Wetland Soils Phosphorus Criteria Development V.D. Nair*, M.W. Clark, K.R. Reddy, and S.G. Haile

Soil and Water Science Department, University of Florida, Gainesville, Florida

The Need for Phosphorus Criteria for Wetland Soils

Materials and Methods

In response to USEPA mandate to establish numeric nutrient criteria under the Clean Water Action Plan, methods to evaluate nutrient enrichment in wetland systems has become a major focus for state environmental resource managers. In this effort, soil P indices may prove useful due to their persistence throughout wet and dry periods in wetlands and the capacity of soils to integrate nutrient condition over time. Indices for soil P in uplands have been well established such as PSR, SPSC, EPC (Nair et al., 2004).This study will investigate the suitability of upland soil PSR indices for wetlands soils and will identify and address limitations in soil extraction procedures posed by the variability in soil moisture content that often occurs in wetlands.

Hypothesis/Objectives

Challenges with Protocol Development

Hypotheses: •Upland soil indices (Nair et al., 2004) can be used for wetland soils•Soil P in relation to extractable Fe and Al can be used to predict limit of P retention in wetland soils.

Overall Objective: Evaluate various regulators of P sorption in wetland soils and develop a criterion for P sorption saturation in the soils

Specific objectives:•Identify a routine soil test P or P-related parameter such as the P saturation ratio (PSR) that could be used as an indicator of P release to the water column in wetland soils, irrespective of location or type of wetland•Develop a protocol for determining water soluble P in wetland soils

Mukherjee, A., V.D. Nair, M.W. Clark, and K.R. Reddy. 2009. J. Environ. Qual. 38:878 - 886.Nair V.D., K.M. Portier, D.A. Graetz, and M.L. Walker. 2004. J. Environ. Qual. 33:107-113.

• Soil samples were collected from three ecoregions: Southeastern Temperate Forested Plains and Hills (IX), Southern Coastal Plain (XII), and Eastern Coastal Plain (XIV)

• Sampling sites in several states occurred in different ecoregions: FL (IX, XII), AL (IX), GA (IX), SC (IX, XII, XIV), IN (IX).

• Sampling at uplands, edge of wetland and interior (center) of wetlands• Additional sampling from isolated wetlands in Florida for developing P criteria• All samples analyzed for moisture, organic matter content ( loss on ignition; LOI), water

soluble P (WSP; 1:10 for dry soils; 1:20 for wet soils), Mehlich 1-P, Fe and Al • M1-PSR calculated as Mehlich 1-P/Mehlich 1 (Fe+Al)

WSP is lower when soil is extracted under wet conditions irrespective of location of soil sampling (Fig. 2)

Note: Data in Fig 1 based on all sampling locations (630 samples). Data used in Fig. 2, 3 and 4 based on soils from Okeechobee isolated wetlands (30 samples)

In theory, we should be able to identify a change point for wetland soils at which any further P addition cannot be assimilated by the mineral soil fraction (Fig. 3). However, P retention within the wetland may still be possible through organic matter accretion.

Possible Solutions

Conclusions

• To use threshold PSR at least as a part of numeric nutrient criteria, a protocol for determination of WSP that is independent of the organic matter concentration has to be developed

• Adjustments need to be made for the moisture content of the soil while determining WSP

• Additional wetland soils representative of several wetlands and P concentration levels must be collected and evaluated to establish a protocol for WSP determination

• A soil-related index of nutrients may prove beneficial for sampling logistics when compared to water column sampling due to its persistence through wet and dry periods as well as less temporal variable in wetlands

• A threshold PSR for wetland soils could be used to assess a point above which additional P loads cannot be assimilated by mineral components of the soil. This would provide environmental managers the realization that any additional P removal by the wetland would have to be through biological uptake and organic matter accretion.

• The threshold PSR could also be used by managers to determine when soil amendments are needed in treatment wetland systems to provide additional P adsorption capacity in the soils

• Specialized procedures for use of PSR on wetland soils will likely be necessary due to high variability in soil moisture content.

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Fig 2. Relationship of WSP (Dry) with WSP (Wet) for soil samples collected from uplands, edge and center of Okeechobee isolated wetlands

Fig 1. Relationship of WSP (Dry) with Phosphorus Saturation Ratio (M1-PSR) based on 630 soil samples from uplands, edge and center of wetlands

Fig 3. Relationship of WSP (wet) with M1-PSR based on 30 soil samples from Okeechobee isolated wetlands

When soil is extracted under wet conditions, WSP determination is affected by the moisture content of the soil (Fig. 4)

Fig 4. Relationship of WSP (wet) with soil moisture content

Acknowledgments: USEPA for funding the Southeastern Wetland Biogeochemical Survey. The Florida Department of Agriculture and Community Services, the Florida Department of Environmental Protection and the South Florida Water Management District for funding the Okeechobee Isolated Wetlands characterization study and this investigation.

Map showing locations of soil sample collections

Field sampling scheme adopted during soil sample collections

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An isolated wetland in the Lake Okeechobee Basin