Potential Internal Loading of Phosphorus in Constructed Wetlands

Construction of wetlands on agricultural lands, especially those used for dairy operations could result in solubilization of P stored in soils and release to the water column. To study the extent of initial P flux during the start-up period, intact soil-cores from land areas used for dairy operations were obtained and flooded with adjacent creek water. In the first 28-day hydraulic-retention period, P concentration in the water column increased up to 218 times due to P flux from impacted soils. A continuous decrease in P flux to the water column up to third cycle (initial floodwater P concentration 0.2 mg L-1), and constant thereafter (initial floodwater P concentration up to 0.4 mg L-1) suggest that the effect of initial floodwater P upon P flux could be limited. The initial release may be due to high concentration of labile P, however, slow dissolution of relatively stable pools of P could maintain steady flux, well above of that observed from non-impacted soils. The water soluble P along with double acid-extractable magnesium (Mg) could explain up to 76 % of the variability in cumulative P flux to the water column. It is apparent that co-occurrence of active adsorption-desorption phenomena due to independent maintenance of P equilibrium by individual P compounds in soils may regulate the P dynamic of the water column