Soil as a Living System

by Leslie Jones Sauer

Soil wears its problems on the surface. Where trampling or high rates of decomposition prevail, the litter layer and topsoil are entirely absent. Until recently, the annual leaf fall in the woodlands of Central Park typically did not accumulate or even persist from one year to the next. With no litter layer, there was no nursery for the next generation of the forest.

Nearly a decade of woodland management is rebuilding the ground layer in Central Park’s woodlands at the north end of the park. The site is becoming increasingly stabilized as erosion is controlled and bare areas are replanted. The many small saplings and seedlings that were planted or that volunteered after exotics removal help to hold the ground. During the icebound 1993-1994 winter season, some remains of autumn’s leaves persisted under the blanket of ice until spring. That was a turning point for the woodlands. The following winter was unusually mild, and by spring 1995 there was a relatively continuous litter layer.

In time, the organic litter on the forest floor will create humus, an organic soil horizon. Within it, most of the life of soil occurs. As organic matter is continually broken down into humus, it becomes incorporated into the mineral layer of the ground surface to build topsoil. Soils are forming all the time, and like vegetation, integrate and express all of the ecosystem’s processes. Soil is a reflection of climate, parent material, topography, vegetation, and time. The layers of soil tell a more recent history than the rocks beneath.

The soil’s abiotic, or nonliving, factors are generally the primary focus of conventional soil assessment. Much of our thinking in the past was oriented toward an “ideal” soil model that balanced sand, silt, clay, pore space, moisture, minerals, and organic matter. These standards determined whether a native soil was judged poor or good, and where soils did not conform to the ideal, soil amendments were used to modify texture, acidity, fertility, or other characteristics. Many early mitigation, stabilization, and restoration projects suffered from this agricultural/horticultural approach. Standard soil specifications, for example, call for routine topsoil stripping, fertilizing, and liming even though many disturbed or made soils are already less acid than in their native condition because of the repeated addition of lime by means of concreate rubble and urban dust.

Most regulations related to development sites, highways, landfills and abandoned mines require from three to six inches of topsoil spread over new soil surfaces before revegetating. That topsoil comes from somewhere, so the restoration of one site frequently means the destruction of another. We need more research on alternatives to topsoil, especially those that reuse waste materials appropriately to amend local soils and that avoid environmentally costly products such as fertilizers and peat. Even where topsoil has been stockpiled on a site before construction, the living organisms it contains die within days.

The Soil Food Web

A food web is the structure of relations among the organisms within an ecosystem based on what each consumes. Primary producers consume water, minerals, carbon dioxide, and a few other things to produce organic matter, which is consumed by most of the rest of the creatures that are, in turn, consumed by still others. Some organisms have very specialized food requirements while others feed quite omnivorously.

Both soil and water are media in which plants and animals live and grow. And in a very real way, both are living systems. One of the most important contributions to the history of water management occurred with a shift in perspective that originated with Ruth Patrick and others. 1 When one views water as a living system, its quality is measured by the richness of its biota instead of physical and chemical factors such as flood levels or biological oxygen demands. Its biological components are a defining measure of health that reflect a more complex array of factors. This same kind of revolution is happening in our perception of soils.

Like aquatic system, [soil system] have a great deal of redundancy. Very simple systems with simple food webs can be drastically altered by the appearance or disappearance of one or a few species. In more complex systems there may be multiple ways in which energy flows through the food web. Thus the more complex systems are said to have redundancy and are not so dramatically changed when a few species change. Many soil components even lie dormant until favorable conditions occur. The full soil structure is not required for most basic soil functions.

Rather than focusing on the nonliving aspects of soil, restoration should enhance its living components, primarily bacteria, fungi, and microfauna. Most of the work of forming humus is done by plant roots and by animal life in the soil, which depend on a permeable soil crust, stratified soil layer, and appropriate amounts of organic matter. There are up to three thousand arthropods per cubic inch of productive soil A litter layer of leaves one-and-one-half inches thick and a yard square might contain five thousand miles of fungal filaments.

