Carbon exchange following conversion from perennial to annual crops at a Canadian prairie site

Agricultural producers may convert perennial lands (hay or pasture) to annual crops when there is an economic or operational benefit. Typically, soils under perennial systems contain more organic carbon because of less disturbance and the development of deeper roots. We followed the process of land conversion being done by a producer in Manitoba, Canada, where a long-term hay/pasture field was tilled and planted with successive annual crops in a rotation of oats and canola. We measured net carbon exchange using eddy-covariance towers simultaneously on 3 adjacent fields of >100 ha in area: a field converted in 2008, a field converted in 2009, and a control perennial field. Fields with annual crops showed greater carbon losses, but removal of hay from the perennial field was an important event in the annual carbon budget. Interestingly, the initial conversion event in mid-summer affected gross primary production with little change to ecosystem respiration. Three years (2009-2011) of flux data demonstrated substantial interannual variability, largely caused by differences in precipitation.