CONTRIBUTION OF NITROGEN SPECIES AND PHOSPHORUS FRACTIONS TO STREAM WATER-QUALITY IN AGRICULTURAL CATCHMENTS

The contribution from agricultural catchments to stream nitrogen and p hosphorus concentrations was assessed by evaluation of the chemical co mposition of these nutrients in agricultural runoff for both surface a nd subsurface flow pathways. A range of land uses (grazed and ungrazed grassland, cereals, roots) in intensive agricultural systems was stud ied at scales from hillslope plots (0.5 m(2)) to large catchment (>300 km(2)). By fractionating the total nutrient load it was possible to e stablish that most of the phosphorus was transported in the unreactive (particulate and organic) fraction via surface runoff. This was true regardless of the scale of measurement. The form of the nitrogen load varied with land use and grazing intensity. High loads of dissolved in organic nitrogen (with >90% transported as NH4-N) were recorded in sur face runoff from heavily grazed land. In subsurface flow from small (2 km(2)) subcatchments and in larger (>300 km(2)) catchments, organic n itrogen was found to be an important secondary constituent of the tota l nitrogen load, comprising 40% of the total annual load.