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

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.