Microbial biomass and metabolic activity in four acid soils

The fumigation method was used to estimate microbial biomass C in four Haplumbrepts developed over different kinds of rock. In order to investigate the relationship between metabolic activity and microbial biomass and population density, CO₂ release from the glucose-enriched and unenriched soils was measured during 28 days of incubation.

Biomass C levels lay between 36 and 112 mg 100 g⁻¹ of dry soil, and made up only a small proportion of total soil C (0.77–1.38%). Only a small fraction of this biomass was detected by counting viables, but the microbial population was nevertheless significantly correlated with the biomass determined by fumigation. Among the physico-chemical properties of the soils, microbial biomass and population size were both chiefly affected (favourably) by humidity, total C and N and Al gel content. Metabolic activity was slight, either because part of the micro-organisms are inactive or because of a limited supply of substrate (the organic matter present may be unsuitable as a substrate or protected from microbial attack). Percentage C mineralization was inversely related to organic matter, silt and Al gel contents, and likewise failed to exhibit positive correlation with respiration, the biomass determined by fumigation or the counted population. The metabolic activity of the biomass appeared to depend upon the quality and nature of soil organic matter rather than its quantity, which nevertheless controlled microbial population size.

Neither microbial biomass estimates nor viable population counts faithfully reflected metabolic activity in the soils.     

Changes in microbial biomass after continuous application of azolla and rice straw in soil

A model experiment was conducted under tropical conditions with a view to evaluating the changes in microbial biomass and nutrient dynamics in upland soil through the continuous application of azolla and rice straw (2 g C kg^-1 soil per each application). Flush decomposition of C was observed immediately after each application and the rate of mineralization did not change appreciably during this period. After flush decomposition, the rate of C mineralization from azolla was higher than that from rice straw until 9 to 13 weeks after each application and thereafter the mineralization rate was similar. The amount of inorganic N released from azolla increased following each application, whereas inorganic N in rice straw plot was immediately immobilized and the rate of immobilization increased until the 3rd application and did not increase further after the 4th application. The amounts of biomass C and N increased immediately after residue incorporation, reached the maximum level one week after each application and declined thereafter. Maximum biomass formation increased until the 2nd application and then the level remained constant. Maximum biomass N formation was higher in azolla than in rice straw after the 1st application, but after repeated applications, the difference became less pronounced: Continuous increase in biomass in a certain week after each application was observed, probably because of the cumulative effects of the previous applications. The increase suggests that continuous application of organic materials may enable to improve the amount of soil microbial biomass.     

Microbial biomass in agricultural topsoils after 6 years of bare fallow

Inherent soil properties have an influence on microbial activity. These effects were measured in a field trial at Weihenstephan with 30 agricultural and 2 vineyard soils from different sites in Bavaria which had been kept under bare fallow for 6 years. The soils represented a wide range of arable soils from a temperate climate. Unaffected by recent differences in climatic conditions or cropping managements, they were used to assess the relationship between microbial biomass C and a broad spectrum of soil physical and chemical properties (clay content 5–63%, pH 4.5–7.5, organic C 0.55–2.93%). Microbial C was measured using the substrate-induced respiration method. In addition, soil catalase activity and the abundance and biomass of earthworms were determined. Among the soil properties, microbial C was most strongly correlated with organic C (r=0.86, n=29). In a comparison of linear regressions between microbial biomass C and organic C for different cropping managements, the slope under bare fallow was lowest, followed by monoculture and crop rotation. The microbial: organic C ratio ranged from 1.1 to 4.3% and was significantly correlated with soil pH (r=0.66). A positive relationship between microbial C and the clay content (r=0.66) was significantly improved when soils with more than 25% clay were excluded (r=0.80). Partial correlation analysis indicated that clay had a direct influence, hardly affected by an intercorrelation with organic C. Catalase activity was highly correlated with microbial C (r=0.95) and, because a rapid and sensitive method of determination is available, was considered suitable for estimating relative amounts of active microbial biomass. A positive relationship between microbial C and the abundance of earthworms indicated interactions between microorganisms and mesofauna.     

