CARBON SEQUESTRATION IN A BROWN CHERNOZEM AS AFFECTED BY TILLAGE AND ROTATION

Soil organic matter is important because it influences the productivit y and physical well-being of soils. Recently, increased attention has focussed on soil organic matter as a possible sink for CO2-C. Despite this interest, there is a lack of data for quantifying the effect of t illage on soil organic matter. Between 1981 and 1994, two tillage expe riments were conducted at Swift Current, Saskatchewan, on Swinton loam , an Orthic Brown Chemozemic soil. Organic C and N were monitored peri odically to quantify the effects of crop rotation [continuous spring w heat (Cent W) (Triticum aestivum L.) vs. fallow-wheat (F-W)] and tilla ge management [no-tillage (NT) vs. conventional tillage (CT) involving primarily use of a cultivator and rodweeder]. The effect of snow mana gement on soil organic matter was also evaluated in one experiment, bu t this factor was not significant. Organic matter changes were mainly observed in the 0- to 7.5-cm soil depth. Carbon and N were greater in both concentrations and amounts in Cent W than in F-W; the latter crop ping system was employed on this land during the previous 70-80 yr. In the 0- to 7.5-cm depth, the amount of organic matter was only moderat ely greater in NT than CT in the Cent W systems while in the F-W syste ms tillage was not significant (P > 0.10). During the 12-yr period, Ce nt W (average of NT and CT) gained about 2 t ha(-1) more C in the top 15 cm of soil than F-W (average of NT and CT), with most of the increa se occurring in the first 5 yr. Further, Cent W (NT) gained about 1.5 t ha(-1) more C than Cent W (CT), and F-W (NT) gained about 0.5 t ha(- 1) more than F-W (CT). When a system that was maintained as Cent W (NT ) for 9 yr was changed to Cent W (CT) for 3 yr and then summerfallowed (CT) for 1 yr, soil organic matter declined (P < 0.05). Our observati ons, supported by calculations based on crop residue production, indic ated that an increase in organic C, averaging about 0.4-0.5 t ha(-1) y r(-1), has occurred in the top 15 cm of soil in Cent W (NT) between 19 82 and 1993. However, because of uncertainty in our estimated C levels at the start of the experiment the nature of the rate of C increase ( linear or curvilinear) is not known.