Effects of hydrology and field management on phosphorus transport in surface runoff

Phosphorus (P) losses from agricultural landscapes arise from the interaction of hydrologic, edaphic, and management factors, complicated by their spatial and temporal variability. We monitored sites along two agricultural hillslopes to assess the effects of field management and hydrology on P transfers in surface runoffat different landscape positions. Surface runoff varied by landscape position, with saturation excess runoff accounting for 19 times the volume of infiltration excess runoff at the north footslope position, but infiltration excess runoff dominated at upslope landscape positions. Runoffdiffered significantly between south and north footslopes, coinciding with the extent of upslope soil underlain by a fragipan. Phosphorus in runoffwas predominantly in dissolved reactive form (70%), with the highest concentrations associated with upper landscape positions closest to fields serving as major sources of P. However, the largest loads of P were from the north footslope, where runoffvolumes were 24 times larger than from all other sites combined. Loads of P from the north footslope appeared to be primarily chronic transfers of desorbed soil P. Although runofffrom the footslope likely contributed directly to stream flow and hence to stream water quality, 27% of runoff P from the upslope sites did not connect directly with stream flow. Findings of this study will be useful for evaluating the critical source area concept and metrics such as the P-Index.