Variability of annual CO₂ exchange from Dutch grasslands

Abstract. An intercomparison is made of the Net Ecosystem Exchange of CO₂, NEE, for eight Dutch grassland sites: four natural grasslands, two production grasslands and two meteorological stations within a rotational grassland region. At all sites the NEE was determined during at least 10 months per site, using the eddy-covariance (EC) technique, but in different years. The NEE does not include any import or export other than CO₂. The photosynthesis-light response analysis technique is used along with the respiration-temperature response technique to partition NEE into Gross Primary Production (GPP) and Ecosystem Respiration (R_e) and to obtain the eco-physiological characteristics of the sites at the field scale. Annual sums of NEE, GPP and R_e are then estimated using the fitted response curves with observed radiation and air temperature from a meteorological site in the centre of The Netherlands as drivers. These calculations are carried out for four years (2002–2005). Land use and management histories are not considered. The estimated annual R_e for all individual sites is more or less constant per site and the average for all sites amounts to 1390±30 gC m⁻² a⁻¹. The narrow uncertainty band (±2%) reflects the small differences in the mean annual air temperature. The mean annual GPP was estimated to be 1325 g C m⁻² a⁻¹, and displays a much higher standard deviation, of ±110 gC m⁻² a⁻¹ (8%), which reflects the relatively large variation in annual solar radiation. The mean annual NEE amounts to –65±85 gC m⁻² a⁻¹. From two sites, four-year records of CO₂ flux were available and analyzed (2002–2005). Using the weather record of 2005 with optimizations from the other years, the standard deviation of annual GPP was estimated to be 171–206 gC m⁻² a⁻¹ (8–14%), of annual R_e 227–247 gC m⁻² a⁻¹ (14–16%) and of annual NEE 176–276 gC m⁻² a⁻¹. The inter-site standard deviation was higher for GPP and R_e, 534 gC m⁻² a⁻¹ (37.3%) and 486 gC m⁻² a⁻¹ (34.8%), respectively. However, the inter-site standard deviation of NEE was similar to the interannual one, amounting to 207 gC m⁻² a⁻¹. Large differences occur due to soil type. The grasslands on organic (peat) soils show a mean net release of CO₂ of 220±90 g C m⁻² a⁻¹ while the grasslands on mineral (clay and sand) soils show a mean net uptake of CO₂ of 90±90 g C m⁻² a⁻¹. If a weighing with the fraction of grassland on organic (20%) and mineral soils (80%) is applied, an average NEE of 28 ±90 g C m⁻² a⁻¹ is found. The results from the analysis illustrate the need for regionally specific and spatially explicit CO₂ emission estimates from grassland.