Lecture 16: Soil Acidity; Introduction to Soil Ecology

Lecture 16: Soil Acidity; Introduction to Soil Ecology

Aluminum and Soil Acidity

Aluminum Toxicity in Acid Soils Tolerant Sensitive Tolerant Sensitive Plants often are sensitive to the presence of dissolved Al 3+ in soil water Al availability is much higher in acidic soils ph 4.4 ph 5.7

Role of Aluminum in Soil Acidity Aluminum is a major component of soil minerals Unlike other ions, when H + exchanges into a clay or adsorbs on its edge, it attacks the mineral structure This releases Al 3+ either to exchange sites or into solution Al 3+ then undergoes hydrolysis, producing H + Al 3+ + H 2 O AlOH 2+ + H + AlOH 2+ + H 2 O Al(OH) 2+ + H + Al(OH) 2+ + H 2 O Al(OH) 3,gibbsite + H + Three H + are produced by every one Al 3+! Hint: This is an important concept!!! Aluminum has a key role in soil acidity!

Aluminum Hydrolysis Products Interfere with Cation Exchange AlOH 2+ and Al(OH) 2+ are cations and can exchange These species strongly bind to negative sites on clays and organic matter These species also polymerize; the polymers bind to negatively charges sites on colloids However, the polymers are not exchangeable They instead block much of a mineral s potential CEC If the ph increases enough, these species precipitate as gibbsite [Al(OH) 3 ], unmasking cation exchange sites This is one reason why CEC often increases with increasing ph

Example Polymers Al Hydroxy-Polymers Block Interlayer Sites in Clay Minerals Al-polymer Al-polymer Al-polymer Al-polymer Figures from: Velde and Meunier (2008) The Origin of Clay Minerals in Soils and Weathered Rocks

Pools of Soil Acidity

Pools of Soil Acidity Total Acidity = Active + Exchangeable + Residual (+ Potential) Active Acidity: H + present in soil solution Exchangeable (Salt-Replaceable) Acidity: Al 3+ and H + that are easily and rapidly exchangeable by other cations Residual Acidity: Al 3+ and H + bound in non-exchangeable forms to clays and OM Can be released slowly in response to ph changes Potential Acidity: Rare; caused by oxidation of sulfide In general: Exchangeable 100-5000x Active Residual 1000-100,000x Active

Relationship between ph and Exchangeable and Residual Acidity Two forms of H + /Al 3+ : Bound and Exchangeable Release of bound H + /Al 3+ increases effective CEC Bound acidity is mostly Al hydrolysis products and polymers that block exchange sites

Effective Versus Potential CEC Laboratory measurements of CEC are typically performed at a standard ph (7 or 8.2) This value is called the potential CEC Potential CEC is used in soil classification However, CEC is ph-dependent! CEC measured at the actual soil ph, which is typically <7, is the effective CEC This is often lower than the potential CEC

High Acid Saturation Promotes Low ph A B C D Acid saturation is closely related to soil ph Soil ph is less sensitive to the absolute acid cation concentration on exchange sites Soils with fewer total acid cations but greater acid saturation (e.g., A vs. B) are more acidic Acid saturation is a better indicator of soil acidity that the amount of exchangeable Al 3+

Potential Acidity from Reduced Sulfur Anaerobic soils contain reduced sulfur compounds (FeS 2, FeS, S) Horizons that contain such compounds are called sulfidic When aerated, often by drainage, the sulfur oxidizes, producing sulfuric acid Horizons having low ph caused by sulfuric acid are called sulfuric Soils acidified by sulfur oxidation are called acid sulfate soils Problem in soils on reclaimed coastal wetlands

Drainage of Costal Wetlands and Weathering of Mine Materials Causes Acidification

Soil ph Buffering

ph Buffering in Soils A: Pure Water B: Moderately buffered soil C: Well buffered soil Buffering: Resistance to ph change

Soil ph Buffering Reactions Aluminum hydrolysis OM protonation or deprotonation Protonation or deprotonation of mineral surface groups Cation exchange Carbonate mineral dissolution or precipitation Soils high in materials that can undergo these reactions have a high buffering capacity

CaCO 3 for Acid Buffering and Neutralization Calcium carbonate present in or added to soils neutralizes acid and buffers the ph CaCO 3 + H + = Ca 2+ + HCO 3-3CaCO 3 + Al 3+ + 3H 2 O = 3Ca 2+ + Al(OH) 3 + 3HCO 3 - CaCO 3 + CO 2 + H 2 O = Ca 2+ + 2HCO 3-2CaCO 3 + NH 4+ + 2O 2 = 2Ca 2+ + NO 3- + 2HCO 3- + H 2 O 2CaCO 3 + H 2 SO 4 = 2Ca 2+ + 2HCO 3- + SO 4 2-

