Effect of fungal to bacterial biomass ratio on the relationship between CO2 evolution and total soil microbial biomass

The relationship between the fungal: bacterial biomass ratio and the metabolic quotient (qCO₂) was studied in three different soils. In addition, the effect of the fungal: bacterial biomass ratio on the relationship between CO₂ evolution and the size of the soil microbial biomass was examined. Soil samples were collected from three experimental fields amended with various organic materials (Yatsugatake, Ibaraki, and Tochigi fields). The range of the fungal:bacterial biomass ratio in the Yatsugatake and Ibaraki fields was small (1.54–2.24 and 1.11–1.71, respectively), but it was large in the Tochigi field (1.18–3.75). We found a high negative correlation between this ratio and the metabolic quotient (qCO₂=2.10–0.361 (fungal:bacterial biomass ratio), R=–0.851, P<0.01) in the Tochigi field. Therefore, we suggest tha qCO₂ decreases with an increase in the fungal:bacterial biomass ratio, which may be due to a higher efficiency of substrate C use by fungal flora in comparison with bacterial flora. In the Yatsugatake and Ibaraki fields, there was a high positive correlation between CO₂ evolution and total microbial biomass. In contrast, no correlation was observed between these two parameters in the Tochigi field, probably reflecting the wide range of values for the fungal:bacterial biomass ratio. From the results obtained, we suggest that the fungal: bacterial biomass ratio is an important factor regulating the relationship between CO₂ evolution and the size of the microbial biomass.     

Effect of liming on some chemical,biochemical, and microbiological properties of acid soils under spruce (Picea abies L.)

Summary Soil pH, total organic C, total N, exchangeable Al, available P, CO₂ evolution, microbial biomass C and N, phosphatase and dehydrogenase activities were determined in acid soils sampled under spruce subjected to acid deposition, before and after liming. A slight decrease in pH values was observed from the edge of a tree canopy to the base of the trunk in acid soils. Liming drastically reduced exchangeable Al and increased CO₂ evolution, microbial biomass, and the metabolic quotient. The microbial biomass C to total organic C ratio increased after liming but did not reach 2%, the average value considered valid in soils where the C content is in equilibrium, that is when C inputs are equal to C outputs. The microbial biomass C:N ratio decreased after liming, thus indicating that bacteria became predominant over fungi when soil acidity decreased. Dehydrogenase activity but not phosphatase activity was increased by liming. The decrease in phosphatase activity was not completely related to the increase in available P, but was also dependent on microbial growth and the decrease in acid phosphatase, the predominant component of acid soils.     

Microbiological and biochemical characteristics of a range of New Zealand soils under established pasture

Twenty one topsoils, selected to represent major New Zealand soil groups were collected from developed, grazed pasture and examined for microbiological and biochemical characteristics. Organic C and total N levels ranged from 2.4–46% (mean 8.7%) and 0.22–2.31% (mean 0.65%), respectively. The characteristics examined included microbial biomass (mean 1240 μg Cg^?1 soil), min-N flush (mean 85 μg N g^?1 soil), microbial P (mean 52 μg P g^?1 soil), min-N and CO₂ produced, nitrification index, urease, protease, phosphatase and arylsulphatase activities. Most of these characteristics were generally higher than the values already published possibly due to high levels of organic matter.Principal component analysis of the correlation matrix showed two distinct groups of characteristics: CO₂ produced, N mineralized, total N, organic C, microbial biomass, microbial P and phosphatase comprising one group and arylsulphatase, min-N flush, urease and nitrification index comprising the other group. Protease activity and soil pH were not strongly associated with each other or the other groups.     