Soil microbial biomass, dehydrogenase activity, bacterial community structure in response to long-term fertilizer management

This study describes the effects of balanced versus nutrient-deficiency fertilization on soil microbial biomass, activity, and bacterial community structure in a long-term (16 years) field experiment. Long-term fertilization greatly increased soil microbial biomass C and dehydrogenase activity, except that the P-deficiency fertilization had no significant effect. Organic manure had a significantly greater (P<0.05) impact on the biomass C and the activity, compared with mineral fertilizers. Microbial metabolic activity (dehydrogenase activity per microbial biomass C) was significantly higher (P<0.05) under balanced fertilization than under nutrient-deficiency fertilization. General bacterial community structure was analyzed by PCR-denaturing gradient gel electrophoresis (DGGE) targeting eubacterial 16S rRNA gene. Mineral fertilization did not affect the DGGE banding pattern, while specific DGGE band was observed in organic manure-fertilized soils. Phylogenetic analysis showed that the change of bacterial community in organic manure-fertilized soil might not be because of the direct influence of the bacteria in the compost, but because of the promoting effect of the compost on the growth of an indigenous Bacillus sp. in the soil. We emphasize the importance of balanced-fertilization, as well as the role of P, in maintaining soil organic matter, and promoting the biomass and activity of microorganisms.     

Reduced CO2 release from decomposing wheat straw under N-limiting conditions: simulation of carbon turnover

The turnover of residue carbon in soil containing little available N can affect the management of crop residues. The effects of N deficiency on CO₂ release from decomposing wheat straw were measured in an incubation experiment and interpreted by computer simulation. Straw with a C:N ratio of 91, incubated for 460 days in sand that was inoculated with a soil suspension, released CO₂ much more slowly than when inorganic N was added to obtain a C:N ratio of 5. The evolution of CO₂ continued longer without added N, approaching the amount released in the high N treatment with time. The simulation model NCSOIL was modified to simulate reduced CO₂ release from decomposing residue when N limits microbial growth by (i) including the decomposers' biomass in the rate of residue decomposition in the form of a Monod-type equation, where the biomass reduced the rate when its concentration was small compared with a saturation constant, and (ii) including formation of a polysaccharide-like pool that received the decomposed C that could not be assimilated by the biomass because of insufficient N. The modified model simulated the reduced CO₂ production in the absence of sufficient N, as a result of a smaller microbial biomass that reduced the rate of residue decomposition, and the formation of polysaccharides as long as N limited synthesis of microbial biomass.     

Modelling mineralization of plant residues on soil: effect of physical protection

A mechanistic dynamic model (Verberne et al. 1990) was used to simulate mineralization of white-clover materials in a loam (25% clay) and a sandy loam soil (5% clay). I tested the model‘s ability to simulate the observed temporal patterns and to take account of altered physical protection as affected by soil compaction or spatial residue distribution. With default parameter values, the model greatly overestimated net N mineralization. The model was very sensitive to changes in the C/N ratio of the microbial biomass. Reducing this value from 8.0 to 6.0 improved the model performance. Nevertheless, initial N mineralization was appreciably overestimated. Two hypotheses may explain the discrepancies: (1) the C/N ratio of the microbial biomass is initially low (3–4) and gradually increases because of a succession from bacterial- to fungal-dominated biomass (H ₁); (2) the C/N ratio of the substrates first attacked by microorganisms, i.e. water-soluble components such as sugars and free amino acids, is higher than the average value (6.0) assumed for the readily decomposable fraction (H ₂). Conceptually, this fraction originally included N-containing polymers (proteins and nucleic acids), which in large part are water insoluble and probably attacked somewhat later than the monomers. Modification of the model, either by implementing a dynamic C/N ratio of the biomass and the effect of faunal grazing or by increasing the C/N ratio of the easily decomposable fraction, improved the model performance substantially. The two hypotheses need to be tested experimentally. The model adequately simulated measured effects of spatial residue distribution and soil compaction on N mineralization after adjustment or parameter values regulating physical protection of microbial biomass and metabolites. Moreover, there was a good agreement between simulated and measured microbial biomass N in the two soils. Received: 9 December 1996     

