Net CO2 exchange and carbon budgets of a three-year crop rotation following conversion of perennial lands to annual cropping in Manitoba, Canada

Eddy covariance flux towers were used to measure net ecosystem production over three adjacent agricultural fields in Manitoba, Canada, from 2009 to 2011. Two fields were converted from long-term perennial hay/pasture to annual cropping, while the third field served as a control field that was maintained as hay/pasture. One converted field had a rotation of oat–canola–oat crops, while the second was hay–oat–fallow. Weather was an important driver of inter-annual variability, with poor yields on all fields in 2011 because of dry conditions in summer, with the summer-fallow condition on one field caused by excess spring moisture not allowing planting. The cumulative net ecosystem production of the oat–canola–oat field showed a net CO2 emission of 100 g C m−2, the hay–oat–fallow field emitted 500 g C m−2, and the hay field gained 550 g C m−2 by the end of the 30-month study period. The hay field had the highest cumulative gross primary production of 2500 g C m−2, whereas the oat–canola–oat and hay–oat–fallow fields had only about 1400 g C m−2. The perennial field had the advantage of both early- and late-season growth when crops were absent on the other fields. The hay and hay–oat–fallow fields had comparable cumulative ecosystem respiration (1400 g C m−2). Manure additions contributed 300 g C m−2 on the two converted fields. With harvest exports and manure additions included, the oat–canola–oat field was a carbon source of 240 g C m−2, the hay–oat–fallow field was a source of 415 g C m−2, and the hay/pasture field was a sink of 120 g C m−2 over the 30-month period.