Investigation of Particulate Phosphorus Mobilised by Run-offin Small Agricultural Catchments

Abstract

Research carried out in this work has contributed valuable insights and new findings into the impact of particulate phosphorus (PP) associated with fluvial suspended sediments, in terms of its sources and speciation dynamics within geologically and hydrologically contrasting agricultural catchment streams. Various conventional and advanced solid phase analytical techniques were used, in order to provide a detailed analysis of the phosphorus (P) fractions associated with fluvial suspended sediments. Three agricultural catchments were monitored which showed seasonal dynamics in nutrient concentrations and export: Bunoke (BE), Ballyboughal (BB), and Tintern Abbey (TTA). Seasonal variations for in-stream soluble reactive P concentrations ranged between 0.05 and 0.2 mg/L at BE, and between 0.015 and 0.2 mg/L at BB and TTA. Dissolved P load transported was calculated to be, approximately, 23.6 tonnes km-2 (from April to November 2019), and 20.4 tonnes km-2 year-1, for BE and BB catchments, respectively. Sediment concentration ranged from 0.9-8.1, 0.4-0.7 and 1.6-11.2 mg L-1 at BE1, BE2 and BE3 respectively; 0.6-0.9, 0.3 and 1.7-3.2 2 mg L-1 at BB1, BB2 and BB3 respectively; and 0.4-1.1 mg L-1 at TTA1. Particulate P daily concentrations varied at the each catchment, with BE having the highest measured PP concentration (BE1: 0.9-2.3 mg g-1; BE2: 1.2-2.5 mg g-1; BE3: 2.1-2.2 mg g-1), followed by TTA (TTA1: 0.16-1.8 mg g-1; TTA2: 0.3-1.2 mg g-1; TTA3: 0.3-1.5 mg g-1) and BB (BB1: 0.5-1.5 mg g-1; BB2: 0.3-0.4 mg g-1). Sequential sediment extractions showed a decrease in residual P, loosely sorbed P, and organic P fractions when comparing a winter period (December 2017) to samples taken in the summer (June 2018) in TTA and BB, while BE saw a decrease in Fe/Mn-bound P and non-reducible P fractions from January to June 2019. In addition, at BB outflow, the dynamics of the PP fraction within fluvial suspended solids revealed that P fraction composition in the sediment significantly changed throughout storm events: during a summerxii storm, before the event, the major detected fraction was composed of organic P and inorganic P afterwards; during a winter storm the inverse was observed. PRes fraction predominated at BB1 and TTA1 and BE2 (1.1, 1.3 and 1.0 mg g-1, respectively); PHCl and POrg were measured the highest at BE1 (0.4 mg g-1) and BE3 (0.3 mg g-1), respectively; PNR was the second most common P pool detected, with BE3 having the highest concentration (0.6 mg g-1); PRedox was the least dominant PP extracted fraction, with BB1 having highest measured PRedox concentration (0.3 mg g-1); PH2O fraction varied greatly with catchment location, with the highest concentrations found at BE2 (0.5 mg g-1), BB1 (0.4 mg g-1), and TTA2 (0.3 mg g-1). Finally, the hydrology and P transport in the rivers monitored catchments were simulated using computer models of which were then used to extrapolate the findings to longer periods. These simulations suggested that BE had the highest yearly in-stream sediment load and that PP is the main contributor to total P found in stream water, largely surpassing total dissolved P.