Organic Carbon: From Tree Leaves To Soil Organisms
Donald H. Marx PhD.
Plant Health Care, Inc.
FROM TREE LEAVES
Let's start this discussion off with describing the most important biochemical process on Earth…photosynthesis!! This is the process by which energy from the sun is captured by the chloroplast in green leaves and used to make organic carbon…the sugar glucose (the photosynthate)…from inorganic carbon dioxide and water. The process releases oxygen from the splitting of the water molecule. When the water molecule is split, high-energy electrons are released and, after many biochemical reactions, they are transferred to and stored in phosphorus-containing organic compounds like ATP. The spending of this stored energy, that can be traced back to the sun, eventually leads to the production of the sugar, glucose. Glucose is found in all living cells of all plants, animals and microbes and, as glucose phosphate, serves as the major substrate for cellular respiration. Nearly all of the energy, sugar and oxygen used by the diverse forms of life…plants, animals and microbes…on Earth come from photosynthesis! Life as we know it could not exist without this biochemical reaction and its end products, organic carbon as glucose, the captured energy from the sun and oxygen! All life, whether it is the smallest microbe or the largest animal, is dependent on the availability and utilization of this carbon, oxygen and energy. Carbon is the most abundant organic chemical on Earth and serves as the "building blocks" for all life. Everything living contains carbon! None of the biochemical reactions in photosynthesis are spontaneous. All reactions are facilitated by a large diverse group of unique proteins- enzymes – that are biological catalysts in virtually all biochemical reactions of all life forms.
The amount of sunlight and its duration has a controlling effect on photosynthesis. Leaves near the top of the tree canopy have a much higher photosynthetic rate than leaves near the bottom of the canopy. This is because there is more light saturation of the chloroplasts in direct sunlight. Inefficient lower branches with relatively few but highly shaded leaves of shade intolerant trees like ash, hickory (pecan), walnut, pine, birch, willow, etc., often do not contribute any new carbohydrates for growth of the main stem. The limited amounts produced in these shaded leaves are mainly used for maintenance of these specific leaves. Shade leaves are normally larger, but thinner, and have fewer stomates. Branches supporting these shade leaves normally shed, i.e. natural branch pruning, because of limited maintenance respiration. At low light intensities and for short durations, the rate of photosynthesis is higher in shade tolerant trees like maple, beech, buckeye, sugarberry and flowering dogwood, than in shade intolerant trees. That's why shade tolerant trees and shrubs can grow and thrive under the closed canopies of shade intolerant trees. These understory plants also benefit from the higher levels of respiratory carbon dioxide emitted from all the roots and organisms in the soil. This additional carbon dioxide somewhat compensates for the effects of reduced sunlight.
* land plants produce about 100,000,000,000 metric tons of carbon each year of which two-thirds is produced by trees? This exceeds that of the entire world by a factor of four, and that of agricultural plants by a factor of more than two.
* one acre of young plantation trees removes about three-fourths of a ton of carbon each year. Old growth forests and their soils emit as much respiratory CO2 as their leaves fix in photosynthesis. You and I each add (via breathing, automobiles, heat, electricity, etc.) about 5 tons of carbon each year!! That means it takes over 6 acres of young productive forests to recycle the CO2 each of us "produces"!
* trees and other land plants capture about 40% and the ocean about 50% of the annual CO2 produced by industry (coal combustion and cement production).
* trees are also important commercially! They produce more than 5,000 wood and paper products…everything from baby food to rayon, and toothpaste to football helmets. Oil and coal are simply very, very old dead trees! Ecologically, trees are essential to land stability and hydrology. Imagine a world without trees…look at Haiti!!
By now you know that organic carbon in its various forms is essential to all life. This means that every living organism must find enough carbon to sustain and reproduce itself. Lets' look at that another way, all other things being equal, when you find a population of organisms, whether they are fish, elephants, butterflies, earthworms or soil bacteria, it is because they have satisfied their carbon-based dietary requirements. They wouldn't be where you found them long without food, i.e. organic carbon in its' many forms.
