Short-term effects of single or combined application of mineral N fertilizer and cattle slurry on the fluxes of radiatively active trace gases from grassland soil

After implementation of legislative measures for the reduction of environmental hazards from nitrate leaching and ammonia volatilisation when using organic manures and fertilizers in Europe, much attention is now paid to the specific effects of these fertilizers on the dynamics of global warming-relevant trace gases in soil. Particularly nitrogen fertilizers and slurry from animal husbandry are known to play a key role for the CH4 and N2O fluxes from soils. Here we report on a short-term evaluation of trace gas fluxes in grassland as affected by single or combined application of mineral fertilizer and organic manure in early spring. Methane fluxes were characterised by a short methane emission event immediately after application of cattle slurry. Within the same day methane fluxes returned to negative, and on average over the 4-day period after slurry application, only a small but insignificant trend to reduced methane oxidation was found. Nitrous oxide emissions showed a pronounced effect of combined slurry and mineral fertilizer application. In particular fresh cattle slurry combined with calcium ammonium nitrate (CAN) mineral fertilizer induced an increase in mean N2O flux during the first 4 days after application from 10 to 300 μg N2O-N m−2 h−1. 15N analysis of emitted N2O from 15N-labelled fertilizer or manure indicated that easily decomposable slurry C compounds induced a pronounced promotion of N2O-N emission derived from mineral CAN fertilizer. Fluxes after application of either mineral fertilizer or slurry alone showed an increase of less than 5-fold. The NOx sink strength of the soil was in the range of −6 to −10 μg NOx-N m−2 h−1 and after fertilization it showed a tendency to be reduced by no more than 2 μg NOx-N m−2 h−1, which was a result of both, increased NO emission and slightly increased NO2 deposition. Associated determination of the N2O:N2 emission ratio revealed that after mineral N application (CAN) a large proportion (c. 50%) was emitted as N2O, while after application of slurry with easily decomposable C and predominantly NH4+-N serving as N-source, the N2O:N2 emission ratio was 1:14, i.e. was changed in favour of N2. Our work provides evidence that particularly the combination of slurry and nitrate-containing N fertilizers gives rise to considerable N2O emissions from mineral fertilizer N pool.