Soil Chemistry

Jennifer D. Knoepp Leonard F. DeBano Daniel G. Neary Chapter 3: Soil Chemistry Introduction _ _ ________ Soil Chemical The chemical properties of the soil that are affected Characteristics by fire include individual chemical characteristics, The chemical characteristics of soils range from the chemical reactions, and chemical processes (DeBano inorganic cations-for example, calcium (Ca), sodium and others 1998). The soil chemical characteristics (Na), magnesium (Mg), potassium (K), and so forth most commonly affected by fire are organic matter, that are adsorbed on the surface of clay materials to carbon (C), nitrogen (N), phosphorus (P), sulfur (S), those contained mainly within the organic matrix of l cations, cation exchange capacity, pH, and buffer power. the soil- for example, organic matter, C, N, P, S. All Some purely chemical reactions occur in soils. These chemical characteristics are affected by fire, although include the exchange of cations adsorbed on the sur the temperatures at which changes occur can vary face of mineral soil particles and humus with their widely. The best estimates available in the literature surrounding solutions. Another predominately chemi forthe threshold temperatures ofindividual soil chemi cal reaction is the chemical weathering of rocks and cal characteristics are given in table 3.1. their eventual transformation into secondary clay The published information describing the effects of minerals during soil formation. During the chemical fire on changes in individual chemical constituents of decomposition of rock material, the soil and its sur soils and organic matter are contradictory and have rounding solution become enriched with several cat often led to differing conclusions about the magni ions. Associated with the chemical interactions during tude and importance of the chemical changes that weathering and soil formation are physical forces actually occur during a fire. Different studies have (freezing and thawing, wetting and drying) and bio concluded that soil chemical constituents increase, logical activities (production of organic acids during decrease, or remain the same (DeBano and others the decomposition of humus) that also accelerate soil 1998). This has been particularly true for studies development. The most common chemical processes reporting changes in N and other nutrients that can occurring in soils that are affected by fire, however, are volatilize readily during a fire (for example, organic those mechanisms that are involved in nutrient avail matter, sulfur, and phosphorus). Differing conclu ability and the losses and additions of nutrients to the sions arise primarily because of the method used for soil. USDA Forest Service Gen. Tech. Rep. RMRS-GTR-42-vol. 4. 2005 53 Table 3.1-Temperature thresholds for several soil chemical characteristics of soil. Soil characteristic Threshold temperature Source OF oc Organic matter 212 100 Hosking 1938 Nitrogen 414 200 White and others 1973 Sulfur 707 375 Tiedemann 1987 Phosphorus and potassium 1,425 774 Raison and others 1985a,b Magnesium 2,025 1,107 DeBano 1991 Calcium 2,703 1,484 Raison and others 1985a,b Manganese 3,564 1,962 Raison and others 1985a,b calculating the chemical constituents. Chemical changes in chemical constituents will first focus on changes can be expressed in either percentages (or the more fundamental changes in chemical constitu some other expression ofconcentrations, for example, ents in wildland ecosystems. ppm or mg/kg) or be based on the actual changes in As a general rule, the total amounts of chemical total amounts of the constituent(for example, pounds/ elements are never increased by fire. The total amounts acre or kg/ha). Before fire, the percent of a given of different chemical elements on a particular burned chemical constituent is usually based on the amount site most likely decrease, although in some cases may contained in a prefire sample that can contain vari remain the same (for example, elements with high able amounts of organic matter. In contrast, follow temperature thresholds such as Mg, Ca, and others ing the fire the percent of the same chemical constitu listed in table 3.1). The fire, however, does change the ent is based on the weight of a burned sample that form of different elements and in many cases makes contains varying amounts of ash along with charred them more available for plants and other biological and unburned organic matter. Thus, the confusion in organisms. A classic example of this is total N con nutrient changes arises because different bases are tained in the ecosystem organic matter (table 3.2). used for calculating the change in a particular chemi When organic matter is combusted, total Non the site cal constituent (that is, based on mainly organic is always decreased, although increases in the avail matter before combustion as compared to