Proprietes des diphenol oxydases extraites des sols

Properties of diphenol oxidases extracted from salts. Salmine and SP-Sephadex C-25 were used to separate the enzyme system associated with humic materials in the neutral extracts of fresh soils (NAFS Extract). Electrophoresis on polyacrylamide gel shows that this preparation is heterogeneous. The elementary analysis of the soil enzyme is C 43·13%; N 5·09%; H 7·21%; O 44·58%. Chromatographic analyses indicate that the soil enzyme contains 53 per cent amino acids, 36 per cent sugars and amino sugars and 10 per cent ammonium and inorganic materials. The soil enzyme has a maximum absorption at 270–280 nm. The soil enzyme degrades the following substrates at the relative rate mentioned in parentheses : d-catechin (298);p-cresol (251); catechol (156); dl-DOPA (100);p-phenylene diamine (59);p-quinol (20) in terms of rate of oxygen absorption. This enzymatic preparation has the properties of an o- and p-diphenol oxidase. The rate of decarboxylation was measured using a radiorespirometer. The following relative values are dl-DOPA-l-¹⁴C (100); dl-tyrosine-l-¹⁴C (35) ; dl-tyrptophan-1-¹⁴C (7); dl-phenylalanine-l-¹⁴C (2). The dl-DOPA-2-¹⁴C was partially degraded to ¹⁴CO₂. The O₂ absorbed and CO₂ (carboxyl) evolved in case of dl-DOPA was in the ratio of 1·8 at 37°C. The activation energy on dl-DOPA was 3·1 and 7·9 kcal/mole/°C for oxygen absorption and decarboxylation respectively. The enzymatic activity on dl-DOPA-l-¹⁴C was optimum in air and inhibited in a N₂ atmosphere. Decarboxylation on dl-DOPA-l-¹⁴C followed the Michaelis-Menten law, from which we found that K_m = 8·3 × 10^?4M for decarboxylation. The oxidative decarboxylation was inhibited by H₂O₂ (74%); KCN (75%); ascorbate (92%); BAL (97%);DIECA(90%).Melanogenesis of dl-DOPA followed first order kinetics. The maximum absorption at 305 nm during melanogenesis shows the formation of dopachrome.     

Chromatographie et purification des diphenol oxydases du sol

Neutral sterile and lyophilized extracts of fresh soil (NAFS Extract) degraded the following substrates: p-cresol: dl-3(3.4-dihydroxyphenylalanine)(dl DOPA): d(+) catechin and p-phenylcnediamine respectively (19.7); (5.3); (5.7) and (4.4) nmoles O₂ mg C^?1 min^?1 as specific activity (sp. act.). NAFS Extract was fractionated by G100 Sephadex column chromatography into two major peaks (K_D ~ 0 and ~ 1) without an increase in sp. act. DEAE DE 52 cellulose chromatography separated NAFS Extract into three fractions. The first fraction was free from humic acid, relatively homogeneous on polyacrylamide gel electrophoresis and had sp. act.: dl DOPA (17.2): d(+) catechin (8.5): p-phenylenediamine (22.9). The o- and p-diphenol oxidases which accompanied this fraction were well separated as complexes on G100 Sephadex column and were liberated by dialysis against distilled water. Following isolation, we obtained an o-diphenol oxidase on dl DOPA (23.4) and a laccase activity on p-phenylenediamine (33.8) in the free state; these activities being associated with nucleoprotein. Fractions (II) and (III) appeared to be relatively homogeneous in the form of “humic acid—enzyme complexes”. Specific activity were high in fraction (III): dl DOPA (10.8): d(+) catechin (0.7); p-phenylenediamine (5.4). The diphenol oxidase activity extracted from soil (NAFS Extract) was treated by salmine and SP Sephadex C25 to remove humic matter. The EFS Extract obtained had the following sp. act.: dl DOPA (14.0); p-phenylenediamine (6.3). This EFS Extract was separated into three fractions by means of G100 Sephadex column chromatography. The Kn were (I) ~0; (II) ~ 0.52; (III) ~ 1.3 respectively. The first fraction showed an increase of sp. act. only with p-phenylenediamine (9.1) and in the following two fractions the sp. act. were not augmented. The first fraction was further fractionated by means of DEAE cellulose chromatography into four fractions: the-first and the second had no activity on dl DOPA and p-phenylenediamine. The third was an o-diphenol oxidase on dl DOPA (11.0) with traces of laccase: p-phenylenediamine (0.7). The fourth was pure laccase: p-phenylenediamine (18.1). These results suggested that electrostatic, covalent and van der Waals forces contributed to the formation of humic acid enzymes complexes, associated in the tetramer to monomer forms of diphenol oxidases.     

