D. C. Coleman

Nitrogen Limitation of Production and Decomposition in Prairie, Mountain Meadow, and Pine Forest

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 of leaves 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.

Trophic interactions in soils as they affect energy and nutrient dynamics. IV. Flows of metabolic and biomass carbon

Flows of biomass and respiratory carbon were studied in a series of propylene-oxide sterilized soil microcosms. One-half of the microcosms received three pulsed additions of 200 ppm glucose-carbon to mimic rhizosphere carbon inputs. Biotic variables were: bacteria (Pseudomonas) alone, or amoebae (Acanthamoeba) and nematodes (Mesodiplogaster) singly, or both combined in the presence of bacteria.Over the 24-day experiment, respiration was significantly higher in the microcosms containing the bacterial grazers. Biomass accumulation by amoebae was significantly higher than that by nematodes. The nematodes respired up to 30-fold more CO2 per unit biomass than did amoebae. Similar amounts of carbon flowed into both respiratory and biomass carbon in microcosms with fauna, compared with the bacteria-alone microcosms. However, partitioning of available carbon by the microfauna varied considerably, with little biomass production and relatively more CO2-C produced in the nematode-containing microcosms. The amoebae, in contrast, allocated more carbon to tissue production (about 40% assimilation efficiency) and correspondingly less to CO2.

Trophic interactions in soils as they affect energy and nutrient dynamics. I. Introduction.

The dynamics of nutrient transformations at the soil-root interface are complex but amenable to controlled experimental study. Using a conceptual model we introduce a series of papers which ascertain the role of microfloral-faunal trophic interactions in carbon, nitrogen, and phosphorus transformations in soil microcosms.

Nitrogen transformations in soil as affected by bacterial-microfaunal interactions

Summary-We investigated the effects of soil microfauna feeding on a soil bacterium ~~se~d~~fl~s cepaciu) and the resultant influence on net N mineralization. As the bacteria1 biomass increased, it assimilated N from the soil. Later, only if this bacterial biomass decreased was N remineralized. Grazing by amoebae (Acanthamoeba pofyphaga) always reduced bacterial biomass, increased respiration, and increased nitrogen mineralization. Grazing by nematodes (~es~djpfogaste~ lheritieri) always reduced bacterial numbers and increased respiration, but only increased N mineralization when nematode populations themselves declined from peak values. A N budget calculated for the populations indicated that the nematode biomass was not sufficient to account for the unmineralized N, so we postulate change in excretory pathways as the bacterial food becomes limiting. This budget further indicated that amoeba1 and bacterial biomass could account for all of the non-mineral N when only these two species were present. Our experiments showed that microfauna can play an important role in N mineralization in soil and that the mechanism for this role is more likely to be through direct excretion by the grazers than through indirect physiological effects on the bacteria.

Trophic interactions in soils as they affect energy and nutrient dynamics. V. Phosphorus transformations

Regeneration of nutrients from relatively nutrient-poor organic residues is essential for overall operation of an ecosystem. Nutrients thus released are, however, inadequate for the needs of the decomposer populations, and a much faster nutrient turnover involving bacterial immobilization and release occurs concurrently. Evidence from aquatic ecosystems indicates that bacteria release little phosphorus, for which they have high demand, whereas bacterial grazers play an important role in regeneration of bacterial phosphorus. Our studies extend these relationships to terrestrial ecosystems. We studied phosphorus immobilization and mineralization in soil incubations, simulating rhizospheres with combinations of bacterial, amoebal, and nematode populations. Bacteria quickly assimilated and retained much of the labile inorganic phosphorus as carbon substrates were metabolized. Most of this bacterial phosphorus was mineralized and returned to the inorganic phosphorus pool by the amoebae. Nematode effects on phosphorus mineralization were small, except for indirect effects on amoebal activity. The observed remineralization may reflect direct excretion by the amoebae, physiological effects on the bacterial populations, or both. These results suggest a major role of microfauna in nutrient cycling.

Effect of the Nematodes Acrobeloides Sp. and Mesodiplogaster Lheritieri on Substrate Utilization and Nitrogen and Phosphorous Mineralization in Soil

Release of nutrients by bacterial—feeding nematodes may result in increased nutrient availability for plants. Effects of two species of bacterial—feeding nematodes on N and P mineralization and C—substrate utilization were examined in soil microcosms. Both nematodes increased the rate of 14C—labelled glucose—C utilization and N and P mineralization after 10d. However, after 65 d incubation, soluble C, Pi and NH4+—N were similar in grazed and ungrazed systems. The bacterial grazers, although not directly affecting total decomposition, act to maintain the dynamic nature of the nutrient cycles. See full-text article at JSTOR

An analysis of food-web structure and function in a shortgrass prairie, a mountain meadow, and a lodgepole pine forest

