Interactions of Bacteria, Fungi, and their Nematode Grazers: Effects on Nutrient Cycling and Plant Growth

Abstract

The most common system responses attributed to microftoral grazers (protozoa, nematodes, microarthropods) in the literature are increased plant growth, increased N uptake by plants, decreased or increased bacterial populations, increased C0 2 evolution, increased N and P mineralization, and increased substrate utilization. Based on this evidence in the literature, a conceptual model was proposed in which microftoral grazers were considered as separate state variables. To help evaluate the model, the effects of microbivorous nematodes on microbial growth, nutrient cycling, plant growth, and nutrient uptake were examined with reference to activities within and outside of the rhizosphere. Blue grama grass (Bouteloua gracilis) was grown in gnotobiotic microcosms containing sandy loam soil low in inorganic N, with or without chitin amendments as a source of organic N. The soil was inoculated with bacteria (Pseudomonas paucimobilis or P. stutzerz) or fungus (Fusarium oxysporum), with half the bacterial microcosms inoculated with bacterial-feeding nematodes (Pelodera sp. or Aerobe/aides sp.) and half the fungal microcosms inoculated with fungal-feeding nematodes (Aphelenchus avenae).Similar results were obtained from both the unamended and the chitin-amended experiments. Bacteria, fungi, and both trophic groups of nematodes were more abundant in the rhizosphere than in nonrhizosphere soil. All treatments containing nematodes and bacteria had higher bacterial densities than similar treatments without nematodes. Plants growing in soil with bacteria and bacterial-feeding nematodes grew faster and initially took up more N than plants in soil with only bacteria, because of increased N mineralization by bacteria, NH 4 +-N excretion by nematodes, and greater initial exploitation of soil by plant roots. Addition of fungal-feeding nematodes did not increase plant growth or N uptake because these nematodes excreted less NH 4 +_N than did bacterial-feeding nematode populations and because the N mineralized by tl;le fungus alone was sufficient for plant growth. Total shoot P was significantly greater in treatments with fungus or Pelodera sp. than in the sterile plant control or treatments with plants plus Pseudomonas stutzeri until the end of the experiment.The additional mineralization that occurs due to the activities of microbial grazers may be significant for increasing plant growth only when mineralization by microftora alone is insufficient to meet the plants' requirements. However, while the advantage of increased N mineralization by microbial grazers may be short-term, it may occur in many ecosystems in those short periods of ideal conditions when plant growth can occur. Thus, these results support other claims in the literature that microbial grazers may perform important regulatory functions at critical times in the growth of plants.

Microbial-faunal interactions in the rhizosphere and effects on plant growth

“…Protozoa and microbial feeding nematodes are known to be the most important grazers of the microflora in terrestrial ecosystems [7,27,36,64,68]. In the presence of microfauna, soil respiration and specific respiration (i.e.…”

Carbon Fluxes in the Rhizosphere

“…Accelerated turnover rates of rhizosphere microbial biomass due to faunal grazing may be another explanation for the low microbial assimilation efficiency of root exudates as measured. Faunal grazing on rhizosphere bacteria and fungi has been suggested as a key factor of the "priming effect" of root exudates (Ingham et al 1985;see Chapters 3and 5). The high population density of bacteria in the rhizosphere (Foster 1988) may attract many grazers.…”

“…The high population density of bacteria in the rhizosphere (Foster 1988) may attract many grazers. The densities of both protozoa and bacterial-feeding nematodes have been shown to be higher in the rhizosphere than in the bulk soil (Ingham et al 1985). Faunal grazing will increase the turnover rate of carbon and nitrogen in the rhizosphere, and subsequently result in a lower amount of exudate carbon or nitrogen in microbial biomass form, and higher amount being released as CO 2 (see Chapter 3).…”

Interactions between mycophagous nematodes, mycorrhizal and other soil fungi

“…However, the results of a later experiment with the same organisms showed that nitrogen mineralization was increased by the grazing of A. avenae on the fungus (Trofymow et al 1983). Ingham et al (1985b) conducted a similar but more detailed study which also included plants. They predicted that in soils where nutrients were limiting, mineralization of phosphorus and nitrogen by saprotrophic fungi and bacteria should provide additional inorganic nutrients for uptake by plants and thus increase plant growth.…”

“…In the present experiment is more likely that the nematodes temporarily reduced the capacity of A. bisporus to release phosphorus or other nutrients from the bran or the soil by damaging the mycelium of this fungus on which they had apparently fed. Trofymow & Coleman (1982) found that A. avenae had a similar negative effect on the mineralization of organic nitrogen by Fusarlum oxysporwn but the results of Ingham et al (1985b) with the same species of nematode and fungus indicated that the influence of mycophagous nematodes on the mineralization of nutrients by saprotrophic fungi is likely to be small.…”