Phosphorus in the soil microbial biomass

Phosphorus in the soil microbial biomass (biomass P) and soil biomass carbon (biomass C) were linearly related in 15 soils (8 grassland, 6 arable, 1 deciduous woodland), with a mean P concentration of 3.3% in the soil biomass. The regression accounted for 82% of the variance in the data. The relationship was less close than that previously measured between soil biomass C and soil ATP content and indicates that biomass P measurements can only provide a rough estimate of biomass C content. Neither P concentration in the soil biomass, nor the amount of biomass P in soil, were correlated with soil NaHCO₃-extractable inorganic, organic or total P.The calculated mean annual flux of P through the biomass (in a soil depth of 10 cm) in 8 grassland soils was large, 23 kg P ha^?1 yr^?1, and more than three times the mean annual P flux through 6 arable soils (7 kg P ha^?1 yr^?1), suggesting that biomass P could make a significant contribution to plant P nutrition in grassland.About 3% of the total soil organic P in the arable soils was in microbial biomass and from 5 to 24% in the grassland soils. The decline in biomass P when an old grassland soil was put into an arable rotation for about 20 yr was sufficient to account for about 50% of the decline in total soil organic P during this period. When an old arable soil reverted to woodland, soil organic P doubled in 100 yr; biomass P increased 11-fold during the same period.     

Limitations when quantifying microbial carbon and nitrogen by fumigation‐extraction in rooted soils

Soil microbial C and N (C_mic, N_mic) estimation by the chloroform fumigation‐extraction method is erroneous in densely rooted soils due to CHCl₃‐labile C and N compounds. The effect of a pre‐extraction with 50 mM K₂SO₄ and a pre‐incubation (conditioning at 25 °C for 7 days) on the flush in extractable, CHCl₃‐labile C (C‐flush) and N (N‐flush) was tested with reference to rooting density (0.3—75 mg root dry matter g^—1) in one arable and 3 grassland soils. In the arable soil and in the second horizon (10—20 cm) of a grassland soil, C‐flush values were not affected by the pre‐extraction. However, the pre‐extraction considerably reduced C‐flush values in the top soils of the grassland (above 10 cm). Only about 42 % was found in the pre‐extracted roots and the rest was lost during the pre‐extraction. The estimated concentrations of N_mic decreased due to pre‐extraction of soil samples with low root biomass. Clearly, the concentrations of N_mic were underestimated by introducing the pre‐extraction. Soil pre‐incubation reduced C‐flush values only slightly, whereas N‐flush values were not affected. It can be concluded that (1) CHCl₃‐labile root C and N is partly extracted with K₂SO₄ after pre‐incubation and (2) CHCl₃‐labile C and N removed with the roots during pre‐extraction is partly derived from microbial biomass. Soils with low rooting density (arable soils, grassland soils below approximately 10 cm depth) should therefore be fumigated and extracted without pre‐extraction. In densely rooted soils, fumigation extraction with and without pre‐extraction probably gives estimates for the minimum and maximum of C_mic and N_mic.     

Determination of soil microbial biomass phosphorus in acid red soils from southern China

A CHCl₃ fumigation and 0.03 M NH₄F-0.025 M HCl extraction procedure was used to measure microbial biomass P (P_mic) in 11 acid red soils (pH <6.0) from southern China and the results compared to those obtained by the commonly-used CHCl₃ fumigation and 0.5 M NaHCO₃ extraction method. Extraction with NH₄F-HCl was found to be more effective and accurate than NaHCO₃ extraction for detecting the increase of P from microbial biomass P following chloroform fumigation due to its higher efficiency in extracting both native labile phosphate and added phosphate (³²P) in the soils. This was confirmed by the recovery of ³²P from in situ ³²P-labeled soil microbial biomass following fumigation and extraction by the NH₄F-HCl solution. Soil microbial biomass P, measured by the NH₄F-HCl extraction method, was more comparable with soil microbial biomass C (with a more narrow C:P ratio range of 4.3 to 22.3 and a mean of 15.6 in the microbial biomass), than that obtained by NaHCO₃ solution (with a mean C:P ratio of 30.7 and a wide range of 14.9 to 48.9). K_p, the fraction of soil microbial biomass P extracted after CHCl₃ fumigation, by the NH₄F-HCl solution was 0.34. The amount of microbial biomass P determined (using K_p =0.34) was 3–400% (mean 131%) higher than that obtained by the NaHCO₃ extraction (using K_p =0.40) for the 11 red soils studied. The results suggest that the CHCl₃ fumigation and NH₄F-HCl extraction method is more reliable for measuring microbial biomass P than the NaHCO₃ extraction method in acid red soils.     

