Top-Down Constraints on Regional-Scale Indirect N2O Emissions.

Nitrous oxide (N₂O) emissions within the US Corn Belt have been estimated to be 200-900% larger than predictions from emission inventories, implying that one or more source categories in bottom-up approaches are underestimated. Our research has focused on constraining the indirect emissions associated with runoff and leaching using bottom-up and top-down approaches. Here we interpret hourly N₂O mixing ratios measured from 2010 to 2015 at a tall tower using a time-inverted transport model and a scale factor Bayesian inverse modeling approach to help constrain direct and indirect agricultural emissions. The tall tower measurements and analyses show large inter-annual variability in N₂O emissions (316 to 585 Gg N₂O-N y^-1). This implies that the regional emission factors are highly sensitive to climate. In the warmest year and spring (2012) of the observational period, the emission factor was 7.5%, nearly double previous reports. Indirect emissions associated with runoff and leaching dominated the inter-annual variability of the total emissions, with results suggesting that indirect emissions represent 40 to 60% of the total agricultural budget—substantially larger than that estimated from bottom-up inventory approaches. These findings are supported with a new bottom-up modeling approach involving the Soil and Water Assessment Tool (SWAT). Here, emissions from zero and first order streams in a catchment in southern Minnesota represented 25 to 50% of the direct N₂O emissions.