Carbon dioxide exchange in a northern Prairie cropping system over three years

Net carbon dioxide ecosystem exchange (NEE) was measured over three years (2006–2008) in a maize/faba/spring-wheat crop rotation in the Red River Valley of southern Manitoba, Canada using the flux-gradient method. The turbulent transfer coefficients were obtained using a similarity-theory, eddy-covariance based aerodynamic method and the carbon dioxide vertical gradient was measured with a tunable diode laser analyzer. The system measured 30-min-average fluxes over each of four plots sequentially to test reduced and intensive tillage treatments. Annual cumulative NEE was −720, 70 and −2400 kg C ha−1 yr−1 in the maize, faba and spring-wheat crop years, respectively. Enhanced respiration in the faba year was largely responsible for the loss, whereas the large gain in the spring-wheat year was a combination of high photosynthetic activity and reduced respiration. The three years experienced sequentially cooler and wetter years that likely affected the crops. After accounting for harvest removals, the net ecosystem loss was 510, 3140 and −480 (gain) kg C ha−1 yr−1 for the respective crop years, summing to a three-year loss of 3170 kg C ha−1. A recent conversion from intensive tillage to a reduced tillage practice showed no difference because of relatively large variability among the four plots.