Interference from plant roots in the estimation of soil microbial ATP,C, N and P

Excised, solution-grown roots of maize or ryegrass added to two pasture soils at the rate of 6.0mg g^?1 and 13.8 mg g^?, respectively, increased the flush (fumigated minus control values) of CO₂-C by up to 1.89-fold, KCl extractable N by up to 1.88-fold, and NaHCO₃ extractable P by 3.28-fold. The ATP content of the soil was increased by up to 1.42-fold. Because of high variability the effect of the roots on the C and N flushes was not significant at P < 0.05.Incubation of the root-amended soils for 7 days at 25°C prior to fumigation much decreased the contribution from the roots to the C and N flush, and to the ATP content. There was, however, still a large significant effect of the roots on the P-flush, this being up to 3 times greater than the equivalent soil without roots.In soil samples with a high viable root density (> 6mg g^?1) such as may occur in dense pastures, greenhouse pot experiments or rhizosphere soil samples, it is recommended that they be incubated for 7 days prior to fumigation and analyses. Without such prior incubation there is the risk that root material may be included in the “microbial” biomass estimations.     

Ergosterol and microbial biomass relationship in soil

Ergosterol and microbial biomass C were measured in 26 arable, 16 grassland and 30 forest soils. The ergosterol content ranged from 0.75 to 12.94 g g^-1 soil. The geometric mean ergosterol content of grassland and forest soils was around 5.5 g g^-1, that of the arable soils 2.14 g g^-1. The ergosterol was significantly correlated with biomass C in the entire group of soils, but not in the subgroups of grassland and forest soils. The geometric mean of the ergosterol: microbial biomass C ratio was 6.0 mg g^-1, increasing in the order grassland (5.1), arable land (5.4) and woodland (7.2). The ergosterol:microbial biomass C ratio had a strong negative relationship with the decreasing cation exchange capacity and soil pH, indicating that the fungal part of the total microbial biomass in soils increased when the buffer capacity decreased. The average ergosterol concentration calculated from literature data was 5.1 mg g^-1 fungal dry weight. Assuming that fungi contain 46% C, the conversion factor from micrograms ergosterol to micrograms fungal biomass C is 90. For soil samples, neither saponification of the extract nor the more effective direct saponification during extraction seems to be really necessary.     

Adenylates as an estimate of microbial biomass C in different soil groups

Adenylate (i.e. adenosine tri- (ATP), di- (ADP) and monophosphates (AMP)) and microbial biomass C data were collected over a wide range of sites including forest floor layers and forest, grassland and arable soils. Microbial biomass C was measured by fumigation extraction and adenylates after alkaline Na₃PO₄/DMSO/EDTA extraction and HPLC detection. Our aims were (1) to test whether the sum of adenylates is a better estimate for microbial biomass than the determination of ATP, (2) to compare our conversion values with those proposed by others, and (3) to analyse whether soil properties or land use form affect the relationships between ATP, adenylates and microbial biomass C. A close relationship was found between microbial biomass C and ATP (r=0.96), but also with the sum of adenylates (r=0.96) within all appropriately conditioned soil samples (n=112). In the mineral soil (n=98), the geometric means of the ATP-to-microbial biomass C ratio and the adenylates-to-microbial biomass C ratio were 7.4 and 11.4 μmol g⁻¹, respectively. The mean ratios did not differ significantly between the different texture classes and land use forms. In the forest floor, the ATP-to-microbial biomass C ratio and the adenylates-to-microbial biomass C ratio were both roughly two-thirds of those of the mineral soil. The average adenylate energy charge (AEC) of all soil samples was 0.79 and showed a strong negative relationship with the soil pH (r=−0.69). However, the AEC is presumably only indirectly affected by the soil pH.     