ashy and able forms of N are likely to occur as is discussed in a unburned materials following a fire). This confusion later section, "Nitrogen." Therefore, managers must between percentages and total amounts was first be alert when interpreting the significance of the reported in a study on the effect of fire on N loss sometimes contradictory changes in different nutri during heating (Knight 1966). This study indicated ents during a fire that are reported in the literature. that the differences between percent N and total The following sections focus on describing these changes amount of N started at 212 °F (100 °C) and became in terms of the underlying chemical processes and to greater until about 932 °F (500 °C). Because of these indicate the management implications ofthese changes difficulties in interpreting concentration and percent in terms ofsoil and ecosystem productivity and postfire age data, the following discussion on the fire-related management. Table 3.2- Effect of burning at low, medium, and high severities on organic matter and mineralizable N in forest soils in northern Idaho. (Adapted from Niehoff 1985, and Harvey and others 1981 ). Organic matter Mineralizable N Treatment Mineral soil Mi neral Organic depths= 2.5-7.5 cm 2.5-7.5 cm 0-2.5 cm Total N Percent Change (%) ppm(mglkg) ppm(mglkg) Change (%) Undisturbed 3.6 0 9.4 68 0 Clearcut No burn 3.9 +8 9.7 97 +22 Low 4.1 +12 9.5 75 +8 Medium 2.8 - 22 9.3 5 - 82 High 0.6 -83 0.7 0 - 99 54 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-42-vol. 4. 2005 Organic Matter and Carbon materials, some of which is usually mixed with the upper mineral soil layers. Many chemical properties and processes occurring • Coarse woody debris that is eventually de in soils depend upon the presence of organic matter. cayed but can remain on the soil surface or Not only does it play a key role in the chemistry of the buried in the mineral soil for long periods. soil, but it also affects the physical properties (see • Charcoal or other charred materials that be chapter 2) and the biological properties (see chapter 4) come mixed with the forest floor and upper of soils as well. Soil organic matter is particularly most layers of the mineral soil. important for nutrient supply, cation exchange capac • The uppermost part of the A-horizon, which is ity, and water retention. However, burning consumes composed mainly of a mixture of humus and aboveground organic material (future soil organic mineral soil particles. matter, including large logs), and soil heating can • A mixture of mineral soil, plant roots, and consume soil organic matter (fig. 3.1). The purpose of biomass (live and dead) that is concentrated the following discussion is to focus as much as possible primarily in the A-horizon but may extend on the purely chemical properties of organic matter downward into the B-horizon or deeper de and on changes that occur as the result of soil heating. pending upon the type of vegetation growing Because organic C is one of the major constituents of on the site. organic matter, the changes in organic matter and organic C during soil heating are considered to be The amount of aboveground and belowground or similar for all practical purposes. ganic matter varies widely between different vegeta tion types depending upon on the temperature and Location of Organic Matter in Different Eco moisture conditions prevailing in a particular area systems-Organic compounds are found in both (DeBano and others 1998). In almost all ecosystems aboveground and belowground biomass where they throughout the world, greater quantities of C (a mea make up the standing dead and live plants and dead sure of organic matter) are found belowground than organic debris (that is, leaves, stems, twigs, and logs) aboveground (fig. 3.2). In grasslands, savannas, and that accumulate on the soil surface and throughout tundra-covered areas, much greater quantities of or the soil profile (DeBano and others 1998). Organic ganic Care found in the underground plant parts than matter found in the soil consists of at least seven aboveground (less than 10 percent of the total C in components, namely: these herbaceous vegetation ecosystems is found • The L-layer, (oi) which is made up of readily identifiable plant materials. • The F-layer, (oe) which contains partially de PLANT BIOMASS composed organic matter but can still be iden Temperate 150 Deciduous tified as different plant parts (needles, leaves, Forest stems, twigs, bark, and so forth). ca 100 • The H-layer, (oa) which is made up of com :c...... C> pletely decayed and disintegrated organic Tundra [ e 50 Savanna 1 ...J 0 0 c.. 0 z 0 al 50 a: uct 100 Tropical Temperate Forest Grassland Boreal 150 Forest 200 SOIL ORGANIC MATTER Figure 3.2-Distribution of C and soil organic matter (including litter) in major ecosystem types of the world. (Adapted from J .

Load more