Life Underground Note: Links to photos of soil organisms can be found at the end of the text.

The soil community of eastern forests is the most underappreciated, undervalued, and under-represented in the education of ecologists and naturalists. So close, but yet so far away, the value of the soil community is not appreciated with unaided human eyes, but with microscopes and molecular analysis. The chemical and physical properties of organic rich forest soil, recycles precious nutrients and minerals, provides adequate moisture and habitat for seeds and spores to germinate, and soil animals to live and prosper, buffer all the organisms from extreme temperatures, and grant a long-term lease for the forest to occupy space.

Forest soil is teeming with life. Each cubic centimeter of soil holds billions of bacterial cells from countless species. Fungi exist as threads and clumps of soft tissue within the soil matrix. Algae, protozoa, slime molds, and arthropods are plentiful. Millipedes, centipedes, salamanders, moles, and chipmunks are among the most obvious of soil life, but it is the billions of microscopic nematodes, springtails, mites, miniscule beetles, beetle larvae, and fly larvae that dominated the soil community and are most important in maintaining the continuity of the forest. A human footprint in the forest literally compresses the life out of millions of microscopic soil organisms.

The soil community operates as a massive machine. The functional components of this machine are analogous to the digestive system of a mammal. Soil organisms shred and eat organic debris, digest organic matter, extract useful energy, and recycling mineral elements to the forest ecosystem. The digestive system analogy works well for understanding how and why organic matter decomposes, but it does not pay full homage for the ecological value and biological diversity of the soil community.

The analogy begins with a bite from an apple. Incisors and canines tear apart and dismember the apple into manageable pieces for our molars. These giant, flat-faced teeth grind and pulverize the apple into microscopic bits. The total surface area of the apple has increased.

Large surface areas increase the exchange properties of materials with the environment. This physical law is observed and appreciated throughout biology. Plant and animal structures with great surface area specialize in managing opportunities for exchanges with the environment. The microvilli of the small intestine increase the area for absorbing nutrients from food, the numerous small alveoli of the human lungs maximize gas exchange, the high surface area of plant root hairs maximize water and mineral ion uptake, large flat ears of African elephants increase the rate of heat exchange with the surrounding air. The increase surface area of the apple improves chemical reactions and the absorptive potential of the digestive system of the mammal.

The mashed apple bits contain soluble sugars, large starch molecules, and tough pieces of skin. The mashed apple is pretreated with an aqueous secretion from the salivary glands containing the enzyme amylase. Amylase cleaves larger starch molecules into simple sugars such as glucose. Together, the simple sugars of glucose and sweet tasting fructose from the apple meat are readily absorbed by the lining of the stomach and small intestine with little effort. The large complex molecules of proteins, pectins, and hemicelluloses require much more chemical treatment and energy to breakdown. The integrity of these molecules persists into the small intestine. The apple mass has now been chemically altered to a low pH and a new array of enzymes clip smaller subunits off of the larger molecules. It takes more energy and work, but amino acids can be retrieved from proteins and absorbed by the high surface area of the small intestine.

Still many large, complex carbohydrates and lipids survive the chemical assault of the stomach and early small intestine. The micro fauna of the animal gut now jump in with their metabolic machinery. We are all born with a sterile intestine. Our mothers provide a healthy inoculum of bacteria that colonizes our intestine. The microbial population digests complex carbohydrates and lipids. The smaller byproducts of microbial digestion are important nutritional molecules that we readily absorb. Without our intestinal micro fauna, all of the apple skin and many other plant tissues would pass through our intestines unaltered.

Figure 1. Flow chart of organic matter (solid brown arrows) element release and recycling (green arrows with border) and organism involved in soil ecology (green ovals).

The soil community functions similar to the mammalian digestive system. Large pieces of organic litter must be shredded and macerated to allow nutrients to be released and increase surface area for chemical digestion. The simple, accessible chemicals and nutrients are quickly absorbed by opportunistic organisms. Mineral ions are either leached from the soil or absorbed by plant roots or mycorrhizal fungi. Simple carbohydrates and starch are easily used by primary detritivores and scavengers. Millipedes and earthworms are two important shredders in the soil community. Each ingests large quantities of leaf matter and digest simple carbohydrates, but also receive nutrition by eating bacteria and fungi on the leaf surface. Large quantities of leaf material are ground and macerated then cast aside in the feces of earthworms and millipedes. The moist and pasty leaf litter now has a large surface area that favors chemical digestion by bacteria and fungi. At least initially, the first bacteria and fungi to colonize macerated litter make use of the smaller and readily available organic molecules. The microfauna secrete enzymes to digest starch, cellulose, and pectin. Only then can nutrients be absorbed and nutrients released to the soil.

