Influence of bacterial-feeding nematodes (Cephalobidae) on soil microbial communities during maize growth

The effect of several bacterial-feeding nematodes of the Cephalobidae family (Zeldia punctata, Acrobeloides nanus and Cephalobus pseudoparvus) on the microbial community of a Sahelian soil (Senegal) was investigated in microcosm. The consequences of the activity of these nematodes on the growth and nitrogen nutrition of young maize plants (aerial biomass, root biomass and nitrogen content) were also estimated. Laboratory-cultured nematodes were inoculated into soil containing maize seedlings where the natural nematofauna had been previously eliminated by alternately freezing and defrosting (five cycles). The microbial compartment of the soil community was characterised through total microbial biomass (using fumigation-extraction), density of bacteria (using colony forming units counts), microbial activity (using alkaline phosphatase) and genetic structure of soil microbial community (using denaturing gradient gel electrophoresis) at sowing and at 12, 26 and 47 days after planting. Final nematode densities in the different treatments (between 4 and 20 Ind g-1 dry soil) demonstrated a high level of reproduction. The different types of nematodes tested induced similar trends in changes in the microbial pool of the soil and in maize seedling growth. Compared to control soils, the presence of nematodes led to an increase (+12%) in plant biomass and reduced concentrations of soil ammonium but had no effect on concentrations of nitrate by the end of the experiment. Sixty-three percent of the inorganic nitrogen initially present in the soil was incorporated into the maize plants with nematodes whereas only 47% was incorporated without nematodes. Nematode activity led to a significant decrease in microbial biomass (-28%) and density of cultivable bacteria (-55%), however, nematodes stimulated bacterial activity (+18%). The effects of Z. punctata were weakest compared to A. nanus and C. pseudoparvus. The presence of nematodes modified the genetic structure of the microbial community essentially by changing the relative abundance of dominant bacterial populations. Among nematode species tested, A. nanus modified the structure of the microbial communities the most compared with control soils without nematodes. Overall, results from this study provide evidence for the ability of microbial feeding nematodes to alter microbial activity, microbial community structure, nitrogen mineralisation and growth of maize seedlings in a Sahelian soil from Senegal, West Africa.