Effect of cattle slurry in grassland on microbial biomass and on activities of various enzymes

We examined the long-term effects of cattle slurry, applied at high rates, on microbial biomass, respiration, the microbial quotient (qCO₂) and various soil enzyme activities. In March, June, July, and October 1991, slurry-amended grassland soils (0–10 cm) contained significantly higher levels of microbial biomass, N mineralization and enzyme activities involved in N, P, and C cycling. With microbial biomass as the relative value, the results revealed that the slurry treatment influenced enzyme production by the microbial biomass. High levels of urease activity were the result not only of a larger microbial biomass, but also of higher levels of enzmye production by this microbial biomass. The ratio of alkaline phosphatase and xylanase to microbial biomass was nearly constant in the different treatments. The metabolic quotient (qCO₂) declined with increased levels of slurry application. Therefore it appears that microorganisms in slurry-amended soils require less C and energy if there is no competition for nutrients. The results of this study suggest that urease activity, nitrification, and respiration (metabolic quotient) can be used as indicators of environmental stress, produced by heavy applications of cattle slurry.     

Enzyme activity and carbon dioxide evolution from upland and wetland rice soils under three agricultural practices in hilly regions

Summary Dehydrogenase activity (a measure of microbial biomass), urease activity and CO₂ evolution were measured in soils planted to rice (Oryza sativa L.) under three different agricultural practices prevalent in hill regions. The effects of hill slope, terrace and valley agriculture were investigated for two cropping seasons. The valleys and terraces were kept flooded during each cropping season while the hill slopes were cultivated with dryland practices. The type of agricultural practice and the date of observation had a significant influence on enzyme activity and CO₂ evolution. A positive correlation was observed between dehydrogenase and urease activity and soil moisture content. Dehydrogenase and urease activity and soil respiration were positively correlated among themselves. The activity of both enzymes and CO₂ evolution were highest in valley soils followed by terrace and hill-slope sites.     

Influence of organic by-products and nitrogen source on chemical and microbiological status of an agricultural soil

We assessed the influence of the addition of four municipal or agricultural by-products (cotton gin waste, ground newsprint, woodchips, or yard trimmings), combined with two sources of nitrogen (N), [ammonium nitrate (NH₄NO₃) or poultry litter] as carbon (C) sources on active bacterial, active fungal and total microbial biomass, cellulose decomposition, potential net mineralization of soil C and N and soil nutrient status in agricultural soils. Cotton gin waste as a C source promoted the highest potential net N mineralization and N turnover. Municipal or agricultural by-products as C sources had no affect on active bacterial, active fungal or total microbial biomass, C turnover, or the ratio of net C:N mineralized. Organic by-products and N additions to soil did not consistently affect C turnover rates, active bacterial, active fungal or total microbial biomass. After 3, 6 or 9 weeks of laboratory incubation, soil amended with organic by-products plus poultry litter resulted in higher cellulose degradation rates than soil amended with organic by-products plus NH₄NO₃. Cellulose degradation was highest when soil was amended with newsprint plus poultry litter. When soil was amended with organic by-products plus NH₄NO₃, cellulose degradation did not differ from soil amended with only poultry litter or unamended soil. Soil amended with organic by-products had higher concentrations of soil C than soil amended with only poultry litter or unamended soil. Soil amended with organic by-products plus N as poultry litter generally, but not always, had higher extractable P, K, Ca, and Mg concentrations than soil amended with poultry litter or unamende soil. Agricultural soil amended with organic by-products and N had higher extractable N, P, K, Ca and Mg than unamended soil. Since cotton gin waste plus poultry litter resulted in higher cellulose degradation and net N mineralization, its use may result in faster increase in soil nutrient status than the other organic by-products and N sources that were tested. Received: 15 May 1996     

Soil CO2 evolution: Response from arginine additions

Short-term response of soil C mineralization following drying/rewetting has been proposed as an indicator of soil microbial activity. Houston Black clay was amended with four rates of arginine to vary microbial responses and keep other soil properties constant. The evolution of CO₂ during 1 and 3 days following rewetting of dried soil was highly related to CO₂ evolution during 10 days following chloroform fumigation (r² = 0.92 and 0.93, respectively) which is a widely used method for soil microbial biomass C, which disrupts cellular membranes. This study suggest that the release of CO₂ following rewetting of dried soil with no amendments other than heat and water can be highly indicative of soil microbial activity and possibly be used as a quantitative measurement of soil biological quality in Houston Black soils.     

