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.     

Decomposition de corps microbiens dans des sols fumiges au chloroforme: Effets du type de sol et de microorganisme

Five microbial species (Aspergillus flavus, Trichoderma viride, Streptomyces sp., Arthrobacter sp., Achromobacter liquefaciens) were cultivated in liquid media containing ¹⁴C-labelled glucose. The decomposition of these microorganisms was recorded in four different soils after chloroform fumigation by a technique related to that proposed by Jenkinson and Powlson, to determine the mineralization rate of microbial organic matter (K_c coefficient). Three treatments were used: untreated soil, fumigated soil alone and fumigated soil supplied with ¹⁴C-labelled cells. Total evolved CO₂ and ¹⁴CO₂ were measured after 7 and 14 days at 28°C.The labelled microorganisms enabled the calculation of mineralization rate K_c (K_c = mineralized microbial carbon/supplied microbial carbon). The extent of mineralization of labelled microbial carbon depended on the type of soil and on the microbial species. Statistical analysis of results at 7 days showed that 58% of the variance is taken in account by the soil effect and 32% by the microorganism effect. Between 35 and 49% of the supplied microbial C was mineralized in 7 days according to the soil type and the species of microorganism. Our results confirmed that the average value for K_c = 0.41 is acceptable, but K_c variability according to soil type must be considered.The priming effect on organic C and native microbial biomass mineralization, due to microbial carbon addition was obtained by comparison between the amount of non-labelled CO₂-C produced by fumigated soils with or without added labelled microorganisms: this priming effect was generally negligible.These results indicate that the major portion of the error of microbial biomass measurement comes from the K_c estimation.     

Determination and use of a corrected control factor in the chloroform fumigation method of estimating soil microbial biomass

Estimates of soil microbial biomass are important for both comparative system analysis and mechanistic models. The method for measuring microbial biomass that dominates the literature is the chloroform fumigation incubation method (CFIM), developed on the premise that killed microorganisms are readily mineralized to CO₂, which is a measure of the initial population. Factors that effect the CFIM have been thoroughly investigated over the last 15 years. A question that still remains after countless experiments is the use of an appropriate nonfumigated control for accounting for native soil organic matter (SOM) mineralization during incubation. Our approach was to add hot-water-leached ¹⁴C-labeled straw to both fumigated and nonfumigated samples assuming the straw would mimic a recalcitrant C substrate fraction of SOM. The ratio of the ¹⁴C evolved from the fumigated sample over the ¹⁴C evolved from the control sample would provide a corrected control value to be used in calculating microbial biomass. This experiment was conducted on soils from forest, agricultural, grassland and shrub-steppe ecosystems. The results clearly indicate that equal recalcitrant C mineralization during incubation is not a valid assumption. The results with these soils indicate than on the average only 20% of the control CO₂ should be subtracted from the fumigated CO₂ for the biomass calculation. The correction value ranged from 18% for agricultural soils to 25% for shrub-steppe soil, with the average correction value being 20%. Our experiments show that corrected biomass values will be 1.5–2 times greater than uncorrected biomass values. In addition using a corrected control improved the 1:1 correlation between the CFIM and SIR methods for these soils.     

Decomposition of carbon-14-labelled wheat straw in repeatedly fumigated and non-fumigated soils with different levels of heavy metal contamination

We analysed the decomposition of ¹⁴C-labelled straw at five different levels of heavy metal contamination (100-20,000 µg total Zn g^-1 soil) in non-fumigated and repeatedly fumigated soils. The soils were not spiked with Zn, but were taken from sites containing different heavy metal concentrations. Zn was only used as a reference and the effects observed are most likely due to this metal. Microbial biomass decreased with increasing heavy metal content of soils, paralleled generally by the decreasing amount of wheat straw ¹⁴C incorporated into the microbial biomass. In addition, the newly synthesised microbial biomass declined more rapidly as the incubation proceeded. In the repeatedly fumigated soils, microbial biomass ¹⁴C corresponded to roughly 50% of the maximum ¹⁴C incorporation of the non-fumigated soil. The relative decline during incubation was similar to that of the non-fumigated soil at the respective contamination level. These results reveal clearly that heavy metal effects on straw decomposition do not depend on the ratio of substrate C to microbial biomass C. In contrast to microbial biomass C, the mineralisation of the wheat straw was not seriously affected by heavy metal contamination. The same was true for all of the repeatedly fumigated treatments, where a much smaller microbial biomass mineralised nearly the same amount of straw as in the non-fumigated soils. However, repeated fumigation caused a strong reduction in the decomposition of soil organic matter. The ratio of CO₂-¹⁴C to microbial biomass ¹⁴C after 60 days was linearly related to the Zn concentration in both non-fumigated and repeatedly fumigated samples, clearly indicating that an additional energy cost is required by soil microorganisms with increasing heavy metal concentrations.     

