Crop Rotation Controls on Carbon Flux Dynamics in the Eastern Canadian Prairies

Long-term flux measurements have helped us understand the dynamics and driving forces of carbon exchange in many ecosystems. We have learned that inter-annual variability in climate can cause a large range in net ecosystem production (NEP), sometimes changing a carbon sink to a source. However, there are additional considerations for many cropping systems in temperate regions. For annual crops, most cropping systems employ rotations of two to more-than-four different crops in sequential years. At any given site, this can make it difficult to differentiate the effects of inter-annual climate variability from crop type on NEP. Further, NEP depends on the previous cropping history because residues can take several years to decompose with a lagging respiration signal. Other complications include that the period of carbon uptake depends on planting and harvesting dates, as well as weed-control strategies. These additional contributions to variability show the need to increase our measurement database for crop systems. Here, we present a decade of NEP measurements that include two crop rotation sequences: one with only annual crops (corn, fababean, barley, rapeseed, barley, wheat, corn, soybean, wheat, soybean) and the other that has a perennial phase (corn, fababean, alfalfa, alfalfa, alfalfa, alfalfa, corn, soybean, wheat, soybean). The data clearly show the early season carbon uptake of perennial crops, and the effect of different planting dates for annual crops. Total annual cumulative carbon ranged from near zero to more than 300 g C m^-2, depending on the crop and year. However, harvest removals caused the field to be a carbon source over this time period. The identification of possible crop rotations as a beneficial management practice is needed to maintain soil carbon in our agricultural soils.