EVALUATING SOIL PHOSPHORUS RETENTION

AND RELEASE POTENTIAL ON LANDS RECEIVING

PAYMENT FOR ENVIRONMENTAL SERVICES.

M. Clark1, P. Bohlen2, S. Shukla3, S. Lynch4,

L. Shabman5, H. Swain6 and E. Boughton7

1 Soil and Water Science Department, University of Florida, Gainesville, FL USA

2 Department of Biology, University of Central Florida, Orlando, FL USA 3 Department of Agricultural and Biological Engineering, University of Florida, Immokalee, FL USA

4 Agricultural - Markets Unit, World Wildlife Fund, Washington, DC USA 5 Resources for the Future, Washington, DC USA

6 Archbold Biological Station, Venus FL USA 7 MacArthur Agro-Ecology Research Center, Lake Placid, FL USA

Concern Over Phosphorus

• Lake Okeechobee was listed in 1998 as being use impaired by nutrients (particularly, phosphorus), dissolved oxygen, ammonia, chlorides, coliforms, and iron. (FDEP)

• Protective numeric criteria of 40 ppb total phosphorus was established for the pelagic zone of the lake

• Corresponding TMDL set at 140 metric tons.

• Most management actions in the watershed need to consider potential effects on phosphorus loads to the lake.

Soil Objective within FRESP

• Original: – Develop a tool to evaluate the potential increase in

phosphorus release from soils in response to implementation of water management alternative.

– Identify high “risk” sites.

• Subsequent: – Identify “opportunity” sites.

– Identify phosphorus “hot” spots within water management area.

– Guide strategies to mitigate P release if “risk” is high.

Phosphorus Dynamics 101

• The majority of phosphorus in the landscape

is stored in soils.

• Phosphorus can bind with other elements in

the soil mainly Iron (Fe+3) and Aluminum (Al+3)

and become permanently or temporarily

unavailable to plants or leaching.

• Accumulated phosphorus (legacy

phosphorus) can later be released depending

on porewater and floodwater phosphorus

concentration, solution characteristics (pH,

redox), availability of Iron and Aluminum to

bind phosphorus and biological uptake and

burial in organic sediment..

• Numerous soil analysis and metrics can be

used to estimate phosphorus availability.

Fe+3

Al3+

Al+3 Diffusion flux

Convective flux

PO4-3

PO4-3

PO4-3

Phosphorus Parameters Evaluated

SRP Soluble Reactive Phosphorus

WSP Water Soluble Phosphorus

TP Total Phosphorus

M1-TP Mehlich-1 extractable phosphorus

M3-TP Mehlich-3 extractable phosphorus

HCl-Tpi Total inorganic phosphorus

HCl-TP HCl extractable phosphorus

M1-PSR Phosphorus Sorption Ratio based on Mehlich-1 extractible phosphorus

M3-PSR Phosphorus Sorption Ratio based on Mehlich-3 extractible phosphorus

HCl-PSR Phosphorus Sorption Ratio based on HCl extractible phosphorus

HCl2-PSR Phosphorus Sorption Ratio based on double HCl extractible phosphorus

EPC Equilibrium Phosphorus Concentration

M1-SPSC Soil Phosphorus Sorption Capacity based on Mehlich-1 extractible

phosphorus

M3-SPSC Soil Phosphorus Sorption Capacity based on Mehlich-3 extractible

phosphorus

Field Soil Sampling

Williamson RanchWilliamson Ranch Syfrette RanchSyfrette Ranch

Center Edge

Upland

Soils Analysis • Samples were analyzed at the State Soil Testing

Laboratory at the University of Florida (mehlich-1 TP,

SPSC), or UF Wetland Biogeochemistry Laboratory

(EPC).

0.0

pp

m

0.1

pp

m

0.5

pp

m

1.0

pp

m

10

pp

m

Determination of

Equilibrium Phosphorus

Concentration

Soil Phosphorus Sorption Capacity

• When SPSC is positive,

soil is a P sink

• When SPSC is negative,

soil is a P source

• SPSC is additive; it is

possible to calculate

SPSC to any specified

depth or area

• Able to predict whether a

given volume or weight of

soil is a P source or sink

y = -11.5x - 4.8

R 2

= 0.8

-1000

-800

-600

-400

-200

0

200

400

600

0 10 20 30 40 50 60

Negative SPSC, n=159

Positive SPSC, n=602

WSP (mg kg -1

)

SP

SC

(m

g k

g

-1 )

