We tried to elucidate the mechanisms of soil disinfestation by a combined treatment of wheat bran amendment, temporary flooding, and tarping with plastic film (reductive soil disinfestation, RSD). In a laboratory experiment, redox potential decreased to under -200 mV in RSD-treated soil and short-chain volatile fatty acids (VFAs), such as acetic and n-butyric acids, were generated at levels of 9.8 to 10.8 mM in the soil solution in 48 hr. VFAs showed high nematicidal activities when second-stage juveniles of Meloidogyne incognita were exposed to them at 10 mM for 24 hr. Acetic and n-butyric acids had nearly equal nematicidal activity and their mixtures showed an additive effect. VFAs increased the nematicidal activity with decreasing pH, indicating that nematicidal activities of VFAs are related to their ionization. Logistic LC₅₀ of nonionized acetic acid was estimated to be 5.6 ± 0.2 (SEM) mM. In a field demonstration, RSD was confirmed to be adequate for nematode control, and 5.7 mM of acetic acid and 1.5 mM of n-butyric acid were calculated in the water-phase of the soil, although the soil pH was not low enough to form nonionized acids at the lethal concentration. These results show that VFAs generated by soil microorganisms in reduced soil probably play an important role in nematode suppression in practice.

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Compared with studies conducted in experimental crop fields, those in commercial fields have been very limited to examining how organic (ORG) and conventional (CONV) farming systems differ in their effects on biological and physicochemical characteristics of soils. To evaluate practical effects of ORG systems, a comparison is essential with “practical” CONV systems where organic amendments are also used in addition to chemical fertilizers and pesticides. We compared nematode community structures between the two systems for commercial tomato production in greenhouses located in northern Japan in 2005 and 2006. We examined if the farming system was still influential even when differences in soil texture, study plot locations and growing seasons among the plots were considered. The density of the predator-omnivore group, with Dorylaimida as the dominant taxon, and Structure Index were always significantly affected by farming system, and their values were greater in ORG. Redundancy analysis in 2005 revealed that farming system could significantly explain the whole community structure, and determined Dorylaimida as an indicator for ORG, while Diplogasteridae and Anguinidae were indicators for CONV. Although redundancy analysis could not significantly explain community structure with farming system in 2006, Dorylaimida and Diplogasteridae were still good indicators for the two systems. Individual variables, such as soil texture and bacterial density, were found to have significant effects on the two nematode taxa, although we could not interpret sufficiently the causal relationships. Further studies should include other measurements such as chemical pesticides applied for better interpretation of the differences between these systems.

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Abstracts of papers presented at the 17th annual meeting of the Japanese Nematological Society in Kumamoto, September 3-5, 2009

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