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This article in AJ

  1. Vol. 69 No. 6, p. 1019-1023
    Received: Feb 7, 1997

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Uptake and Losses of 15N Applied to Annual Grass and Clover in Lysimeters1

  1. M. B. Jones,
  2. C. C. Delwiche and
  3. W. A. Williams2



The California annual grasslands comprise about one-third of the state, and they play a large role in the hydrology of most watersheds in central and northern California. Apparent recovery of applied N has been low, and a large percentage has been unaccounted for in N balance studies. The present experiment was done with 15N to determine the fate of applied N, especially that leached and that volatilized (by difference).

Ammonium sulfate labeled with 15N ammonium chloride was applied to pure stands of soft chess (Bromus mollis L.) and subclover (Trilolium subterraneum L.) growing in lysimeters in a climate with mild, rainy winters and hot, dry summers. Uptake and leaching of N were measured on single and repeated applications for 3 years (mean annual rainfall 1,100 mm). After 3 years of annual fertilization in February (100 kg N/ha) 59% of N applied to grass was removed in forage, 24% remained in the soil and roots, 3% leached, and 14% was not detected (presumed gaseous loss); for clover 49% was removed in the forage, 33% remained in the soil and roots, 9% leached, and 9% was not accounted for. October fertilization at the same rate produced in both species about the same total growth, markedly decreased plant uptake, increased soil-plus-root retention, increased leaching, and reduced unaccounted for N from fertilizer. An October application of 500 kg N/ha resulted in greater gaseous losses of N, and more was lost from clover than grass.

Conclusions: There was modest leaching loss but essentially no gaseous loss from moderate rates of N applied to grass in October; there was almost negligible leaching loss but modest gaseous loss from moderate rates of N applied to grass in February; recovery of fertilizer by clover was poor; and a single large application (500 kg N/ha) resulted in a substantial leaching and gaseous losses under both grass and clover.

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