Plants are the primary producers of organic matter in the forest soil system. Ants and other invertebrates initiate the breakdown of ground-layer litter. Soil microorganisms including fungi, bacteria, protozoa, and actinomycetes continue this process of converting organic matter into soil minerals that in turn become available as nutrients to plants. In food-web nomenclature these organisms are “consumers.” Primary consumers (herbivores) feed directly on the “producer,” which are the plants; secondary and tertiary consumers are predators and parasites, which feed upon each other as well as upon herbivores. Food webs also contain other decomposers and detritivores that feed on litter, such as mites, woodlice, and earthworms. Woodlands typically support more diverse assemblages of soil organisms than grasslands. If soil organisms are included in the species count temperate rainforest are richer in biodiversity than tropical rainforests.

The soil food web performs the primary function of the soil, which is to cycle energy and nutrients, including nitrogen, sulfur, and phosphorus. Native soil systems are very efficient and succeed in recycling, for example, upwards of eight percent of the nitrogen in the system. The cycling of nitrogen is intimately associated with the cycling of carbon, which is tied up largely in organic matter. Nitrogen, in part, determines the rate at which carbon is broken down. Bacteria and fungi take up the nitrogen as they decompose soil organic matter, and some fix atmospheric nitrogen. This nitrogen too is released into the soil to be again available to plants. Nitrogen’s slow release from an organic to an inorganic form, which is available to plants, is called “mineralization.”

The microbial community performs three major functions: as discussed above, conversion of organic nitrogen to a plant-available form such as ammonia; nitrification when ammonia is converted to nitrates; and denitrification when nitrogen is recycled into the atmosphere as a gas. The soil microbial community also contributes to soil stability, another vital function. Fungal hyphae knit bits of organic matter together to create a denser, stronger litter horizon and upper soil horizon.

Not all soil food webs are the same. Fungi appear to dominate in forest soils, bacteria in agriculture soils. Thus, soil communities change over time as the landscape succeeds to forest. The nature of the vegetation determines the nature of the fuel/food available for soil organism. Grassland litter, a relatively easily decomposed herbaceous material, does not typically contribute all of the soil’s organic matter. The extensive root system of grasslands is also a major source of the soil’s organic matter. The roots of grasses exude carbon directly into the soil as sugar, amino acids, and other forms to feed soil fungal associates and activate bacteria and other microbes.

As the landscape matures, the litter becomes more difficult to break down. While herbaceous litter is primarily cellulose, the litter of the forest becomes increasingly higher in lignin, the woody component of plants. Tree leaves have more lignin than grasses, and the leaves of late successional species, like beech (Fagus grandifolia) and oak (Quercus spp.), tulip poplar (Liriodendron tulipifera), and other successional species. In woodlands an important shift occurs as leaf fall and other litter becomes the most important sources of organic matter, rather than the direct contribution of carbon by the roots, as in the grasslands. There are also larger volumes of wood on the ground in the form of fallen twigs and limbs, which directly foster fungi because bacteria are unable to decompose lignin. The mycorrhizal filaments from tree roots reach up into the old wood to extract the valuable nutrients. Insects such as beetles and ants are also able to break down wood. Wood in contact with the soil and standing dead trunks (“snags”) create many opportunities for various wood and soil invertebrates of the forest.

The soil communities continue to change along with the vegetation communities. Over time, the cycling becomes less rapid. In a humus-rich forest soil, the organic matter that remains the longest is the rather stable organic compounds that degrade much more slowly. By then the humus is important more as a site for important chemical processes and for the physical qualities it gives the soil than as a stockpile of nutrients. The humus, for instance, increases the water-holding capacity of the soil.

Another important role of dead wood is to serve as a water reservoir for the forest in times of drought. Dead wood, especially larger logs approaching a foot or more in diameter, soaks up water like a sponge and retains it for long periods. Old logs or stumps make great nursery sites by carrying vulnerable seedlings through dry spells. Salamander populations also depend on large logs for needed moisture, which is, in part, why they are absent so long after clear-cuts and timbering, although they may number one or two per square yard in old-growth forests. Logs increase local storm water retention as well by inhibiting overland flow and by absorbing water in place.

