Lecture 3 Soil Ecological definition of soil

Lecture 3: Soil • Ecological definition of soil • Why are soils so crucial to vegetation studies? • Typical soil profiles – Soil horizon designations in the U. S. Soil Taxonomy • Major processes of soil development • Elements of describing a soil • Soil properties (physical and chemical properties) • Trends in soil properties along toposequences and chronosequences • Soil taxonomy

Ecological definition of soil • Soil is a mixture of mineral and organic materials that is capable of supporting plant life. • Composed of mineral particles, organic matter, water, and air. – Mineral component • • Anchorage for plant roots. Pore space for water and air. Source of many plant nutrients through weathering. Exchange sites for plant nutrients. – Organic component • Source and exchange site for nutrient cycling. • Influences soil structure, pore space, and water holding capacity. • Energy source for soil microbes and other heterotrophs. – Water component • Solvent for many essential plant nutrients. • Maintains equilibrium between cation and anions that are held on exchange sites. – Air component • Contains O 2 for aerobic metabolism of plant roots and soil organisms. • Exchange of CO 2 from soil respiration and which facilitates weathering. • Provides N 2 for N-fixing soil organisms.

Why are soils so crucial to ecological studies? • They provide all or part of all essential factors for plant growth except light. • Rooting material for the plants: the platform on which trophic levels of the ecosystem are built. • Contains most of the decomposers that recycle energy and nutrients of the ecosystem. • Contain the history of the site, which can be interpreted through paleoecological reconstructions. • The soil is an ecosystem in itself (producers, consumers, and decomposers)

The soil ecosystem (Smith 1974)

Typical soil profile University of Hawaii, Maui County, College of Tropical Agriculture and Human Resources

Soil horizons O - organic horizons. A - predominantly mineral horizon that is mixed with humified organic material (an eluvial horizon, i. e. a source of organic material, clay, and cations to lower horizons). E - light colored, bleached mineral horizon underlying the A horizon that occurs only in highly leached acidic soils. B - mineral horizon that shows little or no evidence of the original rock structure and which has been altered by oxidation, and illuviation (addition of minerals, clays, and organic matter from the A horizon). K - a subsurface horizon that is characterized by accumulation of calcium carbonate. Occurs mostly in desert and dry areas. C - a subsurface horizon that is basically the material from which the soil formed (loess, alluvium, till, etc. ). It lacks most of the properties of the A or B horizon, but can be somewhat oxidized (Cox horizon). R - regolith (consolidated bedrock).

Grassland soil: Romania

Deciduous forest soil: northeastern USA

Tropical soil: Kauai

Organic soil: arctic Alaska

Major processes of soil development Physical weathering: The breakdown of rock (the regolith) into finer particles through weathering. Chemical weathering: The breakdown and re-depositing of organic and inorganic substances primarily through the processes of dissolving, leaching, and oxidation. Decomposition: The breakdown of organic mater by bacteria and fungi into simpler organic substances (carbohydrates, lignins, proteins). Mineralization: The ultimate breakdown of organic substances into non-organic substances (minerals, carbon dioxide, water, salts) through the process of decomposition. Nitrification: The transformation by soil bacteria of ammonia compounds into nitrates and nitrites.

Spodosol with a leached E horizon A horizon E Horizon Bh Horizon Bt Horizon C horizon

Describing Soils Charles Tarnocai, Banks Island, July 2003 Photo by D. A. Walker

Elements of describing a soil • General site information (Location, physiographic position, topography, drainage, vegetation, parent material, observor, remarks) • For each horizon: – – – nomenclature depth color consistence texture Other properties (percent gravel and sizes, percent pores and sizes, percent roots and sizes, degree of decomposition of the peat, mottles (size, color, character), silt caps, clay films, carbonate deposits, salts, cementation due to iron or carbonates, p. H),

Soil field description form

Example: Formal soil description based on field description Header information Description of soil horizons: Horizon Depth Color Structure Texture Consistence Other information Sample no.

Example: Formal soil description based on field description Header information Description of soil horizons: Horizon Depth Color Structure Texture Consistence Other information Sample no.

