Phosphorus sorption capacity and exchange by soils from mitigated and late successional bottomland forest wetlands

A central tenet of wetland mitigation is that replacement wetlands can sequester nutrients and perform other functions at the same level as natural wetlands. This study evaluated phosphorus (P) sorption capacity and P exchange in flooded soil microcosms obtained from eight early successional (ES) mitigated and eight late successional (LS) bottomland forest wetlands in western Kentucky, USA. The LS soils had three times greater capacity to remove and retain soluble inorganic P than ES soils, which was mostly due to higher amounts of amorphous aluminum (Al) oxides (oxalate extractable), organically-bound Al (CuCl₂ extractable), and organic carbon in LS soils. Phosphorus exchange rates between the soil and water column were not significantly different in LS and ES microcosms, but rates in both systems were strongly related to the molar ratio of Mehlich III extractable P to Al + Fe in the soil (r²=0.64). Relationships between P sorption/exchange and organic C, Mehlich III- and oxalate-extractable forms of P, Al, and Fe determined in this study could be useful for (i) identifying suitable mitigation sites that would be P sinks rather than P sources to the water column and (ii) determining replacement ratios that would fairly compensate for P retention capacity losses caused by destruction/alteration of Kentucky bottomland hardwood forest wetlands.