Assessing Native Phosphorus Availability in Riparian Soils of Northwestern Vermont.

Factors affecting riparian soil phosphorus (P) availability can be important for nonpoint source P management in watersheds with extensive stream bank erosion.  We investigated P availability at multiple riparian sites in northwestern Vermont to characterize the relationship between soil type, P concentrations, and potential bioavailability.  A subset of samples was analyzed by solution nuclear magnetic resonance spectroscopy (³¹PNMR) and phosphatase enzymes to speciate P and evaluate potential bioavailability.  Both total and Modified Morgan extractable P were greater in the more imperfectly drained, finer-textured soils compared to coarser-textured soils.  This trend was confirmed in a set of soil profile samples from three different watersheds (n = 142).   For these samples, the ratio of oxalate extractable P to total P decreased linearly (R² = 0.69) with increasing sand content.  The ³¹PNMR analysis revealed an array of soil organic P compounds, including the four known stereoisomers of inositol hexakisphosphate, which together comprised approximately 50% of the NaOH-EDTA extractable organic P.  For the same soils, about 70% of the total water-extractable P was unreactive, of which approximately 48% was hydrolyzed to orthophosphate by enzyme hydrolysis.  Results suggest a strong relationship between soil properties and native P concentrations and highlight the need for improved agri-environmental P indices in riparian zones that consider organic P.