Subordinate modifiers for horizons

Example: Formal soil description based on field description Header information Description of soil horizons: Horizon Depth Color Structure Texture Consistence Other information Sample no.

Soil colors: Munsell color system Albert Munsell color system: A color space that specifies colors based on three color dimensions: 1. Hue (principal light wavelength, (5 principal colors and 8 intermediate colors) 2. Value (lightness or darkness 3. Chroma (brightness). It was created by Professor Albert Munsell in the 1900 s, and adopted by the USDA as its official color system in 1930. Munsell Color Company: http: //munsell. com/color-products/

Soil colors: Munsell color system Hue: The dominant color wavelength (Y – Yellow, R – Red, B – Blue, etc. Value: The quantity of light reflected, or the lightness or darkness of the color. Lower numbers (toward the bottom of a page) are used for darker colors, those which are absorbing more light, and higher numbers (as you move to the top of a page) are used for lighter colors, those which reflect more light. Chroma: The purity of the color, or degree of color saturation. Low chroma numbers are dull colors, and it may be difficult to tell when the color actually changes from one color chip to another. High chroma colors are much easier to tell apart because the colors are stronger (more saturated or intense). Color name: Example: Soil color 10 YR 6/3, “pale brown”.

Page from Munsell soil color book

Example: Formal soil description based on field description Header information Description of soil horizons: Horizon Depth Color Structure Texture Consistence Other information Sample no.

Soil structure • • Structure is the bonding together into aggregates of individual soil particles. Organic matter generally promotes the development of spheroidally shaped structures (granular and crumb structures) Clay promotes the development of blocky, angular, prismatic, and columnar structure. Platy structures are related to layering in cementing the soil, or induced by freezethaw processes.

Soil Peds • Individual soil particles are usually bond together into larger units called soil aggregates or soil peds. Subangular blocky Angular blocky Columnar Granular (chocolate cookie crumbs

Combination of mineral, organic, water and air in a loam soil Modified from the Nature and Propreties of Soils 11 th edition, N. C. Brady and R. R. Weil. Prentice Hall. 1996.

Soil particles and void space Bulk density = Dry weight of soil / volume of undisturbed soil in nature

Soil structure relevance to desirability for plant growth

Example: Formal soil description based on field description Header information Description of soil horizons: Horizon Depth Color Structure Consistence Texture Other information Sample no.

Consistence: Field estimate of soil texture

Texture by feel: Consistence http: //www. youtube. com/watch? v=IOya. Bxj 767 s&feature=related

Example: Formal soil description based on field description Header information Description of soil horizons: Horizon Depth Color Texture Structure Consistence Other information Sample no.

Common soil physical properties analyzed in the laboratory • • Soil particle size - texture Bulk density % organic matter Soil water retention properties – – Field capacity - 1/3 atm Wilting point - 15 atm Hygroscopic water - 30 atm Available water = field capacity - wilting point

Soil texture based on soil particle sizes • • Gravel, >2 mm Sand, 2 - 0. 05 mm Silt, 0. 05 - 0. 002 mm Clay, <0. 002 mm Nested sieves to remove gravel and obtain < 2 mm fraction

Soil texture triangle Depicted is a soil that is 15% sand, 15% clay and 70% silt (a silt loam). Modified from Nature and Poperties of Soils 11 th Edition. N. C. Brady and R. R. Weil. 1996. Prentice-Hall.

Soil texture by analysis: Hydrometer method for sand, silt, clay (< 2 mm Fraction) We’ll go over the method in lab.

Particle sizes and surface area of sand, silt and clay

Surface area vs. particle size Fine sand: particle size 0. 5 mm Silt: particle size 0. 05 mm

Percent of sand, silt and clay in various textured soils

Influence of soil organic matter on water holding capacity • Soils that are high in organic content are darker and have higher water holding capacity. • The same amount of water was applied to each container.

Soil moisture retention • Field capacity is the amount of water the soil will hold when freely drained. • Wilting point is the point at which plants can no longer take up water. It is generally considered to be the amount of water held in the soil under 15 atmospheres of pressure, which approximates the wilting point of a large suite of broad leaved plants. • Available water is the amount of water available to most plants for uptake. It is the difference between the wilting point and field capacity. • Hygroscopic moisture is the water that is tightly bound to the soil particles by molecular forces and unavailable to plants. It is the amount of water held in the soil under the 30 atmospheres of pressure.

