Survival strategies of bacteria in the natural environment.

Two different agricultural tillage practices were used to study how changes in the structure of the soil-litter system affected litter decomposition rates, microbial community composition, and soil organic matter dynamics. Surface straw placement results in spatial separation of carbon-rich litter (C:N ratio 80:1) and mineralized soil nitrogen. In contrast, when the litter is plowed into the soil, straw carbon and soil nitrogen are in intimate contact. Our field studies in Colorado showed that fungal biomass in surface- straw treatments was 144% of that in the incorporated-straw treatments, probably because fungi, with their extensive hyphal networks, are able to utilize both the surface straw carbon and the available soil nitrogen. Field studies using '4C-labeled wheat straw showed that a greater proportion of added 14C was retained in the surface-straw treatment than in the incorporated-straw treatment. Maximum net N immobilization was higher and litter de- composition was slower in the surface straw than in the incorporated straw placements both with and without experimental nitrogen addition. Slower litter decomposition of the surface litter may contribute to reduced soil organic matter losses. Soil organic matter losses may also be reduced in no-till systems as a result of the increase in the ratio of fungal to bacterial activity because of the greater growth efficiency of fungi and the accumulation of carbon in the less decomposable fungal biomass. The surface placement of straw in no-till agriculture allowed management of microclimate and microbial populations so that losses of soil organic matter and nutrients were minimized.

https://www.jstor.org/stable/pdfplus/1939274.pdf

Litter placement effects on microbial and organic matter dynamics in an agroecosystem

We conducted field experiments to test the general hypothesis that the com- position of decomposer communities and their trophic interactions can influence patterns of plant litter decomposition and nitrogen dynamics in ecosystems. Conventional (CT) and no-tillage (NT) agroecosystems were used to test this idea because of their structural sim- plicity and known differences in their functional properties. Biocides were applied to ex- perimentally exclude bacteria, saprophytic fungi, and microarthropods in field exclosures. Abundances of decomposer organisms (bacteria, fungi, protozoa, nematodes, microar- thropods), decomposition rates, and nitrogen fluxes were quantified in surface and buried litterbags (Secale cereale litter) placed in both NT and CT systems. Measurements of in situ soil respiration rates were made concurrently. The abundance and biomass of all microbial and faunal groups were greater on buried than surface litter. The mesofauna contributed more to the total heterotrophic C in buried litter from CT (6-22%) than in surface litter from NT (0.4-1/1%). Buried litter decay rates (1.4-1.7%/d) were -2.5 times faster than rates for surface litter (0.5-O.7%/d). Ratios of fungal to bacterial biomass and fungivore to bacterivore biomass on NT surface litter generally increased over the study period resulting in ratios that were 2.7 and 2.2 times greater, respectively, than those of CT buried litter by the end of the summer. The exclusion experiments showed that fungi had a somewhat greater influence on the decomposition of surface litter from NT while bacteria were more important in the de- composition of buried litter from CT. The fungicide and bactericide reduced decomposition rates of NT surface litter by 36 and 25% of controls, respectively, while in CT buried litter they were reduced by 21 and 35% of controls, respectively. Microarthropods were more important in mobilizing surface litter nitrogen by grazing on fungi than in contributing to litter mass loss. Where fungivorous microarthropods were experimentally excluded, there was less than a 5% reduction in mass loss from litter of both NT and CT, but fungi- fungivore interactions were important in regulating litter N dynamics in NT surface litter. As fungal densities increased following the exclusion of microarthropods on NT surface litter, there was 25% greater N retention as compared to the control after 56 d of decay. Saprophytic fungi were responsible for as much as 86% of the net N immobilized (1.81 g /m2) in surface litter by the end of the study when densities of fungivorous microarthropods were low. Although bacteria were important in regulating buried litter decomposition rates and the population dynamics of bacterivorous fauna, their influence on buried litter N dynamics remains less clear. The larger microbial biomass and greater contribution of a bacterivorous fauna on buried litter is consistent with the greater carbon losses and lower carbon assimilation in CT than NT agroecosystems. In summary, our results suggest that litter placement can strongly influence the com- position of decomposer communities and that the resulting trophic relationships are im- portant to determining the rates and timing of plant litter decomposition and N dynamics. Furthermore, cross placement studies suggest that the decomposer communities within each tillage system, while not discrete, are adapted to the native litter placements in each.

