Phosphate oxygen isotope ratios in vegetated riparian buffer strip soils

The oxygen isotopic composition of phosphate (delta O-18(P)) in soils and surface water bodies has been used to trace terrestrial P inputs into aquatic ecosystems. However, enhanced biological activity in vegetated riparian buffer strips (VBSs) may lead to an alteration of delta O-18(P) values. The objective of this study was to assess whether enhanced biological cycling of P in VBS soils can be identified using delta O-18(P) values. For this purpose, we sampled temperate grassland soils at various depths along a VBS to grassland transect. Here, we combined sequential P soil extracts with an analysis of delta O-18(P) values. Soil P pool concentrations tended to decrease significantly along the transect from the VBS to the grassland soils; the strength of this relationship varied with P extract, sample depth, and inorganic or organic bonding form. For the delta O-18(P) values of the 1 M HCl-extractable P we observed a significant negative trend along the VBS to grassland transect, indicating a tendency for accelerated rates of biological cycling of P within the VBS soil profile compared with the upslope soils. We conclude that oxygen isotope-based assessments of P source contributions to freshwater bodies should consider the enhanced biological turnover of P in VBS soils.