The effect of N fertilizer forms on nitrous oxide emissions from UK arable land and grassland

Nitrous oxide emission factors (EFs) were calculated from measurements of emissions from UK wheat crops and grassland, that were part of a wider research programme on N loss pathways and crop responses. Field studies were undertaken in 2003, 2004 and 2005—a total of 12 site-seasons. Nitrous oxide emissions were measured by the closed static chamber method, following the application of various N fertilizer forms (ammonium nitrate (AN), calcium ammonium nitrate (CAN), urea (UR), urea ammonium sulphate and urea ammonium nitrate) at the recommended rates. Emission factors for the growing season (March–September) ranged from less than 0.1–3.9 %. In the 2nd year, measurements continued at three sites until the following February; the resulting annual EFs were one-third greater, on average, than those for the growing season. There was some evidence that N2O emissions from UR were smaller than from AN or CAN, but when this was adjusted for loss of ammonia by volatilization, there was generally little difference between different forms of N. Emissions from UR modified by the addition of the urease inhibitor nBTPT (UR + UI) were lower than corresponding emissions from nitrate forms, except under conditions where emissions were generally low, even allowing for indirect emissions, suggesting that the use of a urease inhibitor can provide some mitigation of N2O, as well as NH3, emissions. The emission data broadly bear out the relationships obtained in earlier UK studies, showing a strong dependence of N2O emission on soil wetness, temperature and the presence of sufficient mineral N in the soil, which decreases rapidly after N application mainly as a result of plant uptake. Overall net mean EFs for the whole season (after subtracting background emissions from unfertilized controls) covered a range wider than the 0.3–3.0 % range of IPCC (2006).