Physical properties of low-lying agricultural peat soils in England
Introduction
The conservation and the sustainable management of peat soil resources rely strongly on effective soil water management (Renger et al., 2002, Wessolek et al., 2002). Falloon et al., 2006 report that peat soils account for approximately 43% of the total soil organic carbon (SOC) pool in Great Britain. The stability of this carbon stock is determined by biogeochemical processes which are governed by oxygen and nutrients availability, soil pH and as well as environmental parameters such as temperature. These factors are influenced by land use and water management practices (Best and Jacobs, 1997) and the increase in aerobic conditions created in the soil after drainage and agricultural tillage operations causes significant increases in peat oxidation relative to saturated soil moisture conditions (Freeman et al., 1993, Moore and Dalva, 1993, Funk et al., 1994, Aerts and Ludwig, 1997). Water table management can potentially be used as an efficient means of controlling the rate of peat mineralisation (Kechavarzi et al., 2007) by altering oxygen availability and damping the effect of temperature variations (Oechel et al., 1998). However, oxygen availability is a function of soil moisture (Chow et al., 2006) and not directly of water table position (Kellner and Halldin, 2002). Soil moisture varies with soil water potential (water release characteristics) which under hydrodynamic conditions changes with time-variable boundaries such as water table position and evapotranspiration. Hence a better understanding of peat hydraulic relations, which govern water flow and storage, is important with regards to water table management as a means of controlling peat mineralisation.
However, peat soils exhibit distinct dynamic physical and hydraulic properties, in that they are very compressible and have a large propensity to shrink and swell under changing moisture regimes (Schwärzel et al., 2002). Shrinkage, although partly reversible, can severely alter the soil hydraulic properties including water retention, hydraulic conductivity and specific yield (Price and Schlotzhauer, 1999, Kellner and Halldin, 2002, Kennedy and Price, 2005). Moreover, drainage of peatlands can lead to subsidence and consolidation and the structural collapse and loss of soil pores as peats become less buoyant and the over-burden experienced by underlying peat horizons increases. In addition, bio-oxidation of soil organic matter leads to irreversible changes in soil physical characteristics including soil structure as evidenced through variations in the hydraulic properties of peat soils at different stages of degradation. Thus, understanding the influence of anthropogenic activities, such as water table management, on the mineralisation process of peat soils also requires knowledge about the influence of shrinkage and long-term changes in peat physical properties on the hydraulic relationships (Weiss et al., 1998).
However, there is a lack of data and models in a functional form describing the hydraulic and shrinkage properties of peat soils (Schwärzel et al., 2002, Wessolek et al., 2002). In addition, the influence of long-term changes in peat soils physical characteristics on hydraulic functions is poorly understood and, as a result, generally discounted in modelling peat soil hydrology (Letts et al., 2000, Kellner and Halldin, 2002). The objective of this study is to investigate the impact that the degradation of lowland agricultural peat soils in the UK has on their physical and hydraulic properties and therefore on their ability to fulfil soil functions such as storing, transmitting and retaining water.
Section snippets
Study sites
Two study areas, of contrasting land use and under water management regime, were used in this work. The first peatland, West Sedgemoor (WSM), covers an area of 1035 ha, with peat ranging from 3–8 m in depth (Burton and Hodgson, 1987). It is grassland where fen peat underlies a thin alluvium and forms part of the River Parrett catchment, which lies in the southern part of the Somerset Levels and Moors of South West England (51°01.00′N 2°56.15′W). The site is on a wildlife conservation area under
Soil physical characteristics
The measured physical characteristics including the peats ranking on the von Post scale are given in Table 1.
At WSM, the peat is capped with an organic mineral soil layer with a thickness ranging from 0.06 to 0.15 m classified as peaty loam (Burton and Hodgson, 1987). This surface layer of soil has relatively lower % SOM content than other peats and according to Burton and Hodgson (1987) should be considered as an organic soil rather than a true peat soil. As a consequence its ranking is not
Conclusions
This study indicates that peats at various degrees of decomposition had distinctly different physical and hydraulic properties. Decomposition and degradation results in lower SOM content, higher bulk density and lower porosity. Intensive drainage and agricultural land use lead to pedogenic alterations and a greater degradation of the soil structure in the upper soil layers. The void ratio of less decomposed peats is higher than that of degraded peats and they have a much higher propensity to
Acknowledgements
This work was supported by the European Union project “Europeat” (QLK5-CT-2002-01835).
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