The organic matter in ecosystems
The organic matter in an ecosystem consists of the organic matter above and below the soil surface. The distribution of organic matter in a ponderosa pine forest ecosystem is shown in Table 9.1. The distribution of organic matter, expressed as organic carbon, was 38 percent in the trees and ground cover and 9 percent in the forest floor. The remaining 53 percent of the organic matter was in the soil and included the roots plus the organic matter associated with soil particles. In the forest there is a continual growth of plants and additions to these three pools of organic matter: standing crop, forest floor, and soil. These three pools of organic matter represent a supply of nutrients for future growth. In grassland ecosystems, much more of the organic matter is in the soil and much less occurs in the standing plants and grassland floor. Although approximately 50 percent of the total organic matter in the forest ecosystem may be in the soil, over 95 percent may be in the soil, where grasses are the dominant vegetation.
DECOMPOSITION AND ACCUMULATION
The primary or original source of soil organic matter (SOM) is the production of the primary producers - the higher plants. This organic material is subsequently consumed and decomposed by soil organisms. The result is the decomposition and the accumulation of organic matter in soils that has great diversity and a highly variable composition.
Properties of Stable Soil Organic Matter
Stable soil organic matter (humus) is a heterogeneous mixture of amorphous compounds that are resistant to microbial decomposition and possess a large surface area per gram. The large surface area enables humus to absorb water equal to many times its weight. During decomposition of plant and animals residues, there is a loss of carbon as carbon dioxide and the conservation and reincorporation of nitrogen into microbial products, which eventually are incorporated into humus. As a consequence, humus contains about 50 to 60 percent carbon and about 5 percent nitrogen, producing a C : N ratio of 10 or 12 to 1. The more decomposed the humus is the lower or narrower will be the C : N ratio. Compared with plant materials, humus is relatively rich in nitrogen and, when mineralized, humus is a good source of biologically available nitrogen. Humus is also a significant source of sulfur and phosphorus. The ratio of C:N:P:S is about 100:10:1:1. Humus contains phenolic groups, 6-C rings with OH, that dissociate H + at pH above 7.0. During humus formation there is an increase in carboxyl groups, R-COOH, and these groups tend to dissociate H+ at pH less than 7: R-COOH = R-C00- + H+
As a result, humus has a considerable amount of negative charge. Cations in the soil solution tend to be weakly adsorbed onto the negatively charged phenolic and carboxyl groups. However, adsorbed cations can be readily exchanged by other cations remaining in solution. The total negative charge of the SOM, or ability to adsorb exchangeable cations, is called the cation exchange capacity (CEC). The CEC of organic matter acts similarly to that of clay particles, in that the charge adsorbs and holds nutrient cations in position to be available to plants, while effectively holding these cations against loss by leaching. The most abundant nutrient ions adsorbed are Ca. ²1 , Mg²+, and K+ . Adsorbed H + affects the pH of the soil solution, depending on the amount adsorbed and the degree of dissociation into the soil solution. Clay and SOM are the source of most of the negative charge in soils. The sum of the CEC of the SOM plus the CEC of the clay is generally considered equivalent to the CEC of the total soil.
Formation of Organic Soils
High organic matter contents in soils are the result of slow decomposition rates rather than high rates of organic matter addition. Organic soils are very common in tundra regions where the rate of plant growth is very slow. In the shallow water of lakes and ponds, plant residues accumulate instead of decomposing because of the anaerobic conditions. Consequently, organic soils may consist almost entirely of organic matter. In some cases, an infusion of mineral sediment from soil erosion may result in the accumulation of some mineral matter, together with the organic matter. Some organic soils, peat, for example, are mined and used for fuel.
Formation of many organic soils consists of the accumulation of organic matter in a body of water. After glaciation, many depressions received clay and other materials that sealed the bottoms and created ponds and lakes. As the plant material accumulated on the bottom of the bodies of water, the water depth decreased and the kinds of plants contributing organic matter changed. Another important organic component is the remains and fecal products of small aquatic animals. Minor climatic changes have occurred since the last glaciation, and this has had an effect on the water levels and types of vegetation and the animal populations. Thus, a series of layers or soil horizons develop from the bottom upward that reflect the conditions at the time each layer was formed. Pollen grains are preserved in the peat, which can be readily identified. The vegetation type can be verified by pollen analysis.
SUMMARY
The total soil organic matter is composed of a labile fraction and a stable fraction. The labile fraction consists of readily decomposable plant, microbial, and animal products. Even though it is a minor amount of the total soil organic matter, labile organic matter is very important in nutrient cycling. The stable organic matter is humus, which contributes to cation exchange capacity, water adsorption, and soil structure stability. Stable organic matter decomposes very slowly, and as a result, releases (mineralizes) nutrients very slowly.
Organic soils consist mainly of organic matter. They have light weight and are susceptible to wind erosion. Most organic soils require drainage for cropping, which aerates the soil and greatly increases the rate of decomposition. The absence or very low content of minerals is associated with very low potassium supplying power. The organic matter content of a soil tends to approach an equilibrium. When the losses of organic matter exceed additions, the soil organic matter content declines. Cultivated soils contain about half as much organic matter as they did before the soils were converted to cropping. The exception is desert soils where cropping may result in an increase in organic matter content. Horticultural peats and composts are organic materials used for mulching and for soil mixes.
(H.D. Foth, Fundamentals of Soil Science)