The seasonality of phosphorus transfers from land to water: Implications for trophic impacts and policy evaluation

The Nitrates Directive regulations are a Programme of Measures under the EU Water Framework Directive to protect waters from agricultural transfers of nitrogen and phosphorus. Soil phosphorus management to an agronomic optimum and closed winter periods for organic and inorganic fertiliser amendments are among a suite of policy measures to curtail diffuse pollution at catchment scale. In this investigation, two intensive grassland and two arable catchments (7–12 km2) in the Republic of Ireland were studied to link a high resolution spatial survey (≤ 2 ha) of soil P availability with P delivery in receiving rivers; monitored on a sub-hourly basis over one year. Data indicated that source risk, as defined by soil P availability and organic P loading, was less important than mobilisation and hydrological transfer potential which increased delivery due to runoff flashiness as described by a hydrological metric during the winter. Overall, however, annual TP loads were low to moderate (0.175 to 0.785 kg ha− 1 yr− 1). The data also highlighted, without exception, the influences of summer background P loading and subsequent ecologically significant P concentrations from persistent point sources. This may have implications for expected ecological status and recovery in these catchments, which appeared more at risk in catchments with little buffering in terms of summer base flow dilution. Wetter winters and drier summers under climate change scenarios would likely increase stream P concentrations both during storms and during baseflows and would be particularly magnified in those catchments with flashy runoff and suppressed baseflow. These seasonal insights into source-to-delivery functions and risk (re)assessment were only possible with high resolution (spatial and temporal) data collection and will be important in influencing expectations of policies that are evaluated at larger scales but with coarser resolution sampling.

► High resolution P data are presented at source (soil) and delivery (rivers) in agri-catchments.
► Lowest perceived P source risk is found to be highest in P delivery load.
► Transport factors including soil permeability appear more important than source pressures.
► Summer river concentrations are significantly high.
► Future hydrological function may intensify diffuse and point P impacts.