Soil bacteria function in the three‐dimensional space in heterogeneous soil complex and their activities depend in part on encountering substrates at the microbial scale. The bacterial density per gram of soil, which is generally measured, does not indicate if bacteria are all in the same location or spread throughout the soil complex. We characterized spatial distribution for how dispersed or aggregated nitrifiers (NH+4 and NO−2 oxidizers) were at a submillimeter scale. The spatial approach was based on the relationship, obtained experimentally, between the percentage of microsamples (50–500 μm diam.) harboring nitrifiers and the volume of the microsamples. The smallest sample size (50‐μm diam.) was considered as an approximation of microhabitat. The simulated spatial pattern of NO−2 oxidizer microhabitats in soil were compared with experimental data. The simulated pattern of NO−2 oxidizer distribution suggested that microhabitats averaged seven NO−2 oxidizers and occurred in preferentially colonized patches that had about a 250‐μm diam. These were randomly distributed and occupied 5.5% of the soil volume. They were functionally connected through microporosity and hence diffusion processes probably controlled the spatial distribution of nirifiers. The nitrifier spatial pattern enabled efficient nitrification because NH+4 and NO−2 oxidizers were near one another. The results showed the potential of our method to study spatial distribution of bacteria at the microhabitat scale.

Most tubificid worms are classified in the same functional group by their similar bioturbation and feeding activities in fine sediments of lakes. The objective of this study was to test the functional redundancy of two genera of tubificids (Limnodrilus and Tubifex) at two densities in coarse sediments using slow filtration columns. We measured the effects of the worms on particle redistribution, organic matter processing, nutrient fluxes, and microbial characteristics. The results showed that Limnodrilus and Tubifex created the same stimulation of aerobic and anaerobic microbial activities in the sediment. However, 50 Tubifex had a greater effect than 50 Limnodrilus on microbial processes. Furthermore, at a density of 100 oligochaetes per column, only Tubifex significantly increased particle redistribution and oxygen consumption in the first centimetre of the sediment. In contrast, Limnodrilus more often modified microbial activity in the deeper layers of the columns. The functional redundancy of the two genera of tubificid worms was validated by most measured processes. However, the variability within functional groups cannot be neglected because each genus had its functional peculiarity in the ecosystem.

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