Calibration model of microbial biomass carbon and nitrogen concentrations in soils using ultraviolet absorbance and soil organic matter

There is a need for a rapid, simple and reliable method of determining soil microbial biomass (SMB) for all soils because traditional methods are laborious. Earlier studies have reported that SMB‐C and ‐N concentrations in grassland and arable soils can be estimated by measurement of UV absorbance in soil extracts. However, these previous studies focused on soils with small soil organic matter (SOM) contents, and there was no consideration of SOM content as a covariate to improve the estimation. In this study, using tropical and temperate forest soils with a wide range of total C (5–204 mg C g^?1 soil) and N (1–12 mg N g^?1 soil) contents and pH values (4.1–5.9), it was found that increase in UV absorbance of soil extracts at 280 nm (UV₂₈₀) after fumigation could account for 92–96% of the variance in estimates of the SMB‐C and ‐N concentrations measured by chloroform fumigation and extraction (P < 0.001). The data were combined with those of earlier workers to calibrate UV‐based regression models for all the soils, by taking into account their varying SOM content. The validation analysis of the calibration models indicated that the SMB‐C and ‐N concentrations in the 0–5 cm forest soils simulated by using the increase in UV₂₈₀ and SOM could account for 86–93% of the variance in concentrations determined by chloroform fumigation and extraction (P < 0.001). The slope values of linear regression equations between measured and simulated values were 0.94 ± 0.03 and 0.94 ± 0.04, respectively, for the SMB‐C and ‐N. However, simulation using the regression equations obtained by using only the data for forest profile soils gave less good agreement with measured values. Hence, the calibration models obtained by using the increase in UV₂₈₀ and SOM can give a rapid, simple and reliable method of determining SMB for all soils.     

Relative importance of substrate type and previous soil management in synthesis of microbial biomass and substrate mineralization

Our aim was to determine whether the soil microbial biomass, which has developed naturally over many years in a given ecosystem, is specially adapted to metabolize the plant‐derived substrate C of the ecosystem within which it developed or whether the nature of recently added substrate is the more important factor. To examine this, soils from three sites in close proximity (woodland, grassland and arable from the Broadbalk Experiment at Rothamsted Research, Harpenden, UK) were each amended with air‐dried wheat straw (Triticum aestivum), ryegrass leaves (Lolium perenne) or woodland leaf litter (mainly Quercus robur and Fagus sylvatica) in a fully replicated 3 × 3 factorial laboratory experiment. The initial mineralization rates (evolved CO₂‐C) were determined during the first 6.5 hours and again, together with the amount of microbial biomass synthesized (microbial biomass C), at 7, 14, 21, 30 and 49 days of incubation. The hourly rate of CO₂‐C production during the first 6.5 hours was slowest following leaf litter addition, while the added grass gave the fastest rates of CO₂‐C evolution both within and between soils. Ryegrass addition to the arable soil led to approximately four times more CO₂‐C being evolved than when it was added to the woodland soil, at an overall rate in the arable soils of 41 μg C g^?1 soil hour^?1. In each soil, the net amounts of CO₂‐C produced were in the order grass > straw > leaf litter. In each case, the amount produced by the added leaf litter was significantly less (P < 0.05) than either the added grass or straw. Overall, the trend was for much slower rates of mineralization of all substrates in the woodland soil than in either the arable or grassland soils. During 49 days of incubation in the woodland and grassland soils, the net total amounts of CO₂‐C evolved differed significantly (P < 0.01), with grass > straw > leaf litter, respectively. In the arable soil, the amounts of CO₂‐C evolved from added grass and straw were significantly larger (P < 0.01) than from the leaf litter treatment. Our findings indicated that the amounts of CO₂‐C evolved were not related to soil management or to the size of the original biomass but to the substrate type. The amount of biomass C synthesized was also in the order grass > straw > leaf litter, at all stages of incubation in the woodland and grassland soil. In the arable soil, the same effect was observed up to 14 days, and for the rest of the incubation the biomass C synthesized was in the order grass > straw > leaf litter. Up to three times more biomass C was synthesized from the added grass than from the other substrates in all soils throughout the incubation. The maximum biomass synthesis efficiency was obtained with grass (7% of added C). Overall, the woodland soil was most efficient at synthesizing biomass C and the arable soil the least. We conclude that substrate type was the overriding factor that determined the amount of new soil microbial biomass synthesized. Mineralization of substrate C by soil microorganisms was also influenced mainly by substrate type and less by soil management or size of original biomass.     

