Interactions Between Bacterial-feeding Nematodes And Bacteria And Their Effects On Soil Nitrogen Mineralization And Root Growth Of Plant Seedlings

Bacterial-feeding nematodes is one of the most important bacterial grazer in soil.Interactions between bacterial-feeding nematodes and microbe and their ecological effectsin terrestrial ecosystems have caused great attention of ecologists due to their high biomassand turnover and the close interactions with microbe. Despite the results were notconsistent up to now, mostly researches found that the direct or indirect effects due tobacterial-feeding nematodes grazing on bacteria increased the nitrogen mineralization andimproved the status of the supply of inorganic nitrogen, so subsequently the plant growthwas stimulated. But this effect must be affected by the species and density of nematode andbacteria and the surrounding environmental condition, such as the humidity. Researchesabout protozoa found that there were hormonal effects on plant growth by protozoangrazing changing the rhizosphere bacterial community to stimulate plant growth-promotingrhizobacteria. Considering the similarly of the physiology and zoology betweenbacterial-feeding nematodes and protozoa, and that the bacterial-feeding nematodes occurat equal or greater biomass in the rhizosphere than protozoa, it is believed that the activityof bacterial-feeding nematodes in the rhizosphere will also stimulate root proliferation byhormonal effects as seen for protozoan grazing.On the base of previous experiments in our lab, the gnotobiotic microcosmexperiments were conducted to study the interactions between bacterial-feeding nematode(Caenorhabditis elegans) and bacteria (Bacillus subtilis) and their effects on soil nitrogenmineralization at different nematode density or different soil moisture content. Then,experiments with natural soil (soil didn't sterile but directly incubated for enriched in soilnative nematodes or inoculated with nematodes) were manipulated to determine the effectsof bacterial-feeding nematodes on the root development of tomato and wheat seedlings. Thehormonal effect by nematode grazing changing the microbiall community was also discussed.The main results were as follows:1. Interactions between bacterial-feeding nematodes and bacteria were different fromthose between other hunters and preys. The grazing of nematodes significantly stimulatedthe growth and activity of the bacteria. Furthermore, the activity of nematodes mainlyenhanced soil nitrogen mineralization, especially remarkably increased the soil NH₄⁺-N andmineral N. But the NO₃^--N was significantly decreased with the inoculation ofbacterial-feeding nematodes in the experiments.2. The effect of bacterial-feeding nematodes on bacteria growth was exhibited a"moderate density" of nematodes, which means when the number of nematodes was morethan this density, the increment of the nematodes on bacteria was decreased. With thetreatments designed in the experiment, the effect of nematodes on bacteria was in the orderof 20 nematodes·g^-1 dry soil＞10 nematodes·g^-1 dry soil＞40 nematodes·g^-1 dry soil. At thesame time, the enhancement of nematodes on soil nitrogen mineralization was alsodecreased in the excessive grazing of nematodes on bacteria, and the increment ofnematodes on nitrogen mineralization was also in the same order of nematode densities asaffected on bacterial population in the experiment.3. Soil moisture content affected the interactions between bacterial-feeding nematodesand bacteria. The stimulated effect of nematodes on bacteria growth was different with thesoil moisture content in the order of 23% water content (w/w) treatment＞17% watercontent (w/w) treatment＞28% water content (w/w) treatment as designed in the experiment.The increment of nematodes on soil nitrogen mineralization was also greater in the 23%water content (w/w) treatment than the other two treatments, while there were nosignificantly difference between those two. These results indicated that at the lower soilmoisture content, effects of nematodes on bacteria growth and soil nitrogen mineralizationwere enhanced with the increasing of soil moisture content. But these effects weredecreased in the soil with too higher moisture content. Nematodes are aquatic and theirsurvival and movement need water films, and they can move quickly in thin water film than in free water. The difference of movement, distribution and activity of nematodes atdifferent soil moisture contents will affect their effects on bacteria growth and soil nutrientmineralization.4. Soil mixed with either pig manure or rice straw was placed in a mesh bag (1mm or5μm), and then surrounded by an outer layer of unamended soil. The greater populations ofbacterial-feeding nematodes that generated in the soil mixed with pig manure or rice strawmigrated through the 1 mm diameter mesh bag into the outer soil, thus giving greaterpopulations than in soil surrounding the control treatment of 5μm diameter mesh bag,through which nematodes cannot migrate. The increased bacterial-feeding nematodes wereprimarily Protorhabditis. sp. There were more nematodes in the pig manure treatment thanin the rice straw treatment. This is probably related to the more rapid bacterialdecomposition of the pig manure, as reflected in its lower C: N ratio. The increase innematode numbers, especially bacterial-feeders, is directly related to the rate ofdecomposition of different organic amendments.5. Tomato and wheat seedlings grown in the soils containing more bacterial-feedingnematodes (no matter by stimulating the native population or by adding additionalnematodes) developed a highly branched root system with longer and thinner roots.6. The contents of IAA and GA₃ were significantly enhanced in the soils containing morebacterial-feeding nematodes compared with the control soils, no matter planted with tomatoor wheat. The mean values of all nematode treatments showed that in the presence of morebacterial-feeding nematodes, IAA and GA₃ increased by 72.7% and 71.4% respectively inthe soils planted with tomato, while in the soils planted with wheat, increased by 90.7% and41% respectively. Bacterial-feeding nematodes also stimulated the total microbial activity(indicated by AWCD) and significantly changed the soil microbial community structure thatthe soil microbe's functions shifted evidently based on the sole carbon use pattern derivedform Biolog method. The increment of IAA and GA₃ in the soils should be caused by thechanges of soil microbial community structure due to the selective grazing of nematodes.Combined these results with the effects of bacterial-feeding nematodes on plant root proliferation, in some extent illustrated that bacterial-feeding nematodes affect plant rootgrowth likely through the hormonal effects by a grazing-induced change of soil microbialcommunity as found in the presence of protozoa.7. Bacterial-feeding nematode's effects on soil microbial community and plant rootgrowth were affected by nematode species. Tomato and wheat seedlings grown in the soilscontaining more native bacterial-feeding nematodes developed a more highly branched rootsystem than those grown in the soils inoculated with C.elegans. The microbial activity wasalso higher in the soil with more native nematodes than with C.elegans and the soilmicrobe's use pattern of the sole carbon was different in the presence of these twonematodes species. While the plant species also affected the interactions amongbacterial-feeding nematodes, microbe and plant root, for different plant will affect the soilmicrobe by releasing different root exudates. That the interactions among soil microfauna,microbe and root were complex.To summarize, the research farther discussed the factors affected the interactionsbetween bacterial-feeding nematodes and bacteria and their effects on soil nitrogenmineralization, and probed into bacterial-feeding nematode's effects on plant root growththrough the hormonal effects by grazing-induced changes of soil microbial community.These findings supported theoretic knowledge and technic to host soil nematodes as animportant bio-resource in soil nutrient management and plant growth. The present studymay be valid not only for theory, but also for future application in agriculture.