Salinity reduces the ability of soil microbes to utilise cellulose

An incubation experiment was conducted to determine the response of soil microbial biomass and activity to salinity when supplied with two different carbon forms. One nonsaline and three saline soils of similar texture (sandy clay loam) with electrical conductivities of the saturation extract (EC_e) of 1, 11, 24 and 43 dS m^?1 were used. Carbon was added at 2.5 and 5 g C kg^?1 (2.5C, 5C) as glucose or cellulose; soluble N and P were added to achieve a C/N ratio of 20 and C/P ratio of 200. Soil microbial activity was assessed by measuring CO₂ evolution continuously for 3 weeks; microbial biomass C and available N and P were determined on days 2, 7, 14 and 21. In all soils, cumulative respiration was higher with 5C than with 2.5C and higher with glucose than with cellulose. Cumulative respiration was highest in the nonsaline soil and decreased with increasing EC, whereas the decrease was gradual with glucose, there was a sharp drop in cumulative respiration with cellulose from the nonsaline soil to soil with EC11 with little further decrease at higher ECs. Microbial biomass C and available N and P concentrations were highest in the nonsaline soil but did not differ among the saline soils. Microbial biomass C was higher and available N was lower with 5C than with 2.5C. The C form affected the temporal changes of microbial biomass and available nutrients differentially. With glucose, microbial biomass was highest on day 2 and then decreased, whereas available N showed the opposite pattern, being lowest on day 2 and then increasing. With cellulose, microbial biomass C increased gradually over time, and available N decreased gradually. It is concluded that salinity reduced the ability of microbes to decompose cellulose more than that of glucose.     

Functional and structural responses of bacterial and fungal communities from paddy fields following long-term rice cultivation

Purpose

Rice paddy soils undergo pedogenesis driven by periodic flooding and drainage cycles that lead to accumulation of organic matter and the stratification of nutrients and oxygen in the soil profile. Here, we examined the effects of continuous rice cultivation on microbial community structures, enzyme activities, and chemical properties for paddy soils along a chronosequence representing 0–700 years of rice cropping in China.

Materials and methods

Changes in the abundance and composition of bacterial and fungal communities were characterized at three depths (0–5, 5–10, and 10–20 cm) in relation to organic carbon, total nitrogen, dissolved organic carbon, microbial biomass carbon/nitrogen, and activities of acid phosphatase, invertase, and urease.

Results and discussion

Both soil organic carbon and total nitrogen increased over time at all three depths, while pH generally decreased. Microbial abundance (bacteria and fungi) and invertase and urease activity significantly increased with the duration of rice cultivation, especially in the surface layer. Fungal abundance and acid phosphatase activity declined with depth, whereas bacterial abundance was highest at the 5–10-cm soil depth. Profiles of the microbial community based on PCR-DGGE of 16S rRNA indicated that the composition of fungal communities was strongly influenced by soil depth, whereas soil bacterial community structures were similar throughout the profile.

Conclusions

Soil bioactivity (microbial abundance and soil enzymes) gradually increased with organic carbon and total nitrogen accumulation under prolonged rice cultivation. Microbial activity decreased with depth, and soil microbial communities were stratified with soil depth. The fungal community was more sensitive than the bacterial community to cultivation age and soil depth. However, the mechanism of fungal community succession with rice cultivation needs further research.

     

Spatial changes of soil fungal and bacterial biomass from a sub-alpine coniferous forest to grassland in a humid, sub-tropical region

