Due to their high levels of soil organic matter, peatlands represent important carbon storages for the global carbon cycle. And even though these ecosystems make up only three percent of the world's land surface (Parish et al. 2008), they store approx. one-third of the total organic carbon (TOC) (Post et al. 1982). All of carbon (C) stored globally in peatlands is estimated to exceed 500 billion tons, which is equivalent to more than half of the amount residing in the atmosphere in the form of carbon dioxide (CO2) (Houghton 2007, Limpens et al. 2008).
The phase of mire formation, and with it the storage of C, began in Berlin, as in the rest of Central Europe, mainly with the end of the last Ice Age (Succow & Joosten 2001).
The activity of biota in peat soils is much reduced by high water levels, all year round causing low levels of oxygen so that dead parts of plants are not fully decomposed. As a result, they are deposited as peat layers, which are sometimes several metres thick (Koppisch 2001a). Compared to mineral soils, these peat soils can generally store vast amounts of C, often greatly in excess of 1,000 t per hectare (Möller et al. 2014).
Through these high proportions of carbon stored and fixed in peat soils, peatlands make a meaningful contribution to climate protection, and thus have had a major role in the cooling of the global climate (Frolking et al. 2001, Akumu & McLaughlin 2013). Due to their capacity to capture and fix atmospheric CO2 (carbon dioxide), the ‚global cooling effect‘ of peatlands accounts for approx. 1.5 to 2 °C over the past 10,000 years (Holden 2005).
Growing peatlands with high water levels still accumulate C up to the present day. However, peatlands are increasingly being aerated by drainage and decreasing water levels caused by agricultural land use and forestry, groundwater extraction for drinking water, or climate-change-induced reductions in rainfall. This leads to an intensification of the activity of soil biota and therefore to peat decomposition and mineralisation. Peatlands change from C sinks into C sources, releasing considerable amounts of CO2 (Koppisch 2001b). Drösler et al. (2013) specifies, for example, the current greenhouse gas emissions from drained peatlands with 0–34 t CO2 equivalents per hectare and year depending on land use, which accounts for a proportion of up to 5 % of the total emissions nationwide.
In the current debate by management and planning councils and agencies about the ecosystem services of peatlands and their effect on climate change, fundamental data are missing. Such data would allow regional or site-specific analyses of historical storages (C pools) or make predictions regarding potential C loss for individual peatlands possible (Sachverständigenrat für Umweltfragen 2012). Therefore, the collection and assessment of data relating to climate proctection services of peatlands and their vulnerability on the basis of different C pools is a focal part of this project.
The assessment of the current climate effect or the greenhouse gas emissions of a peatland in the form of CO2 equivalents can, in principle, be done by a number of existing models (Drösler et al. 2013, Couwenberg et al. 2011). These models require input parameters such as vegetation or water table. Tests conducted for Berlin's peatlands, however, showed that the accuracy of modeling results was highly liable to change depending on regional or site-specific variables. On the one hand, it is not possible for many biotope types in Berlin to assign them to the vegetation units and water levels of the models (e.g. on account of a wide ecological amplitude that is, for example, typical of reeds and tall grass, included in several water levels). On the other, these models assume categories of land use which are frequently not applicable to the peatlands of Berlin, which are not used for agriculture and are usually listed nature reserves. Caused by these uncertainties, analyses produce results that must remain vague regarding the climate effect of individual peatlands. For this reason, region or site-specific interpretations of the current climate effect in terms of net emissions were omitted for this project.
The climate protection service is expressed by the total C stored in all of Berlin's peatlands (their ‚historical‘ C pool). The amount of C stored may vary considerably from one peatland to another. Based on their natural diversity (hydrology, geomorphology etc.) during peatland formation, different soil horizons were formed, consisting of varying peat thicknesses and varying contents of organic C. Thus, peatland or mire types can be categorised according to their formation conditions, for example, as percolation mire, which can store up to ten times more carbon than flat peatlands such as a ‚water rise mire‘ (Zauft et al. 2010). In addition to differences in peat thickness, there are considerable differences also in peat qualities (peat forming plants, degree of decomposition etc.). This is reflected in the substrate-specific C storages and bulk densities of single soil horizons, and likewise is also reflected in the amounts of stored C (Rosskopf & Zeitz 2009). In order to quantify the amount of C stored in Berlin's peatlands, it was important to gather detailed information on each soil profile of every single peatland. To obtain them, all peatland soils were sampled and then systematically classified by substrate and soil types. The data regarding bulk densities and C amounts were taken at representative soil horizon profiles in Berlin. For this purpose, more than 500 peat and gyttja samples were analysed in the laboratory. Parts of the data for bulk density were additionally complemented with pre-existing, backfile data.
To identify environmental relief potentials related to the climate protection services of Berlin's peatlands, it was important to be able to show not only the amount of carbon historically stored in the C pools but also to show what the risk situation is for each peatland currently. To this end, the vulnerable portion of total C was determined. The drained and aerated part above of the median groundwater table is threatened through processes of peat decomposition and the release of C (see figure below).
Additionally, it is important to determine the labile fraction within the vulnerable C, as it is particularly liable to conversion and can easily emitted into the atmosphere, for example in the form CO2 (Khanna et al 2001, Kalisz et al. 2010). This labile part with a high release potential was determined in this project using the hot-water extractable fraction, which is one of the most labile and dynamic fractions within soil organic matter (SOM) (Leinweber et al. 1995, Haynes 2005). During this process, parts of the SOM are weakly hydrolyzed and going into solution (Schulz & Körschens 1998). The hot-water extractable C (Chwe) correlates strongly with the SOM that can easily be mineralised (Behm 1988, Körschens et al. 1998) as well as with the soil microbial biomass (Sparling et al. 1988), and represents the active part of SOM (Schulz 1997). The hot-water extractions were conducted with a method that has been developed for organic soils, with special respect to the high C concentrations in peatland soils (Heller & Weiß 2015).