Asuming-Brempong, S. 1999. The effect of 2,4-D selection on microbial communities in micrososm and field studies and the impact on ecosystem function. Ph.D. Dissertation, Michigan State University, East Lansing, MI, USA.
The effect of long term 2,4-D selection on the microbial community and its impact on nutrient transformation was studied in the field and microcosms. 2,4-D was applied at (0X which is the control), the normal field rate of 1.1 kg/ha (1X), 10X and 100X the normal field rate for ten years. The microbial community was analyzed by cultural methods, reverse sample genome probing (RSGP) and functional gene probing. RSGP showed that the microbial community in the 100X plots had been altered such that there was an enrichment in some 2,4-D degrading populations compared to the control. In such altered microbial communities, microbial biomass, and microbial respiration were reduced. Carbon and nitrogen mineralization were also depressed and the depressive effect lasted for a 30 day incubation. Effect of long term high 2,4-D application did not significantly interfere with nutrient transformation although it did reduce plant productivity which may have contributed to the reduction in soil microbial biomass.
Microcosm studied demonstrated that management history of the soil (40 years plant succession, 10 years no-till, conventional agriculture) did not influence which 2,4-D degraders be3came dominant members in the 2,4-D amended soil. Dominance by Burkholderia sp. was seen in the microcosm studies whilst in the field studies Burkholderia and its close relative Ralstonia sp. were dominant providing evidence of niche partitioning of 2,4-D degrading bacteria in the field. Discontinuing 2,4-D treatment in the southern part of the plots for three years resulted in a decrease in culturable and RSGP detectable population of 2,4-D degrading bacteria although 2,4-d enriched populations were still above control levels three years after 2,4-D added had been suspended. Hence it appears to take many years for perturbed populations to return to their normal densities once the perturbation is removed.
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