Greenhouse gas contributions of agricultural soils and potential mitigation practices in Eastern Canada

Agricultural soils can constitute either a net source or sink of the three principal greenhouse gases, carbon dioxide (CO 2), nitrous oxide (N 2O), and methane (CH 4). We compiled the most up-to-date information available on the contribution of agricultural soils to atmospheric levels of these gases and evaluated the mitigation potential of various management practices in eastern Canada and northeastern USA. Conversion of native ecosystems to arable cropping resulted in a loss of ∼22% of the original soil organic carbon (C)—a release of about 123 Tg C to the atmosphere; drainage and cultivation of organic soils resulted in an additional release of about 15 Tg C. Management practices that enhance C storage in soil include fertilization and legume- and forage-based rotations. Adopting no-till did not always increase soil C. This apparent absence of no-till effects on C storage was attributed to the type and depth of tillage, soil climatic conditions, the quantity and quality of residue C inputs, and soil fauna. Emission of N 2O from soil increased linearly with the amount of mineral nitrogen (N) fertilizer applied (0.0119 kg N 2O-N kg N −1). Application of solid manure resulted in substantially lower N 2O emission (0.99 kg N 2O-N ha −1 year −1) than application of liquid manure (2.83 kg N 2O-N ha −1 year −1) or mineral fertilizer (2.82 kg N 2O-N ha −1 year −1). Systems containing legumes produced lower annual N 2O emission than fertilized annual crops, suggesting that alfalfa ( Medicago sativa L.) and other legume forage crops be considered different from other crops when deriving national inventories of greenhouse gases from agricultural systems. Plowing manure or crop stubble into the soil in the autumn led to higher levels of N 2O production (2.41 kg N 2O-N ha −1 year −1) than if residues were left on the soil surface (1.19 kg N 2O-N ha −1 year −1). Elevated N 2O emission during freeze/thaw periods in winter and spring, suggests that annual N 2O emission based only on growing-season measurements would be underestimated. Although measurements of CH 4 fluxes are scant, it appears that agricultural soils in eastern Canada are a weak sink of CH 4, and that this sink may be diminished through manuring. Although the influence of agricultural management on soil C storage and emission of greenhouse gases is significant, management practices often appear to involve offsets or tradeoffs, e.g., a particular practice may increase soil C storage but also increase emis