 Fungal and bacterial biomass were determined across a gradient from a forest to grassland in a sub-alpine region in central Taiwan. The respiration-inhibition and ergosterol methods for the evaluation of the microbial biomass were compared. Soil fungal and bacterial biomass both significantly decreased (P<0.05) with the shift of vegetation from forest to grassland. Fungal and bacterial respiration rates (evolved CO₂) were, respectively, 89.1 μl CO₂ g^–1 soil h^–1 and 55.1 μl CO₂ g^–1 soil h^–1 in the forest and 36.7 μl CO₂ g^–1 soil h^–1 and 35.7 μl CO₂ g^–1 soil h^–1 in the grassland surface soils (0–10 cm). The fungal ergosterol content in the surface soil decreased from the forest zone (108 μg g^–1) to the grassland zone (15.9 μg g^–1). A good correlation (R ²=0.90) was exhibited between the soil fungal ergosterol content and soil fungal CO₂ production (respiration) for all sampling sites. For the forest and grassland soil profiles, microbial biomass (respiration and ergosterol) declined dramatically with depth, ten- to 100-fold from the surface organic horizon to the deepest mineral horizon. With respect to fungal to bacterial ratios for the surface soil (0–10 cm), the forest zone had a significantly (P<0.05) higher ratio (1.65) than the grassland zone (1.05). However, there was no fungal to bacterial ratio trend from the surface horizon to the deeper mineral horizons of the soil profiles. Received: 30 March 2000     

Nitrogen mineralization,N<Subscript>2</Subscript>O production and soil microbiological properties as affected by long-term applications of sewage sludge composts

A field study was conducted to investigate the long-term effect of surface application of sewage sludge composts vs chemical N fertilizer on total N, total C, soluble organic C, pH, EC, microbial biomass C and N, protease activity, deaminase activity, urease activity, gross and net rates of N mineralization and nitrification, CO₂ evolution, and N₂O production. Soil samples were taken from five depths (0–15, 15–20, 20–30, 30–40, and 40–50 cm) of a long-term experiment at the University of Tokyo, Japan. Three fields have been receiving sewage sludge composted with rice husk (RH), sawdust (SD), or mixed chemical fertilizer NPK (CF), applied at the rate of 240 kg N ha^–1 each in split applications in summer and autumn since 1978. Significantly higher amounts of total N and C and soluble organic C were found in the compost than in the CF treatments up to the 40-cm soil depth, indicating improved soil quality in the former. In the CF treatment, soil pH values were significantly lower and electrical conductivity values were significantly higher than those of compost-treated soils of up to 50 cm depth. Soil microbial biomass C and N, CO₂ evolution, protease, deaminase, and urease activities were significantly higher in the compost than in the CF treatments due to greater availability of organic substrates that stimulated microbial activity. Gross N mineralization rates determined by ¹⁵N dilution technique were eight and five times higher in the SD and RH treatments than in the CF treatment, respectively, probably due to high levels of microbial and enzyme activities. Net N mineralization rates were also significantly higher in the compost treatments and were negative in the CF treatment indicating immobilization. Net nitrification rates were higher in compost treatments and negative in the CF treatment. Nitrous oxide productions from compost treatments were higher than the CF treatment due to the greater availability of mineral N as a result of higher mineralization and nitrification rates and soluble organic C in the former. Most of the measured parameters were highest in the surface soil (0–15 cm) and were significantly higher in the SD treatment than in the RH treatment.     

The impact of long-term CO2 enrichment and moisture levels on soil microbial community structure and enzyme activities

Soil organic matter is the most important reservoir of terrestrial organic C and minor changes in the balance may have a significant impact on the climate. However, the response of microbial decomposers of soil C to global changes is not fully apprehended. This is particularly the case with regard to the interactive effects of the various climatic changes. Here, we present data from the Giessen Free Air CO₂ Experiment (Gi-FACE, University of Giessen, Germany) in which the CO₂ concentration at a grassland site was increased by 20% relative to atmospheric levels during a period of 10 years. The site included a slope that resulted in differences in average soil moisture. The effects of CO₂ and soil moisture on soil microbial community structure, measured by Denaturing Gradient Gel Electrophoresis (DGGE), PhosphoLipid Fatty Acids (PLFA) and enzyme activity profiles were determined. Total carbon, nitrogen and phosphorous contents were also determined. Soil moisture explained a large part of the variance in the microbial community structure data, by affecting fungi and bacteria. Furthermore, the CO₂ treatment had no significant effect on either overall PLFA or DGGE profiles, despite the fact that the fungal:bacterial PLFA ratio was altered. Overall enzyme activity profiles were also only affected by soil moisture levels, although the CO₂ treatment induced a significant increase of the acid phosphatase activity. Finally, neither soil moisture nor elevated CO₂ induced changes in the soil C stock.     

