Variability of annual CO₂ exchange from Dutch Grasslands

C. M. J. Jacobs (1) , A. F. G. Jacobs (2) , F. C. Bosveld (3) , D. M. D. Hendriks (4) , A. Hensen (5) , P. S. Kroon (5) , E. J. Moors (1) , L. Nol (1) , A. Schrier-Uijl (2) , E. M. Veenendaal (2)

1 Alterra
2 WUR - Wageningen University and Research [Wageningen]
3 KNMI - Royal Netherlands Meteorological Institute
4 Free University
5 ECN

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 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 (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). 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 ±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 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, 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.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

C. M. J. Jacobs, A. F. G. Jacobs, F. C. Bosveld, D. M. D. Hendriks, A. Hensen, et al.. Variability of annual CO₂ exchange from Dutch Grasslands. Biogeosciences Discussions, 2007, 4 (3), pp.1499-1534. ⟨hal-00297893⟩