Liming CaCO 3 or other neutralizing materials can be added to raise the ph of acid soils CaCO 3 + H + = Ca 2+ + HCO 3 - This is also used to reduce acid saturation 2X 3 Al + 3CaCO 3 + 3H 2 O = 3X 2 Ca + 2Al(OH) 3,gibbsite + 3CO 2 (g) Product is gibbsite, carbon dioxide gas, and a soil with an increased base saturation Read Box 9.2 (E) or 9.4 (N) to learn how to calculate liming needs

Key Concepts in Soil Acidity, Part 1 Many processes, including rain, generate soil acidity Addition of acidity coupled to leaching leads to a loss of base cations, increase in Al 3+ Important process in weathering and soil formation One source of damage caused by acid rain Aluminum is a critical source of acidity in soil Produced when H + reacts with soil minerals Undergoes hydrolysis to produce H + (3 H + for 1 Al 3+ )

Key Concepts in Soil Acidity, Part 2 Aluminum hydrolysis products interfere with cation exchange Block cation exchange sites on clays and OM, reducing CEC If ph increases from acidic to neutral conditions, these precipitate as gibbsite, freeing up cation exchange sites This is one reason that CEC increases with increasing ph

Key Concepts in Soil Acidity, Part 3 Multiple pools of soil acidity exist and react on different time scales Effective CEC differs from the potential CEC measured at ph 7 or 8.2 Al 3+ hydrolysis and ph-dependent charging of mineral surfaces and OM cause CEC to increase as ph increases Potential acidity associated with sulfide in soils can produce highly acidic soil conditions if exposed to oxygen from the air Liming: Calcium carbonate can be used to neutralize soil acidity, including reducing acid saturation

Soil Biological Diversity

Soil Fauna in Pulp Fiction?

Soil Fauna in Pulp Fiction

Soil Fauna Macrofauna (>2 mm) Mice, ants, termites, earthworms, snails Mesofauna (0.1-2 mm) Mites, pot worms, protura, collembola Microfauna (<0.1 mm) Nematodes, rotifers, amoeba, water bears UC Santa Cruz Banana Slugs Fungi-feeding springtail (collembola) (photo from USDA) Soil Nematodes (photo from USDA)

Soil Flora Macroflora Feeder roots, mosses Microflora (<0.1 mm) Root hairs, algae, fungi, aerobic and anaerobic bacteria, cyanobacteria, actinomycetes, methanotrophic archaea Roots Bacteria coating fungal hyphae (photo from USDA) Fungal Hyphae and Root Hairs

Length Scales of Soil Habitats

Soil Organisms Display a Wide Range of Sizes

Microflora and Earthworms Dominate Soil Biomass

Overview of Soil Ecology

Metabolic Pathways Soil ecosystems are dependent on the interactions of organisms over a wide size range employing a wide array of metabolisms Metabolisms are defined based on the sources of carbon and energy used by organisms Carbon: Source for making biomass Heterotroph: Existing organic carbon compounds Autotroph: Make organic carbon from CO 2 Energy: Source for driving metabolism Chemotroph: Biochemical redox reactions Phototroph: Sunlight through photosynthesis

Metabolic Pathways in Soils Chemoheterotrophs: All animals, plant roots, fungi, actinobacteria (actinomycetes), most bacteria Chemoautotrophs: Bacteria and archaea that respire using inorganic compounds and fix CO 2 into OM; generally anaerobic but some gain energy from oxidizing CH 4 and reduced S compounds with O 2 Photoheterotrophs: A few algal species, purple and green non-sulfur bacteria, heliobacteria; mostly limited to waterlogged soils as all are anaerobic Photoautotrophs: Plants, algae, cyanobacteria

Soil Food Web

Primary Producers In most soil ecosystems vascular plants are the primary producers Create organic carbon and O 2 via photosynthesis Other organisms are important primary producers in some environments Algae and cyanobacteria in arid soils and wetlands Lichens and mosses in cold regions and poorly developed soils

Primary Consumers Herbivores: eat live plants Parasitic nematodes, insect larvae, termites, ants, beetle larvae, mice Detritivores: eat plant debris (detritus) Earthworms, woodlice, millipedes Many organisms that eat detritus are not detritivores, instead primarily feeding on microorganisms that live on detritus Saprophytic Microorganisms: decompose plant and animal debris Fungi and bacteria

Secondary Consumers Carnivores: Eat living animals Centipedes, predatory mites and nematodes, spiders, snails Microbivorous feeders: Eat microflora Collembola (springtails), mites, termites, some nematodes, protozoa Bacteria, fungi, actinomycetes: Eat remains of primary consumers

Example Detrital Food Web: Shortgrass Prairie From: Coleman et al. (2004) Fundamentals of Soil Ecology

Key Concepts in Soil Ecology There are a range of metabolic strategies employed by soil organisms Primary producers are the source of all food in soils These are dominantly vascular plants but may also be algae, bacteria, mosses, and lichen Primary consumers feed off of primary producers or their debris Secondary consumers feed on primary consumers