Loss of Nitrogen From Soil During Nitrification of Urea

Urea enriched with 52.4% 15N was added to an Orthic Black soil (Wellwood) of pH 6.1 at a rate of 20 mg N per 20 g (a 1000 ppm rate of addition was selected to simulate concentrations that occur in the field near fertilizer pellets), and the samples incubated for 3-day intervals in containers which were alternately closed and open to the atmosphere. Analysis of the soil samples indicated gradual nitrification of the urea with the production of nitrite and nitrate. Analysis of the gas following incubation with containers closed indicated that the vessels remained partially aerobic and that 20% of the added urea was denitrified. About 60% of the loss was N2O and N2 which, based on their 15N content, were derived entirely from the enriched NO2−. The remaining 40% of the loss was N2, in which one half of the N came originally from the urea and the other half from a soil source. In further experiments, it was shown that the same gases were produced following the addition of nitrite as NaNO2 but that gas production was decreased by increasing the soil pH. The production of the van Slyke-like N2 appeared to be chemical in nature in that it was unaffected by sterilization. The opposite was true of the N2O and N2 which were derived entirely from the added nitrogen.