An entire soil community revolves around this initial digestion or decomposition of organic matter. Tardigrades, springtails, and other decomposers feed on bits of organic matter, fungi, and bacteria that are primary decomposers. Centipedes, pseudo scorpions, and beetle larvae become major predators in the food chain. All of these arthropods alter the community to some degree and leave their chitinous exoskeleton to the decomposition process upon death. Ants and earthworms are critical and mixing the soil by carrying organic. They transport nutrient rich bits deeper into the soil and bring mineral bearing inorganic soil to the surface where weathering adds to the elemental ions in the soil. The mixing action of earthworms and ants also transports fungal spores and bacteria elsewhere in the soil matrix.

Still the tannins and lignins have hardly been touched by the microbes, but this will soon change. Tenacious bacteria with a different suite of metabolic machinery begin the arduous task of disassembling these molecules for scant amounts of energy and nutrition. The remains of lignin are still large and indigestible, but are reformulated into organic acids that now play a critical role in the physical and chemical properties of the soil. In summary, here are the critical stages of decomposition (1) shredding, maceration and preparation of litter, (2) leaching of elemental ions, (3) chemical extraction of simple, readily available nutrients, and (4) further digestion of large complex biological molecules. The process is accomplished by many organisms that assist and aid in one or more of these stages.

In the deciduous forest, the seasonal shedding of leaves provides the soil, streams, and vernal ponds with the organic building blocks of life. The complex chemistry of leaves and wood envelops the key mineral and organic nutrients to fuel the soil economy. Nitrogen abounds in proteins such as RuBP carboxylase, amino acids, nucleic acids such as RNA and DNA, alkaloids like caffeine, and what may be left over of chlorophyll molecules that originally captured the energy to manufacture the diversity of biological chemicals. Phosphate is found in nucleic acids, energy transfer molecules, and the phospholipids of cell membranes. A plethora of elemental ions, calcium, potassium, magnesium, and sulfur, are found throughout the cells and tissues of leaves and wood. Some of these ions are attached to organic molecules; others are washed free by precipitation. All must reinter the biosphere for organisms to continue to build these important biological molecules, cells, and tissues. Nutrient cycling is the lifeblood of the forest ecosystem.

The organic molecules of leaf and wood litter provide the capital energy investment in soil community. Simple sugars, starch and cellulose are readily available sources of chemical energy that are most easily extracted from litter. Simple sugars such as glucose move quickly into bacteria, fungi, and the gut of many invertebrates such as earthworms and millipedes. Most organisms can process starch and digest it into simple sugars of glucose for quick energy. Cellulose, which provided the structural backbone to plant cells, can only be digested by those bacteria and fungi that secrete a specific enzyme, cellulase. This enzyme methodically clips useable sugars of glucose and maltose from cellulose.

High quality carbon molecules, starch and cellulose, are the first to break down by bacterial and fungal digestion. As much as 87% of a sugar or red maple leaf contains high quality carbon molecules that break down quickly. American beech and red oak leaves, on the other hand, have lower quantity of high quality carbon molecules and have more lignin and tannin. Thus, beech and oak leaves decompose more slowly on the forest floor. As the high quality carbon molecules disappear from the soil surface the concentration of low energy lignin increases.

Other plant molecules require much more time, treatment, and processing to extract energy. The hemicullusoes and pectins are the first to breakdown among this group, but the tannins and lignins are the most enduring. Lignin is a monster molecule. Scientists have worked diligently to identify its chemical structure which to this day can not be man-made in the laboratory. Lignin is hard, durable, and virtually indestructible. Oak and hickory trees produce high lignin wood that is hard, durable, and decay resistant. Trees with low lignin content, such as poplar and red maple, are soft and readily decompose on a woodland floor. Tenacious bacteria work slowly to extract the meager energy resources of lignin. In the long run, the chemical modification of lignin and its incorporation into the soil matrix is critical to the physical and chemical properties of forest soil.

