Wildlife contamination studies found high levels of DDT and associated metabolites in bird eggs from Canadian orchard sites during the early 1990s. The present study investigated local dietary uptake of DDT and geographic variability in tissue concentrations in the same orchards. A soil-earthworm-robin food chain was chosen for study, as early surveys showed that robins contained the highest levels of DDT of several avian species and because published research indicated that earthworms were a probable dietary exposure route. Organochlorine pesticides and PCBs were measured in soil, earthworm, robin egg, and robin nestling samples collected from fruit orchards and reference sites. High average DDE (soil: 5.2 mg/kg; earthworm: 52 mg/kg; robin egg: 484 mg/kg dry weight) and DDT (soil: 9.2 mg/kg; earthworm: 21 mg/kg; robin egg: 73 mg/kg dry weight) concentrations in Okanagan (British Columbia) samples confirmed that previously recorded contamination was common in the region. Concentrations detected in Simcoe, Ontario, orchards were not as high but were still significantly elevated relative to levels in soils and robins from reference areas. Significant positive linear regressions between soil and earthworm concentrations and consistent trends in food chain accumulation suggested that robins were acquiring DDT and metabolite (DDTr) burdens locally. Low concentrations of DDT and DDTr in robin eggs collected from nests in nearby nonorchard and post-DDT orchard habitats suggested that the local sources were in orchards. Persistence of DDT in orchard food chains is likely due to a combination of retarded degradation rates for DDT in soil and its extensive use historically. DDT concentrations in some robin eggs and earthworms were at levels comparable to those observed in field studies where mortality or reproductive effects occurred.

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Many research studies have dealt with the influences of minimum or no-till soil management practices on the major physical, chemical, biological and morphological properties in the soil profile. However little work has been done on the assessment of the rates of changes in pore properties as management practices are converted from conventional to no-till (NT) methods. Short-term changes in soil micromorphology attributed to conversion to no-till from conventionally tilled management are evaluated in this paper. As the number of years in no-till increased there was a decrease in the number pores of 30- to 100-µm diameter in the no-till soils. However, pores from 100- to 500-µm diameter increased in number only after 4 yr of NT. The pores of this size are important for water storage, transmission and root development. The decline in the number of these pores after no-till initiation followed by the increase after 4 yr may explain why crop yields tend to be lower only after the first few years after implementing no-till. The 100- to 500-µm diameter pores may be crucial for the proper development of roots in wheat and corn. The no-till soils had greater numbers of horizontally oriented elongated macropores in the top 5 to 15 cm of the soil profile due to the lack of tillage and annual freeze-thaw processes. These pores may inhibit proper drainage and root penetration. Rounded macropores increased with the number of years the soil was in no-till as these pores were maintained each year due to the lack of tillage and greater faunal activity in the no-till soils. Key words: Micromorphology, image analysis, earthworms, no-till, soil structure

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. 2000. Interpretation of soil enzyme activities in a comparison of tillage practices along a topographic and textural gradient. Can. J. Soil Sci. 80: 71-79. Assessment of the impacts of soil conservation practices requires measurement of the resulting changes in soil quality at scales inclusive of soil variability comparable to that of typical farms. The objective of this study was to compare changes in six soil enzyme activities (urease, glutaminase, phosphatase, arylsulfatase, β-glucosidase and dehydrogenase) and organic carbon (OC) content resulting from implementation of notill along a topographic and soil textural gradient. Activities of β-glucosidase, glutaminase, phosphatase and arylsulfatase, and OC content were greater in coarse-textured soils at a lower slope-position than in fine-textured soil at an upper slope-position. Tillage practice influenced activities of urease, glutaminase, β-glucosidase and dehydrogenase but not OC content. The effect of tillage practice on enzyme activities was influenced by sampling depth and slope-position. Phosphatase and arylsulfatase activities of the Ap horizon behaved as indices of soil organic matter (SOM) content along the topographic gradient. Urease and dehydrogenase activities behaved as indices of soil biochemical activity within the Ap horizon. When interpreted in these terms, soil enzyme activities are comparable to other integrative measurements and as such describe system behavior and attributes. . L'évaluation des impacts des pratiques de travail de conservation du sol suppose qu'on puisse mesurer leurs conséquences sur la qualité du sol et sur leur degré de variabilité à une échelle correspondante à celle de la ferme typique. L'objet de nos travaux était de comparer les changements affectant les activités de six enzymes du sol : uréase, glutaminase, phosphatase, arylsulfatase, β-glucosidase et déshydrogénase, ainsi que la teneur en C organique du sol, résultant de l'application de la culture sans labour selon un gradient topographique et un gradient granulométrique du sol. Les activités de la β-glucosidase, de la glutaminase, de la phosphatase et de l'arylsulfatase et la teneur en C organique (CO) étaient plus importantes dans les sols de texture grossière situés en bas de pente que dans un sol de texture fine situé en haut de pente. Le régime de travail du sol avait un effet sur les activités de l'uréase, de la glutaminase, de la β-glucosidase et de la déshydrogénase, mais pas sur la teneur en CO. Les effets sur les activités enzymatiques variaient selon la profondeur d'échantillonnage et selon l'emplacement dans la pente. Les activités de la phosphatase et de l'arylsulfatase dans l'horizon Ap se comportaient comme des indicateurs de la teneur en matière organique du sol le long du gradient topographique, tandis que les activités de l'uréase et de la déshydrogénase servaient d'indicateur de l'activité biochimique du sol dans l'horizon Ap. Ainsi interprétées, les activités enzymatiques du sol sont comparables aux autres mesures intégrantes et, comme tell...

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