Impact of black carbon addition to soil on the determination of soil microbial biomass by fumigation extraction

The efficiency of the fumigation extraction method on the determination of soil microbial biomass carbon and ninhydrin-N was tested in three different soils (UK grassland, UK arable, Chinese arable) amended with black carbon (biochar or activated charcoal). Addition of activated charcoal to soil resulted in a significant decrease in K₂SO₄ extractable carbon and ninhydrin-N in all three soils, whereas the addition of biochar generally did not. A lower concentration of the extraction reagent (0.05 M vs. 0.5 M K₂SO₄) resulted in a significantly lower extraction efficiency in the grassland soil. The extraction efficiency of organic carbon was more affected by black carbon than that of ninhydrin-N, which resulted in a decreased biomass C/ninhydrin-N ratio. The impact of black carbon on the extraction efficiency of soil microbial biomass depended on the type of black carbon, on the concentration of the extraction medium and on soil type.     

The effects of biocidal treatments on metabolism in soil—V: A method for measuring soil biomass

A new method for the determination of biomass in soil is described. Soil is fumigated with CHCl₃ vapour, the CHCl₃ removed and the soil then incubated. The biomass is calculated from the difference between the amounts of CO₂ evolved during incubation by fumigated and unfumigated soil. The method was tested on a set of nine soils from long-term field experiments. The amounts of biomass C ha^?1 in the top 23 cm of soil from plots on the Broadbalk continuous wheat experiment were 530 kg (unmanured plot), 590 (plot receiving inorganic fertilizers) and 1160 (plot receiving farmyard manure). Soils that had been fallowed for 1 year contained less biomass than soils carrying a crop. A calcareous woodland soil contained 1960 kg biomass C ha^?1, and an unmanured soil under permanent grass 2020. The arable soils contained about 2% of their organic C in the biomass; uncultivated soils a little more—about 3%.     

Essai de correlation entre proprietes biochimiques d'un sol salsodique et sa biomasse

In order to study the influence of salinity on the biological activity of soils, experiments were performed in a saline alkaline soil from Tunisia (mediterranean semi-arid climate) and compared with results of similar experiments performed in a pelosol from a semi-continental climate (Lorraine, France). Both soils had received ¹⁴C-labelled maize straw.The microbial biomass was estimated by a modified Jenkinson's method and also by measurements of ATP content. The microbial activity was determined by measurements of total and ¹⁴CO₂ evolved.The results have shown an inverse correlation between the salinity determined by electrical conductivity and the biomass estimations and its activity. The higher ATP con tent (141.5 ng · 100 g^?1 soil) was observed in the pelosol and the lower (99.4 ng · 100 g^?1) in the saline soil. Simultaneously and respectively in the pelosol and the saline soil the biological carbon evolved was 114 and 54 mgC 100 g^?1 soil and the average rate of ¹⁴C mineralization was 0.82 and 0.45 mgC · 100 g^?1 soil.Results have also shown that CO₂ evolved after sterilization and reinoculation is not only provided by mineralization of microbial biomass during fumigation but also from interaction between organic-matter and CHCl₃; this interaction is more intense in the saline soil.     

A simple method for quantitative determination of ATP in soil

A simple method to measure soil ATP by the luciferin-luciferase system is described. The ATP is extracted from the soil by vigorous shaking with a sulfuric acid-phosphate solution for 15 min. An aliquot of the soil suspension is neutralized with a Tris-EDTA solution and mixed with a special ATP releasing reagent (NRB). ATP is measured after a 10 s exposure to the NRB reagent, followed by addition of luciferin-luciferase and integration over 10 s in a Lumacounter M 2080. The ATP content in soils which had been stored at 5°C for 90 days and then incubated at 25°C for 5 days, ranged from 0.37 to 7.52 μg ATP g^?1 dry wt, with standard deviations less than 10%. There was a close (r = 0.96) linear relationship between ATP content and biomass C determinated by fumigation for this group of soils. The soil biomass contained 4.2–7.1 μg ATP mg^?1 biomass C. The ATP content of the biomass declined during storage at 5°C for 210 days.     

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

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 g 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.     

The effects of biocidal treatments on metabolism in soil—II. Gamma irradiation,autoclaving, air-drying and fumigation

