This study measured the spatial dependence of soil enzyme activities and other properties of the Ap horizon in a Gray Brown Luvisol (Hapludalf). Soil samples were collected at 74 positions along a slope following harvest of soybean [Glycine max (L.) Merr.] and fall tillage. Parameters measured were activity of dehydrogenase, urease, glutaminase, phosphatase, arylsulfatase, and β‐glucosidase; water, organic carbon (OC), mineral N, and inorganic P contents; the light fraction of soil organic matter; and depth of the Ap horizon. Rank correlation indicated significant relationships between water and dehydrogenase, urease, glutaminase, phosphatase, and arylsulfatase activities, and between water and OC content. Depth of the Ap horizon, water content, and arylsulfatase activity were strongly spatially dependent; OC and inorganic P contents and phosphatase activity were moderately spatially dependent. Other properties showed little or no spatial dependence. The ranges of spatial dependence were similar for depth of the Ap horizon, inorganic P content, and phosphatase activity (≈20 m). The range for arylsulfatase activity was 16 m, while that of OC content was 32 m. The relatively long range estimate for water content (98 m) was influenced by a trend along the slope. Maps of water and OC contents and phosphatase and arylsulfatase activities indicated similar spatial patterns along the slope. The magnitude of these soil properties was minimal in the middle or upper portion of the slope, and maximal at the footslope. Similarity in spatial patterns along the slope was interpreted as evidence for influence of water or OC content on amounts of phosphatase and arylsulfatase at that scale.

The fate of atrazine in sediment sampled from two agricultural watersheds in the Great Lakes basin of southern Ontario was studied in laboratory flask incubations of sediment slurry and enrichment cultures. Atrazine at an initial concentration of 10 mg/L disappeared within 15 d in sediment slurries incubated aerobically at 30°C. Because atrazine concentrations were stable in similarly incubated autoclaved slurry and the lag preceding atrazine degradation was abolished by repeated atrazine additions, it was concluded that the herbicide was biodegraded. Rapid atrazine degradation was found in concentrated suspended particulates and surface (0 to 10 cm) sediments, but not in sediments taken from a depth of 20 cm. Atrazine was not degraded under anaerobic or denitrifying conditions. Sediment and enrichment cultures converted [U-ring-'4C]-labeled atrazine to hydrophilic metabolites, tentatively identified as cyanuric acid and urea, and then to carbon dioxide.

The usual model of Histosol subsidence emphasizes the influence of the water table level on peat shrinkage and biochemical oxidation. It predicts a decrease in subsidence rate with time. However, erosion may increase the rate of subsidence significantly. The long‐term subsidence rate of the Histosol under study, where vegetable crops were grown yearly, was estimated at 2.06 ± 0.35 cm/year between 1936 and 1974. In 1974, part of the fields was sown in oats, as a conservation measure, whereas a windbreak was removed in 1976 and unusual meteorological events were recorded in 1974 and 1975. Erosion patterns were detected on the portion of the experimental site where there was no conservation measure. Between 1974 and 1978, the subsidence rate was reduced to 0.99 ± 1.59 cm/year in fields protected against erosion but increased to 4.53 ± 2.29 cm/year in unprotected fields. The decrease in subsidence rate predicted by the model was observed only in fields protected by conservation measures. Consequently, subsidence and erosion should be estimated separately for modelling adequately the subsidence process as a whole.

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