Hydrologic Controls on Phosphorus Speciation and Mobilization in a Subalpine Watershed (East River, Colorado)

The cycling of phosphorus (P) through floodplain environments is critical to ecosystem productivity and has significant implications on both water quality and soil fertility. P export from soils in response to saturation has been well documented, but the relative vulnerability of specific P pools to mobilization remains poorly constrained, as do the mechanisms mediating its release. The prediction of P availability in and export from mountainous floodplain soils is of great importance as global climate change is projected to significantly alter precipitation regimes in alpine systems. This study combined a thorough characterization of P distribution across a hillslope to floodplain transect with a high resolution analysis of P response to saturation, providing insight into both P cycling and flood-induced mobilization in the East River watershed in Gothic, Colorado. P-XANES, 31P NMR, and sequential extractions revealed that P concentrations were greatest in the floodplain and dominated by inorganic P (Pi) bound to primary minerals, while hillslope P was lower and characterized by more bioavailable and organic (Po) pools. Time series analysis of P response to extended water saturation confirmed our hypothesis that P mobilization would occur in two phases, first driven by lysis of microbial cells upon initial rewetting, and later driven by the reductive dissolution of Fe oxides with extended saturation. These findings indicated that (i) the interaction of weathering and hydrology dictate P speciation across the transect; (ii) hillslope P may be concentrated in microbial pools while floodplain P is primarily bound by minerals; (iii) bioavailable P pools may be dominated by Po; and (iv) Po is more vulnerable to mobilization during initial rewetting of dried soils while Pi responds more to changes in redox potential during extended saturation. Results from this study underscore the significance of hydrology in determining P speciation and mobilization, indicating that long-term changes in precipitation may influence P bioavailability and export from alpine watersheds. Understanding the mechanisms by which P is mobilized is therefore of critical importance to better predict soil P response to saturation in a changing climate