Soil organic carbon after twelve years of various crop rotations in an Aridic Boroll

Eliminating summer fallow or establishing perennial grass elicited measurable gains in soil organic C (SOC) after just 6 yr in a crop rotation study on an Aridic Boroll in southern Alberta. This study was resampled after 12 yr to determine if SOC increases were continuing with time and to evaluate the impact of alternative crop rotation and fertilizer treatments on SOC. The crop rotation treatments included in this study were fallow-wheat (FW), fallow-wheat-wheat (FWW), fallow-flax-wheat (FXW), legume-wheat (LW), continuous wheat (W) and continuous grass (G). The gain in SOC due to the elimination of fallow was 1.5 Mg C ha(-1) after 12 yr, no greater than that observed after 6 yr. Soil organic C was the same for all rotations that included fallow (FW, FWW, FXW). Fertilizer treatments that had the greatest benefit on grain yields of annual crops also tended to increase SOC, although differences were barely detectable. The gain in SOC of unfertilized grass compared to the FW rotation was no higher after 12 yr than after 6 yr (3 Mg C ha(-1)). Under fertilized grass, in contrast, SOC continued to increase at a rate of approximately 0.5 Mg C ha(-1) yr(-1). Accumulation of light fraction C accounted for most of the gains in SOC that occurred with elimination of fallow or establishment of grass. These findings suggest that much of the SOC gain due to adoption of C-conserving practices in soils like those of this study may occur early, within the first decade, and consist primarily of decomposable soil fractions. If confirmed, this means that C sequestration in these soils may be comparatively short-lived and vulnerable to future loss.