Assessment of volatilization loss of ammonia from surface-applied urea on forest soil by N15 recovery

Summary Using vertically isolated micro-plots the isotopic recovery technique was tested for assessing the extent of ammonia volatilization loss from N¹⁵-labelled urea applied on the forest floor (Pinus silvestris L.). The size of the ammonia loss was obtained as a difference between the amounts of labelled urea N added and the amounts of labelled N recovered in the soil profile after 13, 31, and 39 days's exposure, respectively. Urea materials of two different pellet size were used: granulated small pellets (280 pellets per gram) and tablets (2.06 g each). The nitrogen application rate was 200 kg N per hectare. The recovery data for 13 days' exposure indicated a volatilization loss, which for the small pellet urea was 24.9 per cent and for the tabletted urea 12.1 per cent. The corresponding figures for the 31 days' exposure, during which the total amount of precipitation was 14 mm, were 15.1 and 26.9 per cent, respectively. The pattern of labelled N distribution in the soil profile examined showed that during the period of exposure in question a leaching loss of labelled N was rather unlikely. It was demonstrated, furthermore, that nitrogen from the tabletted urea had diffused to a greater depth of the soil than that from the small-pellet urea. Nitrogen from the small-pellet urea was to a large extent recovered in the litter layer. On exposure to heavy rain the tabletted urea was subjected to the highest leaching loss. An addition of 10 per cent (w/w) of metaphosphoric acid or sublimed sulphur to the tabletted urea did not result in any further reduction of the volatilization loss. The merits and limits of the isotopic recovery technique are discussed.