•HP-HG exhibits higher P concentrations in all fractions

•NaHCO3 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 Changes in Soil Phosphorus Forms Due to Application of Phosphorus Saturated GypsumSaturated GypsumKaren L. Grubb1, Joshua M. McGrath1, Chad J. Penn2, Ray B. Bryant3

IntroductionAccelerated 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 TechnologyLab for Ag and Environmental StudiesLab for Ag and Environmental Studies

1- University of Maryland Department of Env. Science and Technology2- Oklahoma State University Department of Plant and Soil Sciences3- USDA-ARS Pasture Systems and Watershed Research Laboratory

0.5g of Sample (Soil + Gypsum/P)

diH2O

1.0 M HCl

0.1 M NaOH

0.5 M NaHCO3

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

Effect of Day on Chemically Defined P Fractions

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H2O NaHCO3 NaOH HCl Cumulative P

P (

mg

kg-1

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Day 1 Day 7 Day 119

•NaOH included for comparisons, day involved in 3-way interaction soil*gyprate*psat

•P shift from more labile P (H2O) 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

Combined Effect of Gypsum and P Rate on Chemically Defined P Fractions Averaged across Soil and Date

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H2O NaHCO3 NaOH HCl Cumulative P

P (

mg

kg

-1)

LP-LG HP-LG LP-HG HP-HG

LP = 25% saturationHP = 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)

Effect of Soil Type on Chemically Defined P Fractions in Soil Average over All Dates

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H2O NaHCO3 NaOH HCl Cumulative P

P (

mg

kg

-1)

Sandy Loam Silt Loam

•Cannot show main effect of soil for H2O and NaOH due to interactions

•Due to higher buffer capacity, silt loam has higher extractable P in all fractions except water

Experimental DesignTreatment 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