Source: Krebs (1994); original source Cole and Rapp, 1981, in Dynamic Properties of Forest Ecosystems, D. E. Reichle (Ed). Cambridge University Press, Cambridge.
Source: Krebs (1994); original source Cole and Rapp, 1981, in Dynamic Properties of Forest Ecosystems, D. E. Reichle (Ed). Cambridge University Press, Cambridge.
Soil is the part of terrestrial ecosystems with the slowest turnover time (Table 15.3). This makes it the part that regenerates slowest if disturbed. A forest fire can kill all above-ground life in an area, but if the soil is not deeply burned, the forest can soon grow back. But if erosion carries away the topsoil with its organic matter and nutrients, the area may remain desolate for a long time. Thus, one of the most important goals for conservationists should be preservation of an ecosystem's soil.
The mineral portion of soil is produced by weathering of bedrock, either locally or at a distant location followed by transport by wind or water. Weathering is caused by physical, chemical, and biological factors. Physical weathering is caused mainly by the expansion of water when it freezes in cracks. Chemical weathering is caused by water dissolving the more or less soluble components, and by acids from atmospheric carbon dioxide (which forms carbonic acid in water) or from acid rain. Biological weathering contributes to chemical weathering by adding more CO2 plus organic acids to the water. Roots can help break up rocks by penetrating and forcing open cracks. Lichens produce acids that dissolve rocks.
A well-developed soil is divided into three main layers, or horizons (Figure 15.2). The A horizon, often called the topsoil, is the uppermost portion of the soil. The A horizon is rich in organic matter from decayed plants and animals. It is the site of most of the biological activity in the soil. Below this is the B horizon, which is lower in organic content and contains soil particles that are less weathered than the A horizon. The B horizon is a zone in which minerals, clay particles, and to a lesser extent organic matter are deposited after leaching from the A horizon. Next comes the C horizon, which is made up of partially broken-down and weathered rocks from which the soil of the upper layers is formed. Below this is the bedrock. Some classifications include the layer of decaying plant and animal matter above the A horizon, called the O horizon, which has little mineral matter. The portion of the soil that is subject to the influence of plant roots is called the rhizo-sphere or root zone.
When rocks weather, less soluble components such as silicates remain as solid particles. Particles larger than 20 mm are classified as sand; between 2 and 20 mm as silt, and particles smaller than 2 mm as clay. Soil with a preponderance of larger particles drain well, holding less moisture and nutrients. Clay particles tend to retain both. Mineral surfaces tend to have a net negative charge that enables them to adsorb cationic nutrient ions such as Ca2+, Mg2+, K+, Na+, NH4+, and H+. Anionic nutrients such as NO3~, SO|~, Cl_, and HCO3~ are not strongly adsorbed and leach more easily from soil. PO|~ is an exception since it forms very low solubility precipitates and adsorbs to mineral surfaces containing iron, aluminum, or calcium.
Groundwater Zone Figure 15.2 Soil horizons.
The capacity of soil to adsorb cations is called its cation-exchange capacity (CEC). The CEC describes the soil's capacity to serve as a reservoir for nutrients for plants. Because clay particles have a large surface area per unit mass, they have a relatively high CEC. For example, the CEC of kaolinite clay ranges from 3 to 15 milliequivalents (mEq) per 100 g. Montmorillonite clay, which has about an order-of-magnitude smaller particle size than kaolinite, has a CEC of 80 to 100 mEq per 100 g. The CEC of a whole soil will be strongly affected by its clay content as well as by organic matter (see below). The relative strength of attraction of cations to clay particles is
Thus, acidic conditions displace the other ions from the soil, allowing them to be leached. A low pH can also cause charge reversal on clay particles, enabling adsorption of anionic nutrients. In a process called podsolization, acidity breaks down the clay particles in the A horizon, precipitating their soluble salts in the B horizon. This reduces the CEC of the soil and therefore its capacity to hold nutrients. Podsolization commonly occurs in coniferous forests in cold regions, where the heavy fall of needles produce organic acids as they decay. This type of soil is called spodisol.
In low-lying tropical regions such as the Amazon basin, warm, moist conditions result in faster weathering than erosion. As a result, soil accumulates to great depths. Since the soil at the surface is so far from the parent mineral, the clay is leached out, leaving iron and aluminum oxides. These minerals do not retain nutrients as well as clay and can consolidate into a hard, concretelike material. This is called laterite soil, or latisol.
Soils with about 20% clay and 40% each of sand and silt are called loam. Increasing the fraction of one or the other component produces soils classified as "sandy loam,'' "clay loam,'' "silty clay loam,'' and so on. Soil with a clay content above 40 to 60% has a low permeability to water, which makes it a poor substrate for plants. A very sandy soil does not retain water well and has too low a CEC to store significant quantities of nutrients.