14C-labelled maleic hydrazide degradation in soil: Influence of temperature,moisture, clay and organic material

¹⁴C-labelled maleic hydrazide (MH) was added to each of three soils at a concentration of 4 mg kg^?1, and its degradation measured by the release of ¹⁴CO₂ after 2 days. Between 1 and 30°C, at a constant moisture content (full field capacity), the mean degradation rate increased by a factor of 3 for each temperature increment of 10°C (Q₁₀ = 3). The mean activation energy was 78 kJ mol^?1. Above 35°C, the degradation rate decreased.At soil moisture contents between wilting point and 80–90% of field capacity, the degradation rate doubled with an increase in moisture content of 50% of field capacity (constant temperature, 25°C). Above field capacity, the degradation rate was either unchanged or decreased. Below wilting point the degradation was very slow, even after 2 months.The rate of decomposition of MH at all temperatures and moisture contents was lowest in the soil with the highest content of organic matter and the lowest clay content. This soil had the highest Freundlich K value, and presumably adsorbed MH the most strongly, although the lower clay content may also play a role in the lower decomposing capacity of this soil.     

Kinetics of glucose uptake by soil microorganisms

The kinetics of glucose uptake by soil microorganisms was investigated. Soil amended with an inorganic nutrient solution containing C glucose at concentrations of 2.5, 5.0, 10.0 or 20.0 mmol 1⁻¹ was maintained at 4, 12 or 25°C for varying times. The soil was analyzed for glucose, soluble ^14C, total organic ¹⁴C and evolved ¹⁴CO₂ to develop a carbon balance for the system and to define Michaelis-Menten kinetic parameters (K_m and V_max) for glucose uptake at each temperature.Glucose uptake rates, as measured by the depletion of glucose or soluble ¹⁴C from solution, were similar in soils maintained at 12 or 25°C. Based on the depletion of soluble ¹⁴C, values for K_m were 2.25 and 2.43 mmol I⁻¹ at 12 and 25°C, respectively, while V_max values were 0.25 and 1.61 h¹⁴', respectively. Glucose depletion at 4°C was faster than at 12C, while soluble ¹⁴C was removed at a significantly slower rate, suggesting soluble-C intermediates were produced in the 4°C system. Based on Chromatographie techniques and GC-MS, a soluble ¹⁴C-compound accumulating in the 4°C system was identified as maltose. The conversion of glucose to maltose resulted in K_m and V_max values of 17.29 mmol I⁻¹ and 0.12h⁻¹, respectively, for soluble ¹⁴C depletion and 4.96mmol1⁻¹ and 0.43 h⁻, respectively, for glucose depletion at 4δC. These results demonstrate the need to differentiate uptake rates for the parent compound as well as for transitory intermediates excreted into the growth medium. Evolution of CO₂ was shown to be a poor indicator of the rapid disappearance of glucose in soils.     

The rapid oxidation of atmospheric CO to CO2 by soils

Gas exchange rates over soils were measured in a closed, flowing-gas system. ¹⁴CO was rapidly oxidized to ¹⁴CO₂ with only a minor loss in atmospheric radioactivity. Incorporation of ¹⁴C into the soil was slight and was via ¹⁴CO₂ rather than ¹⁴CO. CO oxidation was a microbial process and no oxidation occurred when soils had been autoclaved. The rate of CO depletion was concentration dependent and followed Michaelis-Menten kinetics. The rate constants K_m and V_max ranged from 18 to 51 μ 1^?1 CO and from 0.58 to 4.35 mg C kg^?1 dry soil h^?1 respectively. The maximum rate of reaction for Hubbard Brook soil was about an order of magnitude greater than any soil previously reported. The oxidation reaction was accompanied initially by a reduction in net soil respiration. This was then followed by a period of high respiration which continued until CO levels were reduced to about 5μll^?1. Thereafter respiration fell below the pretreatment rate and only returned to that rate 45 min after CO had been depleted from the atmosphere. The data suggest that at high CO concentrations (40–100 μll^?1CO) autotrophic carboxydobacteria comprise the main component of the CO-oxidizing population and, as the concentration declines towards ambient levels they are replaced by heterotrophic microorganisms possessing a cometabolic process.     

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.     

