Changes in total, mineralizable and light fraction soil organic matter with cropping and tillage intensities in semiarid southern Alberta, Canada

There has been a trend toward increased cropping intensity and decreased tillage intensity in the semiarid region of the Canadian prairies. The impact of these changes on sequestration of atmospheric CO2 in soil organic carbon (C) is uncertain. Our objective was to quantify the changes in total, mineralizable and Light fraction organic C and nitrogen (N) due to the adoption of continuous cropping and conservation tillage practices. We sampled three individual long-term experiments at Lethbridge, Alberta, in September 1992: a spring wheat (Triticum aestivum L.)-fallow tillage study, a continuous spring wheat tillage study and a winter wheat rotation-tillage study. Treatments had been in place for 3-16 years. In the spring wheat-fallow study, different intensities (one-way disc > heavy-duty cultivator> blade cultivator) of conventional tillage (CT) were compared with minimum tillage (MT) and zero tillage (ZT). After 16 years, total organic C was 2.2 Mg ha(-1) lower in more intensively worked CT treatments (one-way disc, heavy-duty cultivator) than in the least-intensive CT treatment (blade cultivator). The CT with the blade cultivator and ZT treatments had similar levels of organic C. The CT treatments with the one-way disc and heavy-duty cultivator had light fraction C and N and mineralizable N amounts that were about 13-18% lower than the CT with the blade cultivator, MT or ZT treatments. In the continuous spring wheat study, 8 years of ZT increased total organic C by 2 Mg ha(-1), and increased mineralizable and light fraction C and N by 15-27%, compared with CT with a heavy-duty cultivator prior to planting. In the winter wheat rotation-tillage study, total organic C was 2 Mg ha(-1) higher in a continuous winter wheat (WW) rotation compared with that in a winter wheat-fallow rotation. The lack of an organic C response to ZT on the WW rotation may have been due to moldboard plowing of the ZT treatment in 1989 (6 years after establishment and 3 years before soil sampling), in an effort to control a severe infestation of downy brome ( Bromus tectorum L.). Our results suggest that although relative increases in soil organic matter were small, increases due to adoption of ZT were greater and occurred much faster in continuously cropped than in fallow-based rotations. Hence intensification of cropping practices, by elimination of fallow and moving toward continuous cropping, is the mst step toward increased C sequestration. Reducing tillage intensity, by the adoption of ZT, enhances the cropping intensity effect. (C) 1997 Elsevier Science B.V.