Bilan de carbone d'une prairie pâturée en Région wallonne: effets du climat et de la gestion du pâturage

As grassland carbon (C) sequestration can play an important role in mitigating total greenhouse gas (GHG) emissions of livestock production systems (carbon dioxide – CO2, nitrous oxide – N2O, methane – CH4), this PhD investigates the C balance of an intensively managed grazed grassland in the temperate climate of southern Belgium. It analyses more particularly impact of climatic conditions and management practices on the C balance.
The research site is a permanent grassland, covering 4.22 ha, intensively used for more than 50 years for Belgian Blue cattle grazing with the application of organic and inorganic fertilizers. Management practices but also climatic conditions are representative of livestock production systems developed in southern Belgium.
Over a three years study period, soil C stock variations are determined indirectly by measuring the balance of C fluxes at the system boundaries. This approach not only takes into account CO2 and CH4 exchanged with the atmosphere but also organic C imports (manures, complementary feedings), exports (harvests, animal products) and C lost through leaching. CO2 fluxes are measured by eddy covariance while other C fluxes are estimated from data provided directly by the farmer, from punctual measurements and from aboveground herbage mass measurements.
Considering only assimilation and respiration, the ecosystem behaves, on average, as a weak CO2 source, although the interannual variability is large. Interannual variability seems not to be significantly correlated with the main environmental drivers but rather influenced by fertilization management. Taking other C fluxes into account, the site appears as being at equilibrium. Management (organic fertilization), as well as climatic conditions that dictated them (complementary feeds), are the principal factors influencing the C balance. Finally, results show that management practices were the key control of the C flux variability at this grassland.
Grazing impact on CO2 fluxes appears not explicitly at the seasonal and annual scale. It is therefore quantified by developing innovative eddy covariance data analyses and experiments. For that, indirect and direct grazing impacts are separated. Indirect impact results from biomass consumption, excretion deposits and soil compaction by cattle modifying CO2 exchanges. To quantify it, the variation during periods with fixed stocking rate of gross primary productivity at light saturation (GPPmax) and normalised dark respiration (Rd,10) is analysed. On average, GPPmax decreases during grazing periods and increases during non-grazing periods. This could respectively be explained by aboveground biomass reduction and re-growth. In addition, GPPmax variations are negatively correlated to grazing intensity (defined as the product of the stocking rate and the grazing duration). On the contrary, no significant evolution of Rd,10 is found during both grazing and non-grazing periods, probably due to a combination of opposing effects of grazing on the total ecosystem respiration components. Direct impact results from livestock CO2 emissions through respiration that adds to the total ecosystem respiration. It is emphasized through specific designed livestock confinement experiments. Net CO2 exchange is compared on successive days with similar climatic conditions, livestock being confined (≈ 26 livestock units ha-1), or not, in the main wind direction area of the eddy covariance set-up. Results obtained are corroborated by independent estimates based on the C ingested by cattle during confinement.
Finally, the C balance analysis of this particular grazed grassland in southern Belgium reveals that, taking into account the climatic conditions observed and the management practices developed over the three study years, the site is C neutral. This means that GHG emissions linked to fertilization (N2O) and grazing (CH4) are not partly mitigated by soil C sequestration. However, we conclude that, management practices being the main factor controlling the C balance, strategies to enhance soil C sequestration exist. If those kinds of management practices are studied, a global approach, taking into consideration both the C fluxes and other GHG fluxes, would be required to see if they do not induce supplementary N2O and CH4 emissions.