DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY Organic and Conventional Management Effects on Biologically Active Soil Organic Matter Pools M. M. Wander,* S. J. Traina, B. R. Stinner, and S. E. Peters ABSTRACT Specific impacts of organic management practices on soil organic matter characteristics have not been documented. This study tested how organic management practices influence soil fertility by investigat- ing whether 10 yr of organic or conventional management generated differences in biologically active soil organic matter (SOM) pools at the Rodale Institute Research Center's long-term Farming Systems Trial experiment (FST). The experiment included an organically man- aged rotation that was animal based, an organic treatment that was cash-grain based, and a conventional cash-grain-based rotation. The biologically active SOM matter pools of the three FST treatment soils were compared through characterization of soil CO 2 evolution, available inorganic N pools and N mineralization rates, water- dispersible organic carbon (WDOC), and participate organic matter (light fraction). Soils receiving the organic treatments accumulated biologically active C. Accumulated organic matter in the manure- amended soil was the most labile whereas the cover-cropped soil accumulated the most organic matter overall. In the cover-cropped soil, higher total C and N, participate SOM, and reduced WDOC contents indicated that its SOM was more stable than SOM in the other two treatment soils. The conventionally managed soil had the lowest biological activity (N supply and soil respiration rates) and did not accumulate SOM during the 10-yr experiment. Assays that characterize particulate organic matter emerged as the best indices of biologically active SOM because they documented important quality (i.e., biological lability) and quantity aspects of SOM character in the Rodale FST soils. S OIL ORGANIC MATTER is linked to desirable soil physi- cal, chemical, and biological properties and is closely associated with soil productivity (Stevenson, 1982; Tate, 1987). Despite this, there are few explicit organic matter management recommendations. Making recommenda- tions about organic matter management is difficult be- cause accepted measures of SOM quality do not exist. Biologically active SOM is believed to be the key to soil productivity when fertility is biologically mediated. Altered management practices can affect active SOM characteristics and soil quality (nutrient supply and reten- tion characteristics) before net organic matter contents change (Woods and Schuman, 1986; Wander et al., 1993, unpublished data). Hence, farmers switching to organic management practices are encouraged to focus their efforts on active SOM pools (Weil, 1992). Changes in biologically active SOM may be linked to the so-called M.M. Wander, LASAS, Univ. of Georgia, Georgia Station, Griffin, GA 30223-1797; S.J. Traina, Agronomy Dep., Ohio State Univ., 2021 Coffey Rd., Columbus, OH 43210; B.R. Stinner, Ohio Agriculture and Research Development Center, 1680 Madison Ave., Wooster, OH 44690; and S.E. Peters, Rodale Institute Research Center, 611 Siegfriedale Rd., Kutztown, PA 19530. Received 30 Oct. 1992. *Corresponding author. Published in Soil Sci. Soc. Am. J. 58:1130-1139 (1994). transition effect. When farmers convert from conven- tional to organic management practices, they frequently observe a period of suppressed yields followed by re- bounding crop productivity (U.S. Department of Agricul- ture, 1980; Brusko, 1989; Liebhardtetal., 1989). Some farmers attribute this transition, or recovery in crop productivity, to management practices that build soil organic matter quality and enhance the life of the soil. Such a shift in SOM characteristics is not well docu- mented in the scientific literature. How organic manage- ment practices alter biologically active SOM has not yet been established. Active SOM refers to a heterogeneous mix of living and dead organic materials that are readily circulated through biological pools. It is a major soil nutrient reser- voir. Balance between decay and renewal processes in this pool controls nutrient availability and SOM status, i.e., determines whether organic matter is aggrading or degrading overall. Characteristics of the active fraction have been described primarily through the use of C and N isotopes employed as tracers (Jenkinson, 1971; Paul and van Veen, 1978; Gasser, 1979; Jansson and Persson, 1982). Isotopic analyses have generated consistent de- scriptions of soil C and N dynamics that have led to SOM classification based on isotope turnover rates (Jen- kinson et al., 1987; Parton et al., 1987; Jenkinson and Parry, 1989; Paustian et al., 1992). Consequently, the active SOM fraction has been used as a functional descrip- tion of organic materials with turnover tunes of 1 to 2 yr (Ladd et al., 1981; Parton et al., 1987). Although the active fraction has not been fully described in terms of its composition, nor has it been successfully isolated, it has been correlated with several procedurally denned soil fractions (Castellanos and Pratt, 1981; Paul and Voroney, 1983; Janzen, 1987; Sikora and McCoy, 1990). Because organic, sustainable, and low-chemical- input systems rely increasingly on soil nutrient cycling mechanisms, it is critical to understand the relationships between active SOM, total SOM, and nutrient retention and supply characteristics. To effectively manage soil organic matter, indices sensitive to changes in the func- tionally important active SOM fraction must be identified. The Rodale Institute Research Center's FST experi- ment has undergone a typical conversion scenario: the productivity of land converted from conventional to or- ganic management was suppressed for a few years, after Abbreviations: SOM, soil organic matter; FST, Rodale Institute Research Center's long-term Farming Systems Trial experiment; WDOC, water- dispersible organic carbon; ANOVA, analysis of variance; ANCOVA, analysis of covariance; RCF, relative centrifugal force; LF, light fraction; HF, heavy fraction. 1130