Microbial and Faunal Interactions and Effects on Litter Nitrogen and Decomposition in Agroecosystems

The fungi-to-bacteria ratio in soil ecological concepts and its application to explain the effects of land use changes have gained increasing attention over the past decade. Four different main approaches for quantifying the fungal and bacterial contribution to microbial tissue can be distinguished: (1) microscopic methods, (2) selective inhibition, (3) specific cell membrane components and (4) specific cell wall components. In this review, the different methods were compared and we hypothesized that all these approaches result in similar values for the fungal and bacterial contribution to total microbial biomass, activity, and residues (dead microbial tissue) if these methods are evenly reliable for the estimation of fungal biomass. The fungal contribution to the microbial biomass or respiration varied widely between 2 and 95% in different data sets published over the past three decades. However, the majority of the literature data indicated that fungi dominated microbial biomass, respiration or non-biomass microbial residues, with mean percentages obtained by the different methodological approaches varying between 35 and 76% in different soil groups, i.e. arable, grassland, and forest soils and litter layers. Microscopic methods generally gave the lowest average values, especially in arable and grasslands soils. Very low ratios in fungal biomass C-to-ergosterol obtained by microscopic methods suggest a severe underestimation of fungal biomass by certain stains. Relatively consistent ratios of ergosterol to linoleic acid (18:2ω6,9) indicate that both cell membrane components are useful indicators for saprotrophic and ectomycorrhizal fungi. More quantitative information on the PLFA content of soil bacteria and the 16:1ω5 content of arbuscular mycorrhizal fungi is urgently required to fully exploit the great potential of PLFA measurements. The most consistent results have been obtained from the analysis of fungal glucosamine and bacterial muramic acid in microbial residues. Component-specific δ 13 C analyses of PLFA and amino sugars are a promising prospect for the near future.

Quantitative assessment of the fungal contribution to microbial tissue in soil

Microbial biomass C and ATP were measured in 10 forest soils that ranged in pH from 3.2 to 7.2. A close relationship (r = 0.85) between biomass C (measured by the fumigation-incubation method, using the CO2-C evolved from the non-fumigated soil in 10 days as the control) and ATP was obtained in soils with pH ⩾ 4.2. The average biomass ATP concentration was 9.1 βmol ATP g−1 biomass C, similar to that found in earlier studies for soils above pH 4.2. There was also good agreement (r = 0.87) between biomass C as measured by fumigation-incubation and as measured by direct microscopy in soils of pH ⩾ 4.2, with an average ratio of one between the two measurements. However, in soils below pH 4.2, biomass C measured by the conventional fumigation-incubation method was very low compared to that estimated from soil ATP concentration or by direct microscopy.

Results with large inocula were inconclusive but it may be possible to measure microbial biomass in strongly acid soils by adding a large inoculum to the fumigated soil and using the appropriate value of kC for that soil.

If biomass C in the strongly acid soils (pH < 4.2) is calculated from the CO2-C evolved by the fumigated soil alone, without subtracting the control, a mean biomass ATP concentration of 9.2 μmol ATP g−1 biomass C is obtained, a value virtually identical to that measured in the 6 soils of pH ⩾ 4.2 by the conventional method using a control. Biomasses calculated by this modified method in the strongly acid soils were also broadly comparable with those measured by direct microscopy. Soils at or below pH 4.2 contained very high concentrations of KCl-extractable aluminium. It is possible that this is linked to the reduced activity of microbial recolonizers in acid fumigated soils.

Microbial biomass measurements in forest soils: The use of the chloroform fumigation-incubation method in strongly acid soils

SUMMARY: Soil micro-organisms have been counted by a new technique whose essential feature is the suspension of measured amounts of soil in a molten agar gel from which small drops are removed and allowed to solidify as thin films on a haemocytometer slide of known depth. The instantaneous gelation of the agar ensures the fixation of the soil constituents in their original distribution. The films are dried and stained in a solution of acetic-aniline blue and permanent preparations made by subsequent dehydration in ethanol and mounting in euparal. If the suspension is of known dilution, since films of a definite volume contain a known quantity of soil, differential counts of a measured area of film will yield a quantitative estimate of soil micro-organisms.
The distribution of bacteria per microscopic field was found to be complex. The frequencies of bacterial colonies and of pieces of fungal mycelium form a Poisson series; those of the number of bacteria per colony form a logarithmic series; and those of the total number of bacteria per field fall into a negative binomial distribution.
The method appears to be capable of modification by the use of selective nutrient media for determining the quality of the microflora and possibly the percentage viability of the organisms present.

/pdf/a-technique-for-the-quantitative-estimation-of-soil-micro-ywwrt6vp2c.pdf

P. C. T. Jones

Papers

A Technique for the Quantitative Estimation of Soil Micro-organisms: With a Statistical Note by