Tannins are large organic molecules that modify soil properties. Tannins are believed to function as an anti-herbivore compound in plants. Tannins bind proteins and in the gut of some insects, this action ceases digestion and may ulcerate tissues. Other insects overcome this toxin and will eat the leaves and wood without illness. Oak and hemlock are major tannin producers. Tannins leaching from hemlock bark and needles grant streams and lakes a rich brown color. The lakes and ponds of New England forests are colored brown because of hemlock tannins. Hemlock bark was once a major forest product and it provided a rich source of tannins to bind skin proteins in animal hides. The process of tanning leather toughens hides into a long lived, durable product.

Tannins may inhibit the growth of certain bacteria, insects, and the seed germination of woodland herbs. Soil tannins are known to inhibit enzyme activity and slow the decomposition of leaf and wood litter by soil bacteria. Tannins also precipitate proteins in the soil. Thus, high tannin loads in the soil serve to slow decomposition and mineral recycling. Thus, one can observe thick layers of litter in forests dominated by tannin rich species such as hemlock in eastern North America.

Everyone loves a vegetable garden that is dark, crumbly, and soft. We appreciate our intuition that a garden soil rich in biochemically reorganized organic molecules is mineral rich, well aerated, and provides an environment prime for a high yield garden. The long-lasting organic component of soil is refereed to as humus. The undigested organic acids from lignin decomposition carry a negative charge. Humic and fulvic acids act as a magnet for positive charged minerals and elements such as calcium, magnesium, potassium, sodium and aluminum. This chemistry allows soil to aggregate into small clumps or clusters thus contributing to the crumbly nature of soil. A humus-rich, crumbly soil provides abundant pore space for gas exchange and water percolation through the soil. Root hairs, root tips, and microscopic animals easily probe and migrate between soil crumbs. A soil rich in humus retains elements critical for plant growth and fosters a community rich in microscopic and macroscopic animals. Without ample humus, the soil bakes, hardens, looses air/pore spaces, and leaches minerals. Garden plants flourish in soils with ample nutrient rich organic humus.

Elements such as potassium and calcium are quickly released from leaf litter because they are not bound to organic molecules. They recycle into the soil and are reabsorbed by plant roots. Sugar maple, red maple, and white ash accumulate calcium and require more soil calcium for germination, leaf production and growth. This can lead to a “Calcium ion” pump in many soils. As more calcium weather from inorganic soil particles, these trees accumulate more calcium ions. Upon death and recycling, their leaf litter releases calcium which then leads to a net accumulation of calcium in the soil. The calcium ion pump tends to increase the pH of forest soils underneath the canopies of these species. Sugar maple, in particular, has a high demand for calcium during seed germination and sapling establishment. In areas of the U.S. affected by acid precipitation, calcium leaches from the soil quickly. The negative charge of acid rain effectively pulls positive elemental ions such as calcium from the humus. The loss of calcium rich soils has lead to a decline in sugar maple saplings in northern hardwood forests of New York and New England.

Organic matter decays very slowly in the northeast where cool temperatures, and cold winters slow the progress of decomposers. In southern forests, and those of the tropics, organic matter is digested much more rapidly. Warm soils possess far less humus than those of cooler temperate forests. As one would predict, this has a significant impact on nutrient cycling in warm southern and tropical forests. These forests have lower diversity of micro fauna and flora, and invertebrate animals. The soil is more compacted, hard, and does not retain nutrients as well. In tropical forest, most of the elemental nutrition is firmly held in the living biomass. Upon death, the warm, moist conditions favor rapid decomposition and the nutrients are quickly absorbed by fungi and plant roots. Temperate deciduous forests maintain a healthy dose of mineral nutrients in the humus of the soil. In fact, organic matter accumulates at a faster rate than it decomposes in cool deciduous forests. In addition to the local implications of maintain nutrient loads in the soil, cool temperate forests have a more global impact by storing large quantities of organic matter. In temperate zones, decomposition outpaces storage when the forest is removed and the soil is warmed. Thus, forest removal increases atmospheric carbon dioxide in two critical processes. First, the organisms capable of removing carbon dioxide from the atmosphere are removed. Second, warming of exposed forest soils lead to rapid decomposition and accelerated carbon dioxide release.

Nitrogen and phosphorus are more likely to limit growth in terrestrial systems than calcium, potassium, and magnesium. Phosphorus is most often found in minerals that are -3 abundant in Earth’s rocks. Weathering of rocks releases small amounts of phosphates (PO4 ) into water. It is the aqueous phosphate that is readily absorbed by plants and incorporated into nucleic acids, phospholipid membranes, and high energy compounds such as ATP. In the soil, the negatively charged phosphate quickly leaches into ground water if not absorbed by plants. Once in streams and rivers phosphate travels quickly to oceans where it deposits in sediments.

