Impact of grazing on carbon dioxide flux exchanges in an intensively managed grassland

To date, there are few studies assessing the impact of specific management events, particularly grazing, on carbon (C) and carbon dioxide (CO2) fluxes in managed grasslands. Grazing effects are indeed difficult to discern. They vary with the stocking rate and the length of the grazing period. Moreover, they are often masked by environmental responses. The aim of the present study was to assess the impact of grazing on the CO2 fluxes of a grassland grazed by the Belgian Blue breed of cattle.
The research was run at the Dorinne Terrestrial Observatory (DTO), located in the Belgian Condroz (50° 18’ 44’’ N; 4° 58’ 07’’ E; 248 m asl.). The site is a permanent grassland of ca. 4.2 ha subjected to intensive management. Grassland carbon budget at the system boundaries is calculated from Net Ecosystem Exchange of CO2 measured by eddy covariance by taking imports and exports of organic C and losses of carbon as methane into account. After 2 years of measurements, the site was close to equilibrium. If management practices (harvest, fertilization and imports as supplementary feedings) and climatic conditions had a significant impact on the C balance, the impact of grazing was uncertain, especially on CO2 fluxes.
To do this analysis, we distinguished the long term and the short term impacts of grazing on CO2 fluxes. The long term effect results from the biomass consummation by the cattle and from the cattle effluents that modify assimilation and respiration fluxes. This could be quantified only by comparing fluxes before and after grazing periods. The short term impact is due to cattle respiration that is a part of total ecosystem respiration and should be measured in its presence in the field.
For the long term effects of grazing on CO2 fluxes, we analyzed the temporal evolution of gross maximal photosynthetic capacity GPPmax and dark respiration normalized at 10°C (Rd,10). Those parameters were deduced from the response of daytime CO2 fluxes to radiation over 5-day windows. We calculated parameters variations between the beginning and the end of grazing and non-grazing periods (∆GPPmax, ∆Rd,10) and analyzed their dependence to stocking rate. We found a significant decreased of ∆GPPmax that allowed us to quantify the assimilation reduction due to grass consumption by cattle. Discrimination of this impact from flux response to climate was possible only after gathering and treating two years of measurements taken under various climatic conditions. At the opposite, no significant evolution of Rd,10 with the average stocking rate was found.
The short term impacts were an increase of CO2 fluxes in presence of cattle. It could be distinguished and quantified only thanks to confinement experiments. Each experiment extended over two days: the first day, cattle was confined in the footprint of the eddy covariance set-up (1.76 ha, 27 LU ha-1) and the second day, it was removed from it. We compared filtered half-hourly data made at 24h intervals, in the presence or absence of cattle, considering that environmental conditions were equivalent (air temperature, wind speed, radiation and wind direction). Livestock contribution to CO2 fluxes was estimated to be 2.25 ± 0.68 kg C LU-1 d-1.