Organic Soil Oxidation
(Subsidence) in the EAA
Alan L. Wright
University of Florida – IFAS
Everglades Research & Education Center
Belle Glade, FL
HISTOSOLS
• Histosols (organic soils)
is the dominant soil
order within the EAA
• Develop under flooded
conditions
• Organic matter
accretion rate exceeds
decomposition rate
TORRY
LAUDERHILL
TERRA CEIA
PAHOKEE
DANIA
Soil Subsidence in the EAA
• Rate estimated using transect lines
•depth to bedrock
1924 to 1967 - soil oxidation rate = 1.12 in/yr
1967 to 2009 - rate decreased = 0.55 in/yr
• Reasons for decline
•maintenance of higher water tables, BMPs
•increasing recalcitrance of remaining soil organic
matter
•increased mineral content (CaCO3, sand, clay)
Soil Subsidence
• Land use changes
– soil accretion (wetlands) vs. oxidation (drained soil)
– frequency and duration of flooding
• Decomposition of soil organic matter
– microbial decomposition is the major mechanism
– muck fires
– wind erosion
• Accumulation of soil organic matter
– deposition of plant or crop residues
– tillage and residue management
• reduced tillage
• sugarcane harvesting methods, i.e. burning vs. green cane
• EAA subsidence is related to water-table depth
1968
1999
1989
2008
Soils of the EAA
• Naturally limited in most plant nutrients
– supplemental fertilization necessary
• Subsidence changes land use options
–
–
–
–
–
becoming too shallow for some agricultural practices
need for new crop management strategies
calcification of surface soil increases pH
enhanced P retention in forms unavailable to crops
higher fertilization rates needed to offset P fixed in soil
• Muck soils historically had pH of 4.5 to 5.5
• Current typical range for shallow muck soils is 6.5 to 7.5
• Due to tillage operations for crop production, particles of
calcium carbonate are transported from the subsurface
into the root zone
• Additionally, carbonates dissolved in water can move up
in the soil profile due to capillary action, and are often
deposited at or near the soil surface after water is
evaporated
pH changes for muck soils
8
7
a
a
a
0-15 cm
15-30 cm
30-45 cm
a
Soil pH
a
6
b
5
4
Sugarcane
Uncultivated
Strategies Used in Response to Subsiding Soils
• Change fertilization methods
– rates
– modes of application
• banding, split, timing
– sources
• slow-release fertilizers
• Decrease soil pH
– sulfur amendment and field flooding
• Tillage
– carbon sequestration and residue management
– reduce soil disturbance
• New crops and rotations
– altered hydrologic conditions
– rice, sugarcane
• Water management
Subsidence and Greenhouse Gases
•Greenhouse gas emissions related to potential land use changes
- Global Warming Potentials (100 yr)
•Carbon dioxide = 1
•Methane = 21
•Nitrous oxide = 310
•Mass balance of C inputs & outputs
•Wetlands
-CO2, methane, and nitrous oxide emissions
-Soil organic matter accretion
•EAA drained soils – agricultural use
- Minimal information available on greenhouse gas emissions
- CO2 is primary endproduct
- Nitrous oxide emissions ?
deep soils
N fertilizers
- Methane emissions ?
Conclusions
•Soil subsidence rate is declining
•Fertilizer management is increasingly critical
•Options available for minimizing subsidence
• hydrologic conditions
• flood-tolerant crops
• tillage
• residue management ?
•Carbon sequestration and greenhouse gases
• impacts of land use changes in EAA are largely unknown
Questions