Nine years of enriched CO2 changes the function and structural diversity of soil microorganisms in a grassland

To gain insight into microbial function following increased atmospheric CO₂ concentration, we investigated the influence of 9 years of enriched CO₂ (600 μl litre⁻¹) on the function and structural diversity of soil microorganisms in a grassland ecosystem under free air carbon dioxide enrichment (FACE), as affected by plant species (Trifolium repens L. and Lolium perenne L. in monocultures and mixed culture) and nitrogen (N) supply. We measured biomass and activities of enzymes covering cycles of the most important elements (C, N and P). The microbial community was profiled by molecular techniques of phospholipid fatty acid (PLFA) and denaturing gradient gel electrophoresis (DGGE) analysis. The enrichment in CO₂ increased soil microbial biomass (+48.1%) as well as activities of invertase (+36.2%), xylanase (+22.9%), urease (+23.8%), protease (+40.2%) and alkaline phosphomonoesterase (+54.1%) in spring 2002. In autumn, the stimulation of microbial biomass was 25% less and that of enzymes 3–12% less than in spring. Strong correlations between activities of invertase, protease, urease and alkaline phosphomonoesterase and microbial biomass were found. The stimulation of microbial activity in the enriched atmosphere was probably caused by changes in the quantity and kind of root litter and rhizodeposition. The response of soil microorganisms to enriched CO₂ was most pronounced under Trifolium monoculture and under greater N supply. The PLFA analysis revealed that total PLFA contents were greater by 24.7% on average, whereby the proportion of bioindicators representative of Gram‐negative bacteria increased significantly in the enriched CO₂ under less N‐fertilized Lolium culture. Discriminant analysis showed marked differences between the PLFA profiles of the three plant communities. Shannon diversity indices calculated from DGGE patterns were greater (+12.5%) in the enriched CO₂, indicating increased soil bacterial diversity. We conclude that greater microbial biomass and enzyme activity buffer the potential increase in C sequestration occurring from greater C addition in enriched CO₂ due to greater mineralization of soil organic matter.     

The adverse effects of soil salinization on the growth of Trifolium alexandrinum L. and associated microbial and biochemical properties in a soil from Iran

The aim of this study was to determine the effects of increasing concentrations of salt solutions (including 0.12, 2, 6, and 10 dS m⁻¹) on the growth of berseem clover (Trifolium alexandrinum L.) and related soil microbial activity, biomass and enzyme activities. Results showed that the dry weights of root and shoot decreased with an increase in the concentrations of salt solutions. Soil salinization depressed the microbiological activities including soil respiration and enzyme activities. Substrate-induced respiration was consistently lower in salinized soils, whereas microbial biomass C did not vary among salinity levels. Higher metabolic quotients (qCO₂) and unaffected microbial biomass C at high EC values may indicate that salinity is a stressful factor, inducing either a shift in the microbial community with less catabolic activity or reduced efficiency of substrate utilization. Acid phosphatase and alkaline phosphatase activities decreased with increasing soil salinity. We found significant, positive correlations between the activities of phosphatase enzymes and plant's root mass, suggesting that any decrease in the activities of the two enzymes could be attributed to the reduced root biomass under saline conditions.     

Soil microbial metabolic profiles in two geomorphological units in a semistable sand-dune ecosystem

