Plant Sciences, 208 Waters Hall, Columbia, MO 65211, (3) Center for Agroforestry, Univ of Missouri-Columbia, 203 ABNR Natural Resources Building, Columbia, MO 65211

Agricultural practices can stimulate soil nitrous oxide (N2O) and carbon dioxide (CO2) efflux that may contribute to global warming. However, the effects of conservation practices, such as grass and grass-tree (agroforestry) contour buffer strips, on soil CO2 and N2O emissions have not been extensively studied in temperate regions on a watershed scale. The objective of this study was to assess the effects of establishment of grass-tree and grass contour buffer strips on distribution of soil C and N fractions and soil N2O and CO2 efflux in three adjacent agricultural watersheds in northeast Missouri in the north central region of the United States. The three watersheds were in a corn-soybean rotation and were underlain by claypan soils. These soils are characterized by the presence of an argillic horizon that occurs between 13 to 46 cm below the soil surface and restricts drainage and root growth. In 1997, the three watersheds were randomly assigned one of three management systems: 1) cropped in a corn-soybean rotation only, 2) cropped in a corn-soybean rotation interspersed with grass contour buffer strips, or 3) cropped in a corn-soybean rotation interspersed with grass-tree contour buffer strips. In general, soils collected in 2003 from the grass-tree contour buffer strips had the highest levels of soil organic C and N fractions and soil from the cropped watershed had the lowest. Soil pH, bulk density, and organic C and N fractions were not consistently influenced by landscape position. Throughout the watersheds, surface efflux measurements of soil N2O and CO2 were collected regularly during the 2004 corn growing season at the upper, middle, and lower backslope positions within each watershed. The highest soil N2O efflux in all watersheds occurred within the first 40 days after application of N fertilizer. The cumulative soil N2O production from the cropped watershed was 2.3 and 3.8 times greater than the agroforestry and grass watersheds, respectively. Across all three watersheds, the cumulative soil N2O production was smallest in the lower backslope position. The upper backslope position produced 80% greater cumulative N2O than the lower backslope position. Cumulative soil CO2 production was lowest in the cropped watershed compared to the agroforestry and grass watersheds. The lower backslope position across all three watersheds produced 32 and 40% higher cumulative soil CO2 than the upper and middle backslope positions, respectively. A 72-day incubation study was also conducted to determine the effects of soil water-filled pore space (WFPS) (40, 60, 80, and 100% WFPS) and N rate (0 and 1.39 g KNO3 kg-1 soil) on soil N2O and CO2 efflux from bulk soil collected under each management system. The agroforestry soil with added N fertilizer and 80% WFPS had the highest cumulative N2O production. The grass and cropped soils at 80% WFPS with added N produced 32.5 mg N2O-N kg-1 soil and 18.3 mg N2O-N kg-1 soil, respectively. In the incubation study, soils with no N addition produced relatively low amounts of cumulative N2O at the 80% WFPS. The cumulative soil CO2 production was highest in the grass soil compared to the agroforestry and cropped soils regardless of WFPS and N rate. The highest cumulative CO2 production for the grass soil occurred at the 60% WFPS, and was approximately 3 times greater than the agroforestry and cropped soils at the same WFPS. The results of this study with claypan soils indicate that variations in soil WFPS, temperature, vegetation, and in concentrations of soil NO3- and organic C and N fractions due to the presence of the vegetative contour strips were the primary factors affecting observed differences in soil CO2 and N2O efflux among the watersheds. Therefore, establishment of vegetative contour strips could have an effect on production of greenhouse gases in agricultural watersheds with similar soils.

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