The effect of introducing a winter forage rotation on CO ₂ fluxes at a temperate grassland

Temperate grasslands have the potential to sequester carbon, helping to mitigate rising atmospheric CO ₂ concentrations. The ability of grasslands to absorb CO ₂ 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 CO ₂ 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 CO ₂ during all years. However, the magnitude of the CO ₂ uptake varied considerably from year to year, with a maximum net uptake of 1.32kg CO ₂m ^-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 CO ₂ uptake was less than that of the area under grass, but over subsequent months daytime CO ₂ 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 CO ₂-neutral.