Through several important functions, soil plays a critical role in shaping and maintaining terrestrial communities and ecosystems, whether they are natural systems, such as tallgrass prairies and floodplain forests, or highly managed systems, such as agricultural fields, orchards, and backyard lawns.
One important soil function is the harboring of a diverse community of organisms that includes bacteria, fungi, protozoa, nematodes, mites, springtails, millipedes, sowbugs, earthworms, and many others. These organisms are surprisingly abundant, with thousands of pounds, hundreds of species, and billions of individuals per acre in the top few inches of soil. This community drives the decomposition of organic residues; recycles important plant nutrients like carbon, nitrogen, and phosphorus; and contributes to the formation of new soil and soil structure. With these activities, soil organisms contribute to other important soil functions, such as supporting the growth of plants both in natural plant communities and those grown for food, fiber, or energy; and absorbing, neutralizing, and transforming compounds that might otherwise become pollutants in the environment, especially in surface and groundwater.
Appreciation of the soil's importance to community and ecosystem integrity has resulted in efforts to develop methods for assessing and monitoring the condition of the soil and its functions. Assessment involves measuring soil properties at a single point in time, whereas monitoring involves making repeated measurements through time to detect changes in soil properties. In different situations, soil assessment and monitoring will serve different purposes. In highly managed production systems, such as agricultural fields, the information provided by soil monitoring and assessment can guide management decisions that help improve and sustain production in the long term while avoiding harmful environmental effects. In natural systems, the usual goal is to preserve a particular habitat or biological community. Because the balance of soil properties has a strong influence on the structure of the plant community, and thus on the above-ground community of organisms, soil monitoring can provide an early warning of changes that may lead to community or ecosystem degradation, or it may provide clues to the underlying causes of observed degradation. Similarly, soil assessment can help identify conditions in the soil that are acting as a barrier to the successful restoration of degraded communities or systems, thus providing valuable information to guide remediation and restoration efforts. Once such efforts are implemented, soil monitoring can provide the information necessary to document the progress of the project, or provide timely feedback to help refine management efforts.
A terrestrial isopod about 1 cm long. Several species of isopods, living in a range of
habitats, are found throughout Illinois. Common names for these animals are sowbugs, pillbugs, woodlice, slaters, and rolly-pollies.
In North America, several groups and agencies have been involved in developing different soil assessment and monitoring programs. For example, to address issues that affect the sustainability of forest ecosystems, the USDA Forest Service's National Forest Health Monitoring Program (http://willow.ncfes.umn.edu/fhm/fhm_hp.htm) includes measurements of soil aggregation, organic matter content, and acidity in its national forest monitoring network. INHS scientists are presently working with the Forest Service to conduct these measurements at sites in Illinois.
The Soil Quality Institute (http://www.statlab.iastate.edu/survey/SQI/sqihome.shtml) of the USDA's Natural Resource Conservation Service offers a soil quality test kit developed by the USDA Agricultural Research Service. This kit, which includes background information and guidance for interpreting results, was designed primarily for use in agricultural situations, although many of the procedures can be applied elsewhere. Similarly, the goal of the Illinois Soil Quality Initiative (a program led by Professor Michelle Wander of UIUC in collaboration with others; see Web site at: http://www. aces.uiuc.edu/~asap/isqi/isqi.html) is to identify soil quality indices that farmers can use to make management decisions within the context of their own stewardship goals.
A common feature of all of these examples and others is that they emphasize physical and chemical characteristics of the soil, such as pH, bulk density, aggregate stability, and organic matter content. Those working on developing monitoring procedures recognize, however, that the important functions performed by soil result from the activities of living organisms, and that an objective, comprehensive assessment of the soil's condition should include biological measurements. To date, few biological measurements have been developed, and those included in current programs are not detailed (soil respiration or rough estimates of earthworm populations, for example).
A springtail from the superfamily Entomobryoidea. Springtails are
abundant arthropods in soil and litter (up to half a million per
square meter), and are related to the true insects. The specimen
shown here is about one-half mm long.
Biological indicators offer potential advantages for monitoring and assessment of environmental conditions. Whereas precise physical and chemical measurements in the environment may be difficult or costly to perform, successful monitoring of the presence or abundance of species or groups of organisms could be accomplished with relatively little training and expense, as demonstrated in aquatic monitoring programs around the world. And unlike many physical and chemical indicators, which often reflect conditions at a specific point in time, the presence and abundance of organisms integrates information about past and present environmental conditions during the organisms' lifetimes. Also, because living organisms are sensitive to the interaction of many factors in the environment, they have the potential to indicate stresses before they manifest themselves in physical or chemical indicators, or even to indicate environmental stresses that we don't yet recognize as being important.
Because of the diversity of the soil community, ample opportunity exists to develop a comprehensive suite of biological indicators for the soil. Several hurdles must be overcome, however, before biological indicators of soil condition can be fully implemented: (1) many soil organisms have not yet been described, and easy-to-use identification keys are not widely available for many groups; (2) the development of widely applicable and standardized sampling methods, including determination of appropriate levels of sampling effort, will require targeted research efforts across a range of environments; and (3) a more complete understanding of the response of soil organisms to environmental stresses and disturbances, and of the roles of particular species or groups of soil organisms in mediating normal soil function, will make the detection of changes in populations of those organisms more meaningful by suggesting possible causes and effects associated with those changes. INHS scientists are working to address these problems to ensure that Illinois has the best tools available to manage and protect its valuable soil resources.
Ed Zaborski and Chris Johns, Center for Economic Entomology
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