Clay particles Kaolinite Montmorillonite Illite

An idealized cluster of clay particles From Soils and Fertility by Thompson and Troeh, New York: Mc. Graw-Hill 1973. • Shows the card house effect in soil structure, in which positive charges on edge positions are attracted by negative charges on the broad surfaces. • The negative charges provide exchange sites for cations and are thus a nutrient storage facility for cations (e. g. , K, Ca, Mg).

Clay crystal with adsorbed cations From Buckman and Brady

Common soil chemical properties analyzed in the laboratory • p. H • Cation exchange capacity - total amount of cations (including H+) that can be displaced • Base saturation - the percent of the cation exchange complex occupied by exchangeable bases (mostly plant nutrients such as Ca, Mg, Na, K, etc. ) • Nutrients - amounts of macronutrients and micronutrients

Range of p. H in soils

Soil p. H: saturated paste method We’ll go over the method in lab.

Cation exchange capacity (CEC) • A measure of the number of negatively charge sites on soil particles that attract exchangeable cations. • Factors that influence the CEC include clay content, kinds of clay, humus content, and p. H. University of Georgia, extension

Base saturation • The percent of the cation exchange sites that are occupied by exchangeable bases (Ca++, Mg+, K+, Na+) that are important plant nutrients. Ideal percentages for most agricultural purposes

Relationship between p. H and selected plant nutrients and the activity of soil fungi and bacteria • The p. H of the soil largely controls how many cations can be adsorbed on the exchange sites. • p. H also affects the availability of numerous nutrients, and the activity of bacteria and actinomycetes.

Percentage base saturation • Soil p. H is key factor in base saturation. More acidic soils will have more of the cation exchange sites occupied by H+ ions. • Fine-textured soils have greater cation exchange capacity and can therefore carry more cations at the same base saturation than a course-textured soil.

Soil taxonomy • The names of the soil units impart a great deal of information about the soil. • The U. S. Soil Taxonomy is a highly systematic system that is now used in many parts of the world. • It is similar to vegetation classification in that gradual changes from one type to another may be partitioned into discrete types despite the gradient-nature of natural systems.

Great soil groups in the U. S. soil taxonomy Entisols: “new soils with little horizon development (e. g. , soils on floodplains, sand dunes, mountain tops). Vertisols: soils high in clay and salts that crack deeply when dried (many desert soils) Inceptisols: shallow moderately developed soils on new, very cold, or very wet substrates (e. g. , many tundra soils). Aridisols: red desert soils, highly oxidized. Mollisols: prairie soils and soils with dark surface horizons, high in organic mattter and high base saturation. Spodosols: highly leached soils with a distinct bleached (E) horizon (e. g. , podzols of deciduous forests). Alfisols: well developed, noncalcic soils. Ultisols: intensely leached soils of warm climates, with strong clay translocation, and low base content (e. g. , some laterites, red-yellow podzols). Oxisols: high weathered soils of the tropics with high oxides. Histosols: organic soils, peats. Gelisols: soils in permafrost regions.

Formative elements of the names of soil orders

Toposequences • • Toposequences are excellent ways to study the long-term effects on soil properties of water movement from hill crests to foot slopes. This is a toposequence of Alfisols (temperate deciduous forests). The diagram represents the topographic position the profiles might occupy. Note that the strongest soil developmet takes place on well drained sites where weathering is maximum. The least amount of weathering takes place on very poorly drained soils where the wet season water table lies above the surface of the soil. g indicates After Knox 1952. gleying with mottling; t indicates translocated silicate clays.

