Rolston, Dennis (Univ. of California, Land, Air and Water Resources, Davis, CA, 95616; Phone: 530-752-211; Fax: 3530-752-1552; Email: derolston@ucdavis.edu)
D. E. Rolston*, C. Van Kessel, J. W. Hopmans, J. Six, K. T. Paw U, R. Plant, A. P. King
Conservation tillage practices may lead to an increase in soil C, partially offsetting greenhouse gas (GHG) emissions. The overall objectives for this presentation are: (1) To identify and quantify C input pathways and their spatial and temporal variations at the field scale; and (2) To determine the effect of minimum versus standard tillage on the spatial distribution of the controlling factors and resulting short-term rates of GHG fluxes. Our expected results are essential for testing models and for scaling up of C sequestration potential from the field and landscape to the regional level. The research site (30.8 ha) in Yolo County, CA is furrow-irrigated and was in minimum-till wheat prior to the initiation of the experiment. Extensive baseline data on total soil C, N, bulk density, particle size, residue yield, and root biomass were collected before the site was split into two fields to represent the grower’s standard tillage (ST) and minimum tillage (MT) practices. The ST field was tilled in October 2003 and both fields were planted with corn in April 2004. Each field is instrumented with 1) eddy-covariance instruments to measure field-scale carbon dioxide fluxes, 2) with automated chambers for assessing the temporal pattern of carbon dioxide and nitrous oxide fluxes, and 3) with multiple portable chambers to evaluate the spatial characteristics of carbon dioxide, nitric oxide, and nitrous oxide fluxes. The soil profile is also instrumented to measure various physical soil variables, such as soil temperature, water content, and carbon dioxide and nitrous oxide soil air concentrations. For the first year of this long-term study, using the eddy covariance approach, the ST treatment exhibited approximately triple the photosynthetic uptake of the MT treatment for May, but the MT caught up with the ST by July. The differences in corn growth were evident in the visual appearance of the fields and was also reflected in the grain yield with the MT being smaller than the ST. Overall, similar magnitudes and patterns of carbon dioxide fluxes were measured with the chambers and the eddy-covariance method, although the chambers appear to be giving slightly smaller fluxes (measured during the middle of the day) than the nighttime carbon dioxide exchange as measured by the eddy-covariance approach. This difference is most likely due to nighttime plant respiration. Flux data from the automated chambers corresponded to the diurnal patterns determined with the eddy-covariance approach. Very little or no emission of nitric oxide and nitrous oxide occurred during the fallow period. Soon after planting of the corn and fertilization, the mean monthly nitrous oxide fluxes increased gradually from March through June to a maximum of about 0.2 mg N m-2s-1 and then decreased to about half the peak values in July. Only minor differences were apparent in nitrous oxide fluxes between the two tillage treatments. As expected, large emissions occurred directly over the fertilizer band. It is expected that several more years of measurements will be required in order to determine any differences in GHG gas emissions and C sequestration between the two tillage treatments, especially in light of the yield depression for the MT treatment.