Organic agroecosystems have been increasing in acreage worldwide, but are not well-represented in current emissions inventories for the global warming potential greenhouse gases (GHGs) carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4). Monterey Bay, located on the central coast of California, is a highly productive agriculture region from which empirical soil gas flux data are lacking. The objective of this pilot study was to develop GHG estimates for organic row crops in this region, and to investigate the relationships between soil-water content, soil temperature, and soil gas flux in drip-irrigated spinach crops grown at the UCSC Organic Farm. We monitored soil biogenic gas flux using twelve ventilated non-flow-through non-steady-state gas chambers fabricated from PVC, each covering 6.5 m2 of soil surface in the crop row. In order to capture the potential for diurnal changes in soil temperature and moisture we sampled on alternate mornings and afternoons twice a week over the eight week crop cycle. We used polypropylene syringes to sample chamber headspace at regular intervals throughout 30 to 60 minute periods, then transferred the gas samples to pre-evacuated Exetainer vials for later analysis with a Shimadzu 2010 Gas Chromatograph. For each sampling event we bulk sampled approximately 10 soil cores and determined gravimetric soil water content.
Results/Conclusions
Consistent with the literature, results suggest both N2O and CO2 flux increased with greater soil temperature and water content, while CH4 flux generally decreased over time. Cultural practices play an important role in the extent of soil gas emissions, thus it is imperative to develop more accurate estimates of GHG contributions from organic agriculture in order to understand the reduction potential from these agroecosystems. A better understanding of the processes that affect gas flux from agroecosystems will inform both the productivity and sustainability of agriculture.
