Harvest Removals Negate the Benefit of Perennial Forage in Reducing Greenhouse Gas Emissions in An Annual Crop Rotation Within the Red River Valley, Manitoba.

The introduction of perennial forages into annual cropping systems may have the potential to increase soil carbon sequestration and decrease nitrous oxide (N₂O) emissions to the atmosphere. However, there are few quantitative measurements about this impact on the Canadian Prairies. A long-term field experiment to continuously measure carbon dioxide (CO₂) and N₂O fluxes was established at the Trace Gas Manitoba (TGAS-MAN) research site at Glenlea, Manitoba using the flux gradient method with a tuneable diode laser analyzer. The soil is poorly drained clay in the Red River Valley. The field experiment consisted of four 4-hectare plots planted to corn in 2006 and faba bean in 2007. In 2008, grass-alfalfa forage was introduced to two plots (annual - perennial) and grown until 2011 whereas the other two plots (annual) were planted to annual crops: spring wheat, rapeseed, barley and spring wheat in 2008, 2009, 2010 and 2011, respectively. In late September of 2011 the grass-alfalfa forage was killed and in 2012 all four plots were planted with corn. Termination of the grass-alfalfa forage resulted in greater fall CO₂ emissions in 2011, greater spring melt CO₂ emissions and net annual N₂O emissions in 2012 from the annual-perennial plots when compared to the annual plots. 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 CO_2-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 CO_2-eq ha^-1 respectively.  We are currently following the long-term impacts of inclusion of perennial forages in an annual cropping system.