Variability of annual CO₂ exchange from Dutch Grasslands

International audienceAn 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 photosynthesis-light response analysis technique is used along with the respiration-temperature response technique to partition NEE among Gross Primary Production (GPP) and Ecosystem Respiration (Re) and to obtain the eco-physiological characteristics of the sites at the field scale. Annual sums of NEE, GPP and Re 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). The estimated annual Re 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 ±100 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, which implies that on average the grasslands act as a source, with a relatively large standard deviation. 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, standard deviation of annual GPP was estimated to be 171?206 gC m?2 a?1 (8?14%), of annual Re 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 Re, 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, which means that on average the Dutch grasslands behave like a small sink for CO2. The results from the analysis illustrate the need for regionally specific and spatially explicit CO2 emission estimates from grassland