The playas and microbiotic crust-covered interdunes are two geomorphological units in the Hallamish sand-dune field, Israel. As a depositional unit, a lack of vegetation and microbiotic crusts, as well as high contents of fine particles and salinity, may lead to low microbial activity and functional diversity at the playa. The present study was initiated in order to illuminate the spatiotemporal changes in the activity, biomass and metabolic profiles of soil microbial communities at the two habitats. Soil samples were collected from the 0–50-cm depth at the playa and interdune during the wet and dry seasons of 2010. Soil moisture and organic-carbon contents recorded at the playa were 2- to above 10-fold higher than at the crust-covered interdune throughout the study period. Nevertheless, opposite trends were recorded for fungal and bacterial CFUs, as well as microbial basal respiration and biomass, in most soil depths at the playa compared to the crust-covered interdune. The total substrate utilization rates of soil microbial communities fluctuated between 4.9 and 6.4 and between 3.9 and 14.9 μg CO₂–C g^?1 dry soil h^?1 at the playa and interdune during the wet season, while 3-fold higher values were recorded at the two habitats during the dry season. The total substrate utilization rates, carbohydrates and carboxylic-acid utilization rates, and functional diversity of soil microbial communities at the playa, were lower than at the crust-covered interdune at most soil depths, while higher utilization rates of aromatic acid were recorded in at least three soil layers at the playa.     

Evaluation of microbial methods as potential indicators of soil quality in historical agricultural fields

In agricultural ecosystems that have had consistent cropping histories, standard microbial methods may be used to evaluate past and present practices. Our objective was to evaluate several microbial methods that best indicate cropping histories and soil quality on long-term plots. We selected soil microbial carbon (C), phospholipid analyses, direct counts of total fungal and bacterial biomass, and soil enzymes (phosphatases) to measure direct and indirect microbial activity on the Sanborn Field and Tucker Prairie. The Sanborn Field has been under various cropping and management practices since 1888 and the Tucker Prairie is an uncultivated site. Seven different plots were chosen on the Sanborn Field and random samples were taken in the summit area on the Tucker Prairie, which represented a reference site. Soil microbial biomass C, phospholipids, and enzyme activity were reflective of the cropping and management histories observed on the Sanborn Field. Enzymatic activity was highly correlated to soil organic matter. The direct counts of fungal and bacterial biomass showed that fungal populations dominated these soils, which may be attributed to soil pH. Soil microbial biomass C and enzyme assays seemed to be better potential indicators of cropping histories than the other methods tested in the long-term plots.This paper has been assigned by the Missouri Agricultural Experiment Station to Journal Series no. 12043     

The proportional mineralisation of microbial biomass and organic matter caused by air-drying and rewetting of a grassland soil

During the first few days after rewetting of an air-dried soil (AD-RW), microbial activity increases compared to that in the original moist soil, causing increased mineralisation (a flush) of soil organic carbon (C) and other nutrients. The AD-RW flush is believed to be derived from the enhanced mineralisation of both non-biomass soil organic matter (due to its physical release and enhanced availability) and microbial biomass killed during drying and rewetting. Our aim was to determine the effects of AD-RW on the mineralisation of soil organic matter and microbial biomass during and after repeated AD-RW cycles and to quantify their proportions in the CO₂-C flushes that resulted. To do this, a UK grassland soil was amended with ¹⁴C-labelled glucose to label the biomass and then given five AD-RW cycles, each followed by 7 d incubation at 25 °C and 50% water holding capacity. Each AD-RW cycle increased the amount of CO₂-C evolved (varying from 83 to 240 μg g⁻¹ soil), compared to the control with, overall, less CO₂-C being evolved as the number of AD-RW cycles increased. In the first cycle, the amount of biomass C decreased by 44% and microbial ATP by 70% while concentrations of extractable C nearly doubled. However, all rapidly recovered and within 1.3 d after rewetting, biomass C was 87% and ATP was 78% of the initial concentrations measured prior to air-drying. Similarly, by 2 d, extractable organic C had decreased to a similar concentration to the original. After the five AD-RW cycles, the amounts of total and ¹⁴C-labelled biomass C remaining in the soil accounted for 60 and 40% of those in the similarly incubated control soil, respectively. Soil biomass ATP concentrations following the first AD-RW cycle remained remarkably constant (ranging from about 10 to 14 μmol ATP g⁻¹ biomass C) and very similar to the concentration in the fresh soil prior to air-drying. We developed a simple mathematical procedure to estimate the proportion of CO₂-C derived from biomass C and non-biomass C during AD-RW. From it, we estimate that, over the five AD-RW cycles, about 60% of the CO₂-C evolved came from mineralisation of non-biomass organic C and the remainder from the biomass C itself.     

