The percentage of soil pore space filled with water (percent waterfilled pores, % WFP), as determined by water content and total porosity, appears to be closely related to soil microbial activity under different tillage regimes. Soil incubated in the laboratory at 60% WFP supported maximum aerobic microbial activity as determined by CO 2 production and O 2 uptake. In the field, % WFP of surface no-tillage soils (0-75 mm) at four U.S. locations averaged 62% at time of sampling, whereas that for plowed soils was 44%. This difference in % WFP was reflected in 3.4 and 9.4 times greater CO 2 and N 2 O production, respectively, from surface no-tillage soils over a 24-h period as compared to plowed soils. At a depth of 75 to 150 mm, % WFP values increased in both no-tillage and plowed soils, averaging approximately 70% for no tillage compared with 50 to 60% for plowed soils. Production of CO 2 in the plowed soils was enhanced by the increased % WFP, resulting in little or no difference in CO 2 production between tillage treatments. Nitrous oxide production, however, remained greater under no-tillage conditions. Substantially greater amounts of N 2 O were produced from the N-fertilized soils, regardless of tillage practice. Production of CO 2 and N 2 O was primarily related to the % WFP of tillage treatments although, in several instances, soil-water-soluble C and NOf levels were important as well. Calculations of relative aerobic microbial activity between no-tillage and plowed soils, based on differences in % WFP relative to maximum activity at 60%, indicated linear relationships for COj and NjO production between WFP values of 30 to 70%. Below 60% WFP, water limits microbial activity, but above 60%, aerobic microbial activity decreases-apparently the result Of reduced aeration.

Tillage intensity affects soil structure and the loss of soil organic C and N. We hypothesized that no-tillage (NT) and conventional tillage (CT) differentially affect three physically defined particulate organic matter (POM) fractions. A grassland-derived Haplustoll was separated into aggregates by wet sieving. Free light fraction (LF) and intra-aggregate POM (iPOM) were isolated. Natural abundance "C was measured for whole soil C, free LF C, and iPOM C. The mean residence time of soil C under CT (44 yr) was 1.7 times less than in NT (73 yr). The amount of free LF C was 174,196, and 474 g C m~2 for CT, NT, and NS, respectively. Total iPOM C amounts in CT, NT, and NS were 193, 337, and 503 g C nr 2 , respectively. The level of fine iPOM C (53-250 jim) level in macroaggregates (250-2000 urn) obtained after slaking was five times greater in NT vs. CT and accounted for 47.3% of the difference in total POM C between NT and CT. The amount of coarse iPOM C (250-2000 (Jim) was only 2.4 times greater and accounted for only 21% of the difference in total POM C. Sequestration of iPOM was observed in NT vs. CT, but free LF was not influenced by differential tillage. We conclude that differences in aggregate turnover largely control the difference in fine iPOM in CT vs. NT and consequently SOM loss is affected by both the amount of aggregation and aggregate turnover.

This essay summarizes deliberation by the Soil Science Society of America (SSSA) Ad Hoc Committee on Soil Quality (S-581) and was written to spur discussion among SSSA members. Varying perceptions of soil quality have emerged since the concept was suggested in the early 1990s, and dialogue among members is important because, unlike air and water quality, legislative standards for soil quality have not been and perhaps should not be defined. In simplest terms, soil quality is "the capacity (of soil) to function". This definition, based on function, reflects the living and dynamic nature of soil. Soil quality can be conceptualized as a three-legged stool, the function and balance of which requires an integration of three major components-sustained biological productivity, environmental quality, and plant and animal health. The concept attempts to balance multiple soil uses (e.g., for agricultural production, remediation of wastes, urban development, forest, range, or recreation) with goals for environmental quality. Assessing soil quality will require collaboration among all disciplines of science to examine and interpret their results in the context of land management strategies, interactions, and trade-offs. Society is demanding solutions from science. Simply measuring and reporting the response of an individual soil parameter to a given perturbation or management practice is no longer sufficient. The soil resource must be recognized as a dynamic living system that emerges through a unique balance and interaction of its biological, chemical, and physical components. We encourage SSSA members to consider the concept of soil quality (perhaps as a marketing tool) and to debate how it might enable us to more effectively meet the diverse natural resource needs and concerns of our rural, urban, and suburban clientele of today and tomorrow. I NQUIRIES from policymakers, natural resource conservationists, scientists, and administrators regarding the concept of soil quality increased rapidly after the National Academy of Sciences published the book entitled Soil and Water Quality: An Agenda for Agriculture (National Research Council, 1993). In response, Dr. L.P. Wilding, 1994 president of the SSSA, appointed a 14-person committee (S-581) with representatives from all divisions. Appointees were asked to define the concept of soil quality, examine its rationale and justification, and identify the soil and plant attributes that would be useful for describing and evaluating soil quality. The SSSA president and members accepting this committee appointment recognized the emotion and high public visibility being attached to the subject. Simultaneously, several committee members were being asked

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