Productivity and soil nutrient relations of bedwork watermeadows in southern England

Irrigation (or 'floating') of watermeadows in Europe was economically important before the 20th century because it enhanced grazing sward productivity in the spring. Today, few operational systems remain, but interest in floating as a technique is returning as concern grows over floodplain management, the fate of nutrients and the sustainability of agricultural production systems. Investigations into grass productivity, phosphorus and nitrogen relations were undertaken at a surviving watermeadow system, the Britford Site of Special Scientific Interest, Salisbury, England (1995-2001) and at an experimental site at Norwich, England (1998-2001). Irrigation significantly increased late-winter and early-spring grass productivity at Britford in the order of 60-80% over 5 weeks compared with un-irrigated controls. Phosphorus travelled across the system; some included with the clay fraction in suspended sediment, but available phosphorus in the topsoil reduced across the meadows by between one-half and one-third. Nitrate in irrigation water was utilised, but the meadow system did not reduce concentrations of dissolved nitrate. Increased nitrate concentration in soil-water is attributed to mineralisation after drainage. Annual nitrogen cycle process modelling suggests the present levels of production and soil organic matter accumulation are attributable to environmental eutrophication. A conceptual model is presented for enhanced spring growth following floating and it is concluded that, while temperature is the trigger that enhances productivity early in the season, this is achieved by enhancing the mineralisation of soil organic matter and release of available nitrogen. There was no evidence that watermeadows act as a sink for nitrogen mobile in surface waters but there was significant uptake of topsoil available phosphorus.