Status report on the key climate variables technical supplement to the
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- Contributing baseline GCOS observations
- Significant data management issues
- Analysis products
Main climate applicationSoil organic carbon (SOC) represents a major pool of carbon within the biosphere, estimated at about 1.5 x 1018 g globally, roughly two to three times the atmospheric CO2 pool and acting as both a source and a sink for carbon (Borken et al., 2002). Soil carbon is a major component of soil organic matter (SOM) and by far the biggest SOM stores are in boreal and tropical peatlands. The many processes that determine the magnitude of the carbon exchanges between the atmosphere and terrestrial ecosystems are sensitive to climate factors. The rate at which carbon accumulates or is released from terrestrial ecosystems depends not only on the rate of physiological processes but also on the size of constituent soil carbon pools, which have a wide range of turnover times of up to thousands of years (Parton et al. 1995). Changes in the slow turnover pools determine whether terrestrial systems are net sources or sinks of carbon with respect to the atmosphere (Post et al. 1996). It is therefore necessary to monitor fluctuations of these carbon pools as a function of environmental conditions (e.g. precipitation and temperature). A key point is that the accumulation of carbon is in many cases a very slow process, whereas the release from these soil carbon stocks can be almost instantaneous, and over the near term (50-100 years) the potential for loss is significantly greater than the potential for storage. It has also to be considered that even small changes from such a large store can cause a significant change in the atmospheric CO2 concentration. Fires are also responsible for releasing great amounts of carbon from soils, in particular from organic rich peatlands, which has recently been emphasised by observations made for the El Niño year 1997 in Indonesia (Page et al., 2002). Contributing baseline GCOS observationsNone. Other contributing observationsUnfortunately, whereas both above ground biomass and FAPAR will be quantifiable using existing satellite technology in the near future, this is not possible for SOM where ground measurements and models are essential. The Global Terrestrial Observing System (GTOS) initiative within GCOS has strong links to the International Geosphere-Biosphere Programme (IGBP) with one main aim being to facilitate scientific progress in predicting the effects of changes in land-use, agricultural practice and climate on SOM. The specific need for a network of SOM modellers and long-term data holders has been recognised and, to this end, the global Soil Organic Matter Network (SOMNET) was established within the GCOS framework. SOMNET consists of about 30 SOM modellers and incorporates over 120 long-term experimentalists from all around the world. However, to date, there is no readily accessible database on SOM released from this project. There are a number of global networks that systematically collect SOM data. International Soil Reference and Information Centre (ISRIC) with the World Inventory of Soil Emission Potentials (WISE). A database derived from 1,125 soil profiles but containing only carbon values up to 48 kg C m-2. The data are displayed on a 0.5 x 0.5 degree grid of soil organic carbon (kg C m-2) for either the top 0-30 cm or the 0-100 cm soil layers. Data and Information System of the International Geosphere-Biosphere Programme (IGBP-DIS). This database contains values for soil organic carbon (kg C m-2) for the 0-100 cm soil layer with data displayed on a 5 x 5 arc minute grid and maximal values of up to 82 kg C m-2. World-wide Organic Soil Carbon and Nitrogen Data (Zinke et al. 1984). This database has been derived from c. 3,500 soil profiles and contains much higher maximum soil carbon values of up to 432 kg C m-2. The data are displayed on a 0.5 x 0.5 degree grid of soil organic carbon (kg C m-2) for the top 0-100 cm soil layer. Many samples reported in this survey are compiled from the existing literature and did not have uniform soil increment or bulk density determinations. Missing bulk density values were estimated by regression based on organic carbon contents of 1800 profiles of known bulk density. All the above data can all be downloaded from the Oak Ridge National Laboratory (ORNL) – Distribution Active Archive Centre (DAAC): http://www-eosdis.ornl.gov . Summary of global soil carbon content in broadly categorized terrestrial ecosystems (Amthor, J.S. et al., 1998 after Ajtay et al., 1979; Botkin & Simpson, 1990; Gorham, 1995; FAO, 1997). This dataset summarizes the literature values for 16 biomes with carbon contents of up to 455 (g C m-2). The inventory calculations are limited to 100 cm soil depth and it is worth noting that the largest carbon stores, peatlands, are mostly much deeper than this. Nepstad et al. (1994) report that stores of carbon below 1 m depth exceed those in the top 1.0 m in an Amazonian forest. The question remains as to how likely these stores are to be affected by environmental change. Values for all biomes except wetlands and northern peatlands exclude surface litter, but surface litter and standing dead plants may contain from 50 Pg to >200 Pg C globally, with large amounts in some forest ecosystems (see references in Ajtay et al., 1979; Amthor, 1995). Significant data management issuesThere are some global and regional historical archives of SOM data. However, soil organic matter measurement techniques have not generally been standardised and the inventories are frequently based on differing depths and horizon sampling strategies. It is absolutely necessary for any database to: (i) agree on common metadata formats, (ii) define standard data calculation and presentation formats, (iii) assemble more SOM based on these common formats. Analysis productsThe ideal high-level data product is high-resolution (both spatially and by depth) soil carbon measurements. Acquiring large-scale high spatial resolution data is difficult, since in situ measurements are at present the norm, and direct measurement of SOM using Earth Observation (EO) data is not possible. However, EO has a part to play in producing higher resolution modelled SOM products, based on measurements of biomass distribution, precipitation, soil moisture, etc. Furthermore, the use of satellite imagery in order to assess the potential loss of SOM due to fires has recently become a focus of SOM research (Page et al. 2002). Developing and applying such techniques on a global scale will make a significant contribution to our understanding of the CO2 fluxes released by large-scale fires. This will be of particular importance in peatlands, both tropical and boreal, that form the largest terrestrial carbon stocks. Additionally, SOM research has increasingly focused on monitoring both dissolved organic carbon (DOC) and matter (DOM), combining ground measurements with improved EO techniques (e.g., Landsat images with multiband water column reflectance) (Gallie, 1993, 1994, 1997). A primary objective of GCOS should be the mapping of DOC and DOM changes in lakes, rivers and estuaries, which should enable us to detect changes in soil carbon within the connected catchment areas, e.g., increasing microbial activity in peatlands. Although there still remains the problem of knowing how much of this DOC and DOM is mineralised, this is likely to deliver early warning conditions of climate change impacts on global carbon stocks and deserves high priority. An integrated global database of ground measurements together with the development of methods exploiting EO data to improve the resolution of these data should be a clear objective for GCOS. SOM contents also relate to soil nutrient levels and some combination with complementary sources of information might be useful (e.g. phosphorus and nitrogen levels, C/P and C/N ratios, etc.). 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