SummaryThe structure of the below-ground detrital food web was similar in three different semiarid vegetation types: lodgepole pine (Pinus contorta subsp. latifolia), mountain meadow (Agropyron smithii), and shortgrass prairie (Bouteloua gracilis). The densities of component food-web functional groups and the response to removal of component groups, differed however. As measured by biomass, bacteria were dominant in the meadow and prairie, while fungi were dominant in the forest. Resourde-base dominance was reflected in consumer dominance, and both directly correlated with the form of inorganic N present. Bacterial-feeding nematodes were numerically dominant in the meadow and prairie, while microarthropods were dominant in the forest. Ammonium-N was the dominant form in the forest, while nitrate —nitrite-N was the more important form in both bacterial-dominated grasslands.Addition of a biocide solution containing carbofuran and dimethoate reduced the numbers of both microarthropods and nematodes. In the bacterial-dominated grasslands, these reductions resulted in no apparent effect on bacterial densities because one group of bacterial consumers (protozoa) increased following the decrease in bacteria-feeding nematodes, in increased fungal biomass, and in increased soil inorganic N. Conversely, in the forest, following the biocide-induced reduction in consumers, the total fungal biomass decreased, but inorganic-N levels increased. The meadow appeared to be the most resilient of the three ecosystems to biocide disturbance, as both nematode and arthropod numbers returned to control levels more rapidly in the meadow than in the prairie or the forest.

Trophic interactions in soils as they affect energy and nutrient dynamics. III. Biotic interactions of bacteria, amoebae, and nematodes.

Bacteria (Pseudomonas), amoebae (Acanthamoeba), and nematodes (Mesodiplogaster) were raised in soil microcosms with and without glucose additions. Nematode and amoebal grazing on bacteria significantly reduced bacterial populations by the end of a 24-day incubation period. Amoebal numbers decreased in the presence of nematodes with a corresponding increase in nematode numbers which reached a maximum of 230 nematodes/g of soil in the treatment with amoebae and glucose additions. After 24 days the nematode populations in the treatments without carbon additions were dominated by resistant dauer larvae indicating the unavailability of food. Although larval numbers were high in the treatments with glucose additions, the adult component of the population was still increasing at the end of the 24-day experiment. The effect of the presence of amoebae on nematode abundance was of the same magnitude as addition of 600Μg glucose-C.

Effects of streptomycin, cycloheximide, fungizone, captan, carbofuran, cygon, and PCNB on soil microorganisms

Eight biocides were chosen to determine whether they had any effects on nontarget organisms in soil and to what extent they would reduce their target populations under laboratory experimental conditions. A simplified microcosm system was utilized in which reduced species arrays that included field populations of either only bacteria and fungi, or bacteria, fungi, and protozoa (no nematodes, arthropods, or plants) were inoculated into sterilized soil. In a second set of experiments, plants were grown in sterilized soil. A bactericide-streptomycin-four fungicides-cycloheximide, Fungizone (amphotericin B), captan, and PCNB (quintozene)-an acaricide-cygon-an insecticide-nematicide-carbofuran-and an insecticide-diazinon-were used. Each biocide had effects on nontarget organisms although the increases or decreases, with respect to the control, were of only limited duration. Reductions in target groups were typically of longer duration. Streptomycin, applied at 1 mg·g−1 soil, did not decrease bacterial populations during the experimental incubation. At 3 mg·g−1 soil, streptomycin decreased the numbers of bacteria that grew on tryptone agar, but also reduced active hyphae. Fungizone was the most effective of the 4 fungicides tested in reducing active hyphae. Increased bacterial populations were usually observed following fungal reductions. Carbofuran had the fewest effects on the test organisms (bacteria, fungi, and protozoa). Only an initial stimulation of bacterial and fungal populations was observed with cygon although it also increased NH4+-N concentrations in soil during most of the incubation, as did streptomycin and cycloheximide. A transitory increase in fungal populations following a decrease in ciliate numbers was observed in the cygon with grazers treatments. Diazinon reduced all microbial populations and inorganic nitrogen concentrations measured. Cygon and PCNB decreased growth of blue grama plants, while streptomycin reduced shoot weights of blue grama. These results should be useful in assessing the effects of these biocides when applied to more complex systems.

Effects of freeze-thaw stress on bacterial populations in soil microcosms

To test the effect of freezing on soil biota, isolated from the shortgrass prairie of northeastern Colorado, a series of experiments were performed using gnotobiotic soil microcosms.Pseudomonas paucimobilis was used to examine the effects of freezing on bacteria of different growth stages. Secondly, the effect of multiple freeze-thaw cycles was tested on an assemblage of bacterial species. Lastly, the effect of freezing on predator-prey interactions was studied usingP. paucimobilis and an amoebal predator,Acanthamoeba polyphaga. A temperature of −9°C was not detrimental toP. paucimobilis at any growth stage. A single severe freeze-thaw cycle (−27°C to 23°C) resulted in 40–60% mortality ofP. paucimobilis and the mixed bacteria, although additional freezing events did not reduce the populations further. Multiple freeze-thaw cycles (−9°C to 23°C) gave 40–60% mortality ofP. paucimobilis and the mixed bacteria. Predator-prey population cycles were possibly desynchronized by freeze-thaw events.