Phosphorus loss in natural subsurface pathways and capacity of vegetative riparian buffers to reduce subsurface phosphorus export from a corn-soybean rotation field in Indiana

Eutrophication of surface waters remains a worldwide and rising environmental problem despite more the 40 years of research and implementation of mitigation and management strategies. Agricultural nonpoint phosphorus (P) has been recognized as the main cause of impairment of fresh waters in the USA. Questions persist about the potential of natural subsurface pathways to export P from agricultural ecosystems to surface waters in the Midwestern region of the US. The capacity of vegetative riparian buffers to limit P transport in this pathway is also uncertain. A comprehensive understanding of the parameters and mechanisms controlling subsurface P losses from cultivated lands to adjacent water bodies also remains to be achieved. In order to address these questions, forms and concentrations of soil solution P were characterized at 0.3, 0.6, 1, 1.5 and 2.5m depths within of a corn-soybean rotation field, across two riparian buffers and in tile and ditch drainage flows. Field surface and subsurface hydrology and riparian vegetation biomass were also characterized in order to establish a P mass balance. Results suggest that the main mechanisms controlling seasonal patterns of total, total reactive and total unreactive P concentrations in the upper soil profile were freeze-thaw cycles and enhanced mineralization in winter, P fertilizer inputs, large P leaching and elevated biological activity in spring, dry-wet cycles in summer, and P leaching combined with reduced soil P buffering capacity in fall. Mean water table P concentrations remained constant across seasons in the cultivated field indicating that seasonal P losses were mainly controlled by subsurface water discharge. Amount and form of subsurface P export were influenced by enriched top-soils, pathway of infiltration and field pattern of surface runoff. They occurred at concentrations above the minimum level able to accelerate eutrophication of fresh waters. The two riparian buffers were able to significantly reduce subsurface P transport and the proportions of P lost in the reactive most-bioavailable form. Phosphorus removal by the riparian vegetation was larger than potential P loads from the agricultural field. Management of the two vegetated riparian zones could ensure their long-term efficiency to diminish P loads to surface waters