Toposequence from California coast Siera 104. com

General trends in soil properties downslope in most environments • Increasing – – – Depth of O horizon Depth of the A horizon Percent clay in the B horizons Soil moisture p. H Soil nutrients

Toposequences at Imnavait Creek, Alaska, bioclimate subzone E, (ice-rich permafrost) Imnavait Creek, Subzone E, northern Alaska, Photo: D. A. Walker

Imnavait Creek, Ak toposequence

Hill crest soil

Imnavait Creek: Hill Crest 61 Walker, D. A. , Walker, M. D. 1996. Terrain and vegetation of the Imnavait Creek Watershed. in J. F. Reynolds, J. D. Tenhunen (eds. ) Landscape Function: Implications for Ecosystem Disturbance, a Case Study in Arctic Tundra. Springer-Verlag. New York. 120 pp. 73 -108.

Backslope soil

Imnavait Creek backslope 63 Walker, D. A. , Walker, M. D. 1996. Terrain and vegetation of the Imnavait Creek Watershed. in J. F. Reynolds, J. D. Tenhunen (eds. ) Landscape Function: Implications for Ecosystem Disturbance, a Case Study in Arctic Tundra. Springer-Verlag. New York. 120 pp. 73 -108.

Toeslope soil

Imnavait Creek footslope Walker, D. A. , Walker, M. D. 1996. Terrain and vegetation of the Imnavait Creek Watershed. in J. F. Reynolds, J. D. Tenhunen (eds. ) Landscape Function: Implications for Ecosystem Disturbance, a Case Study in Arctic Tundra. Springer-Verlag. New York. 120 pp. 73 -108. 65

Imnvait Creek Toposequence Soil properties Physical properties Chemical properties Walker, M. D. , D. A. Walker, K. R. Everett. 1989. Wetland soils and vegetation, Arctic Foothills, Alaska. USFWS Biological Report 89(7), 90 pp. 66

Imnvait Creek Toposequenc e Nutrient concentration s 67 Walker, M. D. , D. A. Walker, K. R. Everett. 1989. Wetland soils and vegetation, Arctic Foothills, Alaska. USFWS Biological Report 89(7), 90 pp.

Typical toposequence trends at Imnavait Creek: • Increasing: – – – • Depth of A horizon Amount of clay in the B horizons (structure increases) Oxidation (redder colors) Carbonate cementation (in drier soils) Thickness of clay films and silt caps Decreasing – Soil nutrients – p. H Arctic Foothill toposequence: The maximum p. H and nutrients were recorded at the slope shoulder, where water had washed nutrients from upslope. Downslope from here the p. H and nutrients decreased until the fen at the base of the hill where nutrients and water were flowing along the drainage and not from upslope.

Chronosequence often mimics toposequence The soil chronosequence illustrates the gradual formation of the B and C horizons and deepening of the soil profile through time. From Richter and Markewitz. 1997. How deep is the soil. Bio. Science 45: 600 -609.

Micro-topographic elements of icewedge- polygon system • Basins: wet tundra or water. Basin Rim Trough Photo: D. A. Walker • Rims: moist tundra, soil displaced by the ice wedge. • Troughs: wet cracks between rims, position of ice wedge.

Current Active Layer Intermediate Layer of Upper Permafrost Courtesy of Gary Michaelson Buried carbon in the intermediate layer of permafrost table

Sequestered carbon beneath frost boils Burial of carbon from margin of circle to the base of the circle via gravity Carbon-rich horizon at base of nonsorted circle kg OC m-2 Active layer – 37 Permafrost – 19 Total 56

Carbon is concentrated in the cracks between small polygons. Nonsorted circles, Ostrov Belyy, Russia. After removal of top 10 cm of soil. Circles are situated in the centers of 60 -90 -cm diameter nonsorted polygons with cracks.

Movement of organic material along thermal cracks to the base of the active layer. Left and center: Laborovaya, Russia; right: Mould Bay, Canada Photos: D. A. Walker

Large amounts of carbon are sequestered at the top of the permafrost table in the intermediate layer. Major questions: How old is the carbon? How stable is the carbon? Is it susceptible to decomposition if the active layer becomes deeper? Courtesy of Misha Kenevskiy & Yuri Shur

Trends in the North Campus Lands toposequences Footslope, Dwarf shrub, tussocksedge fens, Plots 3, 6, 9, 12 Midslope, Black spruce forests, Plots 2, 5, 8, 11 Eastern Transect Hill crest/shoulder White spruce forests, Plots 1, 4, 7, 10

Soil Map http: //websoilsurvey. nrcs. usda. gov/app/ Muck: Dark, finely divided, well decomposed organic soil material.

