Determination of the carbon budget of a pasture: effect of system boundaries and flux uncertainties

Abstract. Carbon (C) sequestration in the soil is considered as a potential important mechanism to mitigate greenhouse gas (GHG) emissions of the agricultural sector. It can be quantified by the net ecosystem carbon budget (NECB) describing the change of soil C as the sum of all relevant import and export fluxes. NECB was investigated here in detail for an intensively grazed dairy pasture in Switzerland. Two budget approaches with different system boundaries were applied: NECB_tot for system boundaries including the grazing cows and NECB_past for system boundaries excluding the cows. CO₂ and CH₄ exchange induced by soil/vegetation processes as well as direct emissions by the animals were derived from eddy covariance measurements. Other C fluxes were either measured (milk yield, concentrate feeding) or derived based on animal performance data (intake, excreta). For the investigated year, both approaches resulted in a small near-neutral C budget: NECB_tot −27 ± 62 and NECB_past 23 ± 76 g C m⁻² yr⁻¹. The considerable uncertainties, depending on the approach, were mainly due to errors in the CO₂ exchange or in the animal-related fluxes. The comparison of the NECB results with the annual exchange of other GHG revealed CH₄ emissions from the cows to be the major contributor in terms of CO₂ equivalents, but with much lower uncertainty compared to NECB. Although only 1 year of data limit the representativeness of the carbon budget results, they demonstrate the important contribution of the non-CO₂ fluxes depending on the chosen system boundaries and the effect of their propagated uncertainty in an exemplary way. The simultaneous application and comparison of both NECB approaches provides a useful consistency check for the carbon budget determination and can help to identify and eliminate systematic errors.