Influence of organic and mineral N fertiliser on N2O fluxes from a temperate grassland

The aim of this study was to quantify N2O release from temperate grassland receiving different forms of N, and to assess the importance of controlling variables in the environment. In 2002 and 2003 three organic manures (sewage sludge pellets, poultry manure and cattle slurry) and two mineral fertilisers (NH4NO3 and urea) were applied to grassland in SE Scotland. All plots except the control received N fertilisers at a rate of 300 kg available N ha-1 a-1 (available N includes NO3-, NH4+ and urea). Fluxes of N2O were measured using static chambers. Losses of N2O from plots receiving mineral fertiliser were short-lived with a peak value of 388 g N2O-N ha-1 day-1 from NH4NO3. Release of N2O from manure treatments however extended over a longer period and was greater in magnitude. Highest emissions were measured from sewage sludge treatments with a peak value of 3488 g N2O-N ha-1 day-1 and 26 times higher cumulative loss compared to the NH4NO3 treatment in 2002. A residual effect on N2O losses was observed from sewage sludge treatments in 2004 when the plots did not receive any fertiliser. There were considerable variations in fluxes between the years, which could be linked to differences in rainfall and temperature. The type of N applied affected the influence of these factors on N2O emissions. The amount of rainfall several days after the N application did not affect N2O fluxes on NH4NO3 treatments. However, in the sewage treated plots increased correlation coefficients were calculated when rainfall amounts over a longer period of time (up to 20 days) were included. A synergistic effect of rainfall and temperature on N2O fluxes could be observed on all treatments. A significant correlation between N2O fluxes and mineral soil N (NO3- and NH4+) was observed when soil water content was not limiting (r2 = 67%, p < 0.05). Gradual decomposition of organic material led to mineralization and nitrification of residual organic matter and hence to a prolonged accumulation of soil NO3- on manure plots. Nitrate accumulation was greater in the dry year 2003 due to reduced leaching and decreased N2O loss. Most NH4+ was retained in the 0-10 cm layer and only NO3- moved into the lower soil layer. The organic manure treatments did not result in increased grass productivity but did result in higher contents of soil mineral N (NH4+ and NO3-), increased soil and plant total N content and resulted in an overall higher loss of N2O compared to mineral fertilisers.