Thirty to fifty percent of soil volume is occupied by pore space. This fraction is called the porosity. If soil that is initially saturated with water is allowed to drain freely by gravity, a point is reached when the weight of the water is balanced by capillary forces holding the water in the pores. This point is called the field capacity. The moisture content can drop further by the action of air drying or by absorption by plant roots. The water content can be described in terms of the soil water potential or suction pressure, which expresses the pressure needed to extract the moisture from the soil. Free pure water has a soil water potential of zero.
The maximum pull most plants can exert on soil water is —1.5 MPa (about —15 atm). If the soil water potential falls below that level, it is considered to be below the permanent wilting point for that soil, and plants in that soil will be irreversibly wilted. A typical loam containing a fairly even distribution of sand, silt, and clay has a saturation of about 45 g of water per 100 g of dry soil. This corresponds to a porosity of 25%. Its field capacity is about 32 g per 100 g dry weight (d.w.; about 16% by volume), and the wilting point is about 7 g per 100 g d.w. (about 3% by volume). Thus, the amount of usable water in soil at the field capacity is about 25 g per 100 g d.w. (9% by volume). Soils with higher clay content have more of their water bound at the —1.5 MPa water potential, and therefore have a higher wilting point.
The bedrock is the ultimate source of most of the minerals for the ecosystem. Carbon and nitrogen, of course, come mostly from the atmosphere, and mineral input from the atmosphere, from dust, can also be significant. The bedrock provides the macronutrients phosphorus, potassium, calcium, magnesium, and sulfur, plus the micronutrients boron, copper, chlorine, iron, manganese, molybdenum, and zinc. The importance of bedrock is dramatized by the example of serpentine soils, which overlie serpentine rock, a magnesium iron silicate. These soils are deficient in calcium, nitrogen, phosphorus, and molybdenum, and high in magnesium, nickel, and chromium. Serpentine soils have poor plant coverage. What plants exist are characterized by specially adapted vegetation that tolerates these conditions. These plants cannot compete in normal soils, and normal plants cannot survive serpentine conditions.
Living things affect soil properties in other ways besides causing biological weathering. Living things in the soil mix and burrow, changing its structure, and living things contribute organic matter. Soil organic matter consists of plants and animals and the products of their degradation. Most of it is from plant material such as leaf litter in various states of decay. The conversion of dead biomass to soil organic matter is called humifica-tion. Biological materials that are degraded to the point of being unrecognizable are called humic substances (or humus). They represent about 5 to 10% of the dry weight of topsoil.
Humic substances are divided into three groups, based on whether they are soluble in acids, bases, or neither. The groups are called fulvic acid, humic acid, and humins.
Fulvic acid is the fraction that is soluble in strong acid and base solutions; humic acid is soluble in acids but insoluble in bases; humins are insoluble in both. None of these have a definite chemical structure. They are dominated by randomly polymerized phenolic rings with a variety of side groups, many of them acidic, and by numerous cross-links. Molar masses range from about 700 to 400,000, with fulvic acids tending to be the smallest molecules and humins the largest. Their structure makes them resistant to further biodegradation. The presence of large numbers of acidic groups, such as hydroxyl and carboxylate, makes humic substances a substantial component of the CEC of soil.
Humification occurs in several stages. First, organic polymers such as cellulose, hemi-cellulose, and lignin are broken down to monomers such as phenols, quinones, amino acids, and sugars. In the second stage of biodegradation, the monomers polymerize due to spontaneous reactions and due to enzyme catalysis, producing the humic substances. Following humification, the process of mineralization slowly converts humic substances to inorganic materials. In natural ecosystems the formation of humic substances tends to be in equilibrium with their mineralization.
Humification is caused mainly by saprotrophic bacteria and fungi. Sugars, fats, and proteins are decomposed fairly readily. Cellulose breaks down more slowly, and lignin even more slowly. Lignin may be the source of the phenolic rings that form the major building block of humic substances. As was mentioned previously, the fungus P. chrysosporium plays a major role in lignin degradation. The saprotrophic microbes are also important for the organic compounds they release to the soil environment. These include inhibitory compounds such as antibiotics, and stimulating compounds such as vitamins and essential amino acids.
In some ecosystems, organic matter accumulates to high levels. Grasslands produce large amounts of biomass. Humification proceeds rapidly, but mineralization is slow. Forests, on the other hand, recycle nutrients more efficiently. Thus, grasslands typically have five to 10 times as much soil organic matter as forests. This makes them particularly suitable for conversion to agriculture. Oxygen limitations in wetlands may slow both humification and mineralization. Biomass can accumulate to great depths in wetlands, forming peat. Coal is formed when peat is buried under heat and pressure over geologic time.