Nitrogen is an abundant element in Earth’s atmosphere. Ninety-nine percent of Earth’s nitrogen is found as N2 gas in the atmosphere. There it is chemically inert and although 78% of the air we breathe is nitrogen gas, it doesn’t hinder the function of hemoglobin for removing oxygen. The demand for nitrogen by living organisms is very high. Animals obtain nitrogen for nucleic acids and proteins through their diet. The trick is getting nitrogen into plants.

The nitrogen cycle is balanced by soil organisms that have the ability to perform nitrogen fixation. This process is accomplished by specific groups of bacteria and results in the transformation of nitrogen gas to soil ammonia. Without nitrogen fixation, life on land and in the water just would exist. Nitrogen fixation is an energy demanding process. In an industrial setting, ammonia production from atmospheric nitrogen requires temperatures in excess of 900ºF and very high pressures. The process consumes large quantities of fossil fuels but results in nitrogen fertilizers for agriculture. In bacteria such as Rhizobium and actinomycetes (bacteria that behave as fungi), nitrogen fixation relies on the operation of the planet’s second most important enzyme called nitrogenase. All nitrogen found in plants animals, you, and me was initially captured and converted by nitrogenase in bacteria.

Nitrogen fixation is so energy demanding and difficult that bacteria would prefer not to invest in the process. Rhizobium can be coaxed into producing ammonia in the roots of legumes. The resulting symbiosis is one of the most fundamental and important interrelationships in terrestrial ecosystems. Legumes such as clover, alfalfa, beans, peas, cow vetch, coerce Rhizobium by secreting chemical attractants from roots (Figure Root Nodules). The bacteria attach and invade root cells and the plant forms a large protective nodule for the bacterial colony. The plant shuttles carbohydrates to the bacteria to fuel nitrogen fixation. To make the process more efficient a unique pink pigment called leghemoglobin is produced within the nodules to bind free oxygen. This allows nitrogenase to function more efficiently in an oxygen free environment. Leghemoglobin is chemically similar to mammal hemoglobin, but its production is coded by genes that reside both in the legume and the bacteria. It takes one molecule of nitrogenase 1.2 seconds to make one molecule of ammonia. From the standpoint of enzyme kinetics, 1.2 seconds is a virtual eternity as most enzymes complete their catalyzed reactions in millions of a second.

Legumes are recognized as a vital cover crop for agricultural fields and in crop rotation. As legumes and Rhizobium die and decompose ammonia and nitrates are released and replenish nitrogen in the soil. In tropical regions, legumes constitute a large species-rich group of trees. There they are very important in land reclamation following abandonment from agriculture. Shade grown coffee and cacao plantations are most productive underneath a forest canopy rich in legume tree species. A second group of nitrogen fixing bacteria is an unusual filamentous group called actinomycetes. The form of these bacteria resembles the hyphal filaments of fungi whose name carries the “myco” root. Actinomycetes are best known for the antibiotics they produce such as streptomycin, tetracycline, and neomycin. In soils, actinomycetes form symbiotic relationships with non-leguminous flowering plants such as alders, waxmyrtle, Australian pine, and some blackberries. Inca farmers in the Andes mountains learned that terraced gardens bordered with red alder where more productive than those without the alder and its symbiotic actinomycetes.

Plants can also utilize nitrates. Other soil dwelling bacteria convert ammonia to nitrates. Ammonia and nitrate differ in their chemical charges. Ammonia is basic and has a positive charge. Acidic, negatively charged humic acids will hold ammonia ions in the soil. Unlike ammonia, nitrates are negatively charged and not held tightly by soil aggregates. Thus, once bacteria convert ammonia to nitrate, the nitrate must be quickly absorbed by plants or the nitrogen is leached into the water table.