Bacterial,fungal and protozoan responses to chloroform fumigation in stored soil

Microbial biomass estimated by CO₂ evolution following fumigation was 2.5–14.7 times greater than that estimated by direct microscopy in prairie soil. Bacteria, fungi and protozoa were counted by direct microscopy before, during and periodically for 10 days following chloroform fumigation and compared with microbial biomass as estimated by CO₂ evolution and N mineralization following chloroform fumigation. Protozoan populations were reduced to below detection levels immediately after fumigation and remained below detection levels during incubation following fumigation. Bacterial and fungal populations were reduced by fumigation to 37–79% of their original populations but usually recovered to their initial numbers by the second day following fumigation. In one case protozoa contributed up to 74 μg C, or about half of the total microbial biomass, to CO₂ evolution following fumigation.Microbial biomass was estimated in soil wetted to 60% of water-holding capacity (WHC) 1 wk or 1 day before fumigation. Microbial activity changed during the 1 wk incubation before fumigation but not total microbial biomass determined by microscopy.The ratio of CO₂ evolved-to-N mineralized followed fumigation changed in direct proportion to the ratio of fungal-to-bacterial biomass present in the soil before fumigation. Although more experiments with different soils should be performed, these results indicate that CO₂ evolved or N mineralized varies with the ratio of fungal-to-bacterial biomass initially present.     

Microbial biomass and activity indices after organic substrate addition to a selenium-contaminated soil

High concentrations of Se in soil might have negative effects on microorganisms. For this reason, the effect of organic substrate addition (glucose + maize straw) on Se volatilisation in relation to changes in microbial biomass and activity indices was investigated using an artificially Se-contaminated soil. Microbial biomass N was reduced on average by more than 50% after substrate addition, but adenylate energy charge (AEC) and metabolic quotient qCO₂ were both increased. The Se content decreased by nearly 30% only with the addition of the organic substrate at 25°C. No significant Se loss occurred without substrate at 25°C or with substrate at 5°C. In the two treatments with substrate addition, the substrate-derived CO₂ evolution was about 30% lower with Se addition than without. In contrast, Se had no effect on any of the other soil microbial indices analysed, i.e. microbial biomass C, microbial biomass N, adenosine triphosphate (ATP), AEC, ATP-to-microbial biomass C, and qCO₂.     

Do growth yield efficiencies differ between soil microbial communities differing in fungal:bacterial ratios? Reality check and methodological issues

Soil communities dominated by fungi such as those of no-tillage (NT) agroecosystems are often associated with greater soil organic matter (SOM) storage. This has been attributed in part to fungi having a higher growth yield efficiency (GYE) compared to bacteria. That is, for each unit of substrate C utilized, fungi invest a greater proportion into biomass and metabolite production than do bacteria. The assumption of higher fungal efficiency may be unfounded because results from studies in which fungal and bacterial efficiencies have been characterized are equivocal and because few studies have measured microbial GYE directly. In this study, we measured microbial GYE in agricultural soils by following ¹³C-labeled glucose loss, total CO₂-C, and ¹³CO₂-C evolution at 2 h intervals for 20 h in two experiments (differing in N amendment levels) in which the fungal:bacterial biomass ratios (F:B) were manipulated. No differences in efficiency were observed for communities with high versus low F:B in soils with or without added inorganic N. When calculated using ¹³CO₂-C (in contrast to total CO₂-C) evolution, growth yield efficiencies of soils having high and low F:B were 0.69±0.01 and 0.70±0.01, respectively. When soils were amended with N, soils with high and low F:B had growth yield efficiencies of 0.78±0.01 and 0.76±0.01, respectively. Our experiments do not support the widely held assumption that soil fungi have greater growth efficiency than soil bacteria. Thus, claims of greater fungal efficiency may be unsubstantiated and should be evoked cautiously when explaining the mechanisms underlying greater C storage and slower C turnover in fungal-dominated soils.