The bacterial energy channel in the rhizosphere:

Protozoa – bacteria interrelationships as affected by root vicinity and soil depth

Carbon (C) fluxes from plants into soil via rhizodeposition or litter decomposition provide the energy for the decomposer food web and are therefore extremely important for ecosystem functioning. Nevertheless C-fluxes into the bacterial- and fungal-based compartments of the soil food web are poorly understood, and the roles of different biota in the system are still largely unknown. The DFG research program ‘Carbon flow in belowground food webs assessed by isotope tracers’ aims to uncover the major biotic players in these interactions. Our subproject ProtWeb quantifies protozoan effects on carbon fluxes through the bacterial energy channel in the soil food web. Protozoa are major grazers of bacteria in the rhizosphere and detritusphere. Especially at deeper soil layers protozoa may be the only microbial predators. We investigated the protozoan community in a maize field (Zea mays) over a 3 year period along two gradients: from the rhizosphere to bulk soil and from upper to deeper soil layers.

Labeling of maize plants and maize litter with 13-C and subsequent stable isotope probing and high throughput sequencing will tell us which protozoan taxa are the major grazers in rhizosphere and detritusphere systems. Finally, food web models using our measurements of protozoan functional response and production rates may be able to estimate the relative importance of the bacterial- in comparison to the fungal energy channel in this agricultural soil.