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Chemistry of submerged soils
This document discusses soil chemistry in submerged soils. It explains that submergence leads to a lack of oxygen in the soil, causing a shift from aerobic to anaerobic organisms. Anaerobic respiration causes chemical compounds other than oxygen to be reduced in a predictable sequence from the least to most energetically favorable. This results in changes to the chemical forms of elements like nitrogen, iron, and sulfur in the soil. Submergence also typically causes soil pH to become more neutral. Nutrient availability is highest within this neutral pH range common for submerged soils.
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Chemistry of submerged soils
- 1. Soil Chemistry isall about Oxidation and Reduction • Gain of Oxygen • Loss of Hydrogen • Loss of electrons • Loss of Oxygen • Gain of Hydrogen • Gain of electrons OIL RIG
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- 3. μbes don’t havechoices: NO AerobesFacultative anaerobes NO3 - NO NO Fe3+ NO SO4 2- NO CO2
- 4. The Chemistry of SubmergedSoils Nirmal Kumar PhD Scholar Dept of Soil Science IGKV, Raipur Scientist Division of Remote Sensing Applications NBSS&LUP, Nagpur Especially Paddy Soils
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- 6. Submerged Soils: • Waterlogged(Gley) Soils – Saturated with water for a sufficiently long time annually to give the soil the distinctive gley horizons • Marsh Soils – More or less permanently saturated or submerged • Paddy Soils – Wet cultivation of rice • Sub-aquatic Soils
- 7. Waterlogged (Gley) Soils apartially oxidized A horizon high in organic matter mottled zone in which oxidation and reduction alternate a permanently reduced bluish green zone Marsh Soils Plant residues in the surface horizon
- 8. Paddy Soils A. Levellingof the land and construction of levees to impound water B. Puddling (Ploughing and harrowing the water-saturated soil) C. Maintenance of 5-10 cm of standing water during the 4-5 months the crop is on the land D. Draining and drying the fields at harvest E. Reflooding after an interval which varies from a few weeks to as long as 8 months. These operations and oxygen secretion by rice roots lead to the development of certain features peculiar to paddy soils.
- 9. 9 Optimum composition formost cropsPaddy soil
- 10. 10 Can oxygen penetrateinto submerged soil? • There are two ways for O2 to enter submerged soil 1) O2 moves slowly through the water layer and creates a thin surface layer of aerobic soil 2) O2 moves through the porous tissue of aquatic plants (like rice) into the roots and creates an aerobic zone of soil surrounding the roots
- 11. 11 Oxygen movement intosubmerged soil
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- 13. Any Soil FormingProcess??? 1. Reductive eluviation/ Oxidative illuviation Aquorizem Great Group Mn Fe 2. Ferrolysis
- 14. 14 Soil biology aftersubmergence After submergence, soil O2 is depleted. This starts a biological transition: • Aerobic organisms die or become dormant • They are replaced by microorganisms surviving without O2 – facultative anaerobes can live with or without O2 – obligate anaerobes only live where no O2 is present • The predominant anaerobic organisms in submerged soil are bacteria • Some aerobic organisms continue living in the aerobic zones of submerged soil
- 15. 15 Surviving without oxygen •Organisms get energy by respiration • Respiration is the process where energy is created as carbon compounds are broken down • Respiration involves oxidation of carbon compounds and the reduction of: – O2 in aerobic soil – chemical compounds other than O2 in anaerobic soil
- 16. 16 Oxidation and Reductionin an Aerobic Setting • Organic matter in soil gives up 4 electrons (e-) which are received by O2. As a result, O2 is reduced. • Hydrogen ions (H+) react with the reduced O2 to form water (H2O). 4 e- + O2 + 4 H+→ 2 H2O
- 17. 17 Oxidation and Reductionin an Anaerobic Setting • Electrons (e-) from organic matter in soil are accepted by nitrate (NO3 -) instead of O2. • Nitrogen (N) in NO3 - is reduced; the N compound becomes nitrogen gas (N2) • Hydrogen ions (H+) react with oxygen from NO3 - to produce H2O. 10 e- + 2 NO3 - + 12 H+→ 1 N2 + 6 H2O
- 18. 18 A change insoil chemistry • A review of the soil biological transition: – Lack of O2 in submerged soil causes a shift from aerobic to anaerobic organisms – Respiration of anaerobic organisms causes chemical compounds other than O2 to be reduced. • Chemical compounds in soil are reduced in a predictable sequence. – The compound requiring the least energy for reduction is reduced first (i.e. nitrate) – After the first compound is nearly all reduced, the compound requiring the next lower energy for reduction is reduced
- 19. Reaction sequence followingsubmergence Reaction sequence after draining Chemical Reduction Sequence Following Submergence O2 N2 Mn2+ Fe2+ NO3 - MnO2 Fe3+ CO2 CH4 SO4 -2 H2S H2O Slightly Reduced Moderately Reduced Strongly Reduced Oxidized
- 20. 20 Form of compoundsin aerated versus submerged soil Element Aerated soil (Oxidized) Submerged soil (Reduced) Oxygen (O) Oxygen gas (O2) Water (H2O) Nitrogen (N) Nitrate ion (NO3 -) Nitrogen gas (N2) Manganese (Mn) Manganese IV ion (Mn4+) Manganese II ion (Mn2+) Iron (Fe) Iron III ion (Fe3+) Iron II ion (Fe2+) Sulfur (S) Sulfate ion (SO4 2-) Hydrogen sulfide (H2S) Carbon (C) Carbon dioxide (CO2) Methane (CH4)
- 21. 21 Submergence affects soilpH • Soil pH is a measurement of hydrogen ions in the soil and it indicates whether soil is acidic or basic. • Submergence of soil typically causes a shift towards a more neutral soil pH. This is a result of the change in chemical compounds when soil is reduced
- 22. 22 Effect on soilpH when acid, neutral, and basic soils are submerged
- 23. 23 Availability of nutrientsat different pH levels The typical pH range for many submerged soils is 6.5-7. The 6 important crop nutrients in this chart are collectively more available at 6.5-7 compared to more acidic or basic pH values. The width of each horizontal bar represents the plant availability of the identified nutrient within the pH range of 5-9.