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Water table height and nitrate leaching in undisturbed soil columns Elder, Linda A.


Water table control by subsurface drainage has been shown to affect leaching losses of nitrate-nitrogen: a concern both for economic use of fertilizer, and for maintenance of water quality. The effect of water table height on leaching of NO₃⁻-N was investigated in this study in nineteen 15cm x 100cm undisturbed cores of silty clay loam. The experiment simulated fertilization followed by rainfall, then rapid water table rise and fall, under conditions similiar to those experienced in the early spring in the Lower Fraser Valley. In the first part of the experiment, a concentrated solution of KNO₃ and KG (equivalent to 35 kg/ha of N and 22 kg/ha of Cl) was applied to the columns, followed by intermittent leaching with distilled water. Leachate from two depths in each column was collected before and after a period of static water table, and analyzed for NO₃⁻, No₂⁻, NH₄⁺, and Cl⁻. This procedure was repeated without nutrient addition in the second part of the experiment. Chloride was used an inert tracer to follow anion movement and retention within the columns. There was no significant difference in the leachate NO₃⁻ concentration or leachate N/CI ratio from any of the four water table heights tested (15, 35, 55, and 75 cm above drain depth). The NO₃⁻ concentrations and N/CI ratios decreased with depth in the soil columns, indicating removal of N from the percolating soil solution, either by denitrification or immobilization. The variability in leachate concentrations among all columns was very high (eg. for a typical sample time, NO₃⁻-N ranged from 0.01 to 15.72 mg/L, and Cl⁻ ranged from 4.8 to 14.5 mg/L), as was the variability in constant head satiated hydraulic conductivities (range: 1 to 1468 cm/day; CV = 181%), and drainable porosity (range: 2.7 to 10.4%; CV = 39%). Cross sections of columns leached with 1% methylene blue solution did not reveal differences in patterns of water transmission between low and high conductivity columns. Indications were that penetration of dye was greater in columns with higher conductivities, and that preferential flow occurred in all columns examined. Leachate concentrations and N/CI ratios correlated significantly with hydraulic conductivity: Spearman's correlation coefficients were always > 0.8 for samples obtained from the bottom of the columns. However, even when the conductivity was included as a covariate in an analysis of covariance, there was no significant effect of water table height on nitrate leaching.

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