Colloids can be important for facilitated transfer of phosphorus (P) to groundwater (GW) and contribute to elevated P concentrations later delivered to surface water. To assess the role of colloidal P and other P fractions in delivery processes via below-ground pathways, this study investigated the influence of catchment and flow event characteristics on particulate (>450 nm), medium-sized colloidal (200–450 nm) and fine (<200 nm) P fractions in two agricultural hillslopes (TG, TA). Total and dissolved P fractions and their derivatives were also monitored. Samples in both stream and GW were taken weekly during baseflow conditions and every 2 h during storm conditions. Higher frequency monitoring of streamflow was also conducted to delineate hydrological flowpaths and determine P loads and hysteresis processes. Results indicated that during baseflow fine P was dominant in the streams (80 to 100 % of total P) and in shallow GW in TA (83 to 96 %) whereas in TG shallow GW was dominated by PP (55 to 96 %) possibly due to colloidal Fe-P complexes. Similarly, in TG shallow GW was dominated by PP (79 to 81 %) during high flow events. During a larger flow event (within the period of land fertilization) the quickflow pathway (24 % of total flow) delivered 3.2 g ha −1 of PP which was dominant in the stream (44 to 68 %). A smaller flow event (within the period of prohibited land fertilization) facilitated delivery of P via deeper baseflow pathways (87 % of total flow) as fine reactive P (1.3 g ha −1), also dominant in the stream (73 to 78 %). The research indicated a very limited presence of medium-sized colloidal P but a large presence of fine P that may contribute to elevating P concentrations above environmental thresholds. Further work should constrain the controlling factors for colloidal P presence/absence and also on the extent and speciation of coarser and finer fractions in the hillslope to stream continuum.
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We thank the ACP landowners and farmers for their support, field access and fencing permission, the ACP staff David Ryan, Oisín Coakley and Mark Boland for advices, assistance in field setup, sampling, data downloads, and for providing the fertilisation data, Una Cullen for providing the meteorological data and Simon Leach for creating the GIS maps. The laboratory support of Denis Brenan and Maria Radford for water samples analysis is greatly appreciated. The authors also thank the farm staff of Johnstown Castle and John Murphy for help with the GW sampler, Gérard Gruau and Sen Gu from Géosciences Rennes for advice on the soil water traps. Funding was provided by the Department of Agriculture, Food and the Marine through the Teagasc ACP and by the Teagasc Walsh Fellowship Programme.