Effect of slurry and ammonium nitrate application on greenhouse gas fluxes of a grassland soil under atypical South West England weather conditions

Abstract

In this study we evaluated how typical split applications of cattle slurry (SL) or mineral fertilizer (AN) (in spring, summer and autumn) affected greenhouse gas (GHG) emissions from a grassland soil. Field measurements were carried out between May and November 2011 using the closed chamber technique. The experiment was located in the South West of England, an area which is typically characterized by high annual precipitation (>1000mmyear−1) and cool temperatures (average annual air temperature of 9.6°C). The unusual dry climatic conditions observed during the late spring and summer, and the rainfall events identified in autumn affected soil water filled pore space (WFPS) resulting in low nitrous oxide (N2O) fluxes during the experiment. After the first two applications, climatic conditions dried the soil to values below 60%WFPS, the threshold level for losses of N2O by nitrification. In contrast, the frequent rainfall events observed after the third application (in autumn) increased the WFPS and promoted losses of N2O by denitrification. In terms of fertilizer type, AN resulted in higher cumulative N2O emissions compared with SL after the third application, probably because the SL treatment resulted in more anaerobic soil conditions and ammonia (NH3) volatilization resulted in a smaller mineral N pool in the soil available for N2O production and emission. Ammonia (NH3) emission modelling estimated losses of N by volatilization of NH3 between 25% and 38% of N applied after slurry surface broadcast application. Plant N offtake represented nearly all of the total N applied in AN plots following the first two applications and 59% of that applied in the third, whereas in SL plots an average of 64% of the total N applied in the three applications was harvested in the grass. Nitrogen gas (N2) fluxes were not measured but the large rainfall events observed after the third application gradually increased the soil WFPS to saturation and could also have resulted in losses of N by complete denitrification, especially from the AN treatment. Thus, applications of AN and SL resulted in total N2O-N losses during the 6-month measurement period of 0.21 and 0.17kgNha−1, respectively (representing only 0.02% and 0.003% of the N applied). Methane (CH4) production was observed in the first two or three days after SL spreading. For the remaining days, and also in plots treated with AN, the soil acted as a sink of CH4 (consumption). Total net CH4 cumulative values of −0.09 and 0.92kgCH4ha−1 were observed in AN and SL, respectively. CH4 consumption and production rates were related to changes in the %WFPS. Thus, dry soil conditions (below 60%WFPS) enhanced the CH4 consumption in AN plots and reduced the rate of CH4 production in SL plots during May and June. Total net cumulative carbon dioxide (CO2) fluxes of 1.24 and 0.35MgCO2-Cha−1 were observed in AN and SL plots during the 6-months measurements.