Factors influencing the loss of organic carbon from wheat roots

Wheat plants were grown in an atmosphere containing ¹⁴CO₂ at temperatures of 10°C or 18°C for periods from 3–8 weeks. The plant roots were maintained under sterile or non-sterile conditions in soil contained in sealed pots which were flushed to displace respired ¹⁴CO₂. The ¹⁴C content of the shoots, roots and soil was measured at harvest. The loss of ¹⁴C from the roots, expressed either in terms of total ¹⁴C recovered from the pots or ¹⁴C translocated to the roots, ranged from 14.3–22.6%, mean 17.3% or 29.2–44.4%, mean 39.2%, respectively. The presence of soil microorganisms significantly increased ¹⁴CO₂ release from the rhizosphere but had no effect on the ¹⁴C content of the soil. Fractionation of 6 m HC1 hydrolysates from sterile and non-sterile soils showed the presence in all soils of material behaving as neutral sugars and amino acids, in quantities representing 5.9–9.2% and 13.4–17.2% of the soil ¹⁴C content for the sugar and amino acid fractions respectively. It is proposed that a major loss of root carbon resulted from autolysis of the root cortex. Root lysis was increased by soil microorganisms, apparently without penetration of the plant cell walls.     

Rice-wheat productivity and nutrient status in a lantana- (Lantana spp.) amended soil

Imbalanced and inadequate use of chemical fertilizers is responsible for low rice- (Oryza sativa L.) wheat (Triticum aestivum L.) productivity in many resource-poor farmers' fields. Wheat yields in post-rice soils are also constrained due to soil conditions created by puddling in rice, especially in fine to medium textured soils. Organic amendments are known to improve soil productivity under rice-wheat cropping by way of improving physical conditions and nutrient status of the soil, but their availability is restricted. There is a need to identify locally available and cost-effective organic materials, which have minimal alternate uses as fodder and fuel. We evaluated lantana (Lantana spp. L.) residues, a fast-growing weed in nearby wastelands, as a potential soil organic amendment. Yield trends, and soil and crop nutrient status in a 12-year rice-wheat experiment at Palampur, India, involving four levels (0, 10, 20, and 30 Mg ha^-1 year^-1 fresh mass) of lantana addition were investigated. Chopped lantana was incorporated into soil 10–15 days before puddling. Lantana additions at 10, 20 and 30 Mg ha^-1 increased rice yields on average by 18%, 23% and 30%, wheat yields by 11%, 14% and 20%, and total system productivity (rice + wheat) by 15%, 20% and 26% over controls, respectively, and at the same time saved NPK fertilizer. Linear regression analyses over 12 years did not show any change in yield trends of rice and wheat at P =0.05. Continuous cultivation of rice-wheat significantly increased total C, labile C, and other C indices of soils. Total N, Olsen's P, and NH₄OAc-extractable K in the lantana-amended plots were higher than in the controls. Nutrient concentrations in crop biomass, however, remained generally unaffected by lantana treatments. Results suggest that lantana residues, which improved the nutrient status of soil and system yield, have the potential for resource conservation and sustaining rice-wheat productivity.     

The response of cylindrocladium conidia to soil fungistasis

Direct observation of washed conidia of Cylindrocladium scoparium on non-sterile soils, air dried and rewetted immediately before deposition of conidia, indicated that peak germination (33–58%) occurred after 24 h incubation at 26°C. Peak germination on continually moist soils was lower (18–26%) than on rewetted soils. Lysis of germ tubes and germinating conidia on continually moist soils at 26°C was evident with 48 h. Conidia did not germinate on continually moist soils at 6°C and lysis did not become apparent until 168 h. Conidia germinated at a high level (93–99%) in axenic culture in the absence of exogenous C and N sources. The inhibition of conidial germination on soils may be attributed, in part, to the presence of soil volatiles. Germination of conidia placed on washed agar disks and exposed to volatiles from four soils ranged from 51 to 86% of the no-soil controls. Addition of carbon (13 ng C per conidium as glucose) and nitrogen (65 pg N ng^?1 C as NH₄C1) nullified the inhibitory effect of the soil volatiles. Germinability assayed on a selective medium at 26°C of conidia in artificially infested soils (approximately 10⁴ conidia g^?1 soil) decreased progressively during incubation at 26°C from 1 week to 4 months. No germinable conidia were recovered from artificially infested soils after 2 months incubation at 6°C. Conidia of C. floridanum and C. crotalariae responded similarly to C. scoparium in many assays.     

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.     

