Variability of annual CO2 exchange from Dutch grasslands

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, GPPand 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-2a-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.