Decomposition and distribution of residual activity of some 13C-microbial polysaccharides and cells,glucose, cellulose and wheat straw in soil

Studies were made to determine the rate of decomposition of some ¹⁴C-labeled microbial polysaccharides, microbial cells, glucose, cellulose and wheat straw in soil, the distribution of the residual ¹⁴C in various humic fractions and the influence of the microbial products on the decomposition of plant residues in soil. During 16 weeks from 32 to 86 per cent of the C of added bacterial polysaccharides had evolved as ¹⁴CO₂. Chromobacterium violaceum polysaccharide was most resistant and Leuconostoc dextranicus polysaccharide least resistant. In general the polysaccharides, microbial cells, and glucose exerted little effect on the decomposition of the plant products. Upon incubation the ¹⁴C-activity was quickly distributed in the humic. fulvic and extracted soil fractions. The pattern of distribution depended upon the amendment and the degree of decomposition. The distribution was most uniform in the highly decomposed amendments. After 16 weeks the bulk of the residual activity from Azotobacter indicus polysaccharide remained in the NaOH extracted soil. From C. violaceum polysaccharide both the extracted soil and the humic acid fraction contained high activity. About 50–80 per cent of the residual activity from the ¹⁴C-glucose, cellulose and wheat straw amended soils could be removed by hydrolysis with 6 n HCl. The greater part of this activity in the humic acid fraction was associated with the amino acids and that from the fulvic acids and residual soils after NaOH extraction with the carbohydrates. About 8 16 per cent of the activity of the humic acid fraction was present in substances (probably aromatic) extracted by ether after reductive or oxidative degradation.     

Root activity and carbon metabolism in soils

Summary Two different soils were amended with ¹⁴C-labelled plant material and incubated under controlled laboratory conditions for 2 years. Half the samples were cropped with wheat (Triticum aestivum) 10 times in succession. At flowering, the wheat was harvested and the old roots removed from the soil, so that the soil was continuously occupied by predominantly active root systems. The remaining samples were maintained without plants under the same conditions. During the initial stages of high microbial activity, due to decomposition of the labile compounds, the size of the total microbial biomass was comparable for both treatments, and the metabolic quotient (qCO₂-C = mg CO₂-C·mg^–1 Biomass C·h^–1) was increased by the plants. During the subsequent low-activity decomposition stages, after the labile compounds had been progressively mineralized, the biomass was multiplied by a factor of 2–4 in the presence of plants compared to the bare soils. Nevertheless, qCO₂-C tended to reach similar low values with both treatments. The ¹⁴C-labelled biomass was reduced by the presence of roots and qCO₂-¹⁴C was increased. The significance of these results obtained from a model experiment is discussed in terms of (1) the variation in the substrate originating from the roots and controlled by the plant physiology, (2) nutrient availability for plants and microorganisms, (3) soil biotic capacities and (4) increased microbial turnover rates induced by the roots.     

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%.     

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.     

Mineralization of nutrients from soil microbial biomass

Soil samples of parabrown earth and chernozem, each having a different amount of microbial biomass, were used to investigate the contribution of microbial cells to the pool of mobile plant nutrients in soils. The quantities of nutrients mobilized in soils which had been dried or fumigated were closely related to the quantities available in freshly-killed biomass. For the percent of N mineralized from dead microbial biomass in arable soil during 28 days, a “k_N-factor” (28 days) of 0.37 was suggested. In oven-dried (70°C) and air-dried (room temperature) soils, approximately 77 and 55% of the N mineralized after remoistening and incubating at 22°C for 4 weeks came from the freshly-killed biomass. The remaining 23 and 45% were derived from non-biomass organic N fractions of the soils. In fumigation experiments (CHCl₃, 24 h), the amount of P released was closely related to the P content of the soil microbial biomass. The fluctuating amounts of K available after fumigation did not correspond to the amount of biomass killed. A scheme for the transformation of dead microbial biomass-C and -N in arable soil is suggested.     