A & E horizon

Bh horizon

Sink

Sink

Source

Source

0.623

0.112

0.111

.015

.058

.022 .028

.031

.033

.051

.017

.010

.011

.063

.017

Site Average = 0.080 + 0.149

Center Average = 0.148 + 0.224

Edge Average = 0.038 + 0.036

Upland Average = 0.054 + 0.036

EPC Results

.022

>10

>10

>10

>10

>10 >10

>10

>10 5.67

8.23

2.61 >10

>10 >10

>10

>10

Site Average = 8.62 + 2.90

Center Average = 7.51 + 4.32

Edge Average = 8.16 + 3.19

Upland Average = >10

EPC Results

FRESP Site Average EPC

Ranch n EPC

1 15 0.080 + 0.133

2 29 0.460 + 0.802

3 17 0.607 + 0.543

4 57 1.34 + 2.37

5 21 4.56 + 9.40

6 86 5.82 + 8.67

7 56 6.66 + 11.78

8 16 21.68 + 15.84

Application of EPC EPC

0

2

4

6

8

10

12

14

16

18

20

22

0 50 100 150 200 250 300 350

sample ID

EP

C,

pp

m

2 3 4

5 6

8

1

Maximum value 61.2

7

What are thresholds of concern ?

How should they be determine?

Comparison of Soil Parameter with

Edge of Field Water Quality

“Near Field” and “Far Field” Predictors of

Water Column Phosphorus Concentration

“Near Field”

Effectively measured by

soil extractions

EPC, WSP etc.

Edge of

Field

Biological uptake,

Dilution, evaporative

concentration.

additions and deletions

during conveyance “Far Field”

Diffusion flux

Convective flux

Plant decomposition,

manure, etc.

Correlation between Soil Parameters and Edge

of Field 2008-2010 Phosphorus Concentration

all zones Center zone Edge zone Upland zone

Parameter Linear Ln

Transform Linear Ln

Transform Linear Ln

Transform Linear Ln

Transform

SRP 0.119 0.163 0.041 0.024 0.485 0.627 0.418 0.503

WSP 0.011 0.034 0.001 0.002 0.398 0.491 0.282 0.352

TP 0.022 0.01 0.054 0.054 0.023 0.085 0.010 0.030

M1-TP 0.252 0.388 0.317 0.396 0.410 0.505 0.810 0.916

M3-TP 0.139 0.248 0.179 0.231 0.293 0.387 0.576 0.771

HCl-Tpi 0.04 0.127 0.000 0.028 0.028 0.109 0.003 0.323

HCl-TP 0.003 0.044 0.011 0.075 0.046 0.139 0.024 0.071 M1-PSR 0.545 0.594 0.625 0.611 0.600 0.616 0.291 0.335

M3-PSR 0.698 0.673 0.696 0.591 0.673 0.640 0.509 0.497

HCl-PSR 0.394 0.462 0.599 0.612 0.576 0.595 0.223 0.268

HCl2-PSR 0.356 0.437 0.632 0.686 0.455 0.524 0.109 0.178

EPC 0.108 0.165 0.002 0.072 0.316 0.409 0.048 0.135

M1-SPSC 0.567 0.565 0.364 0.304 0.619 0.528 0.449 0.442

M3-SPSC 0.545 0.474 0.383 0.322 0.565 0.440 0.354 0.263

Summary of Soil Metric Selection

• Equilibrium Phosphorus Concentration (EPC) – good predictor of soil<>porewater exchange of phosphorus

– not a good predictor at catchment scale

– costly to measure

• Upland Mehlich-1 Total Phosphorus – very good predictor of catchment scale phosphorus discharge

concentration

– may have limited application for spatial “hot spot” identification

– does not provide estimate of phosphorus storage or release capacity

– commonly collected to determine soil phosphorus fertility

• Mehlich-1 derived Soil Phosphorus Sorption Capacity – good predictor of catchment scale phosphorus discharge

– can identify spatial “hot spots”

– can provide an estimate of additional mass of phosphorus that can be

stored or released from area

Conceptual Guidance for use of

Soil Phosphorus Metrics

• Scales are derived from data collected

during FRESP, but should be

calibrated for area of application.

• Left side scale used to evaluate

individual sampling location “hot spot”.

• Right side scale used to evaluate

overall site condition and need for

additional intervention.

• Mitigation threshold derived from

correlation between M1-TP and edge

of field water column concentration of

0.350 mg L-1.

dis

ch

arg

e p

ho

sph

oru

s

co

nce

ntr

ation

mg

/L

Questions?