Soil microbial biomass and activity in Chinese tea gardens of varying stand age and productivity

Tea (Camellia sinensis) is a globally important crop and is unusual because it both requires an acid soil and acidifies soil. Tea stands tend to be extremely heavily fertilized in order to improve yield and quality, resulting in a great potential for diffuse pollution. The microbial ecology of tea soils remains poorly understood; an improved understanding is necessary as processes affecting nutrient availability and loss pathways are microbially mediated. We therefore examined the relationships between soil characteristics (pH, organic C, total N, total P, available P, exchangeable Al), the soil microbial biomass (biomass C, biomass ninhydrin-N, ATP, phospholipid fatty acids—PLFAs) and its activities (respiration, net mineralization and nitrification). At the Tea Research Institute, Hangzhou (TRI), we compared fields of different productivity levels (low, medium and high) and at Hongjiashan village (HJS) we compared fields of different stand age (9, 50 and 90 years). At both sites tea soils were compared with adjacent forest soils. At both sites, soil pH was highest in the forest soil and decreased with increasing productivity and age of the tea stand. Soil microbial biomass C and biomass ninhydrin-N were significantly affected by tea production. At TRI, microbial biomass C declined in the order forest>low>high>middle production and at HJS in the order stand age 50>age 9>forest>age 90. Soil pH had a strong influence on the microbial biomass, demonstrated by positive linear correlations with: microbial biomass C, microbial biomass ninhydrin-N, the microbial biomass C:organic C ratio, the microbial biomass ninhydrin-N:total N ratio, the respiration rate and specific respiration rate. Above pH_(KCl) 3.5 there was net N mineralization and nitrification, and below this threshold some samples showed net immobilization of N. A principal component (PC) analysis of PLFA data showed a consistent shift in the community composition with productivity level and stand age. The ratio of fungal:bacterial PLFA biomarkers was negatively and linearly correlated with specific respiration in the soils from HJS (r²=0.93, p=0.03). Our results demonstrate that tea cultivation intensity and duration have a strong impact on the microbial community structure, biomass and its functioning, likely through soil acidification and fertilizer addition.     

Microbial biomass,activity, and organic matter accumulation in soils contaminated with heavy metals

Chemical characteristics and some parameters related to biological components were determined in 16 soils from a fairly homogeneous area in the north of Italy, contaminated with different levels of heavy metals. Correlation analysis of the parameters studied showed close positive relationships among the metals and with the organic C content in the soils studied. Negative relationships were observed among the heavy metals, soil respiration, and the ratio between evolved CO₂–C and microbial biomass C per unit time (specific respiratory activity). This was ascribed to an adverse heavy metal effect on the soil microflora, which appeared to increase the accumulation of organic matter as the heavy metal content increased, probably because the biomass was less effective in mineralising soil organic matter under these conditions.     

Seasonal changes of microbial biomass carbon related to climatic factors in soils from karst areas of southwest China

 The seasonal responses of soil microbial biomass C to changes in atmospheric temperature, soil moisture and soluble organic C were studied in soils from the karst areas of southwest China. These soils are relatively weathered, leached and impoverished, and have a low input of plant residues. Over 1 year, an inverse relationship between soil microbial biomass C and atmospheric temperature was found. The highest microbial biomass C occurred in winter and the lowest in summer, and ranged from 231–723 μg g^–1 dry soil. Although there was no obvious relationship between microbial biomass C and soil moisture, a negative correlation existed between microbial biomass C and soluble organic C. In the ecosystem studied, the marked changes in soil microbial biomass C at above 20  °C were ascribed to fluctuations of soil moisture, which were controlled by climatic factors and geomorphic conditions. The patterns of soluble organic C turnover were similar to those of soluble carbohydrate C, both of which were controlled by soil drying-rewetting cycles. It was concluded that the lowest amounts of soil microbial biomass C, measured in the summer, resulted in increases in soluble organic C due to higher turnover rates of the former at warmer air temperatures. Thus, there was a marked seasonal change in soil microbial biomass C. Received: 1 November 1998     

