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Recovery of ¹⁵N-Labeled Fertilizer from Manured and Sludge-Amended Soil¹

ABSTRACT

A greenhouse pot experiment was conducted with various rates of manure (0, 20, 40, and 60 metric tons/ha) and sludge (0, 10, 20, and 30 metric tons/ha) combined factorially with 0, 50, 100, 150, and 200 mg N/kg soil as (NH₄)₂SO₄ labeled with ¹⁵N, using barley and sudangrass as test crops. Most of the N from (NH₄)₂SO₄ was recovered by the first cutting of barley forage (from 37.2 to 70.2%); the N from (NH₄)₂SO₄ recovered by sudangrass, the last crop of the cropping sequence, ranged from 0.7 to 8.9%. No significant effect of both wastes and (NH₄)₂SO₄ rates on the overall inorganic N fertilizer recovered by the crops was found. The difference method underestimated the (NH₄)₂SO₄-N recoveries by the crops under sludge treatments. The depression in the mineralization rate of sludge by the (NH₄)₂SO₄ applied was proposed to account for these discrepancies.

A fertilizer N balance, in which the sum of (NH₄)₂SO₄-N removed by the crops plus the N accumulated in the soil was subtracted from the (NH₄)₂SO₄-N input, was used to estimate the fertilizer N losses. These losses were 7.8, 9.1, and 13.1%, respectively, for control (unamended), manure and sludge treatments.

With increasing rates of (NH₄)₂SO₄, the nitrogen "A" values defined as the N available in the soil in terms of N from (NH₄)₂SO₄, increased in the control, remained almost constant with manure, and decreased with sludge treatments.

¹ Contribution from the Dept. of Soil and Environ. Sci., Univ. of Calif., Riverside, CA 92521.

² Formerly Graduate Student, now Researcher in the Instituto Nacional de Investigaciones Agrarias, Moncada (Valencia), Spain, and Professor of Soil Science, Univ. of Calif., Riverside, respectively.

Received for publication January 12, 1978. Accepted for publication May 19, 1978.