Measurements of Animal and Area Related Greenhouse Gas Exchange on Pasture using Eddy Covariance

Up to now the scientific investigation and description of the agriculture related greenhouse gas (GHG) exchange has been largely separated into (a) direct animal related and (b) ecosystem area related processes and measurement methods. An overlap of the two usually separated topics occurs for grazed pastures, where direct animal and pasture area emissions are relevant. In the present study eddy covariance (EC) flux measurements on the field scale were combined with footprint modeling and with animal position information. The experiment was performed on a pasture field in Switzerland under a rotational full grazing regime with dairy cows. The exchange fluxes of CH4, CO2, and N2O were measured simultaneously over the entire year. It was found that the observed CH4 emission fluxes correlate well with the presence of cows in the flux footprint. When converted to average emission per cow, the results agreed with published values from respiration chamber experiments with similar cows. However, the results partly depended on the distance of the source and the type of footprint model used. For CO2 a sophisticated partitioning algorithm was applied to separate the pasture and animal contributions, because both were in the same order of magnitude. The N2O exchange fully attributable to the pasture soil showed considerable and continuous emissions through the entire seasonal course mainly modulated by soil moisture and temperature. The resulting GHG budget shows that the largest GHG effect of the pasture system was due to enteric CH4 emissions followed by soil N2O emissions, but that the carbon storage change was affected by a much larger uncertainty. The results demonstrate, that with some animal position information and adapted partitioning methods, the EC fluxes can be partitioned adequately into animal and area related sources/sinks.