The regulating service called filtering function focuses on the self-purification powers of soils and waters, and contributes to water quality. Water purification by filtering is a major service of soils, however, the filtering mechanism of peat soils differs from that of mineral soils. The „filtration capacity“ for dissolved and suspended substances, for example, is expressed using the water conductivity of substrates (kf-value, or coefficient of permeability) by the assesment of soil functions in Berlin. The lower the kf-value, the longer the duration of filtration, and the better the filtration effect (Environmental Atlas Berlin 2015). This approach is unsuited to peat soils, because the surface-near water table of peatlands is usually equivalent to the groundwater table. Against this background, peat cannot render any filtering function, as no filter length is available. An exception are percolation mires, which can absorb dissolved substrates through their lateral flow of groundwater from the source to the receiving water. This type of water filtration, however, has only marginal significance for Berlin, and is found only in minor areas of the Tegeler Fließ.
Nonetheless, undisturbed or near-natural peatlands are highly effective as nutrient sinks. When peat forms, dissolved substances, mostly nitrogen and phosphorus compounds, are captured and deposited in the peat. Their capacity for „internal oligotrophication“ (Succow 2001) by taking up and recycling these compounds during peat growth is very important for the filtering effect of the peatland (Dierßen & Dierßen 2001). Berlin's peatlands do not just store huge amounts of carbon, they also contain 100–300 kg of total nitrogen (Nt) per hectare, depending on peat and gyttja thicknesses. In Berlin, fen peats with a low degree of decomposition usually contain 2 to 3 % Nt, while peats of transition bogs, within the same range of decomposition, always have N contents that are significantly below 2 % . The formation of organic rich gyttja sequences in flooded, rewetted peatlands is highly significant with a view to the sink function for nitrogen and phosphorus. Investigations of 12 sites in North-East Germany provided annual accumulation rates of 96 kg N/ha or 9 kg P/ha (Cabezas et al. 2014). The C sink function is also very high with an annual value of 1.338 kg C/ha. However, the accumulation rate cannot serve as basis for a conclusive judgement about the climate service alone, as flooded areas simultaneously carry the risk of increased methane emissions, which in turn have a negative impact on the greenhouse gas balance (Augustin & Chojnicki 2008).
Particulate loads are deposited in the pores, captured in the peat body or decomposed. In the peatland Langes Luch/Grunewald, a substrate composition consisting of peat and gyttja is found below the current peat forming horizon. It contains more than 5 to 10 times higher loads of heavy metals compared to other horizons, depending on which elements are focused on. These substrates represent the historical filtering service which is connected to an artificial water supply through the formerly polluted water of the river Havel.
Analogous, drained peatlands are characterized by their release of matter. Permanent release occurs during the mineralisation process of drained and aerated peat soils (Balla & Quast 2001), which has the inherent risk of gaseous dissolved or particulate emissions.
In Berlin, the release of sulfates which occurs through the oxidation of ferrous sulphides when peat mineralises Zak et al. (2008) is of particular interest, as it can influence the drinking water quality. The river Spree (160 mg/l) as well as the bank filtrate of the well galleries already show comparatively high sulfate concentrations from anthropogenic sources (e.g. from the waste of building sites and war debris, or the legacy of lignite coal mining). It cannot be ruled out that the mandatory limits for drinking water (240 mg/l) can temporarily be exceeded.
Soluble inorganic nitrogen compounds released by peat mineralisation are hazardous to surface water and groundwater, while the climate is additionally impacted by emissions from nitrous oxide (N2O), a very potent greenhouse gas (Dierßen & Dierßen 2001). It is possible, though, to reduce the net N mineralisation of drained fen peats quite significantly through rewetting. In a peatland in Schleswig-Holstein, the rate of N of 15–25 kg/ha annually could be brought down to under 5 kg/ha by raising the groundwater table from 60 to 10 cm below the surface (Reiche 1996). Parameters for rates of N emissions out of peatlands or their catchments that are being used for intensive agriculture (Holsten & Trepel 2012) were unsuited for application to urban landscapes and could not be used, as the peatlands of Berlin are situated either within the urban area or inside of forests.
The eutrophication risk from dissolution of phosphate following rewetting (Zak & Gelbrecht 2008) is not considered for the filtering function, as a peatland's current state is assessed and no projections are made. Nonetheless, there are important interdependencies, and they will be described as „Risk Factors“ of rewetting on the pages regarding the Adaptation Strategy.
In the development of the indicator, specification of exact numbers for substance deposition rates was knowingly omitted, as this would have required an exact knowledge of entry and discharge paths, substrate composition as well as exact knowledge of all important processes involved. The three-grade rating scale, which was used for assessment instead, is based on the properties of peatland ecosystems to accumulate substances during peatgrowth and to release them during peat decomposition. As this assessment system is also intended for application in landscape planning and management, the mapping was supplemented with possible discharge/deposition paths.