Comparing Nitrous Oxide Emissions from Paired No-Tillage and Conventional Tillage Agricultural Fields in the Northwest US: Insights from a Year of Intensive Monitoring

Abstract:

Agricultural soils are the main anthropogenic source of nitrous oxide (N₂O), a potent greenhouse gas (GHG) and ozone depleting substance. Due to a high degree of both spatial and temporal variability coupled with limited availability of high-precision N₂O sensors, emissions of N₂O are difficult to quantify at the regional and field levels, scales important for determining best management practices. This study combined the use of automated static chambers and the flux gradient micrometeorological technique to continuously monitor emissions of N₂O over two canola fields with differing tillage management: no-tillage and conventional tillage. Each site was instrumented with an array of sixteen chambers for the entire 2015 crop year (1 Oct – 30 Sept), and the N₂O emissions were measured with the flux gradient method from 1 April thru 30 September.

The chamber measurements indicated cumulative annual emissions of 6.0 and 3.1 kg N₂O-N ha^-1 for the conventional tillage and no-tillage sites, respectively, or 4.8% and 2.5% of applied fertilizer N. Emissions at the conventional tillage site were very low until the field was planted and fertilized, when emissions increased dramatically and stayed high until crop senesce. The growing season (1 April – June 15) accounted for 80% of total measured N₂O losses (4.8 kg N₂O-N ha^-1). In contrast, the no-till site was characterized by consistent moderate emissions, and no spike after planting and fertilization was observed. The growing season only accounted for 30% of the total emissions (1.0 kg N₂O-N ha^-1). However, even sixteen chambers may not properly capture hot spots of emissions, and the spatially integrated flux gradient results did not corroborate the chamber results. The total emissions measured by the flux gradient method over the growing season were 1.6 and 1.4 kg N₂O-N ha^-1 for the conventional tillage and no-till sites, respectively. Further work on integrating the two techniques will be necessary to optimize characterization of the spatial and temporal variability of N₂O emissions in this region.