Lee, Juhwan (University of California, DavisDepartment of Plant Sciences, Davis, CA, 95616; Phone: 530-754-7537; Email: ecolee@ucdavis.edu)

Spatial Variability of Greenhouse Gas Emissions and Their Controlling Factors in an Agricultural Landscape

J. Lee *, C. Van Kessel, D.E. Rolston, J. Six, A.P. King

We determined the singular and interactive effects of soil texture, soil moisture and tillage on the flux of CO₂, N₂O, and CH₄ in order to understand differences in greenhouse gas (GHG) emissions between minimum and standard tillage systems (MT versus ST) at 28 ha field scale. We also quantified the spatial variability of controlling factors on GHG emissions across a farmer’s fields. In 2004, 40 soil cores were taken to a depth of 0-15 cm at regular spatial intervals along two 360-m transects which represented a range of variation in soil texture and moisture. The cores were incubated at 25°C at field moist state for 10 days, and then were wetted to 60% of water holding capacity (WHC) and incubated again for additional 10 days. At field moist state, the global warming potential (GWP) was, on average, greater in the ST than MT soil cores, and CO₂ production rates were greatest contributor to the GWP of both the ST and MT soils. However, spatial variability of GHG emissions was large at the field scale, masking tillage-induced differences in the emissions. Upon wetting the soil cores to 60% WHC, both CO₂ production rates and N₂O emission rates drastically increased, but more in the MT than ST cores. Compared to the ST soils, the MT soils with low clay content lost more C and N in the form of GHG as moisture content increased. Most of the spatial variability is primarily explained by differences in tillage and soil texture (e.g., clay content) and to a lesser degree by differences in soil C and N content as well as moisture, indicating an interaction between tillage, soil texture, and moisture content in determining GHG emissions. Microbial biomass, mineral N, and dissolved organic C were also key controlling factors for N₂O emission rates.