An increasing body of evidence suggests that microorganisms are far more sensitive to heavy metal stress than soil animals or plants growing on the same soils. Not surprisingly, most studies of heavy metal toxicity to soil microorganisms have concentrated on effects where loss of microbial function can be observed and yet such studies may mask underlying effects on biodiversity within microbial populations and communities. The types of evidence which are available for determining critical metal concentrations or loadings for microbial processes and populations in agricultural soil are assessed, particularly in relation to the agricultural use of sewage sludge. Much of the confusion in deriving critical toxic concentrations of heavy metals in soils arises from comparison of experimental results based on short-term laboratory ecotoxicological studies with results from monitoring of long-term exposures of microbial populations to heavy metals in field experiments. The laboratory studies in effect measure responses to immediate, acute toxicity (disturbance) whereas the monitoring of field experiments measures responses to long-term chronic toxicity (stress) which accumulates gradually. Laboratory ecotoxicological studies are the most easily conducted and by far the most numerous, but are difficult to extrapolate meaningfully to toxic effects likely to occur in the field. Using evidence primarily derived from long-term field experiments, a hypothesis is formulated to explain how microorganisms may become affected by gradually increasing soil metal concentrations and this is discussed in relation to defining “safe” or “critical” soil metal loadings for soil protection.

Toxicity of heavy metals to microorganisms and microbial processes in agricultural soils: a review.

Well-constrained carbon:nitrogen:phosphorus (C:N:P) ratios in planktonic biomass, and their importance in advancing our understanding of biological processes and nutrient cycling in marine ecosystems, has motivated ecologists to search for similar patterns in terrestrial ecosystems. Recent analyses indicate the existence of “Redfield-like” ratios in plants, and such data may provide insight into the nature of nutrient limitation in terrestrial ecosystems. We searched for analogous patterns in the soil and the soil microbial biomass by conducting a review of the literature. Although soil is characterized by high biological diversity, structural complexity and spatial heterogeneity, we found remarkably consistent C:N:P ratios in both total soil pools and the soil microbial biomass. Our analysis indicates that, similar to marine phytoplankton, element concentrations of individual phylogenetic groups within the soil microbial community may vary, but on average, atomic C:N:P ratios in both the soil (186:13:1) and the soil microbial biomass (60:7:1) are well-constrained at the global scale. We did see significant variation in soil and microbial element ratios between vegetation types (i.e., forest versus grassland), but in most cases, the similarities in soil and microbial element ratios among sites and across large scales were more apparent than the differences. Consistent microbial biomass element ratios, combined with data linking specific patterns of microbial element stoichiometry with direct evidence of microbial nutrient limitation, suggest that measuring the proportions of C, N and P in the microbial biomass may represent another useful tool for assessing nutrient limitation of ecosystem processes in terrestrial ecosystems.

https://www.cfc.umt.edu/research/biogeochemistry/files/publications/BGC_85_232%20copy.pdf

C:N:P stoichiometry in soil: is there a “Redfield ratio” for the microbial biomass?

B6nassy, C., 1955. R6marques sur deux Aphelinid6s: Aphelinus mytilaspidis Le Baron et Aphytis proclia Walker. Annls l~piphyt. 6: 11-17. Lord, F. T. & MacPhee, A. W., 1953. The influence of spray programs on the fauna of apple orchards in Nova Scotia II. Oyster shell scale. Can. Ent. 79: 196-209. Pickett, A. D., 1946. A progress report on long term spray programs. Rep. Nova Scotia Fruit Grow. Ass. 83 : 27-31. Pickett, A. D., 1967. The influence of spray programs on the fauna of apple orchards in Nova Scotia XIV. Can. Ent. 97: 816-821. Tothill, J. D., 1918. The predacious mite Hemisarcoptes malus Shimer and its relation to the natural control of the oyster shell scale Lepidosaphes ulmi L. Agric. Gaz. Can. 5 : 234-239.

http://link.springer.com/content/pdf/10.1007/BF01976688.pdf

Soil enzymes: Kuprevich, V. F. & Shcherbakova, T. A.: Translated from the Russian edition (1966), published by the Indian National Scientific Documentation Centre, New Delhi 1971. 392 pp., offset printing from type script, paper cover. Obtainable from U.S. Department of Commerce, National Technical Information Service, Springfield, Va. 22151

The kEC value (=extractable part of microbial biomass C) of the fumigation-extraction (FE) method was assessed on the basis of 153 soils (94 arable, 46 grassland and 13 forest soils) by indirect calibration using the fumigation-incubation (FI) method. Sixty-six soils were investigated for the first time and the data on a further 87 soils were obtained from the literature. The single kEC values ranged from 0.23 to 0.84. A split according to the form of land use resulted in a significantly (Scheffe, P = 0.05) lower kEC value for the arable soils (0.42; n = 94) in comparison to those for the grassland (0.49; n = 46) and the forest soils (0.51; n = 13). This difference is mainly due to the significant effects of the respiration rate measured in non-fumigated control samples of the FI method which was used for calibration of the kEC value. For that reason, I investigated the effects of incubation temperature (22°, 25° and 28°C) on biomass C data obtained by the FI method, and thus on the kEC value of the FE method, and discuss further problems of direct and indirect calibration. Based on experimental and literature data, I conclude that the kEC values of Vance et al. (Soil Biology & Biochemistry19, 703–707, 1987) and Wu et al. (Soil Biology & Biochemistry22, 1167–1169, 1990) remain valid. A kEC value of 0.38 can be recommended for C analysis by dichromate consumption and a kEC value of 0.45 for that by UV-persulfate or oven oxidation.

