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Phosphorus dynamics in the ditch system of a restored peat wetland

Niedermeier, A., Robinson, J.S.
Agriculture, ecosystems & environment 2009 v.131 no.3-4 pp. 161-169
peatlands, wetlands, land restoration, water table, drainage channels, drainage water, floods, biogeochemical cycles, soil fertility, soil transport processes, eutrophication, water quality, phosphorus, soil matric potential, soil hydraulic properties, hydraulic conductivity, peat, rain, losses from soil, leaching, water pollution, England
The restoration of wetlands as bird habitats often involves the maintenance of a fluctuating water regime by careful, localised ditch water management using pumps and sluices. However, there is evidence in the literature to suggest that alternate flood/drainage cycles can accelerate nutrient cycling and transport within the soil and, therefore, pose a threat to water quality through the process of eutrophication. This study focused on the dynamics and losses of soil P in a recently re-wetted, eutrophic fen peat developed on alluvium in south west England. During the 2-year study (2001 and 2002), soil water tensiometry revealed that the field water table (fluctuating annually between +20 and -60cm relative to ground level) was extensively influenced across most of the 8.4ha field site by the management of the adjacent ditch water levels. This conservation-led, prescribed water balance was facilitated by the high hydraulic conductivity (1.1x10⁻⁵ ms⁻¹) of the lower (70-140cm), degraded layer of peat. However, only during a 7-day period of water table drawdown by intermittent pump drainage, approximately 45gha⁻¹ of dissolved reactive P (DRP) entered the pumped ditch from the field via this degraded layer. Summer rainfall events >35mmd⁻¹ also coincided with significant peaks in ditch water P concentration (up to 200μgL⁻¹ DRP). Even larger peaks (up to 700μgL⁻¹ DRP) occurred with the annual onset of autumn reflooding. These episodic P loss events pose a serious potential threat to biological water quality.