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Effect of Rainfall and Subsequent Drying on Nitrogen and Phosphorus Changes in a Dryland Fallow Loam¹

ABSTRACT

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: NO₃–N = –0.55 – 1.11 M, (r = –0.94**), and NO₃-N = –0.45 - 0.56 M, (r = –0.81*) in the second- and first-year fallow, respectively; (M = % moisture and = 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 NO₃-N and 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 IP in the 0- to 2.5-cm segment was directly related to rainfall (r = 0.98**), to M (r = 0.97**), and to bacteria (r = 0.88**), and inversely related to NO₃-N (r = –0.76*).

¹ Contribution from the Research Station, Research Branch, Canada Agr., Swift Current, Sask.

² Soil Chemist, Soil Microbiologist, Soil Chemist, and Agricultural Meteorologist, respectively.

Received for publication May 24, 1973. Accepted for publication August 9, 1973.