Phosphorus dynamics in the ditch system of a restored peat wetland

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 −60 cm relative to ground level) was extensively influenced across most of the 8.4 ha field site by the management of the adjacent ditch water levels. This conservation-led, prescribed water balance was facilitated by the high hydraulic conductivity (1.1 × 10−5 m s−1) of the lower (70–140 cm), degraded layer of peat. However, only during a 7-day period of water table drawdown by intermittent pump drainage, approximately 45 g ha−1 of dissolved reactive P (DRP) entered the pumped ditch from the field via this degraded layer. Summer rainfall events >35 mm d−1 also coincided with significant peaks in ditch water P concentration (up to 200 μg L−1 DRP). Even larger peaks (up to 700 μg L−1 DRP) occurred with the annual onset of autumn reflooding. These episodic P loss events pose a serious potential threat to biological water quality.