Greenhouse gas emissions from a managed grassland

Managed grasslands contribute to global warming by the exchange of the greenhouse gases carbon dioxide, nitrous oxide and methane. To reduce uncertainties of the global warming potential of European grasslands and to assess potential mitigation options, an integrated approach quantifying fluxes from all three gases is needed. Greenhouse gas emissions from a grassland site in the SE of Scotland were measured in 2002 and 2003. Closed static chambers were used for N2O and CH4 flux measurements, and samples were analysed by gas chromatography. Closed dynamic chambers were used for soil respiration measurements, using infrared gas analysis. Three organic manures and two inorganic fertilizers were applied at a rate of 300 kg N ha−1 a−1 (available N) and compared with a zero-N control on grassland plots in a replicated experimental design. Soil respiration from plots receiving manure was up to 1.6 times larger than CO2 release from control plots and up to 1.7 times larger compared to inorganic treatments (p<0.05). A highly significant (p<0.001) effect of fertilizer and manure treatments on N2O release was observed. Release of N2O from plots receiving inorganic fertilizers resulted in short term peaks of up to 388 g N2O-N ha−1 day−1. However losses from plots receiving organic manures were both longer lasting and greater in magnitude, with an emission of up to 3488 g N2O-N ha−1 day−1 from the sewage sludge treatments. During the 2002 growing season the cumulative total N2O flux from manure treatments was 25 times larger than that from mineral fertilizers. CH4 emissions were only significantly increased (p<0.001) for a short period following applications of cattle slurry. Although soil respiration in manure plots was high, model predictions and micrometeorological flux measurements at an adjacent site suggest that all plots receiving fertilizer or manure acted as a sink for CO2. Therefore in terms of global warming potentials the contribution of N2O from manure treatments becomes particularly important. There were considerable variations in N2O and CO2 fluxes between years, which was related to annual variations in soil temperature and rainfall.