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Influence of Soil Moisture Tension on Nitrate Accumulation in Soils¹

ABSTRACT

The influence of soil moisture tension on NO^-₃-N accumulation was studied in limed, silt loam, loessial derived soils to which a non-limiting source of NH⁺₄-N was added. This was done by saturating the soils in an NH⁺₄ solution, then subjecting the samples to 0, 0.1, 0.33, 1, 5, and 15 bars of soil moisture tension in conventional positive pressure equipment, and incubating the samples in a saturated atmosphere for varying intervals, to characterize the NO^-₃-N accumulation curve with time. The maximum slope of the line gave an estimate of the K_M value or maximum rate of NO^-₃-N accumulation in ppm/day. The K_M values in all soils were greatest at 0.1 bar tension, with K_M values decreasing as soil moisture tension increased or decreased. The relative rates of NO^-₃-N accumulation, R_M values or moisture rate indexes, varied much less than K_M values among soils. The mean R_M values at 0.1, 0.33, 1, 5, and 15 bars tension were 1.0, 0.71, 0.53, 0.29, and 0.13, respectively. The moisture rate index curve can be used to estimate the amount of NO^-₃-N accumulation in soils similar to those used in this study at all soil moisture tensions between 0.1 and 15 bars if the rate of NO^-₃-N accumulation is known at any one tension within the above range. The moisture rate index curve of this study and the temperature rate index curve of a previous paper are used in the previously proposed equation, N = K_FR_K(t – t_Fr_t), to estimate NO^-₃-N accumulation in soils under varying conditions of temperature and soil moisture tension.

¹ Contribution from the Department of Agronomy, University of Illinois, Urbana.

² Associate Professor of Soil Microbiology, Department of Agronomy, University of Illinois, Urbana.

Received for publication September 30, 1968. Accepted for publication December 6, 1968.