118—Fairbanks silt loam, 12 to 20 percent slopes • • • • Elevation: 499 to 1, 998 feet Mean annual precipitation: 10 to 14 inches Frost-free period: 80 to 120 days Extent: 65 to 80 percent of the map unit Landform: hills Position on slope: backslopes Slope shape: convex, linear Slope range: 12 to 20 percent Parent material: loess Hazard of erosion (organic mat removed): by water—severe; by wind—severe Runoff: medium Drainage class: well drained Flooding: none Depth to high water table (approximate): April- Sept. —more than 72 inches Ponding: none Available water capacity (approximate): 12. 2 inches Representative Profile: • Oi— 0 to 3 inches; light olive brown slightly decomposed plant material, high permeability • A, Bw— 3 to 30 inches; grayish brown or light olive brown silt loam, moderately high permeability • C— 30 to 72 inches; light olive brown silt loam, moderately high permeability Minor Components: • Fairbanks, slopes less than 12 percent, and similar soils: 0 to 15 percent of the map unit • Fairbanks, slopes more than 20 percent, and similar soils: 0 to 15 percent of the map unit • Minto and similar soils: 0 to 5 percent of the map unit Steese and similar soils: 0 to 10 percent of the map unit • Soil-related factors: excess slope, frost action

Tall & short forest: 104—Chatanika mucky silt loam, 3 to 7% slopes • • • • • Elevation: 499 to 1, 998 feet Mean annual precipitation: 10 to 14 inches Frost-free period: 80 to 120 days Extent: 70 to 80 percent of the map unit Landform: hills Position on slope: footslopes, toeslopes Slope shape: linear, concave Slope range: 3 to 7 percent Parent material: colluvium and/or loess Depth to permafrost: 12 to 39 inches Hazard of erosion (organic mat removed): by water—moderate; by wind—severe Runoff: very high Drainage class: poorly drained Flooding: none Depth to high water table (approximate): April-May— 0 inches; June-Sept. — 8 inches Ponding: frequent Available water capacity (approximate): 4. 3 inches Vegetation: black spruce forest • Representative Profile: – Oi— 0 to 4 inches; very dark grayish brown slightly decomposed plant material, high permeability – A— 4 to 6 inches; grayish brown mottled mucky silt loam, moderately high permeability – C/Ag— 6 to 21 inches; very dark grayish brown silt loam, moderately high permeability – Cfg— 21 to 72 inches; very dark grayish brown permanently frozen material, impermeable

Tussock site - 133—Goldstream peat, 0 to 3% slopes • • • • • Elevation: 397 to 1, 201 feet Mean annual precipitation: 10 to 14 inches Frost-free period: 80 to 120 days Extent: 70 to 85 percent of the map unit Landform: valley floors Slope shape: concave, linear Slope range: 0 to 3 percent Parent material: organic material over loess Depth to permafrost: 14 to 24 inches Hazard of erosion (organic mat removed): by water—slight; by wind—slight Runoff: very high Drainage class: very poorly drained Flooding: none Depth to high water table (approximate): April-May— 0 inches; June-Sept. — 0 to 8 inches Ponding: frequent Available water capacity (approximate): 3. 6 inches Vegetation: black spruce woodland • Representative Profile: – Oi— 0 to 9 inches; gray mucky peat, high permeability – A— 9 to 12 inches; dark brown mucky silt loam, moderately high permeability – Bjjg— 12 to 20 inches; very dark grayish brown silt loam, moderately high permeability – Cfg— 20 to 72 inches; gray permanently frozen material, impermeable

Literature • • Klinger, L. F. 1996. Coupling soils and vegetation in peatland succession. Arctic and Alpine Research, 28: 380 -387. Van Cleve, K. , L. A. Viereck, and C. T. Dyrness. 1996. State factor control of soils and forest succession along the Tanana River, Alaska, U. S. A. Arctic and Alpine Research, 28: 388 -400.