Preferential leaching in large undisturbed soil blocks from conventional tillage and no-till fields in southern Alberta

There is a concern that adoption of conservation tillage practices such as no-till may increase preferential leaching of water and chemicals to the gr oundwater. Evidence from previous studies of long-term (since 1968) convent ional tillage (CT) and no-till (NT) fields in southern Alberta suggests tha t tillage practice has the potential to influence preferential leaching. Ho wever, no studies have been done to test this hypothesis. Our objective was to utilize the two-region (mobile/immobile water) model of solute transpor t to compare preferential leaching in CT and NT fields of clay loam texture . Four large (46 x 46 x 51 cm) undisturbed blocks of soil were excavated fr om each tillage field (unreplicated) during the fallow phase of a wheat-fal low rotation. The soil blocks were transported to the laboratory, stored un der drying conditions (32 months), and miscible displacement experiments co nducted under saturated, steady-state conditions. Because of the long stora ge period, the focus of our study was on long-term (e.g,, structure, earthw orm burrows, old root channels) rather than short-term (e.g., tillage of su rface soil) tillage effects on preferential leaching Breakthrough curves (B TCs) for chloride were derived, and the modified convection-dispersion equa tion fitted to the experimental data using a two-region model (CXTFIT-Versi on 2.0) to allow estimation of the mobile water fraction (beta) or extent o f preferential leaching. Breakthrough curves for both tillage fields exhibi ted early initial breakthrough, a rapid rise in tracer concentration, a shi ft of the ETC peak to the left of one pore volume, and a slow decline ("tai ling") toward zero concentration for the descending limb of the curve. Reas onably good fits were obtained for fitting of the two-region model to the B TCs, as indicated by correlation coefficients ranging from 0.60 to 0.84. Me an values for the mobile water fraction were similar for the CT field (0.78 ) and NT field (0.80), suggesting no difference in preferential leaching of chloride. We hypothesize that the extent of preferential leaching in earth worm burrows (Aporrectodea caliginosa) in the NT field and leaching in old root channels in the CT field may have been similar. Further research on re plicated plots is needed to examine short-term effects of tillage practice (e.g., tillage of surface soil) as well as the individual contribution of e arthworm burrows and old root channels, to preferential leaching under CT a nd NT.