Assessment of nitrogen losses from urea and an organic manure with and without nitrification inhibitor, dicyandiamide, applied to lettuce under glasshouse conditions

Urea and organic manures such as 'Garden galore' (GG) are used to supply nitrogen (N) in vegetable farming and floriculture systems in New Zealand. However, a significant amount of the applied N is lost to the atmosphere via nitrous oxide (N2O) and ammonia (NH3) emissions, and leached to surface and ground water as nitrate (NO3-) contributing to environmental degradation such as global warming and eutrophication. One of the mitigation options to reduce these losses is to use nitrification inhibitors (NI). Glasshouse and laboratory incubation experiments were conducted under controlled moisture and temperature conditions to determine the effects of an NI, dicyandiamide (DCD), on N losses from urea and GG applied to lettuce grown in a Manawatu sandy soil. Nitrogen and DCD were applied at the rates of 9 and 1.3 g/m(2), respectively, and the gaseous emission of N2O and NH3 were monitored over a 5-week period using a closed-chamber technique. At the end of the experiment the lettuce plant shoots and roots were harvested, and analysed for N concentration. Soils were leached with deionised water and leachates were analysed for ammonium (NH4+) and NO3-. The results showed greater loss of N as NH3 than N2O and the effect was more pronounced in the case of urea. Addition of DCD significantly reduced N2O emissions from both urea and GG, and increased NH3 emissions from both urea and GG, with the increase being significant only for urea. Addition of DCD maintained higher soil NH4+ concentration and lower NO3- concentration than without DCD. Overall, DCD was effective in reducing N losses of N2O emissions and NO3 leaching. Urea application resulted in shoot tip burning and the symptoms were enhanced with the addition of DCD. There was no significant effect of DCD addition on lettuce yield.