长期施肥处理对玉米生长期潮土微生物生物量和活度的影响

以1989年建立的中国科学院封丘农田生态系统国家试验站的长期定位试验为平台,研究经18a连续不同施肥处理后玉米季土壤微生物生物量碳氮和微生物活度的动态变化及其与土壤有机碳之间的相互关系,并探讨施肥措施对土壤微生物及其活性的影响。施肥处理包括:(1)有机肥(OM);(2)1/2化肥和1/2有机肥(1/2OM+1/2NPK);(3)氮磷钾肥(NPK);(4)氮磷肥(NP);(5)磷钾肥(PK);(6)氮钾肥(NK);(7)不施肥,即对照(CK)7个处理。结果表明,微生物生物量碳氮和微生物活度在玉米生长期内均有明显的时间变异性,其中微生物生物量碳与微生物活度的动态变化比较一致,其间的极显著相关关系表明潮土微生物生物量碳的变化可以在很大程度上代表土壤微生物活度的变化。施肥制度显著影响微生物生物量碳氮和微生物活度的变化,总体趋势为OM1/2OM+1/2NPKNPKNPPKNKCK,表明OM有利于保持土壤的生物化学环境及促进土壤的生物学活性;与OM处理相比,化学肥料的长期施用有降低土壤微生物生物量和微生物活度的趋势,尤其是缺素处理的表现更为明显,其中以缺磷处理的表现最为严重。土壤微生物生物量碳氮、微生物活度与土壤有机碳变化均呈极显著正相关。     

Impact of agricultural practices on the size and activity of the microbial biomass in a long-term field experiment

The Dehérain long-term field experiment was initiated in 1875 to study the impact of fertilization on a wheat-sugarbeet rotation. In 1987, the rotation was stopped to be replaced by continuous maize. Crop residues were soil-incorporated and the mineral fertilization was doubled in some plots. The impact of those changes on the microbial biomass and activity are presented. In spring 1987, the soil was still in a steady-state condition corresponding to the rotation. The microbial biomass was correlated with total organic C and decreased in the order farmyard manure>mineral NPK>unfertilized control. Microbial specific respiratory activity was higher in the unfertilized treatments. The soil biomass was closely related to soil N plant uptake. In 1989, after 2 years of maize and crop residue incorporation, the steady-state condition corresponding to the previous agricultural practices disappeared. So did the relationship between the biomass and total organic C, and the soil N plant uptake. Biomass specific respiratory activity increased because of low efficiency in the use of maize residues by microbes under N stress.     

Reponse de la biomasse microbienne a l'adjonction au sol de materiel vegetal marque au 14C: Role des racines vivantes

