Nitrous oxide (N2O) is a potent greenhouse gas (GHG) produced in soils primarily through the microbial processes of nitrification and denitrification. It is the major GHG emitted by US agriculture, with annual emissions from cropland greater than 1 million metric tons. Soil management activities, including nitrogen (N) fertilizer application, are the largest contributor to N2O emissions in the US, accounting for around 80% of total N2O emissions from the agricultural sector. Due to the high global warming potential (GWP) of N2O, every ton of avoided emissions can be considered equivalent to sequestering approximately 300 tons of carbon dioxide (CO2) in the soil.

The most readily accessible management options for reliable mitigation of agricultural N2O production likely involve increasing N use efficiency at the cropping system–level, through the manipulation of external N inputs, such as synthetic N fertilizer. Quantifying the relationship between N fertilizer rate and N2O emissions under varying management practices is vital to help better elucidate the agricultural N2O budget and propose strategies for N2O mitigation. This relationship is typically assumed to be linear and insensitive to increasing N rate, and indeed in this form, is the basis for estimating country–wide inventories of agricultural N2O emissions using the most recent IPCC methodology. Recent studies at the Kellogg Biological Station (KBS) in SW Michigan however, suggest that a non–linear relationship may be more applicable, particularly at rates which exceed crop N demand for maximizing yield.

To further refine this relationship and test the hypothesis that N2O fluxes respond to fertilizer N in a non–linear manner, we established automated flux chambers at KBS to monitor near-continuous N2O emissions from a field scale, rain-fed N fertility gradient planted to winter wheat in 2007 and corn in 2008. Fluxes of N2O were measured from between 6 to 8 fertilizer-N rate treatments (0, 34, 67, 101, 134, 168, 202 and 246 kg N ha-1) up to 8 times daily. As a methodological comparison, manual sampling of non–automated (static) chambers was also carried out during 2008. In both years and sampling systems, fluxes of N2O were related to measured soil and environmental variables and crop grain yield.

In winter wheat and corn cropping systems, average fluxes of N2O increased with increasing N rate and showed a non–linear relationship between rate and total emissions. Emissions of N2O were low at fertilizer N rates which were below or coincident to those which optimized crop yield, but sharply increased thereafter. This threshold response of N2O emissions to fertilizer N addition provides evidence to suggest that a substantial decrease in N2O flux from cropland agriculture could be achieved with moderate reductions in N fertilization and little or no yield penalty.

In this poster, results from the 2007–2009 field seasons will be presented, along with comments relating to the potential for the introduction of an agricultural N2O reduction protocol within the burgeoning carbon and nutrient trading markets.