Getting Closer to GHG Neutrality: Inclusion of Perennial Forage in Cropping Systems.

Perennial legume forages have the potential to increase soil carbon sequestration and decrease nitrous oxide (N₂O) emissions to the atmosphere when introduced into annual cropping systems. However, little is known about what short-term effect the return to annual cropping following termination of perennial legume forage would have on CO₂ and N₂O emissions. A long-term field experiment to continuously measure CO₂ and N₂O fluxes was established at the Trace Gas Manitoba (TGAS-MAN) Long Term Greenhouse Gas Monitoring Site in Manitoba using the flux gradient micrometeorlogical technique. The field experiment consists of two cropping treatments; annual cropping with inclusion of a grass-alfalfa forage phase (annual-perennial) and continuous grain-oilseed (annual) cropping. Over seven crop years (2006-2012), the annual-perennial system increased carbon uptake by 3.4 Mg C ha^-1 and reduced N₂O emissions by 3.0 Mg CO₂-eq ha^-1 compared to the annual system. However after accounting for harvest removals both the annual and annual-perennial systems were net carbon sources of 5.7 and 2.5 Mg C ha^-1 and net GHG sources of 38 and 24 Mg CO₂-eq ha^-1 respectively.