¹⁴C and ¹⁵N-labelled immature wheat straw was incubated in the laboratory for 450 days in either a sandy soil or a clay soil, under controlled conditions of temperature and humidity. One-half of the treatments were cropped 4 times in succession with spring wheat. After each harvest, the roots and shoots were removed from the soil. The remaining treatments were kept bare, without plants. After 277 days, 1% unlabelled wheat straw was again mixed with the soils. Microbial biomass was measured after 0, 25, 53, 80, 185, 318 and 430 days, using the fumigation technique. This paper presents the ¹⁴C-data.The half-life of the labelled compounds in soil was from 60 to 70 days. After 430 days about 10% more labelled C remained in bare soil than in cropped soil. Labelled biomass carbon reached its maximum before day 25. By then 50% of the biomass-C was labelled and the biomass represented 20% of the total labelled C remaining in the soils. This percentage decreased slowly to 15% after 430 days in bare sandy soil and to 17% in bare clay soil. A second incorporation of plant material, this time unlabelled, did not appreciably alter the shape of the curve representing the decrease of labelled C in biomass, expressed as % of the total remaining labelled C. Total biomass-C (labelled + unlabelled) in cropped soil was sometimes higher and sometimes lower than in bare soil. However, the labelled C/total C ratio in biomass was always lower; in cropped soils than in soils without plants, clearly showing the effect of rhizodeposition. From days 25 to 430 an increasing difference appeared between the ratio labelled C/total and C in CO₂ and the corresponding ratio labelled C/total C in biomass. In CO₂-C the ratio diminished rapidly, in biomass-C it remained at a high level, most probably indicating a lower turnover of C in resting but living microorganisms. Other explanations are also discussed. The amount of CO₂-C released mg^?1 of biomass-C was higher in cropped than in bare soil, presumably because the microorganisms were activated by the living (or dying) root system.     

Near infrared reflectance spectroscopy for determination of organic matter fractions including microbial biomass in coniferous forest soils

The objective of this work was to investigate the usefulness of near infrared reflectance spectroscopy (NIRS) in determining some C and N fractions of soils: labile compounds, microbial biomass, compounds derived from added ¹³C- and ¹⁵N-labelled straw. Soil samples were obtained from a previous experiment where soils were labelled by addition of ¹³C- and ¹⁵N-labelled wheat straw and incubated in coniferous forests in northern Sweden (64-60°N) and south France (43°N). The incubation lasted three years with 7-9 samplings at regular time steps and four replicates at each sampling (204 samples). Samples were scanned using a near infrared reflectance spectrophotometer (NIRSystem 6500). Calibrations were obtained by using a modified partial least squares regression technique with reference data on total C and N, ¹³C, ¹⁵N, control extract-C, -N, -¹³C and -¹⁵N, fumigated extract-C, -N, -¹³C and -¹⁵N, biomass-C, -N, -¹³C and -¹⁵N contents. Mathematical treatments of the absorbance data were first or second derivative with a gap from 4 to 10 nm. The standard error of calibration (SEC)-to-standard deviation of the reference measurements ratio was ≤0.2 for 10 models, namely total C and N, ¹³C, ¹⁵N, control extract-C, fumigated extract-C and -N, biomass-C and -N and biomass-¹⁵N models and therefore considered as very good. With an R²=0.955, the fumigated extract-¹⁵N model is also good. The standard error of performance calculated on the independent set of data and SEC were within 20% of each other for all the best equations except for the biomass-¹⁵N model. The ability of NIRS to detect ¹³C and ¹⁵N in total C and N and in the extracts is noteworthy, not because of its predictive function that is not really of interest in this case, but because it indicates that the spectra kept the signature of the properties of the organic matter derived from the straw even after two- or three-year decomposition. The incorporation of the ¹³C in the biomass was less well predicted than that of the ¹⁵N. This could indicate that the biomass derived from the straw was characterised by a particular protein or amino acid composition compared to the total biomass that includes a large proportion of dormant micro-organisms. The predictive ability of NIRS for microbial biomass-C and -N is particularly interesting because the conventional analyses are time consuming. In addition, NIRS allows detecting analytical errors.     