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.     

Mineralization of plant-incorporated residues of 14C-isoproturon in arable soils originating from different farming systems

¹⁴C-isoproturon residues were incorporated in wheat plants by growing seedlings for 18 days in quartz sand with nutrient solution which was treated with ring-labeled ¹⁴C-isoproturon, resulting in ¹⁴C-concentration equivalent to 15.4 nmol isoproturon per g dry shoot mass. The residues were characterized by extraction and HPLC-analysis, and were shown to consist of unchanged isoproturon, soluble metabolites (monodemethyl-isoproturon, didemethyl-isoproturon, 1-OH-isoproturon, 2-OH-isoproturon, 2-OH-monodemethyl-isoproturon, 2-OH-didemethyl-isoproturon, isopropenyl-isoproturon and unidentified metabolites), as well as nonextractable residues. Dried plant samples containing these residues were mixed with soil samples originating from different farming systems, and mineralization to ¹⁴CO₂ was determined in a closed aerated laboratory system. In addition, the microbial biomass and bioactivity of soils were estimated by determination of substrate-induced heat output, basal heat output, metabolic heat quotient, total adenylate content and adenylate energy charge. Significant positive correlations between ¹⁴CO₂ production or adenylate content and microbial biomass were observed in three soils; ¹⁴CO₂ production and total microbial biomass were highest in soil samples from organic farming. Soil samples from a former hops plantation contaminated with copper from previous fungicide applications did not fit this correlation, but exhibited a higher mineralization capacity per unit of microbial biomass. Our results indicate that general soil microbial parameters in many cases are insufficient to describe the influence of biotic factors on the fate of pesticides in soil.     

Significance of microbial biomass for carbon and nitrogen mineralization in soil

C and N mineralization was quantified in an incubation experiment with two samples containing different amounts of microbial biomass. The samples from two layers (0–20, 20–30 cm) of an arable luvisol from loess were fertilized with nitrate, mixed with ¹⁴C-labelled straw and incubated for 52 days at different O₂ levels. Decreasing O₂ concentrations (21, 2, 1 and 0% O₂) in soil conducted a decrease in C and N mineralization. More C and N were mineralized in samples with a higher initial microbial biomass. The differences in microbial biomass were still present at the end of the experiment, but more proliferation was detected in samples with the lower initial microbial biomass, leading to equal ratios between microbial biomass-C and soil organic C in both soils.     

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.     

Maintenance carbon requirements of actively-metabolizing microbial populations under in situ conditions

For estimating turnover rates of soil microbial populations or for energy balance studies maintenance coefficients of microorganisms grown in vitro have generally been used. To study maintenance carbon requirements under in situ conditions the biomasses of two agricultural soils (Cambisol, Chernozem) and a beech forest soil (Rendzina) were activated metabolically and the CO₂ production rate at 22°C recorded every hour. In soil samples treated with increasing amounts of glucose the concentration was determined that still yielded a maximal initial respiratory response but where the CO₂ rate remained constant over 8–13 h. Decrease of CO₂ production after that time was directly related to glucose exhaustion. The amount of glucose-C which kept the CO₂-C rate at the previous level was regarded as the maintenance ration. Values for the maintenance coefficient m (mg glucose-C mg^?1 biomass-C) were 0.012 h^?1 for the two agricultural soils and 0.03 h^?1 for the forest soil. The metabolic quotient qCO₂ and maintenance values were identical for both agricultural soils and the qCO₂ values were the same in the three soils used. No net growth was observed during the experimental period using bacterial plate counts and nalidixic acid treatment as test measurements.The determined m values reflect all individual maintenance requirements of species belonging to the total active biomass pool in these particular soils and correspond to known values from in vitro studies. The relationship between annual input of carbon and the maintenance requirements of actively-metabolizing biomasses are discussed.     