Litter from different sources varies in the quantity of biological nitrogen. Green leaves are a rich source of temporary nitrogen. The nitrogen is locked into chlorophyll and photosynthetic enzymes such as RuBP Carboxylase. After leaves die, these molecules quickly denature and the nitrogen is lost. Tilling green leaves into your garden soil is a positive method of adding soil nitrogen. The green manure improves the population of decomposing bacteria which in turn act as a temporary storage facility for nitrogen. As bacteria slowly die, their remains release nitrogen which is used by plants. Supplementing garden soil with wood chips or organic matter low in nitrogen slows the decomposition process and nitrogen becomes scarce in the soil. Elm and maple leaves have a higher ratio of nitrogen relative to carbon in comparison with oak, hickory, and beech leaves. Leaves with high C:N ratios (>50:1) decompose far more slowly than leaves with low C:N ratios. It also follows that these slow decomposing species produce leaves and wood with higher lignin content and, therefore, are more resistant to decay and decomposition in forest soils.

We have determined that litter and humus provide vital roles for establishing a healthy soil ecosystem and nutrient cycling. The role of humus in the soil is augmented by the animals that its very existence supports. The nematodes, pillbugs, earthworms and other require crumbly, moist, aerated soil to survive. These organisms tunnel further into the soil and transport organic matter and humus to deeper depths. This action further richens the soil by transporting nutrient rich organic matter to plant roots, aerating more soil, and maintaining the crumbly matrix of soil.

In Europe, earthworms have long been recognized as a biological conveyor belt for organic matter in soil. As a young naturalist, Charles Darwin estimated that 10 tons of soil was brought to the surface by earthworms on each acre of meadow soil within a period of a year. In forests of eastern North America this service is provided by many soil invertebrates and small mammals. Earthworms have only recently invaded North American forest ecosystems and their presence is destructive. The rich fauna and flora of under story communities in eastern deciduous forests rely on a thick bed of litter and humus. Many spring wildflowers rely on seed dispersal and germination in the organic rich zone and rarely send their roots into inorganic horizons of soil strata. Slow and deep decomposition is the rule of thumb for a species rich forest ecosystem in eastern North America.

The earthworm invasion in North American forests is permanently altering forest structure. Earthworms are large detritivores and they speed up the rate of decomposition in eastern forest. The earthworm effect, in turn, diminishes the depth of litter and humus in the soil, reduces soil carbon, and speeds up the rate of nutrient loss from the soil. Earthworms assimilate very little of the leaf litter that they digest. To accommodate low digestive efficiency, earthworms ingest many times their body mass in leaf litter on a daily basis. Earthworms ingest both organic matter and mineral soil. Their gut possesses both cellulase and chitinase to digest cell walls of both fungi and plants as well as arthropod exoskeletons. Their processing and transport of large leaf litter quantities to moist areas of the soil facilitates microbial decomposition of the leaf litter. The details on earthworm invasions on northern hardwood forests are still being assessed by ecologists, but the implications on diversity, nutrient cycling, and carbon storage are likely to be significant.

Native North American earthworms were extirpated from glaciated regions over the past 25,000 years. Many of the earthworms present in modern forest of eastern North America are escapees from Europe and Asia. Their transoceanic travels likely occurred via multiple routes including transport of agricultural plants with soil and exchange of ship ballasts. Today, humane releases of fishing worm bait by fishermen also contributes to the earthworm invasion. Two of the more common exotic earthworms are Lumbricus terrestris and L. rubellus, but it is likely that at least 45 other species have made North American forest home since colonists first set foot on American soil. Soil Community Members. Arthropods (Phylum Arthropoda) are the most numerous animals in temperate forest soil. Arthropods are characterized by segmented body plans, jointed appendages, and a chitinous exoskeleton. Members of this phylum found include insects, spiders are relatives, centipedes, millipedes, and pill bugs. Other phyla may represent fewer species, but these still bear an important ecological service in the forest. These groups include tardigrades, nematodes, earthworms, and gastropods (Phylum Mollusca).

Phylum Tardigrada (waterbears, Figure Non-arthropods) are a curious group of microscopic animals related, but separate, to arthropods. Tardigrades are slow, lumbering creatures found on moist surfaces. They have a chitinous exoskeleton, but eight un-jointed legs with each bearing four claws. They have a fairly narrow tolerance of living conditions, but are extremely resistant to drought, radiation, and temperature extremes. They are most likely to be found in patches of moss or on lichens in the forest. They eat protozoan and surface microbes.

