This thesis focuses on the potential mobilization of phosphorus (P), in particular DNA-phosphorus (DNA-P) and phospholipid-phosphorus (PLD-P) from soil in a grassland catchment. DNA-P and PLD-P are among the most labile and biodegradable (and hence interesting) of organic phosphorus compounds. This study was conducted to assess the flow of these compounds along the continuum from soils to surface and sub-surface transport pathways and ultimately into the stream water channel, within the River Eden catchment in Cumbria, England. The aims of the study were to: (i) quantify the magnitude of different P compounds, including DNA-P and PLD-P, in grassland agricultural soils; (ii) determine the forms and concentrations of P compounds in surface and subsurface transport pathways; (iii) quantify the amounts of P fractions in the water column and the bed sediments of streams in the River Eden catchment. The average concentration of total P in soils ranged from 822 to 1792 mg kg¯¹. DNA-P represented between 5% and 17% of total soil P in the study areas; PLD-P accounted for less than 1%. Large concentration ranges of total P (0.012-224 mg L¯¹) across different transport pathways were observed. Most of the organic P in these transport pathways was in particulate form. DNA accounted for 5-25% of total particulate organic P and PLD accounted for 1-7% across the transport pathways. In the water column of streams, DNA-P represented 13 to 23% and PLD-P presented 4 to 7% of the total particulate organic P. DNA-P and PLD-P also accounted for considerable proportions of total P in the streambed sediments, ranging from 2 to 15% and 1 to 2%, respectively. Both DNA-P and PLD-P could have the potential to be important pools of P to support plant nutrition, as well as potential contributors to P transfer and therefore water pollution risks.