Export of Total, Particulate, and Apatite Phosphorus from Forested and Agricultural Watersheds

Concentrations of apatite phosphorus (P) have been shown to be greater in the soils of agricultural and disturbed watersheds than in soils of forested watersheds. This observation suggests that changes in the relative concentration of apatite P in the sedimentary record of lakes might be a useful marker for land use changes in the watershed where sediments are dominated by allochthonous loading. To determine if P export in streams is indicative of soil P in the watershed, we measured the export of total, particulate, and apatite P in first-order streams draining four forested and four agricultural watersheds in western Pennsylvania. Streams draining agricultural watersheds exported significantly more total suspended solids (TSS) and more total, particulate, soluble, and apatite P, particularly during periods of high discharge, than did streams draining forested watersheds. Total suspended solids exported from forested watersheds contained similar amounts of apatite P (μg apatite P g TSS), as did agricultural watersheds in spite of lower soil concentrations. However, since agricultural watershed exported almost four times the mass of TSS, the net export of apatite P was greater than from forested streams. These results confirm the increased apatite P export from agricultural watersheds and suggest that inferences drawn from the profiles of apatite P in the sedimentary record of lakes may reliably indicate historical disturbances in watersheds caused by agriculture or other erosional events.