Reducing Nitrous Oxide Emissions from US Row Crop Agriculture through N Fertilizer Management

Nitrous oxide (N₂O) is a potent greenhouse gas (GHG) produced in soils primarily through the microbial processes of nitrification and denitrification. It is the predominant GHG emitted by US agricultural activities, with annual emissions from cropland greater than 1 Tg. Soil management activities such as nitrogen (N) fertilizer application are the largest contributor (78 %) to US N₂O emissions.

Due to the high global warming potential of N₂O, every ton of avoided N₂O emissions is equivalent to approximately 300 tons of sequestered CO₂. Increasing crop N-use efficiency by altering N management practices is likely the most promising strategy for mitigating agricultural N₂O emissions across a range of cropland systems.

Annual inventories of N₂O emissions from agricultural soil are typically calculated by assuming a constant linear relationship between total N inputs and subsequent N₂O emissions. However, our previous research found evidence of a non-linear relationship, with N₂O emissions sharply increasing above the N fertilizer rate at which crop yields were optimized. This implies that a substantial decrease in N₂O flux from cropland could be achieved with moderate reductions in fertilizer N input and little yield impact.

To better refine this relationship we established automated flux chambers at the Kellogg Biological Station (KBS) in southwest Michigan to monitor near-continuous N₂O emissions from a field scale, rain-fed N fertility gradient planted to corn. Fluxes of N₂O were measured from 8 fertilizer-N rate applications (0, 34, 67, 101, 134, 168, 202 and 246 kg N ha^-1) every 4 hours and related to soil N, moisture, temperature and crop yield.

Results from the 2008 field season will be presented, along with progress on, and the challenges associated with, implementing potential fertilizer N reduction strategies including the introduction of an agricultural N₂O reduction protocol within a cap and trade system.