Measurement of Net Global Warming Potential in Three Agroecosystems

Abstract

When appraising the impact of food and fiber production systems on the composition of the 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 carbon pools in the soil can sequester CO₂ while common 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. Trace gas flux and soil organic carbon (SOC) storage data from long-term studies, a rainfed site in Michigan that contrasts conventional tillage (CT) and no-till (NT) cropping, a rainfed site in northeastern Colorado that compares cropping systems in NT, and an irrigated site in Colorado that compares tillage and crop rotations, are used to estimate net GWP from crop production systems. Nitrous oxide emissions comprised 40–44% of the GWP from both rain-fed sites and contributed 16–33% of GWP in the irrigated system. The energy used for irrigation was the dominant GWP 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 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 y⁻¹. Carbon accrual rates were about three times higher in the irrigated system. The rainfed systems had been in NT for >10 years while the irrigated system had been converted to NT 3 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.