Eutrophication resulting from phosphorus (P) accumulation in water systems has been a worldwide concern for three decades. Agricultural soils are known to be an important non-point source of P in waterways. The objectives of this research are to identify agricultural management practices that reduce the risk of P loss from soils, and to investigate the underlying mechanisms of P retention and loss from soils. In the short term (4 years), dissolved P loads were not affected by tillage and were similar in corn (in a continuous corn rotation) and soybean (in a soybean/corn rotation) production systems. Soils amended with composted cattle manure had a greater P load than chemically fertilized soils. On average, 30% of the total P leached was in organic P forms, indicating that organic P compounds could also be problematic to water systems. Although manure application improved soil aggregation and thus may increase P retention by avoiding soil erosion, P loss through subsurface flow by leaching may be substantial. A simple soil test, either Mehlich-3 P or P saturation ratio can predict the P leaching potential, but water ponding on the surface of agricultural land could significantly affect the accuracy of the prediction. / The P adsorption data was fit with the Langmuir 2-surface model, which predicted that up to 90% of the native adsorbed P was distributed on the high-energy surface. Native adsorbed P in manured soils was weakly retained, as the binding strength coefficient was 50 times less in manured than chemically fertilized soil. This findings was confirmed by measuring P desorption, which showed that P desorption rate was almost 3 times greater from manured soils than from chemically fertilized soils. Manuring alters soil particle surfaces by increasing negative charge. This is the direct reason for less P adsorption and greater P desorption by manured soils. / The Langmuir 2-surface model and the adapted non-ideal competitive adsorption (MICA) model were equally good at modeling P adsorption data. However, the NICA model is more robust and can predict phosphate adsorption with changing soil solution pH. The simultaneously modeling of P adsorption and hydroxyl adsorption with the NICA model makes it a promising tool for analyzing competitive adsorption among anions in soils.

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In 1991, a project was initiated at Quinn and Ormstown, two sites with soils representative of Quebec's present-day agriculture. Three field management techniques were studied related to their possible impact on the soils: tillage, rotation, and N fertilization. The three treatments were randomly assigned in two blocks of a split-split-plot design. The main treatment, tillage, had two levels, conventional tillage (CT) and no-till (NT). The sub-treatment was represented by five 4-year rotations, starting from 1991 as follows: CCCC (continuous corn), CSAC (corn-soybean-alfalfa-corn), CSCS (corn-soybean-corn-soybean), SCSC (soybean-corn-soybean-corn) and SSSS (soybean monocrop). The third factor was N fertilization, at three different rates: 0, 90, and 180 kg/ha, excepting the continuous soybeans which received lower rates of, respectively, 0, 20 and 40 kg N/ha. Soil sampling was performed in 1994, during the last year of the 4-year rotations, at two different moments over the growing season, June and August, for all analysed soil properties, except aggregate stability and organic matter for which samples were taken only in September. (Abstract shortened by UMI.)

Corn (Zea mays L.) production under monoculture and conventional-tillage management may cause soil degradation and nitrate (NO$ sb3 sp-)$ pollution. This study was conducted from 1991 to 1993 to evaluate the impact of conventional-tillage (CT) and no-tillage (NT) practices under corn-soybean (Glycine max L. Merill) rotations (C-S-C and S-C-S), continuous corn (C-C-C) and continuous soybean (S-S-S) systems on optimum N fertilization rates, yield and soil residual N levels. Field experiments were carried out on a Ste. Rosalie clay (Humic Gleysol) and an Ormstown silty clay loam (Humic Gleysol). Overall, tillage had little effect on soil and crop N levels or grain yields. Residual soil NO$ sb3$-N in the fall was related to fertilizer N rates in C-C-C, but not with S-S-S or soybean in rotation. Residual NO$ sb3$-N values after soybean were high and at zero added N were equivalent to 90 kg N with C-C-C. Consequently, soybean contributed the equivalent of 90 kg N ha$ sp{-1}$ to subsequent corn. Corn yields following soybean were higher than following corn, and less fertilizer N was required following soybean than following corn.

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No description available.

Corn (Zea mays L.) production under monoculture and conventional tillage systems may land to soil degradation and nitrate (NO$ sb3 sp-$) pollution of surface and groundwater. This study evaluated the impact of no-till and conventional tillage, monoculture corn and soybean, corn-soybean rotations and corn-soybean-alfalfa rotations, and three fertilizer N rates applied to monoculture corn or soybean, and corn in rotation, on soil NO$ sb3$-N and NH$ sb4$-N levels. Experimental sites were a Ste-Rosalie clay (Humic Gleysol) and an Ormstown silty clay loam (Humic Gleysol). Results obtained from fall 1992 to spring 1995 showed that tillage had no effect on soil N levels under corn. Soil N levels under corn showed a linear response to added N, specially in the fall. Soil N levels under corn did not change considerably during the non-growing season. Higher spring NO$ sb3$-N levels, particularly in plots under conventional tillage and alfalfa or soybean, compared to fall values indicated greater nitrification and/or mineralization than denitrification, immobilization or leaching. Low NH$ sb4$-N levels indicated that nitrification processes were active. No consistent estimation could be made of fertilizer N credits for corn from either soybean or alfalfa based on NO$ sb3$-N levels in soil.

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