Changes in bacterial communities and Azospirillum diversity in soil fractions of a tropical soil under 3 or 19 years of natural fallow
2002
Chotte, J.L. | Schwartzmann, A. | Bally, R. | Monrozier, L.J.
The effects of 3 and 19 yr natural fallows on soil structure and soil bacteria were assessed by aggregate size fractionation of a tropical sandy soil (0-10 cm). Bulk unfractionated soil and fractions including particulate coarse organic residues (COR), and four fractions (>2000, 50-2000, 2-50 and <2 micrometer) from each treatment were analysed for total heterotrophic bacteria, free N-fixers and the diversity of Azospirillum species. The potential N2 fixation (acetylene reduction activity, ARA) was also measured using unfractionated soil and soil fractions. Long-term fallow (19 yr) under Pennisetum was found to stimulate aggregation, while all clay particles were easily dispersed from the 3 yr fallow soil. Hot spots of potential N2 fixation were observed in coarse soil fractions (>50 micrometer), suggesting that these microhabitats were favourable to active N2 fixers. In contrast, more than 70% of the N2 fixing microorganisms and 90% of the recovered Azospirillum were isolated from the dispersible clay fraction (0-2 micrometer). Diversity of Azospirillum species was assessed by hybridisation with specific genetic probes on colonies within each fraction. This approach revealed the abundance of Azospirillum irakense in the 3 yr fallow soil fractions only and a selective effect of fallow on Azospirillum brasilense/Azospirillum amazonense genomic species in the 19 yr fallow soil. These changes would remain hidden, if the investigation had been restricted to the unfractionated soil.
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