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 decreas ed tillage intensity in the semiarid region of the Canadian prairies. The impact of these changes on sequestration of atmospheric CO2 in soi l organic carbon (C) is uncertain. Our objective was to quantify the c hanges in total, mineralizable and Light fraction organic C and nitrog en (N) due to the adoption of continuous cropping and conservation til lage practices. We sampled three individual long-term experiments at L ethbridge, Alberta, in September 1992: a spring wheat (Triticum aestiv um L.)-fallow tillage study, a continuous spring wheat tillage study a nd a winter wheat rotation-tillage study. Treatments had been in place for 3-16 years. In the spring wheat-fallow study, different intensiti es (one-way disc > heavy-duty cultivator> blade cultivator) of convent ional tillage (CT) were compared with minimum tillage (MT) and zero ti llage (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 culti vator) than in the least-intensive CT treatment (blade cultivator). Th e 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 cul tivator had light fraction C and N and mineralizable N amounts that we re 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 increa sed total organic C by 2 Mg ha(-1), and increased mineralizable and li ght fraction C and N by 15-27%, compared with CT with a heavy-duty cul tivator 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 es tablishment and 3 years before soil sampling), in an effort to control a severe infestation of downy brome ( Bromus tectorum L.). Our result s 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 mo ving toward continuous cropping, is the mst step toward increased C se questration. Reducing tillage intensity, by the adoption of ZT, enhanc es the cropping intensity effect. (C) 1997 Elsevier Science B.V.