The Effect of Biochar on Marsh Phosphorus Retention: Implications for P Biogeochemical Ecosystem Services from Prescribed Burns

Mid-Atlantic tidal salt marshes provide a number of ecosystem services including the storage of phosphorus (P) within their soils, which reduces nutrient export to estuaries. The storage of P in marsh soils is vegetation-specific, and management efforts targeting the removal of the invasive common reed, Phragmites australis may reduce the P ecosystem service. The use of prescribed burns to remove P. australis introduces biochar into marsh soils, which has been shown to sorb excess P in agricultural and wastewater systems. Biochar additions with burning may thus recoup P storage services lost via P. australis replacement. Organic and inorganic total P content and relative P mobility (determined by sequential extraction) were measured along with a suite of physical and biogeochemical parameters in field collected soil cores from a marsh that had been recently burned and two comparator marshes. Neither burn frequency nor quantities of black carbon, a biochar proxy, were positively associated with P content in any of the P pools indicating that burns are not enhancing P storage in these marshes. Biochar added to experimental plots similarly showed no P storage benefit over control plots. Instead, environmental (e.g., hydrological, vegetation, soil pH, metal content) factors dominated P concentration variations in this work. The lack of an added P storage observed in these data is indicative that biochar properties vary depending on their production conditions, and low temperature (< 400 °C) prescribed burns of P. australis are unlikely to yield biochars with pH and cation exchange properties favorable for the enhanced P sorption.