Organic Soil Oxidation Organic Soil Oxidation (Subsidence) in the EAA(Subsidence) in the EAA

Alan L. Wright

University of Florida – IFASEverglades 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

TERRA CEIA

PAHOKEE

LAUDERHILL

DANIA

• 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 in the EAA

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

1989

1999 2008

1968

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

Soil

pH

4

5

6

7

8

a a a

aa

b

Sugarcane Uncultivated

0-15 cm15-30 cm30-45 cm

pH changes for muck soils

Strategies Used in Response to Subsiding Soils

•• Change fertilization methodsChange fertilization methods–– ratesrates–– modes of applicationmodes of application

•• banding, split, timingbanding, split, timing–– sources sources

•• slowslow--release fertilizersrelease fertilizers

•• Decrease soil pHDecrease soil pH–– sulfur amendment and field floodingsulfur amendment and field flooding

•• Tillage Tillage –– carbon sequestration and residue managementcarbon sequestration and residue management–– reduce soil disturbancereduce soil disturbance

•• New crops and rotationsNew crops and rotations–– altered hydrologic conditionsaltered hydrologic conditions–– rice, sugarcanerice, sugarcane

•• Water managementWater management

•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 soilsN fertilizers

- Methane emissions ?

Subsidence and Greenhouse Gases

•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

Conclusions

Questions