Eddy covariance

The second original feature of her approach is the eddy covariance technique used to measure the CO₂ flux exchange between the ecosystem and the atmosphere. This relies on a piece of equipment that seems very rudimentary in appearance involving a three-dimensional sonic anemometer and a gas analyzer set up in the centre of the plot which measure wind-speed at a high frequency and the concentration of CO₂ in the air, which makes it possible to deduct the net flux exchanged between the ecosystem and the atmosphere. In reality, this technique offers the enormous advantage of taking account of the temporal and spatial variability of CO₂ fluxes. While this measurement method requires an enormous amount of work with regard to the ex-situ processing of data, it remains the most complete technique available to follow the long-term development of gas fluxes from several months to several years. It also makes it possible to take account of particular episodes of CO₂ emission or storage, for example, during dry periods or when all the animals are concentrated in a particular zone of the grassland. The measurement system is completed by a micrometeorological station which makes it possible to study the effects of climatic conditions on CO₂ fluxes. The station measures temperature and humidity of the air and soil and also radiation, atmospheric pressure and precipitation. These variables constitute the main meteorological drivers likely to influence fluxes.

The first result of this study, spread over a three-year period, concerns the intra and inter-annual variability of the net CO₂ flux. Unsurprisingly, the active growth period of the vegetation which begins in the spring is characterized by a large accumulation of CO₂ by the ecosystem. This is evidently due to the effect of photosynthesis. From the month of August onwards, the drop in temperatures and solar radiation decrease - and eventually stop - the assimilation of CO₂ by the vegetation. “In autumn, the net CO₂ flux is dominated by the respiration of the plants and the soil”, continues the young researcher. “From the month of March, photosynthesis gains the upper hand on respiration. On an annual scale, it is a fact that a grassland is an ecosystem that captures more CO₂ than it emits. In the jargon of the specialists, it acts like a sink of CO₂. In our case however, which concerns a three-year period, the meadow studied generally behaved like a weak source of CO₂”. The net annual cumulated CO2 flux  varies strongly from one year to another with the meadow acting as a source of CO₂ and sometimes like a CO₂ sink. The results show that it is probably the farmer’s management (fertilization type and stocking rate) which, for the three years studied, is responsible for this strong variability as no significant impact of climatic conditions on CO₂ fluxes were able to be demonstrated significantly. Another observation which must be mentioned with regard to CO2 fluxes is that the role played by animal respiration has little significance with regard to the total CO₂ flux: the animals only account for around 10% of the total respiration of the ecosystem.

A positive result

The second observation concerns the result of the pastureland carbon balance: at the end of the three years of the study, and taking uncertainties into account, this was seen to be carbon neutral which means that, on average, the meadow did not store any carbon in the soil or release any into the atmosphere. Was this surprising? Yes and no.  "At first glance, yes, because a majority of studies conducted in Europe have led to the conclusion that grasslands store carbon on average But it is also possible to answer that this is not surprising because, according to the pedoclimatic conditions (climate and soil) and the management practices, the storing potential of these ecosystems can vary strongly, so much so, that it is not unusual to see meadows act as sources of carbon during certain years".