Subsurface phosphorus fluxes in a well‐drained forest soil are small and dominated by particulates

Understanding ecosystem P cycling and the various pathways by which P is transported is crucial for sustainable nutrient management. Colloid‐facilitated transport of P is a poorly understood subsurface P flux pathway that may be particularly important in ecosystems with coarse‐textured soils and minimal surface runoff. This study examines subsurface transport of dissolved (as defined as P <3 kDa) and particulate P in well‐drained forest soils of two coniferous (pine) sites and one deciduous (oak) site. Total, dissolved and particulate (>0.45 μm and >3 kDa) groundwater P fluxes were quantified, as well as surface total P fluxes via litterfall and litter leachate. Our central hypothesis is that particulate and/or colloid‐associated subsurface P flux exceeds that of dissolved P flux as a result of dissolved P sorption to mobile and stationary Fe‐ and Al‐rich solid phases present in the soil. Results show that dissolved P flux (<3 kDa) was below detection in groundwater samples from all study sites, with the P within groundwater being dominated by particulates >0.45 μm. Overall, P fluxes from subsurface P pools (soils, groundwater) at all sites were one to three orders of magnitude lower than surface P inputs from litterfall and litter leachate, suggesting a highly efficient system for P conservation with minimal P losses below the root zone.