Development of ergosterol, microbial biomass C, N, and P after steaming as a result of sucrose addition, and Sinapis alba cultivation

A pot experiment was carried out to monitor the recovery of a steaming-reduced microbial biomass (C, N, and P) and fungal ergosterol by sucrose addition. The second objective was to investigate the recovery of a steaming-reduced microbial biomass by white mustard (Sinapis alba) cultivation and its interactions with microbial residues, freshly formed from sucrose addition. Thirty days after steaming, the soil microbial biomass C and N was still significantly reduced by 80%, leading to a rather constant microbial biomass C/N ratio around 7 throughout the experiment. The steaming-induced decreases of microbial biomass P and ergosterol were only roughly 50%, leading to a decrease in the microbial biomass C/P ratio and an increase in the ergosterol-to-microbial biomass C ratio. Sucrose addition led to a 25% reduction in the ergosterol-to-microbial biomass C ratio. Mustard cultivation had significant positive effects on microbial biomass C, N, P, and ergosterol, but the effects were smaller than those of sucrose addition. Cultivating mustard had no significant effects on the C loss or on the incorporation of sucrose C into the microbial biomass. In contrast, the application of sucrose led to a significant decrease in the mustard shoot biomass and especially in the mustard root biomass.     

Microbial biomass and mineralization-immobilization of nitrogen in some agricultural soils

Summary The chloroform fumigation-incubation method (CFIM) was used to measure the microbial biomass of 17 agricultural soils from Punjab Pakistan which represented different agricultural soil series. The biomass C was used to calculate biomass N and the changes occurring in NH₄ ⁺-N and NO₃ ^–-N content of soils were studied during the turnover of microbial biomass or added C source. Mineral N released in fumigated-incubated soils and biomass N calculated from biomass C was correlated with some N availability indexes.The soils contained 427–1240 kg C as biomass which represented 1.2%–6.9% of the total organic C in the soils studied. Calculations based on biomass C showed that the soils contained 64–186 kg N ha^–1 as microbial biomass. Immobilization of NCO₃ ^–-N was observed in different soils during the turnover of microbial biomass and any net increase in mineral N content of fumigated incubated soils was attributed entirely to NH₄ ⁺-N.Biomass N calculated from biomass C showed non-significant correlation with different N availability indexes whereas mineral N accumulated in fumigated-incubated soils showed highly significant correlations with other indexes including N uptake by plants.     

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.     

Microbial biomass nitrogen pool in soils from a warm temperate grassland,and from deciduous and evergreen forests in Chiba,central Japan

We evaluated the status of the microbial biomass N pool in grassland, and in deciduous and evergreen forest soils in Chiba, central Japan. Microbial biomass N, a labile fraction of total N in the soil, ranged from 6.96 g N m^-2 (15 cm depth) in the grassland to 24.8 g in the deciduous and 20.7 g in the evergreen soils, on a landscape basis. Thus the pattern in the grassland and in the forest soils differed. The N flush measured by a fumigation-incubation method indicated that in the grassland soil microbial biomass N was underestimated by a factor of 2.6 compared with the results from a fumigation-extraction method, because of heavy N immobilization in the microbial biomass. This was in contrast to results from the forest soils, which did not immobilize N. Thus, the forest soils were in a steady-state condition compared with the grassland which formed a seral phase in the ecological succession. Simple correlation coefficients indicated a significant positive relationship between biomass N and organic C in the soil and the N concentration in the litter, the main component of organic matter in the soils of the three ecosystems.     

Microbial biomass in organic soils: Estimation of biomass C,and effect of glucose or cellulose amendments on the amounts of N and P released by fumigation

The microbial biomass C in forest humus from beneath Scots pine (Pinus sylvestris L.), Japanese larch [Larix kaempferi (Lamb.) Carr.] and surface soil from Calluna heath, was estimated by both direct microscopy and by the substrate-induced-respiration (SIR) method of Anderson and Domsch. There was reasonable agreement between the methods with the larch and heath land samples, but with the pine, the SIR method gave much lower estimates of the biomass.Total biomass C was greatest in the larch samples and in all sites was dominated (>89%) by fungi.The effect of glucose, cellulose and NPK salts on the SIR biomass C and on the amounts of N and P released by CHCl₃-fumigation of the pine and larch humus, and a mineral soil, were compared. In general, the effects of glucose and cellulose were similar—the biomass C was increased, but the “flushes” of soluble N and inorganic P decreased. The addition of NPK along with the C substrate did not increase the flushes of N and P.The SIR method appears a useful technique to estimate the biomass C on acid organic soils, but the flushes of N and P were not a reliable measure of the nutrient content of the biomass, especially on C-amended samples.     

