Biological indices of soil quality: an ecosystem case study of their use

Soil quality indices can help ensure that site productivity and soil function are maintained. Biological indices yield evidence of how a soil functions and interacts with the plants, animals and climate that comprise an ecosystem. Soil scientists can identify and quantify both chemical and biological soil-quality indicators for ecosystems with a single main function, such as agricultural lands and forest plantations. However, quantifying these indices in complex ecosystems - that have multiple uses or goals such as maintaining biodiversity, aesthetics, recreation, timber production and water quality - is much more difficult. In an ecosystem context all components - plants, animals and humans - interact with the soil differently, making soil quality indices variable. These interactions result in a combination of biological processes that make each ecosystem unique. We examined the soil and site quality of five forest stands (xeric oak-pine; two mixed hardwood, cove hardwood, northern hardwood), within the 2185-ha Coweeta Hydrologic Laboratory. An initial rank of soil quality based on soil chemical and physical properties was assigned. The ranking was then compared with four common groups of soil biological indicators: (1) nitrogen availability; (2) litter decomposition; (3) soil microarthropod populations; and (4) carbon availability. We also examined estimates of overstory productivity, overstory biodiversity and total aboveground productivity for each site as indices of site quality. We found that soil and site quality rankings varied with the indicator, showing that the soil or site of greatest quality may change depending on the use or goal of the ecosystem under examination. (C) 2000 Elsevier Science B.V. All rights reserved.