Source to Impact: Phosphorus Trends Across the Nutrient Cascade in an Arable Watershed in Response to P Mitigation Measures.

Capturing temporal on farm management practices and environmental processes that generate and attenuate phosphorus (P) within a watershed provides the unique advantage of examining how P changes along the nutrient continuum in response to source P mitigation measures that are implemented to improve water quality status. The aim of this research was to observe over a four year period the efficacy of the current Irish P regulation measures, (introduced under the EU nitrates Directive) in an intensive arable-dominated (mostly malting spring barley) agricultural watershed. The watershed is 11 km², located in the south-east of Ireland and mostly on well-drained cambisol soils. In 2009 and 2013 soil surveys for plant-available P of all farmland in the catchment were carried out at approx. 2 ha sample areas. Annual nutrient input data from the farms within the watershed were collected. Concurrently, high temporal resolution monitoring of water discharge and P concentration was conducted at the watershed outlet across four hydrological years (April to March). Ecological impact surveys were carried out at four sites within each watershed in May and September across the observed four year period (2009-2013). Importantly, the proportion of farmland area within this watershed with excessive soil P concentrations decreased by 8.2% (20% to 11.8%) and continued to decline, as the proportional area below the national crop agronomic optimum thresholds (<6 mg P l^-1) increased from 57% to 68%. This general decline in plant available P was also reflected in the relativity unchanged P input average of 28.8 kg ha^-1(n = 4 years). Observed during the closed period to nutrient application, the total reactive P (TRP) concentrations estimated for transfer pathways to the surface water were highest in the quickflow and interflow, but have notably declined to concentrations below the Environmental Quality Standard (EQS) threshold of 0.035mg/l since 2012. Additionally, total P (TP) concentrations, mostly in the quickflow pathway, also declined during this period. Over the study period fluctuations in TRP and TP concentrations made it difficult to define a clear trend, however annual mean TRP concentrations were within the EQS threshold. Although there were some seasonal trends, there was no clear indication of improvement in the ecological quality status in this watershed. Overall, it appeared that the P mitigation measures had a positive effect within this watershed, especially evident in the source component of the P transfer continuum. This study highlights the need for careful consideration of lag-times throughout the continuum in defining the responsiveness of P mitigation measures in delivering improved water quality status within similar watersheds.