Tillage effects on carbon fluxes in continuous wheat and fallow-wheat rotations

The traditional cropping system in semiarid regions of the Canadian prairies involves frequent summer fallowing with several tillage operations to control weeds during the fallow period. Recently, there has been a trend toward reduced tillage and more intensive cropping, but the impact of this shift in management on sequestration of atmospheric CO2 remains uncertain. In 1995 and 1996, we measured fluxes of CO2 in a tillage experiment that had been initiated in 1982 on a silt loam (Typic Haploboroll) in southwestern Saskatchewan. The experiment comprised two spring wheat (Triticum aestivum L.) rotations (continuous wheat [Cont. W] and fallow-wheat [F-W]), each with conventional tillage (CT) and no-till (NT) treatments. In Cent. W, CO2 fluxes tended to be lower under NT than under CT (mean annual flux was approximate to 20 to 25% less for NT than CT). In F-W, tillage effects on mean annual CO2 flux were significant (P < 0.05) in the wheat phase only (NT <approximate to> 10% less than CT). Tillage had negligible effect on C inputs in crop residues. Lower CO2 fluxes under NT than under CT were attributed to slower decomposition of crop residues placed on the surface of NT soil than when they were incorporated. With good growing conditions (and thus large inputs of residues) between 1989 and 1996, there was an accumulation of partially decomposed residues on the surface of NT soil. Carbon in surface residues represented about one-half of the C gained by NT soil. In Cent. W, surface residue C (in 1996) amounted to 3.6 t ha(-1) under NT vs 1.4 t ha(-1) under CT. Residue C amounts were smaller in the F-W system: 1.7 t ha(-1) (NT) and 0.7 t ha(-1) (CT). Based on our results, producers on medium-textured soils in the semiarid Canadian prairies who switch from the traditional wheat production system (conventionally tilled fallow-wheat) to continuous no-till cropping could, potentially, sequester 5 to 6 t C ha(-1) in soil organic matter and surface residues in 13 to 14 yr.