The fumigation-extraction method to estimate soil microbial biomass: Calibration of the kEC value

Geomycology: biogeochemical transformations of rocks, minerals, metals and radionuclides by fungi, bioweathering and bioremediation

We analysed soils of Lower Saxony, Germany, variously contaminated with sewage sludge, sediment, and dump material from mining and factory residues, to assess the effects of different sources of heavy metals and different metal species (total, EDTA and NH4NO3 extractable) on activity (basal respiration), biomass (microbial C, adenylates) and community structure (ergosterol) of soil microorganisms in relation to important chemical soil properties. The contents of biomass C, adenylates and ergosterol did not necessarily decrease with increasing heavy metal content, reflecting the importance of other environmental factors, e.g. differences in C input. The biomass C/soil C ratio ranged from 0.19 to 2.10% and decreased in the order: sludge>sediment>dump contamination, in accordance with the increasing heavy metal load. The ergosterol/biomass C ratio, which in contrast increased in the orders/sludge<sediment<dump contamination, varied between 0.45% and 3.10% and was on average 1.09%, i.e. it was much higher than in uncontaminated soils, indicating a change in community structure towards fungi. This ergosterol/biomass C ratio was most markedly affected by the concentration of NH4NO3-extractable Pb. The adenylate energy charge, ranging from 0.49 to 0.79, was mainly affected by the three Cu species and showed a strong positive correlation with the metabolic quotient qCO2. The average ATP/biomass C ratio was 7.9 µmol g–1 biomass C. It was the only ratio that remained unaffected by any of the soil properties or heavy metal species.

Different sources of heavy metals and their long-term effects on soil microbial properties

Estimation of soil microbial biomass C in the presence of living roots by fumigation-extraction

We measured microbial biomass C and soil organic C in soils from one grassland and two arable sites at depths of between 0 and 90 cm. The microbial biomass C content decreased from a maximum of 1147 (0–10 cm layer) to 24 μg g-1 soil (70–90 cm layer) at the grassland site, from 178 (acidic site) and 264 μg g-1 soil (neutral site) at 10–20 cm to values of between 13 and 12 μg g-1 soil (70–90 cm layer) at the two arable sites. No significant depth gradient was observed within the plough layer (0–30 cm depth) for biomass C and soil organic C contents. In general, the microbial biomass C to soil organic C ratio decreased with depth from a maximum of between 1.4 and 2.6% to a minimum of between 0.5 and 0.7% at 70–90 cm in the three soils. Over a 24-week incubation period at 25°C, we examined the survival of microbial biomass in our three soils at depths of between 0 and 90 cm without external substrate. At the end of the incubation experiment, the contents of microbial biomass C at 0–30 cm were significantly lower than the initial values. At depths of between 30 and 90 cm, the microbial biomass C content showed no significant decline in any of the four soils and remained constant up to the end of the experiment. On average, 5.8% of soil organic C was mineralized at 0–30 cm in the three soils and 4.8% at 30–90 cm. Generally, the metabolic quotient qCO2 values increased with depth and were especially large at 70–90 cm in depth.

Activity and biomass of soil microorganisms at different depths

Thirty-eight soils from forest sites in central Germany dominated by beech trees (Fagus sylvatica L.) were sampled to a depth of about 10 cm after careful removal of the overlying organic layers. Microbial biomass P was estimated by the fumigation — extraction method, measuring the increase in NaHCO3-extractable phosphate. The size of the microbial P pool varied between 17.7 and 174.3 μg P g-1 soil and was on average more than seven times larger than NaHCO3-extractable phosphate. Microbial P was positively correlated with soil organic C and total P, reflecting the importance of soil organic matter as a P source. The mean microbial P concentration was 13.1% of total P, varying in most soils between 6 and 18. Microbial P and microbial C were significantly correlated with each other and had a mean ratio of 14.3. A wide (5.1–26.3) microbial C: P ratio indicates that there is no simple relatinship between these two parameters. The microbial C: P ratio showed strong and positive correlations with soil pH and cation exchange capacity.

Microbial biomass phosphorus in soils of beech ( Fagus sylvatica L.) forests

The fumigation-extraction (FE) method, applied after extraction of inorganic N and sieving was used to measure soil microbial biomass C, N and carbohydrate C contents at a depth of 0–10 cm under wheat every week in a field study lasting from October 1990 to November 1991 A minimum of 180 μg microbial biomass Cg−1 soil and a maximum of 363 μg g−1 were recorded, the corresponding values for biomass N being 35μg g−1 soil (= 53 kg ha−1) and 66μg g−1 soil (= 99 kg ha−1) An increase was observed from December 1990 to November 1992, with minor fluctuations during periods with strong variations in weather conditions: cold and very wet in winter, extremely dry in summer All three biomass components measured in the 05 M K2SO4 extracts of the FE procedure were significantly correlated The intercepts of the linear relationships between microbial biomass C, N and carbohydrate C were non-significant, indicating constant component ratios The amount of carbon additionally made extractable by fumigation consisted of 17% carbohydrate C The biomass C:N ratio fluctuated closely around a mean value of 55

Brunk Meyer

Papers

Different sources of heavy metals and their long-term effects on soil microbial properties

Estimation of soil microbial biomass C in the presence of living roots by fumigation-extraction

Activity and biomass of soil microorganisms at different depths

Microbial biomass phosphorus in soils of beech ( Fagus sylvatica L.) forests

Time-course of the soil microbial biomass under wheat: A one year field study