Heat output of the soil biomass

Amending soils with glucose (5 mg g^?1) resulted in an immediate increase in microbial activity and within 30 min the rates of heat output and respiration at 22° C were increased by up to 17.8 and 23.4 times, respectively. The increased rate of heat output remained stable for up to 6 h and there was good correlation with the amount of CO₂ respired. The soil biomass was calculated by the method of Anderson and Domsch (1978). The rate of heat output of the biomass varied in different soils and ranged from 11.5 to 83.7 Jh^?1 g^?1 biomass C. In glucose-amended soils, however, the rate of heat output was much more consistent; the soils were in two groups having between 169–265 Jh^?1g^?1 biomass C or 454–482 J h^?1 g^?1 biomass C, both the latter two soils were from pasture. The increased rate of heat output from the amended soils was lower than expected from the respiration rate and the heat of oxidation of glucose, suggesting that a proportion of the CO₂ respired was from catabolism of substrates other than glucose. Use of ¹⁴C-glucose confirmed that between 57–91% of the CO₂ was derived from the glucose substrate.     

Estimating microbial biomass N in soils with and without living roots: Limitations of a pre-extraction step

Special net-closed soil containers were used in a pot experiment with low and high plant densities to give soil samples with and without roots. Soils from the containers were analysed either by the fumigation-extraction method or by a modified procedure starting with a pre-extraction and sieving step to remove plant roots from the samples. In the extracts NO ₃ ^- -N, NH ₄ ⁺ -N, organic N, and total N were measured. Microbial biomass N was calculated from the differences in total N in fumigated and unfumigated soils. Different plant densities had almost no influence on the values of the N compounds using either method. In soils with roots, significantly more organic N (and total N) was found by the fumigation-extraction method compared to soils without roots while no differences were obtained using pre-extractions and sieving. Though the organic N content in pre-extracts from soils with roots was significantly higher than from soils without roots, the NO ₃ ^- -N and NH ₄ ⁺ -N content was lower. Significant differences in biomass N in soils with and without roots were found only with the fumigation-extraction method. Biomass N levels calculated using the results after pre-extraction and sieving were about 50% lower than levels detected using fumigation-extraction alone. With the use of special net-closed soil containers, not only were soil samples produced with and without roots, but it was also possible to induce a rhizophere in the soils. A comparison of the two methods using these soils clearly demonstrated that the method used has profound influence on the final biomass N results. While higher biomass levels were found by fumigation-extraction in soils with roots, because root N becomes extractable after fumigation, the use of a pre-extraction and a sieving step may underestimate the total biomass N content due to the pre-extraction of microbial N (especially from rhizosphere microorganisms) from the sample. Nevertheless, pre-extraction and sieving followed by fumigation-extraction does seem to be the preferable method for biomass N measurement in comparative studies, because in most cases only minor errors will occur.     

Turnover of root-derived material and related microbial biomass formation in soils of different texture

Wheat plants were grown on two soils of different texture, a sandy soil and a silty clay loam, in an atmosphere containing ¹⁴CO₂. The ¹⁴C and total C content of the shoots, roots, soil rhizosphere CO₂ and soil microbial biomass were measured 21, 28, 35 and 42 days after germination. There was a pronounced effect of soil texture on the turnover of root-derived C through the microbial biomass. Turnover was relatively fast and at a constant rate in the sandy soil but slowed down in the clay soil, following an initial high assimilation of root products into the microbial biomass.Four percent of the total fixed ¹⁴C was retained in the clay loam after 6 weeks compared with a corresponding value of 1.2% for the sandy soil. The proportion of fixed ¹⁴C recovered as rhizosphere CO₂ at each of the sampling times was relatively constant for the sandy soil (ca 19%) but decreased from 17% at day 28 to 11% at day 42 in the clay soil. The proportion of total fixed ¹⁴C in the soil biomass as measured by a fumigation technique increased to a maximum value of 20% after 6 weeks in the sandy soil but decreased in the clay soil from 86% at day 21 to 26% after 42 days plant growth.     

