Evaluations of soil organic carbon (SOC) stocks are often based on assigning a carbon density to each one of a number of ecosystems or soil classes considered, using data from soil profiles within these categories. A better approach, in which the use of classification methods by which extrapolation of SOC data to larger areas is avoided, can only be used if enough data are available at a sufficiently small scale. Over 190 000 SOC measurements (0–24 cm) have been made in the Flemish cropland (the Northern part of Belgium) in the 1989–2000 period. These SOC data were grouped into 3‐year periods and as means plus standard deviation per (part of) community (polygons). This large dataset was used to calculate SOC stocks and their evolution with time, without data extrapolation. Using a detailed soil map, larger spatial groups of polygons were created based on soil texture and spatial location. Linear regression analysis showed that in the entire study area, SOC stocks had decreased or at best had remained stable. In total, a yearly decrease of 354 kton OC yr−1 was calculated, which corresponds with a net CO2 emission of 1238 kton CO2 yr−1. Specific regions with a high carbon sequestration potential were identified, based on SOC losses during the 1989–2000 period and the mean 1999 SOC content, compared to the average SOC content of soils in Flanders with a similar soil texture. When restoring the SOC stocks to their 1990 level, we estimated the carbon sequestration potential of the Flemish cropland soils to be some 300 kton CO2 yr−1 at best, which corresponds to a 40‐year restoration period. In conclusion, we can say that in regions where agricultural production is very intense, carbon sequestration in the cropland may make only a very modest contribution to a country's effort to reduce greenhouse gas emissions.

The net ecosystem exchange (NEE) of CO 2 between temperate forests and the atmosphere governs both carbon removal from the atmosphere and forest growth. In recent years, many experiments have been conducted to determine temperate forest NEE. These data have been used by forest modellers to better understand the processes that govern CO 2 fluxes, and estimate the evolution of these fluxes under changing environmental conditions. Nevertheless, it is not clear whether models capable of handling short-term processes, which are mostly source-driven, can provide an accurate estimate of long-term forest growth, which is potentially more influenced by sink-and phenology-related processes. To analyse the interactions between short-and long-term processes, we developed the ASPECTS model, which predicts long-term forest growth by integrating, over time, hourly NEE estimates. Validation data consisting of measurements of NEE by eddy-covariance and forest carbon reservoir estimates were obtained from mixed deciduous and evergreen experimental forests located in Belgium. ASPECTS accurately estimated both: (1) the NEE fluxes for several years of data; and (2) the amount of carbon contained in stems, branches, leaves, fine and coarse roots. Our simulations demonstrated that: (1) NEE measurements in Belgian forests are compatible with forest growth over the course of the 20th century; and (2) that forest history and long-term processes need to be considered for accurate simulation of short-term CO 2 fluxes.

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-Belgian forests covered 693 181 ha in 2000, representing 22.7% of the total land area. As no biomass or carbon stock data are included in the Flemish and Walloon regional forest inventories, species-specific wood densities, biomass expansion factors s.s. and carbon content values were critically selected from the literature. Based on these conversion and expansion factors, and on data from the forest inventories, the total C-stock in the living biomass of productive Belgian forests was assessed at 60.9 Mt C in the year 2000. The overall mean C-stock amounted to 101.0 t C ha -1 . This value was in the higher range of values reported for the neighbouring countries, mainly due to a high mean growing stock in the Belgian forests (261.9 m 3 ha -1 ). The conversion from wood volume to wood biomass based on wood density values reported in the literature appeared to introduce the largest variability in the assessment of the carbon stocks. Additional measurements of wood densities in Belgian forests could help to reduce the uncertainty related to this factor. Because of the time-consuming and destructive character of the determination of biomass expansion factors s.s. (BEFs), the establishment of new BEFs does not have the highest priority in the framework of improving the assessment of the biomass carbon stock in the Belgian forests. As the median C-content value for all species except beech was equal to the default IPCC-value of 50% carbon in dry matter, it seems appropriate to use this value for future calculations.forest inventory / carbon stock / biomass expansion factor / wood density / aboveground and belowground biomass / carbon content Résumé -Estimation des stocks de carbone dans la biomasse des forêts en Belgique. La forêt belge couvrait 693 181 ha en 2000, ce qui représente 22,7 % de la surface totale du pays. Les inventaires forestiers flamands et wallons n'incluent pas de mesures directes de biomasse ou de stock de carbone. Pour calculer les stocks de carbone dans les arbres forestiers à partir des volumes de bois fort, les infradensités du bois, les facteurs d'expansion de la biomasse et les teneurs en carbone ont été sélectionnés dans la littérature. En 2000, les stocks de carbone dans la biomasse des forêts belges productives étaient de 60,9 Mt C, soit 101,0 t C ha -1 . Cette dernière valeur est relativement haute comparée avec celles observées dans les forêts des pays avoisinants, à cause d'un grand volume de bois fort dans les forêts belges (261,9 m 3 ha -1 ). L'infradensité du bois semble être le facteur qui introduit la plus importante variabilité dans le calcul des stocks de carbone dans la biomasse. Des mesures complémentaires des infradensités du bois des essences les plus communes en Belgique pourraient aider à réduire significativement les incertitudes sur les mesures des stocks de carbone dans les forêts. La détermination expérimentale des facteurs d'expansion est coûteuse en temps et se base sur des analyses destructives. Ainsi, l'obtention de facteurs d'expansion propre à la situati...

-Different European Biomass Expansion Factors (BEFs) were compared for the inventory-based quantification of total aboveground and belowground biomass in forests. Therefore a qualitative analysis is performed on the biomass results obtained through the BEF approach and those from experimentally established allometric relations based on destructively sampled and fully excavated trees. Total organic carbon (OC) stock in aboveground and belowground living biomass of Flemish forests amounts to 12 Mt on average, with a significantly larger OC stock per hectare in deciduous forests compared to coniferous or mixed forest types. Total forest biomass seems to be fairly well approximated by a multiplication of the standing stock with either one of the applied BEFs. However an indication of the volume and age class for which the BEFs are established and a refined diameter-volume-biomass relation for oak trees in Europe, are required to gain more accurate results.