The effect of flood irrigation and nitrogen source on the fate of nitrogen fertilizer applied to pasture

Abstract

The effects of flood irrigation on soil inorganic nitrogen (N) and on the recovery of 15N from 15N-labelled fertilizers were studied in two pasture experiments. In the first, changes in soil inorganic N were measured during a flood irrigation cycle after fertilization with 0 or 100 kg N ha-1 as ammonium nitrate (NH4NO3). In the second experiment, the recovery of 15N from 15N -labelled fertilizers (50 kg N ha-1 as NH4NO3, urea or urea plus nitrapyrin) was determined in field microplots under three irrigation regimes (0, 6 and 24 h ponding).Soil inorganic N in the unfertilized pasture remained relatively low (< 12 kg N ha-') during the irrigation cycle with ammonium (NH4+) the dominant form of inorganic N. In pasture fertilized with NH4N03 there was a rapid decline in both NH4+ and nitrate (NO3-) in the soil during the 48 h period after the first irrigation. The decline in NH4+ was less rapid than that of NO3-. Presumably immobilization, nitrification and pasture uptake contributed to the disappearance of NH4+ from the soil.In the second experiment total recovery of applied 15N ranged from 65 to 91%, with about half recovered from the pasture and half from the soil plus roots. The lowest recoveries of applied N occurred with NH4NO3with 24 h ponding.Recovery of the urea nitrogen was unaffected by ponding time, making it a more efficient form of N to apply to irrigated pastures.