1. HP-HG exhibits higher P concentrations in all fractions NaHCO 3 pool exhibits slightly different trends, with LP-HG having significantly higher P concentrations than LP-LG Objective Determine effect of land application of spent gypsum from ditch filters on soil P forms. Changes in Soil Phosphorus Forms Due to Application of Phosphorus Saturated Gypsum Karen L. Grubb 1, Joshua M. McGrath 1, Chad J. Penn 2, Ray B. Bryant 3 Introduction Accelerated eutrophication of the Chesapeake Bay has become a large concern over the past few decades. Direct removal of phosphorus from surface water using by-product materials that have the capacity to sorb phosphorus (known as Phosphorus Sorbing Materials- PSMs) have potential as BMPs in agricultural drainage ditches. Gypsum filters designed to remove phosphorus (P) from agricultural drainage are currently being evaluated in Maryland (USA). A laboratory incubation study was performed to determine how land-application of the P saturated gypsum would affect soil P forms and availability and to evaluate the effectiveness of P saturated gypsum as a fertilizer material. Spent ditch filter gypsum was simulated by saturating FGD gypsum to two levels on a mass basis of P, and applied to two soil types, a silt loam and a sandy loam at both a high and low rate. The treated soils were incubated at 25° C and samples were collected 0, 1, 7, 28, 63, 91, 119, and 183 days after amendment. Changes in chemically defined P forms in the soil are presented. Department of Environmental Science and Technology Lab for Ag and Environmental Studies 1- University of Maryland Department of Env. Science and Technology 2- Oklahoma State University Department of Plant and Soil Sciences 3- USDA-ARS Pasture Systems and Watershed Research Laboratory P Pool Extracted Labile P –Easily extractable, most available to plants and algae, P in solution Plant available P – loosely held Fe/Al and organic P Fe- and Al-Bound P and associated with humic compounds Strongly held P – phytate P, mineral P, held in internal structures Methods: Soil Phosphorus Fractionation Results NaOH included for comparisons, day involved in 3-way interaction soil*gyprate*psat P shift from more labile P (H 2 O) to more insoluble forms over time Conclusions to Date HP-HG tends to have more extractable P in all pools in sequential extraction At agronomic acceptable Ca rates, P saturation levels did not significantly alter soil test P concentrations P trapped by PSMs can be returned to the land in a potentially less mobile form but may also be plant available LP = 25% saturation HP = 75% saturation LG = 5.6 Mg gypsum ha -1 HG = 22.4 Mg gypsum ha -1 P and Ca application rate 123235LP-LG 4928420HP-HG 4928140LP-HG 1232105HP-LG (kg-Ca ha -1 )(kg-P ha -1 ) Cannot show main effect of soil for H 2 O and NaOH due to interactions Due to higher buffer capacity, silt loam has higher extractable P in all fractions except water Experimental Design Treatment Factors Gypsum- 2 Levels (mass basis of P) Low Rate of Gyp (5.6 Mg gypsum ha -1 ) High Rate of Gyp (22.4 Mg gypsum ha -1 ) P Saturation- 2 Levels (based upon P isotherms) 25% saturation (6.25 mg P g gyp -1 ) 75% saturation (18.75 mg P g gyp -1 ) Soil Type- 2 Levels Sandy Loam- Galestown Sandy Loam Silt Loam- Mattapex Silt Loam Time- 3 Levels Day 1 Day 7 Day 119 2 x 2 x 2 x 3 factorial structure The factorial combinations applied to experimental units as randomized complete block design (RCBD) in 4 blocks (incubators) Treatment Combinations