Fate of 15N labelled urine on four soil types

A field lysimeter experiment was conducted over a 406 day period to determine the effect of different soil types on the fate of synthetic urinary nitrogen (N). Soil types included a sandy loam, silty loam, clay and peat. Synthetic urine was applied at 1000 kg N ha^-1, during a winter season, to intact soil cores in lysimeters. Leaching losses, nitrous oxide (N₂O) emissions, and plant uptake of N were monitored, with soil ¹⁵N content determined upon destructive sampling of the lysimeters. Plant uptake of urine-N ranged from 21.6 to 31.4%. Soil type influenced timing and form of inorganic-N leaching. Macropore flow occurred in the structured silt and clay soils resulting in the leaching of urea. Ammonium (NH ₄ ⁺ –N), nitrite (NO ₂ ^- –N) and nitrate (NO₃ ^-–N) all occurred in the leachates with maximum concentrations, varying with soil type and ranging from 2.3–31.4 g NH ₄ ⁺ –N mL^-1, 2.4–35.6 g NO ₂ ^- –N mL^-1, and 62–102 g NO ₃ ^- –N mL^-1, respectively. Leachates from the peat and clay soils contained high concentrations of NO ₂ ^- –N. Gaseous losses of N₂O were low (<2% of N applied) over a 112 day measurement period. An associated experiment showed the ratio of N₂–N:N₂O–N ranged from 6.2 to 33.2. Unrecovered ¹⁵N was presumed to have been lost predominantly as gaseous N₂. It is postulated that the high levels of NO ₂ ^- –N could have contributed to chemodenitrification mechanisms in the peat soil.