CO₂ exchange and carbon balance in two grassland sites on eutrophic drained peat soils

Abstract. In this study we investigated the role of intensive and extensive dairy farm practices on CO₂ exchange and the carbon balance of peatlands by means of eddy covariance (EC) measurements. Year long EC measurements were made in two adjacent farm sites on peat soil in the western part of the Netherlands. One site (Stein) is a new meadow bird reserve and is managed predominantly by mowing in June and August. The second site (Oukoop) is an intensive dairy farm.

Maximum photosynthetic uptake of the grass sward (range 2 to 34 μmol CO₂ m⁻² s⁻¹) showed a close and similar linear relationship with Leaf Area Index (LAI; range 1 to 5) except in maturing hay meadows, where maximum photosynthetic uptake did not increase further. Apparent quantum yield varied between 0.02 and 0.08 (mean 0.045) μmol CO₂ μmol⁻¹ photons at both sites and was significantly correlated with LAI during the growth season. Ecosystem Respiration at 10°C (R₁₀) calculated from the year round data set was 3.35 μmol CO₂ m⁻² s⁻¹ at Stein and 3.69 μmol CO₂ m⁻² s⁻¹ at Oukoop.

Both sites were a source of carbon in winter and a sink during summer with net ecosystem exchange varying between 50 to 100 mmol CO₂ m⁻² d⁻¹ in winter to below −400 mmol CO₂ m⁻² d⁻¹ in summer. Periodically, both sites became a source after mowing. Net annual ecosystem exchange (NEE) for Stein was −5.7 g C m⁻² a⁻¹ and for Oukoop 133.9 g C m⁻² a⁻¹.

When biomass removal, manure applications and estimates of methane emissions ware taken into account, both eutrophic peat meadows are a strong source for C around 420 g C m⁻² a⁻¹.