The effect of introducing a winter forage rotation on CO2 fluxes at a temperate grassland

Temperate grasslands have the potential to sequester carbon, helping to mitigate rising atmospheric CO2 concentrations. The ability of grasslands to absorb CO2 is influenced by site elevation, soil type, management practices, climate and climatic variability. There is a need for long-term observations and field experiments to quantify the effects of the key drivers of management and climate variability. This paper presents over 4 years of eddy covariance measurements of CO2 flux over a managed temperate grassland site in south-east Ireland. For the first 2 years the entire study area was under grass. During the second 2 years a winter forage crop was grown over part of the site. The site was found to have a net uptake of CO2 during all years. However, the magnitude of the CO2 uptake varied considerably from year to year, with a maximum net uptake of 1.32 kg CO2 m−2 in 2004, a year with no winter forage crop. Net uptakes were much lower in the 2 years of mixed grass and kale cultivation, but detailed analysis of the measurement footprint and statistical comparisons showed that this was not due to the introduction of the forage rotation. For a short period following sowing of the forage crop, daytime CO2 uptake was less than that of the area under grass, but over subsequent months daytime CO2 uptake of the kale areas recovered strongly and exceeded that of the grass areas. The net effect over the year following kale planting is close to CO2-neutral.

► Four years of eddy covariance measurements at this temperate pasture show a consistent net annual CO2 uptake of up to 1.32 kg m−2.
► Footprint modelling has been used to isolate portions of the eddy covariance measurement under different crop cover.
► Use of look-up tables based on PAR and soil temperature allowed statistical comparison of fluxes as well as gap-filling.
► Areas under winter kale emit more CO2 than areas under grass immediately after kale sowing.
► After several months, aggregate CO2 fluxes from both treatments are similar.