Experiments have shown that invertebrates play a major role in humification. Important groups include protozoans, nematodes, ostracods, snails and slugs, and earthworms. They act not primarily by digesting the biomass, but more by breaking up the biomass into smaller pieces, increasing its surface area, and by grazing the bacteria and fungi, which increases their activity. This is similar to the role of invertebrates found in biological wastewater treatment processes.
Humic substances are a major source of CEC in soil and increase its water-holding capacity as well as its nutrient capacity. Humic substances form chemical complexes, called chelation, with metal ions. This increases the bioavailability of the metals. Metals may be less toxic when chelated. For example, copper toxicity to phytoplankton depends on the free copper ion concentration. In marine environments the same copper concentration will be less toxic close to shore, where humic substances are more available, then in the open ocean. Soil organic matter also improves the soil structure, especially its friability. Friability is the ability of a soil to crumble. This property makes it easier for roots to penetrate. The increased CEC and friability are the primary reasons that we add composted manure to gardens and agricultural fields, not because of the nutrients they carry.
Agriculture removes organics by harvesting. This removes nutrients and reduces the generation of humic substances. Tilling increases erosion, removing existing topsoil. Adding fertilizers compensates for nutrient loss, but not for soil structure. A new development is no-till farming, in which weeds are controlled by the use of chemical herbicides instead of plowing. Crop residues and winter cover are left on the field as a mulch, which controls soil temperature and evaporation of moisture. The residues add to the production of humus and reduce erosion.
Agriculture, deforestation, and overgrazing all expose bare soil and increase the rate of erosion. Some estimate that as much as 1% of Earth's topsoil is lost each year. By one estimate, 25% of all farmland in the United States is losing topsoil at a rate exceeding a "tolerable" 1 in. every 34 years.
Humic and fulvic acids lower the pH of the soil. Another organic acid produced by degradation of biomass is oxalic acid, (COOH)2. Oxalic acid can form complexes with otherwise insoluble iron and aluminum. The complex then leaches to lower soil horizons, where the metals are precipitated upon oxidation of the oxalic acid to CO2. Oxalic acid also catalyzes the chemical weathering of rocks. Oxidation of sulfide minerals such as iron pyrite produces sulfuric acid. Soil pH was once thought to be a major factor controlling plant distribution. However, now it is known that although some plants have narrow pH tolerances, many are not strict. Even in some of those with strict limitations, it may be due to the effect of pH on nutrient availability rather than the effect of pH on the plant directly.
Animals are an important component of the soil community (Figure 15.3). Nematodes, annelids, and insects form the base of the detritus food web, along with fungi and bacteria. They aid in humification by digesting and reducing the size of biomass particles. Nema-todes, or roundworms, are microscopic worms that are very abundant in soil. Up to 3 billion can be found 1 m3 of soil. Ants and earthworms are important for their role in aeration of the soil, which they do by their burrowing activity. Mammals such as moles and shrews also form large burrows.
Earthworms are particularly important to the soil. Besides their function of aerating the soil by burrowing, they process the soil through their gut. Finally, they mix or till the soil by ingesting soil at depth, then discharging it at the surface when they come out at night. The discharged material can often be seen as twisted strands of soil, called castings, on the surface. Darwin spent a significant amount of time studying the role of earthworms in the years after he published The Origin of Species. He found that rocks in a field gradually became buried as soil was moved from underneath to atop.
Soil typically has 106 to 109 bacteria per gram. Most are associated with the decaying biomass in the O and A horizons. In the A horizon, more are associated with roots of plants than in free soil. Bacteria and fungi that perform the initial degradation of biomass include Pseudomonas, Bacillus, Penicillium, Aspergillus, and Mucor. About 10 to 33% of the bacteria are Actinomycetes, especially the genera Streptomyces and Nocardia. Azotobacter and some of the anaerobic Clostridium species can fix atmospheric nitrogen in free soil.
Chemolithotrophs include the nitrifiers Nitrospira, Nitrosomonas, Nitrobacter, and the Thiobacillus, the last of which oxidizes inorganic sulfur and ferrous iron. In agriculture, nitrification can increase leaching of nitrogen. Addition of nitrification inhibitors increases crop yield 10 to 15% and reduces groundwater pollution by nitrate.
Most kinds of fungi can be found in soil, usually in the top 10 cm, where they may constitute a major fraction of the soil biomass. Protozoans also occupy the upper layers;
populations are typically from 104 to 105 per gram of soil. They function as predators on bacteria and algae. Algae exist mostly on the surface, but they also have been found to exist heterotrophically at depths up to 1 m. Mycorrhyzal fungi are an important group, as has been discussed (see Section 10.7.4).
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