Respiration and mineralisation of N were measured in a set of contrasting soils that had either been autoclaved, air-dried, fumigated (with chloroform or methyl bromide) or exposed to gamma radiation. The soils used were a manured and an unmanured arable soil, an acid and a neutral woodland soil, an arable sandy soil and an organic soil under grass. With the exception of the acid woodland soil, the flushes of decomposition (i.e. the increases in O₂ consumption, CO₂ evolution and N mineralisation that occurred when the treated soil was inoculated and incubated for 10 days) were in the order: air-drying < CH₃Br ? CHCl₃ < irradiation < autoclaving. All of the treatments, except air-drying, decreased the ratio (C mineralised after treatmcnt)/(N mineralised after treatment). All of the treatments increased the amount of 1N K₂SO₄ extractable organic C, autoclaving causing by far the greatest increase.Neither of the fumigants increased respiration in the acid soil over the whole 10 day period, although N mineralisation was slightly increased. Irradiation, air-drying and autoclaving did, however, produce a flush in the acid soil, the order being: irradiation < air-drying < autoclaving. A soluble substrate, extracted from yeast cells by ultrasonic disintegration, decomposed to about the same extent in neutral and in acid soil. When ¹⁴C labelled glucose was added to the acid soil and incubated for 52 days, the retention of labelled C was slightly greater (31·6%) than in a comparable near-neutral soil (28·8%). However, the flush that followed fumigation of the acid soil was only half that in the near-neutral soil, suggesting that less biomass is formed under acid conditions. Liming increased the size of the flush in an acid soil.For soils from the same field but under different management, the size of the flush caused by CHCl₃ is in the order: grassland > cropped arable > bare fallow. The flush is much more sensitive to differences in soil management than is the total amount of soil organic matter; a fallowed soil lost half its organic C in 10 yr whereas the increase in respiration that followed fumigation fell to one-seventh its original value. Two Nigerian soils behaved similarly; a soil that had been 2 years under cultivation contained only 16% less total organic C than an adjacent soil still under secondary forest, yet the flush in the cultivated soil was half that in the forest soil. The amount of substrate metabolised during the flush is thus very sensitive to changes in soil management that alter the amount of fresh organic matter entering the soil each year.     

Microbial biomass and activity in soils amended with glucose

Four contrasting soils were amended with glucose at concentrations up to 10 mg g^?1 soil. The soils were incubated at 22°C for 14 days and the biomass determined at various times by chloroform fumigation or substrate-induced respiration. The adenosine triphosphate (ATP) content or the amylase and dehydrogenase activities were also determined. The size of the increases in biomass, ATP content and the enzyme activities was generally related to the amount of glucose added. The initially higher ATP levels quickly declined, and apparent substrate conversion figures up to 84% indicated that substrate-induced respiration overestimated the biomass. There were generally no significant correlations between ATP, biomass or enzyme activities.     

Estimation of the adenylate energy charge in unamended and amended agricultural soils

To determine relatively low concentrations of adenine nucleotides in agricultural soils a NaHCO₃-based extradant was developed and compared with the trichloroacetic acid-paraquat-phosphate extradant. The new medium, consisting of chloroform, sodium bicarbonate, phosphate and adenosine (pH 8.0) gave soil extracts which could be investigated without further neutralization and dilution. ATP was measured directly in the soil extracts by the luciferin-luciferase system. ADP and AMP were estimated after their enzymatic conversion to ATP by standard methods. The quantities of nucleotides corrected for recovery of standards were used to calculate the adenylate energy charge (AEC) from the formula AEC = [ATP] + 1/2[ADP]/[ATP] + [ADP] + [AMP], The AEC was estimated in six unplanted soils from agricultural fields. A very similar energy charge of 0.3-0.4 was found in all soils sampled which indicates a low metabolic activity of the soil population. Two other soils with a pronounced difference in biomass-C content were used to investigate the influence of different amendments on the AEC. In an experiment with low glucose supplements up to 500 μg C g^?1 soil, the soil with the low biomass-C (a cambisol) showed a distinct increase of the AEC from 0.34 to 0.50, whereas the soil with the high biomass-C content (a phaeozem) increased its AEC only slightly from 0.32 to 0.37. In another experiment with high glucose supplements the phaeozem reached its maximum AEC value of 0.56 after the addition of 4000 μg Cg^?1 soil. An amendment with 8000 μg C g^?1 soil gave no further increase. In the combisol the addition of 1000 μg C g^?1 soil increased the AEC to 0.61. Higher supplements gave only a slight further increase to a maximum value of 0.67 after the addition of 8000 μg C g^?1 soil. The same AEC value was reached when the cambisol was amended with a mixture of organic substrates at a concentration of 10,000 μg C g^?1 soil.     

