November has a certain scent to it. More pronounced when wet, it is the bitter odor of decomposing leaves. Along with this fragrance is the odor of acorns and their tannins. The great process of recycling of nutrients; the breaking down of large molecules into smaller molecules, the decomposition of carbohydrates, proteins and sugars into water, gases, minerals and nitrates once again begins its annual cycle.
Every fall, an average of one pound of leaves and debris fall on every square yard in the forest. Under this litter are the remains of last years leaves, animal carcasses and animal waste products. Numerous holes in the leaves are evident from the rich community of decomposers living in the soil. In fact, representatives of all kingdoms are found in the soil environment – plants, animals, fungi, protozoa, and bacteria.
At the base of this food chain are the bacteria and fungi. In fact, these two groups consume 80-90% of the energy contained within the litter. The remaining fuel is primarily eaten by earthworms, isopods (pill bugs), millipedes, springtails and mites. Others feed on the bacteria and fungi, including many mites, springtails, ants, termites, roundworms and protozoans. Still other generalists feed on both the bacteria, fungi and organic matter, including many roundworms, beetle mites, springtails, and fly and beetle larvae. These, in turn, are fed upon by predaceous spiders, mites, centipedes, beetles, roundworms and predatory mites.
We now know that the biomass that exists underground may comprise four times more biomass than what we see above ground. Such fossorial wildlife includes algae, which can be found at densities as many as a hundred million cells per gram of soil. These fuel packages provide a basis of life in the soil. To give you an ideal of the subterranean populations, here is a recipe for a typical woodland soil. In each square yard of soil, add 100 - 500 earthworms, 500 – 200,000 arthropods (including 500 to 5,000 springtails), and 20 million nematodes (roundworms). And, to each teaspoon, add: 1,000 – 1 million protozoa, 100 million to 1 billion bacteria and 60,000 yards of fungal hyphae. It is with numbers like these that we might appreciate this subterranean food web as being so crucial to the health of our forest ecosystems. It is this staggering community of life that creates the final product of organic decomposition found beneath the shallow surface litter; that being humus.
Leaves are difficult to decompose. Besides large hard to breakdown molecules of lignin and cellulose and protective waxes, leaves have various levels of tannin, which must be leached out. Slow moving streams draining areas with high tannic leaves, such as oaks, will frequently have tea-colored water. Once a majority of these protective tannins have leached out of the leaves, an assortment of bacteria and fungi enter the leaves through holes created by a myriad of microscopic invertebrates. The softer, simpler compounds of the leaf are first eaten, leaving only the thicker, tougher lignin-filled veins to create a skeletonized leaf.
Leaf decomposition is also largely affected by the carbon to nitrogen ratio found within the leaves. Carbon constitutes much of the hard to decompose cellulose material of leaves, while nitrogen makes up much of the easier to digest proteins. The easiest leaves to break down are alders (15:1 ratio), sugar maple (20:1), and elms (28:1). These leaves can decompose in a year or two. Others, such as basswood (37:1) and some oaks (47:1) take two or three years to decompose, while some with a high ratio of carbon to nitrogen, such as some oaks (60:1), beech (51:1) and pine needles (66:1) may take more than three years to be totally digested and excreted as humus products. Compare this to animal carcasses and waste products (not including bones), with a ratio of 3-5:1, which is rapidly decomposed in a matter of weeks during summertime.
The high C:N ratio of pine needles and short growing season (along with the associated low pH, which restricts the number of bacteria which can exist), explains why the litter layer in northern boreal forests is so thick. Conversely, in the warm tropical forests, once the temperature exceeds an average of 80 degrees, the rate of decomposition actually exceeds the rate of litter deposition, such that practically all of the organic matter of a tropical forest is tied up in the living plants of the forest. Humus never really has a chance to accumulate. Unfortunately, when the forests are burned and the soil exposed, the rains leach away the few remaining nutrients beyond the reach of the plant roots.