TO SOIL ORGANISMS
This leads us now belowground. What happens to the organic carbon that is shed aboveground (branches, leaves, flowers, bark, etc.) forming the forest floor. This is nature's mulch. What happens to the carbon that is shed by the root system? Roots shed whatever tissue is no longer functioning (old fine roots and bark) and exude spent organic compounds just like the aboveground tree parts. These are recycled by various organisms. The number of species of organisms involved in this belowground carbon cycle is staggering! Because of this great diversity it is nearly impossible to isolate individual groups of soil organisms and identify precisely their part in the carbon nutrient cycle. This carbon cycle is a chain of events, each event with a different cast of characters (or organisms). It's a succession. Each stage is based on the chemical form of organic carbon now available. Each group of organisms "eats" what they can, they leave or die and then another group follows and eats…and on and on…until all of the carbon is converted back to CO2 and H2O! We're now back at the starting line for the carbon cycle to repeat itself! However, this may take a thousand years before the toughest carbon compounds, like lignin, are Gaseous nitrogen can be fixed symbiotically by nodulating bacteria (as with legumes) or fixed by free-living bacteria in the soil. The nodulating bacteria obtain their organic carbon nutrition directly from their organic union with plant host. The free-living bacteria obtain their carbon nutrition from the organic matter in the soil or from sloughed root cells or root exudates (i.e. rhizodeposition). The nitrogen fixed by these specific bacteria is eventually released as either ammonium or nitrate into the soil. These are the main forms of nitrogen absorbed by plant roots. This fixation of atmospheric nitrogen is the main way that "new" nitrogen is added naturally to plant ecosystems. Nitrogen is essential to the total organic carbon decomposition process. Carbon: nitrogen ratios in the soil between 15 and 30 to 1 are ideal. Greater ratios, like that found in raw wood chips (300:1), actually cause loss of nitrogen (denitrification) from the soil when applied as a mulch.
Actinomycetes are a unique group of microbes that actually link the bacteria and the fungi. They are saprophytic and decompose organic matter. Many live exclusively in the rhizosphere and give soil the earthy odor. Many have been isolated from soil and found to produce antibiotics. Streptomycin comes from the actinomycete, Streptomyces and actionomycin comes from Actinomyces. Their main functions in soil health are the antibiotics affecting root disease pathogens and their ability to decompose organic matter.
Other microbes, involved in this belowground carbon cycle, are fungi. Fungi are especially significant in acidic soils because many bacteria are adversely affected by acid soils. They produce enzymes more capable of decomposing structural components of the shed plant material like cellulose and lignin in woody debris than do most bacteria. There are thousands of these wood decaying, saprophytic fungi. Many produce large conks on living trees and on woody debris on the forest floor. Bacteria are also intimately involved with these fungi in a succession resulting in wood decay. Thousands of other fungi, like the molds Penicillium and Aspergillus, are saprophytes also. They decompose the simple carbon compounds like sugars in various organic matter in soil. Some produce antibiotics, like penicillin, that can reduce the populations of harmful bacteria and fungi. Others, like species of Trichoderma and Gliocladium, may produce effective antibiotics but also may directly attack and parasitize mycelia of pathogenic fungi and, thus reduce the incidence of root disease.
Algae represent another population of soil microbes that have important functions in soil. However, their numbers are far fewer than bacteria and fungi. They occur mostly in moist soils and their numbers decrease rapidly with soil depth because sunlight and photosynthesis is reduced except on the soil surface. They are highly susceptible to soil disturbance. Some algae fix atmospheric nitrogen and produce mucigel that contributes to soil aggregation.
Now let's discuss the major symbionts of plants…mycorrhizal fungi. Over 95 percent of the green plants of the world form symbiotic relationships with mycorrhizal fungi. These unique, root-inhabiting fungi colonize either the outside of fine absorbing roots (ectomycorrhizae) or the inside of the roots (endomycorrhizae). Ectomycorrhizae occur on about 10 percent of the world Mycorrhizae are able to absorb, accumulate and transfer essential elements and water to plants more rapidly and for longer periods of time than nonmycorrhizal roots. From a practical perspective, it would require approximately 100 times more sugars and energy from photosynthesis for a tree to form enough nonmycorrhizal absorbing roots to produce the same surface area formed by the mycelia of mycorrhizal fungi and the mycorrhizae. Trees and other plants are simply not able to produce 100 times more photosynthate; thus, they evolved a dependency on mycorrhizae. Mycorrhizae live and function longer than nonmycorrhizal absorbing roots, increase the tolerance of their tree host to drought, soil compaction, high soil temperatures, heavy metals, soil salinity, organic and inorganic soil toxins and extremes of soil pH. They also depress many root diseases caused by pathogenic fungi and nematodes. Recently, mycorrhizal plants were found to suppress the attacks by certain foliar insects by increasing the natural defense chemicals produced by healthy plants. Mycorrhizal fungi of any type do not significantly decompose soil organic matter but may acquire certain elements, such as nitrogen and phosphorus, from the organic matter and share them with their plant host. An important prerequisite to remember for mycorrhizal development is that nonwoody, susceptible roots must be preformed before they can be colonized by the fungi and become mycorrhizae. Susceptibility means that the simple sugars required by these fungi are available to them in these roots. Remember, everything is based on availability of essential carbon.
In natural forests and grasslands, many species of mycorrhizal fungi share common plant hosts and form a continuous, interconnecting network of mycelia on roots between the plants. It has been shown that dominant forest trees in full sunlight will actually transfer sugars through a common ectomycorrhizal fungal mycelial network to roots of adjacent understory trees that are shaded and produce less photosynthate. The dominant trees can function as nurse trees to the understory trees and improve growth and competitive abilities of the smaller trees. Photosynthate transfer between grasses sharing a common VAM fungal mycelial network has also been reported.