Residue Placement and Fungicide Effects on Fungal Communities in Conventional and No-Tillage Soils

We investigated the effects of the fungicide captan (cis-JV-trichloromethylthio-4-cyclohexene-l,2-dicarboximide) and long-term conventional (CT) and no-tillage (NT) management on the size and composition of fungal communities from crop residues and mineral soil. Captan and reference treatments were established in CT and NT plots on a Hiwassee sandy clay loam soil (clayey, kaolinitic, thermic Rhodic Kanhapludult). At intervals, grain sorghum [Sorghum Bicolor (L.) Moench] litterbags and soils were collected for enumeration and identification of fungal hyphae and fungal colony-forming units (CPU) and to determine residue decomposition rates. Buried residues (CT) had higher densities of fungal hyphae but fewer fungal CPU than surface residues (NT). Buried residue decay rates (it = 0.0136 d~') were 3.4 times faster than surface residues (k = 0.0040 d'). Fungal hyphae were more abundant in NT than CT mineral soils but there were no differences in fungal CPU. Sorghum residues had a common succession of fungi including phyllosphere colonizers (e.g., Altemaria alternate, Epicoccum nigrum) and other common decomposer fungi (Cylindrocarpon, Fusarium, Mucor, and Phoma). Fungal communities were differentiated into surface residue (e.g., Alternaria, Epicoccum) and soil (e.g., Aspergillus, Trichoderma) specialists, while buried residues lacked a specialized community. Rhizopus arrhiais and Aspergillus niger showed a statistical preference for CT soils, while Penicillium verruculosum was most common in NT. Captan reduced fungal CFU and hyphal densities in residues of both NT and CT, resulting in 23 and 7% reductions in their decay rates, respectively. We found no evidence for differential effects of captan on the composition of the fungal communities in residues or soils. C TILLAGE PRACTICES and the management of crop residues are generating increasing interest. Under CT practices, plowing results in mixing of the soil profile and the burial of crop residues. With NT management the soil is not plowed and crop residues accumulate on the soil surface as a mulch. These differences in soil disturbance and residue placement and their effects on soil physical and chemical properties (Phillips and Phillips, 1984) can influence the composition, distribution, and activity of soil microbial communities (Doran, 1980a; Lynch and Panting, 1980; Rice and Smith, 1982; Groffman, 1985). Knowledge of these influences can be important to understanding nutrient cycling (Hendrix et al., 1986; Andren et al., 1990; Beare et al., 1992) and organic matter dynamics (Doran, 1980b; Holland and Coleman, 1987) in different tillage and residue management systems. Fungi are often the largest component of the microbial biomass in arable soils (Anderson and Domsch, 1975). The size and composition of soil fungal communities are related to the quantity and quality of organic matter inputs (Martyniuk and Wagner, 1978; M.H. Beare and D.C. Coleman, Institute of Ecology, Univ. of Georgia, Athens, GA 30602; B.R. Pohlad, Division of Life Sciences, Ferrum College, Ferrum, VA 24088; and D.H. Wright, Dep. of Biology, Emory Univ., Atlanta, GA 30322. Received 2 Apr. 1992. *Corresponding author. Published in Soil Sci. Soc. Am. J. 57:392-399 (1993). Schniirer et al., 1985). Although several studies have reported mycofloristic surveys from cultivated soils (e.g., Miller et al., 1957; Warcup, 1957), relatively few have compared the composition of fungal communities under different tillage regimes. Wacha and Tiffany (1979) found few differences in the composition of fungal communities from plowed and notillage soils in Iowa after 4 yrs of treatment. Most other comparisons of tillage effects on soil fungi have focused on selected organisms, in particular phytotoxic (Norstadt and McCalla, 1969) or phytopathogenic (Ploetz et al., 1985; Rothrock and Hargrove, 1988) fungi. Several studies have described microbial colonization of crop residues (Walker, 1941; Harper and Lynch, 1985) and their relationships to residue decomposition (Broder and Wagner, 1988; Magan et al., 1989; Widden et al., 1986). Others have shown that the composition and activity potentials of residue-borne microbial communities can be important predictors of residue decay rates (Neely et al., 1991). In CT and NT soils the "native" placements of residues (i.e., buried in CT, surficial in NT) significantly alters residue-associated microenvironments (van Doren and Allmaras, 1978). Water content, temperature, and nutrient proximity are among the most important variables that differ with residue placement (Beare et al., 1992). These variables also affect the spore germination, hyphal growth, and cellulolysis of residue-borne fungi (Magan and Lynch, 1986), which, in turn, influence the colonization and growth of opportunistic decomposers (Gochenaur, 1981) and, consequently, the succession of fungal species (Cooke and Rayner, 1984). Relatively few studies, however, have compared the size and composition of residue-borne fungal communities in different tillage systems (Bowen and Harper, 1988). Furthermore, no studies have described relationships between soiland residue-borne fungal communities as they are influenced by tillage. Crop residues also harbor facultatively saprophyticphytopathogenic fungi (Boosalis et al., 1967; Ploetz et al., 1985). Some studies have shown greater pathogenicity in reduced-tillage than in conventional-tillage soils (Sumner et al., 1981; Cook et al., 1980), while others report few differences with tillage (Rothrock and Hargrove, 1988). With increasing emphasis on surface residue management over plowing, removal, or burning, fungicides are receiving wider use for the control of phytopathogenic fungi (Sumner et al., 1981; Elmholt and Smedegaard-Petersen, 1988; Pimentel et al., 1991). Surprisingly, relatively few studies have considered the impact of fungicides on communities of nontarget saprophytic fungi (Domsch, 1970; Agnihotri, 1971; Wainwright and Pugh, 1975). Abbreviations: CT, conventional tillage, NT, no-tillage; CFU, colony-forming unit; AFDM, ash-free dry mass; ANOVA, analysis of variance. 392 Published March, 1993