![]() The available water capacity (AWC) is defined as the volumetric soil water content between matrix potentials at −60 and −15 000 hPa ( Schwärzel et al., 2002). ![]() We suggest a combination of saturated hydraulic conductivity ( K s) with available water capacity as core parameters of soil-water-interactions to characterize a given site in a hydrological sense ( Table 1). This classification scheme is not based on an expert system (e.g., von Post degradation scheme Von Post, 1922) but on independently measured bulk density, which assures easy applicability and-more importantly-comparability with different studies. We created five classes of peat soils from pristine (P) to extremely degraded (E). We categorized degraded peat soils according to their function in the water cycle. Classification of Peat Soil Hydraulic Function With increasing bulk density, from 0.20 to 1.0 g cm −3, K s almost remains constant with a large variance ( Liu and Lennartz, 2019). A negative linear relationship was observed between log 10 K s and bulk density. With an increase in bulk density, from 0.01 to 0.2 g cm −3, K s decreased dramatically ( Figure 2B), because macroporosity is markedly reduced with peatland degradation. With increasing bulk density from 0.2 to 1.0 g cm −3, macroporosity remains constant because of the formation of secondary macropores (e.g., root channels Figure 1 Liu and Lennartz, 2015).Ī strong negative linear relationship was observed between total porosity and bulk density ( R 2 = 0.82, p 50 μm) and bulk density ( Figure 2A). Macropores in low to moderately degraded peat soils (e.g., bulk density <0.2 g cm −3) are formed by the undecomposed parent plant material, which functions as a channel/pipe system ( Figure 1). The relationship between physical properties and peat degradation has been studied ( Boelter, 1969 Schindler et al., 2003). Here, we propose bulk density as a proxy for peat degradation ( Liu and Lennartz, 2019). Drainage of peatland accelerates carbon mineralization, resulting in a higher bulk density and a lower porosity. Therefore, greater saturated hydraulic conductivity values ( K s) are observed in pristine peat than in degraded peat ( Figure 2B). These macropores facilitate water movement and solute transport ( Quinton et al., 2009 Rezanezhad et al., 2016). The most extraordinary feature of pristine peat is its high porosity, which easily exceeds 90 vol% with a dominance of macropores (>50 μm Figure 2A). Pristine peat is formed of decayed plants and characterized by a low density and high organic matter content (e.g., >90 wt% Figure 1). Soil Structure and Hydraulic Functions of Peat The classification scheme shall be further developed and may serve as a decision support tool for peatland restoration projects. We established a rating scheme that takes soil degradation into account and classifies the water related ecosystem services provided by peat soils. We, also, identify soil physical parameters in order to estimate the filter and buffer potential of peat soils. We combine key properties such as available water capacity and hydraulic conductivity to classify peat soils with respect to their function in the water cycle. Little is known about the function of peat soils with respect to water quantity and quality ( Baveye et al., 2016 Rabot et al., 2018 Vogel et al., 2018). Drainage leads to subsidence of peat deposits by 0.5–4 m ( Wösten et al., 1997 Pronger et al., 2014), and oxidation of peat organic matter from 100 to 20 wt% ( Rezanezhad et al., 2016 Liu and Lennartz, 2019), causing a loss in their water storage and water filter function. The drained fraction can be as high as 95% (e.g., Northern Germany). It is estimated that 15% of global peatlands have been drained and are currently being used for agriculture and forestry ( Joosten and Clarke, 2002). Drainage of peatlands induce aerobic conditions, which leads to carbon mineralization, peat degradation and concomitant emissions of carbon dioxide (CO 2) to the atmosphere. Peatlands cover ~3% of the Earth's land area, but store ~30% of the global soil carbon (C), 10% of the global soil nitrogen (N), and 10% of global fresh water ( Joosten and Clarke, 2002 Limpens et al., 2008).
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