Fungi in general foster acid soil conditions, whereas bacteria can increase alkalinity. The bacteria and their predators in grasslands help maintain the soil’s pH and the form in which nitrogen is made available, as well as nutrient cycling rates that work to the advantage of grasses. Where fungi are more abundant, as in natural forests, the nitrogen is converted to ammonium, which is strongly retained in the soil system. The bacteria-dominated systems, the bacteria convert nitrogen to nitrate instead of ammonium. Nitrate leaches more easily from soils than ammonium; however, the growing patterns of grasses tolerate this condition. But when woodland soils become bacteria dominated, rapid leaching may leave most native old-growth species poorly nourished while invasive exotics and some early successional natives are flush with nutrients. Some species are more sensitive than others to soil nutrition. Conifers do not grow in bacteria-dominated soils, whereas agricultural crops cannot be grown in fungi-dominated soils. Indeed, in woodlands, a high ratio of bacteria to total biomass is an indicator of disturbance.These factors, which seem to depend on soil organisms, play a greater role in succession than previously recognized.3

Damaged Soil Systems

Soils are far more damaged and damageable than we realize, but the problem is often hidden. The cumulative effects on forest systems and other environments of acid rain, nitrogen deposition, global warming, ozone thinning, unnecessary grading, and storm water changes have left a legacy of severely altered soil conditions and totally modified soil food webs. The consequences and remedies are still largely unknown.

Many of these changes are so pervasive that we take them for granted. Take earthworms, many non-native, which now are abundant throughout the urban forest system. In fact, they are not part of the historic community of living creatures in native forests and are typically associated with more disturbed landscapes. Earthworms in general increase soil fertility by initiating the breakdown of organic matter, aerating and mixing the upper soil, and creating a micro environment that stimulates the bacteria that convert ammonium to nitrate. High earthworm populations also foster nitrification by supplying the oxygen necessary to convert ammonium to nitrates. They take a system already disturbed by added nitrogen and push it farther from normal by consuming the litter layer five times as rapidly as fungi and converting excess food into nitrate. The same kind of self-reinforcing cycle can be seen when aquatic systems fill with algae.4 Each shift in the soil character will in turn ripple through the entire system. Unfortunately, in many woodlands that look mature because they have larger trees, there is a lag in the succession of the soil, which may still be dominated by earthworms and bacteria and impoverished in terms of types of fungi, invertebrates, and other, more efficient paths for nutrient cycling.

References

  1. Ruth Patrick, Natural and abnormal communities of aquatic life in streams. Via 1, Ecology in Design (University of Penn., Grad. School of Fine Arts, 1968), 37.
  2. M.J. McDonnell, S.T.A. Pickett, and R.V. Pouyat, Application of the ecological gradient to the study of urban effects. In Humans as Components of Ecosystems: The Ecology of Subtle Effects and Populated Areas, ed. G.E. Likens and W.J. Cronon (New York: Springer-Verlag, 1993) 175-189.
  3. E.R. Ingham, Restoration of soil community structure and function in agriculture, grassland, and forest ecosystems in the Pacific Northwest. Proceedings, Society for Ecological Restoration Conference (Seattle, 1995), 31.
  4. W. Nixon, As the worm turns. American Forests (1995) 101(9): 34-36.

About the Author

Leslie Sauer is principal and landscape architect with Andropogon Associates, Ltd., based in Philadelphia, PA, and adjunct professor at the University of Pennsylvania. This article is adapted from The Once and Future Forest: A Guide to Forest Restoration Strategies. The book was developed by Andropogon Associates and is based on their approach to integrating environmental protection and restoration with landscape architecture and design. Their work on Central Park’s North Woods is only one of many restoration projects. This excerpt was printed in the Summer 1999 Arnoldia, magazine of the Arnold Arboretum and is reprinted by permission of Arnoldia, the author, and Island Press.