Assessing the potential for nutrient leaching from beneath grazed leguminous swards at four European sites

This study reports the leaching of nitrogen (N), phosphorus (P) and potassium (K) from beneath grazed leguminous swards established at two Mediterranean and two north-western European sites. The purpose of the experiment was to provide scientific evidence on whether moderately extensive systems of animal production based on legumes could be environmentally sustainable, to comply with the Water Framework Directive. There were differences in the legume species and the grazing animal between sites, reflecting differences in local management and climatic conditions. There were also differences in the methods used to assess nutrient leaching, again reflecting soil and hydrological conditions present. There were no general differences between the quantities of nutrient leached at Mediterranean and north European sites over the 3 year experiment and these quantities, up to 6 kg ha-1 year-1 for nitrate-N, were similar to those reported from earlier studies of leaching beneath grass-white clover swards at temperate sites. Temporal variability in leached nutrient concentrations and amounts was greater at the Mediterranean sites. There were no discernible differences in the effects of sward treatment on leaching. The proportion of N leached in organic combination or as ammonium was greater than with N-fertilized grass systems. The quantities of P leached were relatively high, ranging from 0·1 to 3 kg ha-1 year-1 for soluble reactive P, reflecting the practice of high levels of fertilizer application to these systems. Reduction of P transfer to water from these systems will be critical for future compliance with the Water Framework Directive.