Effect of Rainfall and Subsequent Drying on Nitrogen and Phosphorus Changes in a Dryland Fallow Loam
1973
Campbell, C. A. | Biederbeck, V. O. | Warder, F. G. | Robertson, G. W.
³⁶Chloride placed at 15- or 30-cm depth in 15-cm diameter cylinders of fallow loam was leached to at least 50 cm during and immediately following rainfall > 1.75 cm. As the soil dried following rainfall ³⁶Cl moved upwards. Nitrate content of leachates from a lysimeter experiment corroborated the leaching aspects of the ³⁶Cl expcriment. In the 0- to 2.5-cm segment of a second-year fallow loam which was sheltered from rainfall, moisture was below the wilting percentage and NO₃−N and bacterial numbers declined as the soil gradually dried out. In the 2.5- to 15-cm segment, moisture was in the available range, yet moisture and NO₃-N changes were small. In unsheltered fallow loam, NO₃-N production in the 0- to 2.5-cm depth was primarily a function of soil moisture change: ΔNO₃−N = −0.55 − 1.11 ΔM, (r = −0.94**), and ΔNO₃-N = −0.45 - 0.56 ΔM, (r = −0.81*) in the second- and first-year fallow, respectively; (M = % moisture and Δ = daily change). The increase in NO₃-N during drying seemed to be more a result of upward movement than of nitrification. There was a negative linear relationship between ΔNO₃-N and Δ bacteria in the 0- to 2.5-cm soil segment. Sodium bicarbonate soluble inorganic P (IP) generally exceeded NaHCO₃ soluble organic P (OP). In second-year fallow, P was unaffected by environmental conditions. In first-year fallow ΔIP in the 0- to 2.5-cm segment was directly related to rainfall (r = 0.98**), to ΔM (r = 0.97**), and to Δ bacteria (r = 0.88**), and inversely related to ΔNO₃-N (r = −0.76*).
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