Tardigrades are best known for extreme survival strategies. Their cuticle is permeable to water and stressed tardigrades can dehydrate into small rounded bodies. Re-hydration of the tardigrades usually reactivates cellular machinery and brings the sleepy water bears back to life. Popular literature reports that tardigrades have been returned to life following desiccation on herbarium specimens that were pressed and dried more than 100 years ago. A recent “out-of- this-world” study demonstrated that cryptobiotic tardigrades can survive both the vacuum of space and radiation. Tardigrade adults were desiccated and transported aboard a European Space Agency rocket. Tardigrades exposed only to the vacuum of space had re-hydration survival rates comparable to control tardigrades. Tardigrades exposed to the vacuum of space and solar UV radiation had significantly lower survival rate, but at least a small percentage of one tardigrade species survived both conditions.

Phylum Nematoda (nematodes, Figure Non- arthropods) are simple tube-like worms. They are exceedingly common and have many ecological roles. Nematodes are numerically the most abundant animal group in soil samples. Some nematodes are parasites on plants or animals. Other free-living nematodes act like decomposers, but likely derive most of their nutrition by feeding on fungi or bacteria. Nematodes will appear translucent to slightly whitish, thread-like, and tubular. Nematodes have a rapid life cycle and reproduce five to six times during a growing season.

Phylum Annelida (earthworms) are segmented worms. They are decomposers and scavengers in forest floors. Earthworms had been previously viewed as a healthy component to garden ecosystems. Ecologists now believe that most North American forests are not native habitats to earthworms. North American forests usually have a rich layer of litter that slowly decomposes and provides suitable habitat for many forest herbaceous perennials. Earthworms accelerate decomposition, reduce forest litter and recycle nutrients faster than is intended in the forest. Earthworms appear to have direct effects on soil chemistry and biodiversity.

Phylum Molusca (snails slugs) are primarily herbivores. They often eat dead plant material, but can be significant pests on herbaceous living plants. Slugs often eat a variety of forest fungi and may be important spore dispersers. Gastropods secrete slime from their foot that cushions and lubricates their movement across the forest floor. Slime trails are often seen across vegetation and fungi fruiting bodies. Gastropod mucous proteins may play a role in stabilizing and improving soil structure. Unlike millipedes, slugs and snails possess an arsenal of enzymes capable of hydrolyzing large complex polysaccharides and cellulose

Arthropods. Class Diploda (millipedes) are terrestrial arthropods with multi-segmented cylindrical bodies. Each body segment possesses two pairs of short legs. Antennae are short with seven segments. Millipedes are found under logs and leaf litter. They feed on decaying plant matter although they may acquire their nutrition from the bacteria and fungi on plant surfaces. They produce a lot of waste that is very important in ecosystem nutrient cycling. The waste is macerated leaf litter and the increased surface area promotes digestion by bacteria and fungi. Millipedes produce a foul odor that is emitted from openings along the body side.

Class Chilopoda (centipedes) are terrestrial arthropods with multi-segmented flattened bodies. Centipedes have only one pair of legs per body segment and they are quick and agile. Antennae are significantly longer than millipede antennae. Centipedes are predaceous on soil dwelling insects. Their bite with modified forelegs releases a toxin that quickly paralyzes prey. In some centipedes, the toxin includes serotonin and histamine which cause pain and proteolytic enzymes to help dissolve prey. Larger centipedes may inflict painful bites to humans. Centipedes lack a waterproof exoskeleton and as a result seek moisture laden areas of the soil.

Class Symphyla (Symphylids) are herbivores with unpigmented bodies similar to centipedes. Symphylids have 15-22 bodies segments with a total of 10-12 leg pairs. Antennae are relatively long and most definitely beaded. Symphylids are uncommon but prefer organically rich soil. They are grazers on the roots of annual plants and may present considerable problems in agricultural settings.

Class Isopoda (Pillbugs, sowbugs, Figure Miscellaneous Soil Invertebrates) are small soil crustaceans with flattened oval- shaped bodies. They are more closely related to lobster, crab, crayfish, and shrimp than to insects and arachnids. Pillbugs lack a carapace and their seven cephalothoracic segments possess leg-like structures. Pillbugs are scavengers of organic material. Isopods exchange gas through gills and require high humidity to survive. Thus, in my garden where water is plentiful, they are frequently found underneath pots that remain very moist. The pillbug response to predator threat is to roll into a tight ball and remain very still.

Class Arachnida (spiders, ticks, harvestmen, and pseudoscorpions, Figure Miscellaneous Soil Invertebrates) are common terrestrial arthropods distinguished by possessing four pairs of segmented legs. The forest soil is loaded with arachnids. Mites are among the most numerous and least understood creatures of forest soil.