Establishment of denitrification capacity in soil: Effects of carbon,nitrate and moisture

The effects of moisture, NO₃^?1 concentration and C addition on changes in denitrification capacity and total microbial biomass in a clay loam soil were investigated. Denitrification capacity was evaluated with an anaerobic slurry technique. Total microbial biomass was measured by CHC1₃ fumigation and by extraction of microbial ATP. The results indicate that denitrification capacity and total microbial biomass were increased only by the C addition; differences in NO₃^?1 concentration and moisture had no effect in this agricultural soil. The increase in denitrification capacity could be attributed solely to microbial growth, since the ratio of denitrification capacity to total microbial biomass remained constant and the increased respiration from the C amendment did not increase anaerobiosis. The results also show that denitrifiers compete as effectively for added C as do other heterotrophs.     

Microbial biomass estimations in soils from tussock grasslands by three biochemical procedures

Microbial biomass was determined by three biochemical procedures in nine topsoils from a climosequence in tussock grasslands. The pH values of the samples ranged from 4.4 to 6.2 and organic C contents from 2.5 to 20.0%. When determined by a chloroform-fumigation procedure, contents of biomass C and mineral-N (Min-N) flush ranged from 530–2780 and 59–167 μgg^?1 dry soil respectively. Adenosine 5'-triphosphate (ATP) content ranged from 2.2 to 10.7 μg g^?1 dry soil. All three estimates were significantly correlated with each other and with several soil properties, including organic C and total N contents and CO₂ production. They were not significantly correlated with any climatic factor.In spite of these significant correlations, the ratios of the biomass estimates varied appreciably in the different soils. The ratios of biomass C/Min-N flush ranged from 7.8 to 22.8 (average 12.5), biomass C/ATP from 163 to 423 (average 248) and Min-N flush/ATP from 12 to 35 (average 22). These ratios were mostly higher than those found elsewhere for Australian and English soils. The high biomass C/ATP and Min-N flush/ATP ratios did not appear to originate from inefficient extraction of “native” ATP or from the soils' P status. Based on these results, care in the use of factors for obtaining soil microbial biomass content from Min-N flush or ATP values is indicated.     

Correlation between four methods to estimate total microbial biomass in stored,air-dried and glucose-amended soils

Correlation between the microbial volume, chloroform fumigation (CO₂-C flush), substrateinduced respiration (SIR) and ATP content methods to estimate microbial biomass was assessed on three New Zealand soils (two grassland, one arable) under three different treatments (stored, air-dried and glucose-amended). There were significant, positive correlations between all methods, r = 0.69–0.88, which were improved, r = 0.71–0.96, if the data for air-dried or glucose-amended soils were excluded from the analyses. The best agreement was between CO₂-C flush and ATP and the worst between CO₂-C flush and microbial volume. Exclusion of air-dried soil data improved these correlations.Estimates of microbial biomass for each soil often differed significantly between the four methods, when conversion factors cited in the literature were used. Ratios (i.e. conversion factors) between CO₂-C flush and ATP or SIR, or SIR and volume, were different to those cited in the literature, and only similar if specific data were excluded.We recommend that a minimum of two and preferably three methods be used to quantify the microbial population of soil, and that emphasis should be placed on the relative differences within and between soils using data which have not been converted to biomass C. Conversion of data to biomass C may result in substantial errors.     

Estimation of microbial biomass and activity in soil using microcalorimetry

Relationships between the rate of heat output from soil, the rate of respiration and the soil microbial biomass were investigated for 25 soils from northern Britain. The rate of heat output, measured in a Calvet microcalorimeter at 22°C, correlated well with the rate of carbon dioxide respiration. The average amount of heat evolved per cm³ of gas respired. 21.1 J cm^?3, suggests that the biomass metabolism was largely aerobic. The rate of heat output per unit of total microbial biomass was remarkably uniform over a wide range of soils, but showed differences depending upon whether the soil had been stored or amended. Mineral soils that had been stored at 4°C had the lowest heat output, 12.0 mW g^?1 biomass C, compared with a mean of 20.4 mW g^?1 biomass C for freshly-collected soils. Amendment with glucose (0.5% w/w) caused an immediate increase in respiration and heat output, up to 59.4 mW g^?1 biomass C for stored soils and 188.2 mW g^?1 biomass C for freshly collected soils. There was a consistent relationship between the biomass and the rate of heat output from freshly collected and amended mineral and organic soils which gave a linear fit using log transformed data: y= 0.6970+ 1.025x (r= 0.98, P < 0.001) (y=log₁₀ biomass C, μgC g^?1; x=log₁₀ rate of heat output at 22°C, μW g^?1). The overall relationship between biomass and the rate of heat output for all the amended samples was: 1 g biomass C= 180.05 ± 34.61 mW.