Effects of mitigation measures on phosphorus loss across the transfer continuum from soil to water in a monitored dairy grassland catchment

In many countries with intensive agriculture, water quality is a major issue and phosphorus (P) loss from soils to water is a major pressure. In Ireland, the EU Nitrates Directive Regulations aim to minimise these losses. This study measured the effects of P source management on P transfer across the nutrient transfer continuum from soils to water and subsequent water quality and agronomic impacts in a dairy-dominated, highly stocked and intensively monitored 7.6 km2 grassland catchment with mostly free draining soils over three years. Monitoring included farm P management, surface soil P concentrations, ground- and stream-water concentrations and stream flow. Reduced P source pressure was indicated by: a) lower farm-gate P balances (2.4 kg ha-1 yr-1), higher P use efficiencies (89%) and lower inorganic fertilizer P use (5.2 kg ha-1 yr-1) relative to previous studies, b) almost no P application during the winter to avoid incidental P transfers, and c) decreased proportions of soils with excessive P concentrations (32% to 24%). Over the same period, milk outputs of 14,585 l ha-1 and gross margins of €3,130 ha-1 indicated that production and profitability remained comparable with the top 10% of dairy farmers nationally. Declines in delayed flow and interflow pathway P concentrations during the winter months indicated some response in P delivery in surface water. However, delayed baseflows in the wetter third year resulted in elevated P concentrations and, overall, there were no clear trends in stream biological quality. This suggests that the impact of policy measures may be felt sooner closer to the source end of the nutrient transfer continuum, in soil P concentrations, for example, and a time lag may occur at the other end in P delivery to streams and stream biological quality, with implications for time frames of policy efficacy and policy monitoring.