Harvestmen or Daddy long-legs (Order Opiliones) are scavengers or herbivores. They are best identified by their extremely long legs and segmented abdomen. Ticks and mites have a large oval-shaped abdomen that is unsegmented. The abdomen shares a broad boundary with the thorax. Ticks and mites are most often parasites on birds and mammals although a few are plant parasites. Plant mites will form irregular shaped galls on leaves. Many soil mites feed on soil fungi.

Spiders (Order Araneae) are predators on insects. Their large, round abdomen, shows a distinct constriction where it joins the thorax and small spinneret structures posterior. The venomous bite of spiders contains proteolytic enzymes to digest proteins. Spiders lack compound eyes and instead have eight separate eye clusters. Spider eyes shine a green color at night in the forest at night with a flashlight. This is a wonderful activity to demonstrate the abundance of spiders and their crypticity.

Pseudoscoprions (Order Pseudscorpioliones) are predators with an enlarged pair of pedipalps for grasping prey. The have an elongated pear-shaped body and a small head. They hunt in absolute darkness by detecting vibrations in the soil. Springtails are likely a major prey item for this group. Pseudoscorpions lack physical structure for detecting light and sound. They move slowly and frequently hitch rides on other arthropods through the soil.

Ticks and mites (Order Acarina, Figure Mites) have a large oval-shaped abdomen that is unsegmented. The abdomen shares a broad boundary with the thorax. Ticks and mites are most often parasites on birds and mammals although a few are plant parasites. Nevertheless, soil samples are usually rich in mites. They have diverse diets, but many are generalist on detritous. Some mites likely derive their nutrition from surface microbes. Like many other groups of detritivores, mite primarily fragment litter and organic matter and do little to chemically alter organic soil. Plant mites will form irregular shaped galls on leaves. Many soil mites feed on soil fungi.

Class Insecta (three segmented bodies, three pairs of legs) Order Collembola (springtails and snow fleas, Figure Springtails) are small wingless insects found in forest soil. They feed primarily on fungi and decomposing matter. Springtails are known for their forked extension on the tip of the abdomen called a furcula. Normally the fucula rests upon the ventral surface of the abdomen. It can be rapidly extended to project the insect upward at great velocity. Species found in deeper layers of forest soil generally have smaller fucula and not perception of light. Those near the surface have space for springing around the soil and opportunity to perceive light. As such, they have no need for large furcula and eyes.

Springtails are normally less than 1 mm in length. Snow fleas have a low tolerance for water inundation, hence they emerge and may be seen in early spring as the snow melts in northern forests. I have always been fascinated with the numbers and diversity of springtails in forest soils. Some springtails are pigmented purple, red, or brown. Some move quickly and others move slowly. They are by far the easiest and most numerous soil invertebrates in forest soil.

Order Diptera (Flies, Mosquitoes, midges, and gnats, Figure Diptera). This order of insects has a wide-range of ecological roles in terrestrial ecosystems. In forest soil, fly larvae are important decomposers and recyclers of organic matter. The legs of fly larvae are far less developed than those of beetle larvae, another group of insects that are common in forest soil. Adult flies can be identified by possessing a single pair of wings with the hind-wings modified into halteres. These structures are small “lollipop” or knob-like structures on the thorax. They may function by providing gyroscopic stability during flight.

Order Hymenoptera (Bees, ants, and wasps). Ants are by far the most numerous insect on forest floors. They have several ecological roles and may collect nectar, predate other insects, or scavenge along the forest floor. They perform important tasks that directly improve aeration and nutrient cycling in the forest. Ants are prairie dogs of the woodland. They actively bring inorganic soil particles to the surface and carry decomposing bits of matter into the soil whether nutrients can be released to the roots of plants. The act of tunneling improves soil aeration and allows metabolic activity of fungi and bacteria to proceed more quickly.

Order Coleoptera (Beetles, Figure Coleoptera) are important forest soil insects. Adult beetles are easily recognized by the thickened forewings, or elytra, which protects the thin membranous hind wings. The elytra is often iridescent or brightly colored. Many soil beetles, however, are darkly colored and most active at night. Adults have several ecological roles. Many feed or collect feces or carrion for egg and grub development. Other beetles are predators on other insects on the ground or those that collect near carrion or feces. Beetle grubs develop in the soil and are likely predatory on other insects and insect grubs. Beetle grubs are easily distinguished from annelids by possessing a distinct abdomen and three pairs of tiny legs on the thorax, and short antennae.

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