Active N(2)O emission from bacterial microbiota of Andisol farmland and characterization of some N(2)O emitters.

Andisol in farmland located in Hokkaido, Japan, is known to actively flux nitrous oxide (N(2)O) during the spring to summer seasons. Using a culturing system which mimics farm soils, nitrous oxide (N(2)O) emission potentials of the soils or soil microorganisms were investigated. A total of thirty-three soil samples from the farmland showed high N(2)O production potential, of which the maximum level of N(2)O emission was 3.69 μg per ml of the cultured medium per day (ml(-1) d(-1)) in the assay system. However, only three eubacteria (Leptothrix sp., Paenibacillus sp., and Streptomyces sp.) were isolated as culturable N(2)O emitters among a total of 92 bacterial isolates and 2 fungi obtained from the assayed soil suspensions. N(2)O production from all the isolated N(2)O emitters was more active within a weakly acidic region (pH 4.5-5.0) than neutral regions. However, unlike N(2)O emitters isolated from tropical peat soils, they did not respond to supplemental 0.5% sucrose. In the acetylene inhibition assay for the evaluation of complete denitrification, Leptothrix sp. P3-15D and Streptomyces sp. M2-0C indicated that these culturable N(2)O emitters are not effective denitrifiers but weak N(2)O emitters in the Andisol. Conversely, Rhodococcus sp. that was isolated from the Andisol collected in another season using a KNO(3)-enriched plate, showed 3.2-fold higher N(2)O emission with 10% C(2) H(2). Instead of the culturable bacteria, it is probable that the N(2)O emitters in viable but non-culturable (VBNC) state or obligately anaerobic denitrifiers are the major contributors to N(2)O emission from the vitric Andisol.