Wetland Soil Phosphorus and Phosphorus Sorption Capacity in Wastewater-impacted and Unimpacted Wetlands, NH USA

The long-term impact of treated wastewater effluent on the soil in a 7.3 ha emergent marsh in southwestern New Hampshire, USA, known locally as Beaver Dam Marsh (BDM), was evaluated by comparison to four nearby unaffected wetlands. We hypothesized that 40 years of wastewater input would result in a higher phosphorus saturation ratio (PSR), lower soil phosphorus storage capacity (SPSC), and higher total phosphorus (TP) in the soils of BDM. Samples of the surface soil and the subsoil were collected and analyzed for TP via colorimetric analysis and extractable phosphorus, aluminum and iron via Mehlich-3 extraction paired with Inductively Coupled Plasma (ICP) spectroscopy. The surface soil of BDM showed no significant difference in PSR (p = 0.90) and SPSC (p = 0.79) while the subsoil exhibited a significantly lower PSR (p = 0.029) than the unaffected wetlands. Additionally, there was a significantly lower TP by dry mass in BDM surface soil than the reference wetlands (p = 2.3 × 10 ^− 4 ). The volumetric TP was also lower in BDM; however, this difference was not statistically significant. Phosphorus sequestration within the organic biomass of the invasive Phragmites australis is likely a contributing factor to the lower dry-mass TP. On-going inputs of treated wastewater, low soil pH, as well as saturated conditions, appear to be interfering with formation of inorganic phosphorus complexes with iron and aluminum. Despite some spatial variation within BDM, the wetland soil appears unable to retain any additional phosphorus from the ongoing inputs of treated wastewater.