SOIL ORGANISMS IN ORGANIC AND CONVENTIONAL CROPPING SYSTEMS - Organic Agricultural Practices - page 49

have, in microcosmos studies, demonstrated the role microarthropods in
soil organic matter decomposition process. A low fungivore density (Col-
lembola) has a stimulating effect on microbial respiration, whereas high
densities inhibited microorganism respiration Barsdate et al, 1974; Hanlon
& Anderson, 1979).
Mites and insects, belonging to various families, were the two main
groups of arthropods found in the soil in 1993 and 1994 (Tables 1 and
2). In general, rates and numbers of individuals from these groups were
higher in the organic cropping system, refl ecting on Shannon's diversity
indices, which were higher in the organic system on all sampling dates
(Figure 4), but not on the soil organic matter decomposition (Figure 3).
The largest populations of insects were from the Order Collembola ,
and the number of individuals found in the organic system was three times
as high as that in the conventional system, during the fi rst nine months
(Table 1). During the following six months, the number of collembolans
remained 20% higher in the organic cropping system than in the conven-
tional (Table 2). These data agree with El Titi & Ipach (1989), who verifi ed
larger populations of collembolans for the low-input system than for the
conventional. Collembolans contribute to the soil's abilitity of suppressing
plant pathogens such as Rhizoctonia solani, Fusarium oxysporum f . sp.
vasinfectum , and Pythium (Wiggins & Curl, 1979; Curl et al., 1985a, b;
Rickerl et al., 1989; Lartey et al., 1994), because these organisms are, for
the most part, mycophagous, modifying the community of fungi. Because
in this work the practices in the organic system stimulated the community
of collembolans, it can be inferred that these organisms are responsible,
at least in part, for the suppression ability in soils enriched with organic
matter. Still, in regard to insects, the number of individuals was low for the
rest of the orders (Tables 1 and 2).
During the fi rst nine months of evaluation (Table 1), for both crop-
ping systems, the largest mite population was of the superfamily Orib-
atuloidea , followed by the family Galumnidae and by the superfam-
ily Passalozetoidea , all in the suborder Oribatida and with similar
behavior between cropping systems. In the suborder Gamasida the most
abundant population was Laelapidae and in Actinedida the most abun-
dant was Pygmephoridae , both more numerous in the organic system.
Populations in the suborders Acaridida and Ixodida were very small.