Modification of the original chloroform fumigation extraction technique to allow measurement of δC of soil microbial biomass carbon

In studies of the soil microbial biomass C by the chloroform fumigation extraction (CFE) technique, biomass C is routinely extracted using 0.5 M K₂SO₄ solution. The excessive amounts of salts contained in the extracting solution pause a significant challenge in using ¹³C isotope techniques to study the nature of C in the soil microbial biomass. This is because the salts can affect the oxidation process and therefore hamper accurate mass spectromic analysis of dried extracts. In spite of this, no standard protocol exists for preparing the K₂SO₄ extracts for ¹³C isotope analysis. We have modified the original CFE method to allow measurement of the δ¹³C of soil microbial biomass C by using 2 M KCl instead of the usual 0.5 M K₂SO₄ solution to extract biomass C. Excess salts were removed by dialysis in 100 molecular weight cut off membranes, after which the extracts were freeze-dried and their δ¹³C measured using a mass spectrometer. The soil microbial biomass C and δ¹³C of 2 M KCl extracts were compared with those of 0.5 M K₂SO₄ extracts. There was excellent agreement between organic C and δ¹³C estimates for dialyzed 2 M KCl and 0.5 M K₂SO₄ extracts, but the speed of dialysis for the latter was very slow, making use of the former more rapid. These results suggest that in procedures where oxidation with potassium dichromate is not critical to analysis of soluble C, 2 M KCl may be used in place of 0.5 M K₂SO₄ to extract soil microbial biomass C for δ¹³C measurements. The new procedure is relatively easy and rapid for obtaining indices for both pool sizes and turnover rates of soil microbial biomass C and provides a promising approach to study soil organic C.     

Adenylates in the soil microbial biomass at different temperatures

Five soils from temperate sites (Germany; 2 arable and 3 grassland) were incubated aerobically at 5, 10, 15, 20, 25, 35, and 40 °C for 8 days. Soils were analysed for soil microbial biomass C, biomass N, AMP, ADP, and ATP to determine whether the increase in the ATP-to-microbial biomass C ratio with increasing temperature was either due to an increase in the adenylate energy charge (AEC) or de novo synthesis of ATP, or both. Around 80% of the variance in microbial biomass C and biomass N was explained by differences in soil properties, only 7% by the temperature treatments. Averaging the data of all 5 soils for each incubation temperature, the microbial biomass C content decreased with increasing temperature from 15 to 40 °C continuously by 2.5 μg g⁻¹ soil °C⁻¹ after 8-days' incubation. However, this decrease was not accompanied by a similar decrease in microbial biomass N. The average microbial biomass C/N ratio was 6.8. Between 54 and 76% of the variance in AMP, ADP, ATP and the sum of adenylates was explained by differences in soil properties and between 14 (ADP) and 27% (ATP) by the temperature treatments. However, temperature effects on AMP and ADP were variable and inconsistent. In contrast, ATP and consequently also the sum of adenylates increased continuously from 5 to 30 °C followed by a decline to 40 °C. The AEC showed similarly a small, but significant increase with increasing temperature from 0.73 to 0.85 at 30 °C. Consequently, the majority of the variance, i.e. roughly 60% in AEC values, but also in ATP-to-microbial biomass C ratios was explained by the incubation temperature. The mean ATP-to-microbial biomass C ratio increased from 4.7 μmol g⁻¹ at 5 °C to a 2.5 fold maximum of 12.0 μmol g⁻¹ at 35 °C. This increase was linear with a rate of 0.26 μmol ATP g⁻¹ microbial biomass C °C⁻¹. The energy for the extra ATP produced during temperature increase is probably derived from an accelerated turnover of endocellular C reserves in the microbial biomass.