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Soil Phosphorus Storage Capacity: An Innovative Technology for Water Quality Protection in Agroforestry

Systems

February 12, 2014

Vimala NairSoil and Water Science Department, Univ. of Florida

Co-authors: PKR Nair, R Mosquera-Losada, G-A Michel,

D Howlett, and P Nkedi-Kizza

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Outline…

• Phosphorus Saturation Ratio (PSR)•PSR and the Soil P Storage Capacity (SPSC)

– Determination of SPSC• Application of the PSR/SPSC tool for water quality

protection in tree-based agricultural systems– Silvopastures in Florida (Ultisols/Spodosols)– Coffee Plantations in Uganda (Oxisols)– Dehesa in Spain (Alfisols)– Simulated Silvopastures in Spain (Inceptisols)

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Phosphorus Saturation Ratio (PSR)

20

10

0 0.1 0.2 0.40.3 0.5

25

5

15

PSR

WS

P, m

g/kg

PSR = Ex-P/ [ExFe + ExAl]Ex = Oxalate, Mehlich 1 or

Mehlich 3 Threshold PSR“change point”

The “change point” is determined statistically

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Threshold PSR

• Ex-P/ [ExFe + ExAl] (Ex = Extractable)• Change point = 0.10 (Confidence interval = 0.05 – 0.10)

0

5

10

15

20

25

0 0.125 0.25 0.375 0.5PSR

OX

Wa

ter

So

lub

le P

(m

g k

g-1) Surface Horizon

Subsurface Horizon

Nair, V.D., K.M. Portier, D.A. Graetz, and M.L. Walker. 2004. J. Environ. Qual. 33:107-113

Threshold PSR

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The Approach – SPSC

• SPSC can also be expressed in mmoles P kg-1, or kg P ha-1

• SPSC is additive; SPSC for horizons within a sandy soil can be added providing a single value for a designated depth

• Used to predict the lifespan of a P application site

(mg P kg-1)

Sink when soil PSR < 0.1 (positive SPSC)Source when soil PSR > 0.1 (negative SPSC)

31*2756 úû

ùêë

é + AlOxalateFeOxalateSPSC = (0.1 – Soil PSR) * 31*

2756 úû

ùêë

é + AlOxalateFeOxalateSPSC – Soil PSR) *

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SPSC and Water Soluble P (WSP)

• Location: Suwannee River Basin, Florida, USA

• Soil: Entisols & Ultisols• When SPSC is positive, soil

is a P sink• When SPSC is negative,

soil is a P source

0 10 20 30 40 50 60

-1000

-800

-600

-400

-200

0

200

400

600

f(x) = − 12.2320249998226 x + 2.54064096528334R² = 0.87158886739138

Positive SPSCNegative SPSC

Water soluble P (mg kg-1)

SP

SC

(m

g k

g-1

)

n = 604

Sink

Source

Chrysostome, M, V.D. Nair, W.G. Harris, and R.D. Rhue. 2007. Soil Sci. Soc. Am. J. 71:1564–1569.

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Soils under Coffee in Uganda• Location: Uganda• Soils: Oxisols• Coffea canephora and

Coffea arabica plantations in various parts of Uganda

• Threshold PSR = 0.1• When SPSC is positive, soil

is a P sink• When SPSC is negative,

soil is a P source

y = -26x - 14R² = 0.76

-600

-400

-200

0

200

0 5 10 15 20

SPSC

(mg

kg-1

)

Water Soluble P (mg kg-1)

Negative SPSC

Positive SPSC

-5

0

5

10

15

20

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Wat

er S

olub

le P

, mg

kg-1

M3-PSR

Threshold PSR

Sink

SourceNkedi-Kizza, P. and V.D. Nair (unpublished data)

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Silvopasture on Ultisols in Florida• Location: Suwannee

County, Florida, USA• Soil: Ultisols• SPSC calculated to 1 m

depth

Treeless Pasture

Silvopasture

Treatment SPSC(kg P ha-1)

Silvopasture 342

Treeless pasture -60*

Michel, G.-A., V.D. Nair, P.K.R. Nair. 2007. Plant Soil. 297:267-276.

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Simulated Silvopastures in Spain• Location: Castro de Rey

Farm, Galicia, Spain.• Soil: Inceptisols• Mean SPSC in different soil

depths up to 100 cm between pooled Betula alba and Pinus radiata silvopasture treatments versus pasture

Sand/Silt/Clay

Howlett, 2007

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Dehesa System, Extremadura, Spain

• Location: St. Esteban Farm, Extremadura, Spain

• Soils: Alfisols• SPSC in different soil

depths up to 100 cm at 2, 5, and 15 meters from individual Quercus suber trees

Sand/Silt/Clay

Howlett, 2007

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Silvopasture on Spodosols in Florida

Pasture without trees

Pasture with trees

0

10

20

30

40

50

60

70

80

90

100

-300 -200 -100 0 100 200 300 400

Dept

h, cm

Soil P storage Capacity (SPSC), kg ha-1

Pasture with treesPasture without trees

Additional SPSC underpasture with trees

Soil P storage capacity (SPSC) of soil profiles to a meter depth in a pasture with trees vs. a bahiagrass pasture without trees. Nair, V.D., P.K.R. Nair, R. S. Kalmbacher, and I.V. Ezenwa. 2007. Ecological Engineering 29:192-199.

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Summary & Conclusions• A threshold PSR identifies the soil PSR at which the water-

extractable P begins to increase • SPSC is calculated based on a threshold PSR value• When SPSC is positive (below the threshold PSR), soil is a P

sink; when SPSC is negative (above the threshold PSR), soil is a P source

• The PSR/SPSC approach can be used to predict P storage and loss from agriculture and agroforestry systems

• In the presence of trees, the P storage within a soil profile is greater than that without any trees under comparable ecological settings

“Trees for Life“: Trees remove excess P from soils

Tree-based agricultural systems provide a greater environmental service in regard to water quality protection

compared to treeless ones

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Thank You!