Phosphorus Sorption Dynamics in Soils and Coupling with Surface and Pore Water in Riverine Wetlands

Adsorption to soils is one of the dominant mechanisms of P storage in wetlands. We examined P sorption dynamics in soils collected at 12 sample points with diverse hydrology, geomorphic position, mineralogy, and plant communities in two riverine wetlands in northern Minnesota and Wisconsin. Phosphorus sorption parameters from these 12 sample points were correlated with corresponding biogeochemical variables and subsequently extrapolated across 157 sampling points in the two wetlands, based upon a large spatial dataset. We then used a series of single and stepwise regressions to determine the best set of predictive variables for surface water, soil, and plant P pools. Intrasite variation in P sorption dynamics was greater than intersite variation between the two wetlands and rivaled the variation found in the literature for both upland and wetland soils. An essentially constant final P concentration occurred at moderate P additions (≤32 μmol P L⁻¹), indicating extreme soil buffering capacity of porewater P concentrations. Spatial variation in soil P pools across each wetland were predicted very well in stepwise regressions, particularly in the summer (R ² = 0.49–1.00). Variables that were important in explaining this variation included the amount of P sorbed at equilibrium, maximum P sorption capacity, percentage of P sorption sites occupied at equilibrium, organic matter content, bulk density, and oxalate-extractable Fe and Al content. Phosphorus concentrations in surface water were predicted less well by stepwise regression (R ² = 0.04–0.46), suggesting only weak-to-moderate spatial coupling between soils and surface-water P dynamics. Plant P pools were predicted poorly. Our results indicate the importance of geochemical sorption in controlling P dynamics in riverine soils. We suggest that nutrient studies in spatially diverse wetlands must be designed in a manner that adequately captures the rich spatial dynamics of the system.