Phosphorus Contents and Loads at the Outlet of an Agroforestry Catchment in Northwestern Spain

Phosphorus (P) export in watershed runoff can accelerate the eutrophication of fresh waters. Intensification of crops and animal farming in many areas has created regional imbalances in P inputs and P outputs in farm practice. The aim of this study was to assess the temporal changes in concentration and losses of total P (TP), total sedimentary P (TSP), and total dissolved P (TDP) at the outlet of an agroforestry catchment 36.3km2 in size located at northwestern Spain. The study data sets range from January 1999 to December 2005. Phosphorus load of surface water sampled at the catchment outlet was strongly affected by agricultural practices. Most particulate SP was exported during storm flow events. The TSP contents ranged up to 1063g/L, and TDP contents ranged up to 259g/L. However, in most of the study events particulate P was less than 300g/L and during base flow less than 50g/L. Total and particulate P showed a very close relationship, but there was no consistent relationship between water discharge and total P or dissolved P. A few events of intensive precipitation are responsible for the transport of suspended sediments and particles, mobilizing sediments from different source areas. The mean yearly TP export was 0.80kg/ha and annual dissolved TDP export was approximately 0.30kg/ha.