Abstract—Using a Pseudomonas denitrifying bacterium, which had been isolated from Japanese Andisol corn farm as an active nitrous oxide (N2O) emitter and likely to be missing nosZ gene, we investigated denitrification-regulating activity of some cover crops or green manure plants against this N2O emitter. In the preliminary screening, root exudates from the 10 seedlings of yellow flowering leaf mustard (Brassica juncea) and crimson clover (Trifolium incarnatum) showed 50% repression of the N2O emission by an incomplete denitrifier Pseudomonas sp. 05CFM15-6D. When direct extracts of the seedling roots with MeOH were assayed, however, only the seedlings of B. juncea showed a remarkable inhibition of bacterial cell growth and N2O emission at concentration equivalent to 10 seedlings. The root extract from B. juncea equivalent to 2 to 4 seedlings maintained inhibiting activity toward N2O emission, while it did not affect bacterial cell growth. Conversely, water-soluble fraction from aboveground of European small radish (Raphanus sativus var. sativus) sprouts showed statistically significant acceleration of N2O emission (P<0.01) with slight but insignificant cell growth activation. As some secondary metabolites are uniquely contained in these cover crops, suppressing or accelerating activity of such phytochemicals in N2O emission was also investigated. Allyl isothiocyanate at 30 µM markedly inhibited N2O emission of the Pseudomonas denitrifier but not suppressed its cell growth. In contrast, methyl isothiocyanate sinigrin, cyanamide, and betanin did not affect on N2O emission of the denitrifier at 150-300 µM.
Keywords—cover crops, Brassica juncea, N2O emission, allyl isothiocyanate, Raphanus sativus var. sativus.
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