Bacterial-feeding nematodes, one of the primary grazers of soil bacteria, affect root growth in several plant species. Most researches realized that the effects of bacterial-feeding nematodes on plant root growth resulted from direct or indirect nutrient effects (mainly nitrogen mineralization). Bacterial-feeding nematodes grazing on bacteria accelerate bacterial turnover and increase the turnover of soil organic matter, increased the nitrogen mineralization and improved the supply of inorganic nitrogen, and subsequently the plant growth was stimulated. Several researchers, however, have claimed that this process may not fully explain the mechanism of bacterial-feeding nematode stimulating plant growth. Considering the similarly of the physiology and zoology between bacterial-feeding nematodes and protozoa, and that the bacterial-feeding nematodes occur at equal or greater biomass in the rhizosphere than protozoa, it is believed that the activity of bacterial-feeding nematodes in the rhizosphere will also stimulate root proliferation by hormonal effects as seen for protozoan grazing.
To study the impacts andmechanisms of bacterial-feeding nematodes on the growth of plant,namely Arabidopsis thaliana L. (Ecotype columbia ), a selected soil sample (sandy-loam alluvial soil) was mixed with pig manure and placed in mesh bags. In order to get greater populations of bacterial-feeding nematodes (SM1), the diameters of the openings were 1mm and 5μm. Then the mesh bags were buried under the soil (SM5) which was surrounded with outer layer of un-amended soil for comparison and so as to determine the effects of bacterial-feeding nematodes on the root development of the plant seedlings. Nematodes were able to migrate through the 1 mm diameter mesh bag into the outer soil, thus giving greater populations than the soil surrounded by a control treatment with the mesh bag openings 5 μm in diameter, through which nematodes could not migrate.
After 35 days incubation, the outer soil contained a 4.1-fold increase of nematode numbers in the 1mm treatment compared to the 5μm treatment. The root parameters (total length, average diameter, total surface area and number of tips) of the seedlings grown in the outer soil were measured after 15 days. Soil NH₄⁺-N and NO₃^--N contents were determined using a continuous flow auto-analyzer (AA3, Germany). Soil auxins were measured using high performance liquid chromatography (HPLC). Soil bacterial community structure was analyzed by a Community-level physiological profile (CLPP), using Biolog ECO microplates (Biolog, Hayward, CA, USA).
Results showed thatthe Arabidopsis thaliana seedlings grown in the soil with more bacterial-feeding nematodes (SM1) could develop a highly branched root system with longer roots and bigger surface area. Soil NH₄⁺-N and auxin (GA₃ and IAA) contents were increased significantly in the presence of more nematodes. Furthermore, the potential for utilization of substrates of the Biolog system obtained by BIOLOG ecoplate assay indicated that the presence of nematodes made large differences in the catabolic capability of soil microbial communities. The mechanism was likely to include the nutritional effects and hormonal effects as well.