Mineralization of bacteria and fungi in chloroform-fumigated soils

It has been suggested by others that the size of the flush of mineralization caused by CHC1₃ fumigation can be used to estimate the amount of microbial biomass in soils. Calculation of biomass from the flush requires that the proportion of CHCl₃-killed cell C mineralized be known. To determine this proportion, 15 species of [¹⁴C]labelled fungi and 12 species of [¹⁴C]labelled bacteria were added to four types of soil and these were fumigated for 24 h with CHC1₃, reinoculated with unfumigated soil, and incubated at 22°C for 10 days. The average percentage mineralization of the fungi was 43.7 ± 5.3, while the average for the bacteria was 33.3 ± 9.9. Using a 1:3 ratio for distribution of total biomass between the bacterial and fungal populations, respectively, it was calculated that the average mineralization of both types of cells was 41.1%. In experiments conducted to determine if CHC1₃ vapour alters stabilized microbial metabolites or dead microbial cells in a manner which makes them more susceptible to degradation, it was found that both fumigated and unfumigated dead fungal materials mineralized to the same extent in soil during 10 days of incubation.     

Sulfur transformations in relation to carbon and nitrogen in incubated soils

Changes in biomass-S in relationship to biomass-C and N were evaluated, and the transformation of ³⁵S-labelled SO₄^2? among organic matter fractions were followed during incubation of a Black Chernozemic (Udic Haploboroll) and Orthic Gray Luvisol (Typic Cryoboralf) soils. There was a net immobilization of S with and without the addition of cellulose or sulfate after 64 days. In contrast, a net mineralization of N occurred. Cellulose decomposition rates responded to supplies of S available for new microbial cell synthesis.Fluctuations in the amounts of biomass-S during incubation of both soils followed biomass-C and biomass-N changes and C/S and C/N ratios of the biomass ranged between 47–121 and 4.9–7.7, respectively. Microbially-incorporated S was found concentrated within the biomass or partially transformed into soil organic matter.Fractionation of soils after incubation, by a 0.1 m NaOH-0.1 m Na₄P₂o₇ extraction-separation technique showed significant increases in the C and N contents of the conventional humic acid (HA-A) and fulvic acid (FA-A), and humin (<2 μm) fractions. Biomass C accounted for 20–64% of the observed increases in these fractions suggesting that the differences were due partly to transformed microbial products and partly to microbial cell organic constituents released on lysis of cells during incubation. In contrast to C and N, the contents of total S and HI-reducible S increased in the FA-A fraction only and accounted for 45–76% of the immobilized labelled S.     

Repeated CO2 pulse-labelling reveals an additional net gain of soil carbon during growth of spring wheat under free air carbon dioxide enrichment (FACE)

Rising levels of atmospheric CO₂ have often been found to increase above and belowground biomass production of C3 plants. The additional translocation of organic matter into soils by increased root mass and exudates are supposed to possibly increase C pools in terrestrial ecosystems. Corresponding investigations were mostly conducted under more or less artificial indoor conditions with disturbed soils. To overcome these limitations, we conducted a ¹⁴CO₂ pulse-labelling experiment within the German FACE project to elucidate the role of an arable crop system in carbon sequestration under elevated CO₂. We cultivated spring wheat cv. “Minaret” with usual fertilisation and ample water supply in stainless steel cylinders forced into the soil of a control and a FACE plot. Between stem elongation and beginning of ripening the plants were repeatedly pulse-labelled with ¹⁴CO₂ in the field. Soil born total CO₂ and ¹⁴CO₂ was monitored daily till harvest. Thereafter, the distribution of ¹⁴C was analysed in all plant parts, soil, soil mineral fractions and soil microbial biomass. Due to the small number of grown wheat plants (40) in each ring and the inherent low statistical power, no significant above and belowground growth effect of elevated CO₂ was detected at harvest. But in comparison to ambient conditions, 28% more ¹⁴CO₂ and 12% more total CO₂ was evolved from soil under elevated CO₂ (550 μmol CO₂ mol⁻¹). In the root-free soil 27% more residual ¹⁴C was found in the FACE soil than in the soil from the ambient ring. In soil samples from both treatments about 80% of residual ¹⁴C was found in the clay fraction and 7% in the silt fraction. Very low ¹⁴C contents in the CFE extracts of microbial biomass in the soil from both CO₂ treatments did not allow assessing their influence on this parameter. Since the calculated specific radioactivity of soil born ¹⁴CO₂ gave no indication of an accelerated priming effect in the FACE soil, we conclude that wheat plants grown under elevated CO₂ can contribute to an additional net carbon gain in soils.