Submitted to: Nutrient Cycling in Agroecosystems
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/29/2004
Publication Date: 9/30/2005
Citation: Mosier, A.R., Halvorson, A.D., Peterson, G.A., Robertson, G.P., Sherrod, L.A. 2005. Measusrement of net global warming potential in three agroecosystems. Nutrient Cycling in Agroecosystems. pp. 72: 67-76. Interpretive Summary: Agricultural soils are sources and sinks for atmospheric greenhouse gases, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Storage of atmospheric CO2 into stable organic fractions in the soil can remove CO2 from the atmosphere while normal crop production practices generate N2O and decrease the soil sink for atmospheric CH4 in upland soils. The overall balance between the net exchange of these gases constitutes the net global warming potential (GWP) of a crop production system. In upland cropping systems changes in soil organic matter content, the CO2 emitted from fertilizer production, transport and application, and N2O emissions are the major components of net GWP. The total 'greenhouse effect' in a cropping system needs to be considered in developing new management systems to limit net GWP while maintaining crop production, and not just single components independently. This paper describes the determination of net GWP in rainfed agroecosystems in Colorado and Michigan and an irrigated system in Colorado. Soil organic C storage, energy use in fertilizer production, and field N2O emissions are the main contributors to net GWP in the rain-fed cropping systems. The energy required for irrigation is a major part of GWP in the irrigated systems. In all sites, differences in SOC storage dictated whether the system was a net source or sink for CO2 equivalents.
Technical Abstract: When appraising the impact of food and fiber production systems on the composition of earth's atmosphere and the 'greenhouse' effect, the entire suite of biogenic greenhouse gases -- carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) -- needs to be considered. Storage of atmospheric CO2 into stable organic fractions in the soil can sequester CO2 while normal crop production practices can produce CO2, generate N2O, and decrease the soil sink for atmospheric CH4. The overall balance between the net exchange of these gases constitutes the net global warming potential (GWP) of a crop production system. N2O emission is typically the single greatest source of GWP. Trace gas flux and soil C storage data from-long term studies, a rainfed site in Michigan, and a rainfed and an irrigated site in Colorado, are used to estimate net GWP from crop production systems. N2O emissions comprised approximately 40% of the CO2 equivalent emissions from both rain-fed sites. N2O emissions contributed 16-33% of GWP in the irrigated system. The energy used for irrigation was the dominant CO2 equivalent source in the irrigated system. Whether a system is a sink or source of CO2, i.e. net GWP, was controlled by the rate of soil organic carbon (SOC) storage in all sites. SOC accumulation in the surface 7.5 cm of both rainfed continuous cropping systems was approximately 1100 kg CO2 equivalents ha-1 yr-1. Carbon accrual rates were about three times higher in the irrigated systems. The rainfed systems had been in NT for > 10 years while the irrigated system had been converted to NT three years before the start of this study. It remains to be seen if the C accrual rates decline with time in the irrigated system or if N2O emission rates decline or increase with time after conversion to NT.