Micrometeorological Determination of N2O and CO2 Fluxes for Five Years of a Cropping System in the Red River Valley, Manitoba,.

Studies have shown that including perennial forages in rotations can increase soil carbon (C) and lower nitrous oxide (N2O) emissions when compared to continuous annual cropping. Although there is evidence that perennials may reduce net greenhouse gas (GHG) emissions from agricultural systems, there is a lack of simultaneous carbon dioxide (CO2) and N2O flux measurements from forage crops. The study is part of a long-term experiment that began in the Fall 2005 to determine GHG budgets for cropping systems in the Red River Valley, Manitoba. Located at the University of Manitoba’s National Centre for Livestock and the Environment, south of Winnipeg, Manitoba, the site consists of four 4-ha plots with a history of annual rotation management. Nitrous oxide and CO2 emissions were measured continuously using the flux gradient micrometeorological method. Two cropping rotations, annual-perennial and continuous-annual, each with two replicate plots were examined. The CO2 global warming potential equivalent of cumulative N2O flux over the common annual crops in both rotations over the first three study years amounted to 5.4 t CO2-eq. ha-1. Net CO2 exchange for the site, with accounting for C removed during harvest, amounted to a loss of 11.6 t CO2 ha-1. Thus, the cumulative N2O flux exacerbated the global warming potential equivalent to 47% of that of C loss at the site. In the year of forage establishment (annual-perennial rotation) the perennial plots accumulated 1250 ±250 kg C ha-1 (no forage removed), and emitted 0.9 ±0.2 kg N2O-N ha-1. The continuous-annual plots were a net C sink of 480 ±110 kg C ha-1, after accounting for harvested spring-wheat grain and straw, and released 2.6 ±0.5 kg N2O-N ha-1. The annual-perennial rotation reduced net GHG emissions overall due to the combined effect of reduced N2O emissions (lack of N fertilizer application) and greater CO2-C uptake (longer growing season). Cumulative GHG fluxes (CO2 and N2O) will also be presented for the four years of this study contrasting the continuous-grain and annual-perennial rotation systems.