Low amounts of herbivory by root-knot nematodes affect microbial community dynamics and carbon allocation in the rhizosphere

Increased carbon translocation to the rhizosphere via 'leakage' induced by low amounts of plant parasitic nematodes can foster microorganisms. The effects of the root-knot nematode Meloidogyne incognita on microbial biomass (C-mic) and community structure (phospholipid fatty acids) in the rhizosphere of barley were studied. Inoculation densities of 2000, 4000, and 8000 nematodes were well below the threshold level for plant damage. A (CO2)-C-13 pulse-labelling was performed to assess the distribution of assimilated C-13 in the rhizosphere. Infection with M. incognita increased the carbon concentration in shoots, and enhanced root biomass slightly. The presence of nematodes did not affect microbial biomass, but significantly changed the allocation of the recent photosynthate. Less plant carbon was sequestered by microorganisms with increasing nematode abundance. Microbial community structure was distinctly altered in the early stages of the plant-nematode interactions. Both, bacteria and fungi, showed a positive response with 2000, and a negative one with 4000 and 8000 M. incognita added. The results suggest that low-level root herbivory still imposes a considerable carbon demand, and that proliferation of microorganisms due to increased rhizodeposition may be short-termed. The carbon flow to rhizosphere microbial communities is likely dependent on the specific nematode-plant association and the developmental stage of the nematode in the host.