By Georgine Yorgey
Reprinted from: WSU CSANR Perspectives on Sustainability
If you are interested in ensuring that farming is climate friendly you are likely to start thinking about nitrous oxide (N2O). Nitrous oxide is a powerful greenhouse gas (298 times as powerful as carbon dioxide, over a 100-year time frame). And nitrous oxide from agricultural soils is the single biggest contributor to agriculture’s direct greenhouse gas emissions, as estimated through inventories of greenhouse gas emissions. In Washington State, it was estimated that nitrous oxide from soils accounted for 46% of direct greenhouse gas emissions from agriculture in 2008.1 However, these estimates rely on “default” assumptions about nitrous oxide emissions that were developed from global data – and a review of existing experimental data in our region suggests these defaults may not be appropriate in our region.
Nitrous oxide emissions occur in agricultural soils (and also non-agricultural soils) when microbes in the soil transform nitrogen from one form to another, specifically during the processes of nitrification and denitrification. However, more nitrous oxide is produced under some conditions than others – for example, when nitrogen is added to soils (as in most farming systems), and when oxygen in soils is limited (for example, when soils are saturated with water from rainfall or melting snow).
Most work on nitrous oxide in the Pacific Northwest has been done since 2000 (through CSANR’s Climate Friendly Farming project, among others). And there’s not an overwhelming quantity of data. However, existing data suggest that emissions from inland PNW croplands may be on the low side compared to other regions of the U.S. and world. The data below were collected as part of three webinars that Kristy Borrelli, Chad Kruger, and I presented recently about nitrous oxide emissions and nitrogen management in PNW croplands (accessible here). On the left, you can see the Intergovernmental Panel on Climate Change (IPCC)’s “Tier 1 Emissions Factor”, indicating that in the absence of more specific data, it should be assumed that 1% of N applied as fertilizer is emitted as nitrous oxide. This number is based on a global review of nitrous oxide emissions data in agricultural systems. In recognition of high amounts of variability in the data, they suggest an uncertainty range of 0.3-3% (shown in the graph with the error bars).
Just to the right, you can see data from a several conventional and no-till dryland cropping rotations in Bozeman, MT (Dusenbury et al., 2008). The next two measurements are from dryland winter wheat in Washington State. At the far right are two measurements from irrigated systems, representing measurements in a sweet corn-sweet corn-potato rotation. Error bars here indicate the range of values found across the different years and rotations studied.
In addition to these experimental results, modeling results (discussed in the webinar “Nitrous Oxide Emissions in Inland Pacific Northwest Cropping Systems” here and available in full detail here) also suggest that emissions that are on the low side compared to IPCC estimates.
Ongoing work in the Pacific Northwest, through the REACCH project, the site specific climate friendly farming project, and others, is seeking to confirm this tentative conclusion. Methods being used include experimental efforts using sophisticated flux towers along with the chamber methods that are captured in the data shown above, as well as modeling efforts. The answers that we get will likely have implications for how we might mitigate nitrous oxide emissions in our region.
If emissions are fairly low, one implication is that any efforts to reduce nitrous oxide emissions through management should focus on strategies that offer strong co-benefits such as raising yields or saving water. This is because with lower overall emissions, any strategies that reduce greenhouse gas emissions will also have relatively smaller incentives (whether through carbon credits or some other structure). So strong co-benefits will likely be important for adoption.
1 WA Department of Ecology. 2010. Washington State Greenhouse Gas Emissions Inventory, 1990-2008. Department of Ecology, Olympia, WA.