Session 6.5 soil phosphorus storage capacity

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Session 6.5 soil phosphorus storage capacity

  1. 1. Soil Phosphorus Storage Capacity: An Innovative Technology for Water Quality Protection in Agroforestry Systems Vimala Nair Soil and Water Science Department, Univ. of Florida Co-authors: PKR Nair, R Mosquera-Losada, G-A Michel, D Howlett, and P Nkedi-Kizza February 12, 2014 1
  2. 2. 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) 2
  3. 3. Phosphorus Saturation Ratio (PSR) 25 PSR = Ex-P/ [ExFe + ExAl] Ex = Oxalate, Mehlich 1 or Mehlich 3 WSP, mg/kg 20 Threshold PSR “change point” 15 10 5 0 0.1 0.2 0.3 PSR 0.4 0.5 The “change point” is determined statistically 3
  4. 4. Threshold PSR Water Soluble P (mg kg -1 ) 25 Surface Horizon 20 • Ex-P/ [ExFe + ExAl] (Ex = Extractable) • Change point = 0.10 (Confidence interval = 0.05 – 0.10) Subsurface Horizon 15 Threshold PSR 10 5 0 0 0.125 0.25 0.375 0.5 PSR OX Nair, V.D., K.M. Portier, D.A. Graetz, and M.L. Walker. 2004. J. Environ. Qual. 33:107-113 4
  5. 5. The Approach – SPSC SPSC = (0.1 – Soil PSR ) * – Oxalate Fe 56 Oxalate Al * 31 27 (mg P kg-1) Sink when soil PSR < 0.1 (positive SPSC) Source when soil PSR > 0.1 (negative 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 5
  6. 6. SPSC and Water Soluble P (WSP) 600 SPSC (mg kg-1) 400 200 Positive SPSC n = 604 Negative SPSC Sink 0 -200 0 20 40 -400 -600 -800 -1000 Source Water soluble P (mg kg-1) 60 y = -12.2x + 2.54 R² = 0.87 n = 147 • 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 Chrysostome, M, V.D. Nair, W.G. Harris, and R.D. Rhue. 2007. Soil Sci. Soc. Am. J. 71:1564–1569. 6
  7. 7. Soils under Coffee in Uganda Water Soluble P, mg kg-1 20 15 Threshold PSR 10 5 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 -5 M3-PSR SPSC (mg kg-1) 200 0 0 5 10 -200 -400 -600 7 Sink 15 20 • 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 Negative SPSC Source Positive SPSC y = -26x - 14 R² = 0.76 Water Soluble P (mg kg-1) Nkedi-Kizza, P. and V.D. Nair (unpublished data)
  8. 8. Silvopasture on Ultisols in Florida Treeless Pasture • Location: Suwannee County, Florida, USA • Soil: Ultisols • SPSC calculated to 1 m depth 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. 8
  9. 9. 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 9
  10. 10. 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 10
  11. 11. Silvopasture on Spodosols in Florida Soil P storage Capacity (SPSC), kg ha -1 -300 -200 -100 0 100 200 300 400 0 10 20 Pasture with trees Depth, cm 30 40 Pasture with trees Pasture without trees 50 60 Additional SPSC under pasture with trees 70 80 90 Pasture without trees 100 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. 11
  12. 12. Summary & Conclusions • A threshold PSR identifies the soil PSR at which the waterextractable 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 12
  13. 13. Thank You! 13

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