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.
The responses of decomposition and primary production to nitrogen supply were investigated in a shortgrass prairie, a mountain meadow, and a lodgepole pine forest. Nitrogen (N) supply was increased by applying ammonium nitrate, or decreased by applying sucrose. The litterbag technique was used to follow decomposition ofleaves of the dominant plants: blue grama (Bouteloua gracilis) from the prairie, western wheatgrass (Agropyron smithii) from the meadow, and lodgepole pine (Pinus contorta) from the forest.Soil from beneath the litterbags was sampled at the time of litterbag retrieval in order to detect interactions between decomposition and properties of the underlying soil. There was no consistent effect of soil properties on decomposition rate, but there was a significant effect of litter type on N mineralization in the underlying soil.Decomposition was fastest in the forest, intermediate in the prairie, and slowest in the meadow. Blue grama decomposed faster than the other litters. Each litter type decomposed faster than expected when placed in its ecosystem of origin. This interaction suggests that decomposers in an ecosystem are adapted to the most prevalent types of litter.Nitrogen supply had a small but significant effect on decomposition rate. Within an ecosystem, there was a positive association between decomposition and accumulation of N within the litter, but this relationship was 'reversed when comparing across ecosystems, possibly because of the overriding effects of differences among ecosystems in abiotic factors.Aboveground net primary production was estimated in the grasslands by a single harvest at the end of the growing season, and growth increment of boles was measured in the forest. These indices of primary production showed a greater relative response to N fertilization than did decomposition, suggesting that primary production is the more N-limited process.
Summary A mixture of bacteria and a vesicular‐arbuscular mycorrhizal fungus isolated from field‐collected sods of blue grama (Bouteloua gracilis (H.B.K.) Griffiths) were tested for their interaction in the rhizosphere of pot‐grown blue grama plants. Populations of the inoculated bacterial species and actinomycete populations, as influenced by the presence or absence of Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe, were enumerated by dilution plate counts from rhizosphere and non‐rhizosphere soil samples. Total bacterial counts and the population of one bacterial species in the non‐rhizosphere soil of pots containing plants were significantly greater than in soil of pots without plants. The population of two bacterial species and actinomycetes were not significantly different in the non‐rhizosphere soil of both mycorrhizal and non‐mycorrhizal plant treatments when compared to the soil of pots without plants. In the rhizosphere of mycorrhizal plants, the total bacterial population and colony counts of one of the four bacterial isolates, when expressed as colony‐forming units (CFU) per gram of root dry weight, were significantly reduced compared with controls. The numbers of CFU per gram of rhizosphere soil of one bacterial species were significantly increased by the presence of the mycorrhizal fungus. Although no significant negative correlation was observed between populations of bacterial species in the rhizosphere soils, significant positive correlations between specific bacterial populations depended on whether or not the roots were mycorrhizal.
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