Perennial Grain Crops Reduce N 2 O Emissions Under Specific Site Conditions

Abstract

Perennial grain crops represent a novel hybrid between annually harvested grain crops and perennial forage crops, which are seeded once and grow for multiple subsequent seasons. Previous research has shown comparatively reduced nitrous oxide (N2O) emissions from perennial forage crops relative to annual grain crops, however, the effect of perennial grain cropping on N2O emissions is unclear. We quantified field N2O emissions along an experimental continuum of perenniality (perennial forage, perennial grain, fall grain, spring grain and fallow) established at two sites within Alberta, Canada with contrasting soils: Luvisolic at the Breton site and Chernozemic at the Edmonton site. We used static chambers and a micrometeorological technique based on an open-path Fourier-transform infrared gas sensor (OP-FTIR). Perennial grain crops reduced cumulative N2O emissions at the Breton site by 60% and 94% in years two and three of the study, respectively (Ps < 0.0001). Conversely, no reduction in N2O emissions by the perennial grain crop relative to the annual crop was evident at the Edmonton site. Correlation analyses encompassing both sites revealed that the average root density from 0-60 cm was negatively correlated with soil available nitrogen (N) (0-15 cm depth) in years one (Ps < 0.01) and two (Ps < 0.05). Moreover, in year two, root density was negatively correlated with cumulative N2O emissions, specifically at the Breton site (P < 0.01). Results suggest that the enhanced root density of perennial crops reduced soil N availability at the Breton site, which translated into reduced cumulative N2O emissions in year two. Notably, increased root density did not correlate with reduced N2O emissions at the Edmonton site, suggesting that factors such as increased soil clay and carbon content in the Chernozemic soil overrode crop controls on N2O emission. Further, OP-FTIR measurements at the Breton site were in general agreement with static chamber measurements, which collectively informed that the bulk reduction in cumulative N2O emissions occurred during spring thaw. Overall, the ability for perennial cereal grain crops to reduce N2O emissions relative to annual crops was site-specific.