Impact of leaching and denitrification on temporal distribution of nitrate in several Manitoba soils

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Grift, Sheri L.
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Leaching and denitrification are governed by several factors including soil management, moisture, aeration, organic carbon and soil temperature. The interaction of these factors and their effects on the processes of denitrification and leaching influence the temporal and spatial distribution of nitrate in soils. Nitrate leaching in the field was monitored on an Orthic Black loamy fine sand (Stockton soil), an Orthic Black loam (Wellwood soil) and a Gley d Rego Black Almasippi sand. Four treatments were examined: fertilizer applied (100 kg ha-1 N as NH4NO3) on fallow, fertilizer applied and seeded to wheat, no fertilizer applied on fallow and no fertilizer applied on wheat. Samples were taken in 15 cm increments to a depth of 120 cm and analyzed for moisture and nitrate content, and groundwater samples were analyzed for nitrate content. Laboratory studies of nitrate disappearance rate were conducted on the Stockton and Almasippi soils using soil slurries incubated on a shaker at one of five temperatures: 5, 11, 16.5, 20.5, and 26.5C. The amounts of NO3-N remaining in the slurries after set incubation periods were graphed as a function of time. Although nitrates moved mainly with soil water, soil nitrate contents of planted and fallow treatments were significantly different, while fertilizer application had no significant effects on nitrate distribution and profile content at the application rates used. Laboratory studies on denitrification rates indicate that for the Stockton profile, the biological activity decreased exponentially with depth and increased with increasing temperature according to the Arrhenius equation. The Almasippi profile exhibited two rates of biological activity: one for the 0-15 cm depth and another for the 15-120 cm depth, presumably due to the rapid decline in soil organic matter content below 15 cm in this profile.