New Insights into the Role of Soil Microorganisms in Pesticide Degradation
and Nitrogen Cycling in Agricultural Land
Kanako Tago (Institute for Agro-Environmental Sciences, NARO)
tago@affrc.go.jp
Agricultural activities, such as the application of insecticides and fertilizers, affect microbial
populations and the way they function in the soil. Repeated applications of pesticides have been shownto induce rapid microbial degradation of the pesticide, thus reducing their efficacy. The application oflarge amounts of nitrogen fertilizers to agricultural fields influences nitrogen cycling, especiallynitrification and denitrification, and results in environmental pollution such as nitrate leaching and theproduction of nitrous oxide (N2O). The objective of this study was to reveal the diversity of soil
microorganisms and their role in the degradation of insecticides and nitrogen fertilizers.
1. Dynamics of insecticide-degrading bacteria in an agricultural ecosystem
Phylogenetic and functional characteristics of soil bacteria that degrade an organophosphorus
insecticide fenitrothion were investigated. The results showed that bacteria involving both completeand partial degradation of fenitrothion exist in soil. Most of the degraders were classified into thegenus Burkholderia. Metabolism of fenitrotion and its intermediate methylhydroquinone werecatalyzed by hydrolase and two oxygenases, respectively.
To understand changes in the composition of the bacterial community during insecticidedegradation, population dynamics of the soil bacteria that degrade fenitrothion were examined. The
population density of fenitrothion-degraders belonging to the genus Burkholderia increased withrepeated applications of fenitrothion. The number of dominant species among the degraders declinedwith the increasing density of the degraders, and eventually one species predominated. This process
can be explained by the competitive exclusion principle.
Some of the soil-borne fenitrothion-degrading Burkholderia were able to colonize the symbioticorgans of stinkbugs and make the host insects fenitrothion-resistant. The transmission process of thesymbiotic degraders from soil to stinkbugs was investigated. The results showed that symbiotic
degraders quickly developed in soil as a result of fenitrothion applications [1], and that plants and soilscould be permanent reservoirs of these symbiotic degraders in agricultural fields exposed tofenitrothion [2].
2. Ecology of denitrifying bacteria in paddy-soybean rotation fields
It has been observed that a change of land use from rice paddy to soybean results in an increase in
N2O emissions. The phylogenetic and functional features of the denitrifiers that are active in paddy
and soybean field soils were compared. The denitrifying communities were composed ofphylogenetically diverse bacterial strains. The denitrifiers producing N2O, N2, and a mixture of N2O
and N2 coexisted in the same soil with no relation to the phylogenetic classification or the land use [3].
References[1] Tago K., Kikuchi Y., Nakaoka S., Katsuyama C., and Hayatsu M.: Molecular Ecology 24: 3766–3778 (2015).
[2] Tago K., Okubo T., Itoh H., Kikuchi Y., Hori T., Sato Y., Nagayama A., Hayashi K., Ikeda S., andHayatsu M.: Microbes and Environments 30: 29–36 (2015).
[3] Tago K., Ishii S., Nishizawa T., Otsuka S., and Senoo K.: Microbes and Environments 26: 30–35(2011).