The fate of catechol in soil as affected by earthworms and clay

The effect of endogeic earthworms (Octolasion tyrtaeum) and the availability of clay (Montmorillonite) on the mobilization and stabilization of uniformly ¹⁴C-labelled catechol mixed into arable and forest soil was investigated in a short- and a long-term microcosm experiment. By using arable and forest soil the effect of earthworms and clay in soils differing in the saturation of the mineral matrix with organic matter was investigated. In the short-term experiment microcosms were destructively sampled when the soil had been transformed into casts. In the long-term experiment earthworm casts produced during 7 days and non-processed soil were incubated for three further months. Production of CO₂ and ¹⁴CO₂ were measured at regular intervals. Accumulation of ¹⁴C in humic fractions (DOM, fulvic acids, humic acids and humin) of the casts and the non-processed soil and incorporation of ¹⁴C into earthworm tissue were determined.Incorporation of ¹⁴C into earthworm tissue was low, with 0.1 and 0.44% recovered in the short- and long-term experiment, respectively, suggesting that endogeic earthworms preferentially assimilate non-phenolic soil carbon. Cumulative production of CO₂-C was significantly increased in casts produced from the arable soil, but lower in casts produced from the forest soil; generally, the production of CO₂-C was higher in forest than in arable soil. Both soils differed in the pattern of ¹⁴CO₂-C production; initially it was higher in the forest soil than in the arable soil, whereas later the opposite was true. Octolasion tyrtaeum did not affect ¹⁴CO₂-C production in the forest soil, but increased it in the arable soil early in the experiment; clay counteracted this effect. Clay and O. tyrtaeum did not affect integration of ¹⁴C into humic fractions of the forest soil. In contrast, in the arable soil O. tyrtaeum increased the amount of ¹⁴C in the labile fractions, whereas clay increased it in the humin fraction.The results indicate that endogeic earthworms increase microbial activity and thus mineralization of phenolic compounds, whereas clay decreases it presumably by binding phenolic compounds to clay particles when passing through the earthworm gut. Endogeic earthworms and clay are only of minor importance for the fate of catechol in soils with high organic matter, clay and microbial biomass concentrations, but in contrast affect the fate of phenolic compounds in low clay soils.     

Fate of bound methyl parathion residues in soils as affected by agronomic practices

The fate of [ring-¹⁴C]methyl parathion in a silt loam soil was monitored during a 49-day incubation. After this period, 54% of the initial ¹⁴C remained in the soil; of this, 13% was soxhletextractable with methanol and 87% was bound residue. Soils were then treated with inorganic and organic amendments and incubated for an additional 70 days. Release of methyl parathion bound residues could not be demonstrated, but both bound and extractable ¹⁴C were mineralized to ¹⁴CO₂, CO₂ was evolved slowly and continuously by the controls and where soil was amended with H₂SO₄, (NH₄)₂SO₄, NH₄OH, chitin, oat seedlings or oat straw. Glucose and asparagine caused higher rates of ¹⁴CO₂ production. HgCl₂ gave very high initial rates of ¹⁴CO₂ loss; the rate declined to that of the control only after 9–10 weeks. The lime treatment exceeded the controls after 1 week, declining only slightly with time. The effects of sewage sludge and dairy manure were similar to the controls except that: sludge caused a very high initial release of ¹⁴CO₂, and both treatments gave an unaccountable loss of ¹⁴C, perhaps as ¹⁴CH₄ resulting from the formation of anaerobic conditions. By 70 days, amounts of extractable ¹⁴C and bound ¹⁴C had both declined twice as rapidly in certain soils as in unamended controls.Studies carried out with soxhlet-extracted soils, containing only bound residues, indicated that the soil microflora able to mineralize bound residues without any appreciable buildup of ¹⁴C activity in the extractable phase.     

Respiratory substrate availability plays a crucial role in the response of soil respiration to environmental factors

We examined the response of the temperature coefficient (Q₁₀) for soil respiration to changes in soil temperature and soil moisture through a laboratory incubation experiment. Two types of soils differing in vegetation and moisture status were collected and incubated under two temperatures (10 and 30 °C) and two soil moisture regimes (35 and 75% of water holding capacity, WHC) for 5 weeks. Before and after the incubation experiment, the temperature coefficient of soil respiration was measured using soda-lime method by changing temperature in a water bath. For both soils, the mean Q₁₀ values of the respiration rate were 2.0 in the 30 °C and 2.3 in the 10 °C soil treatments. Higher temperature with lower soil moisture treatment significantly decreased the Q₁₀ value, whereas lower temperature with higher soil moisture treatment significantly enhanced the Q₁₀ value (ANOVA, p < 0.05). These results indicate that soils became less sensitive to temperature when incubated under higher temperature with higher moisture conditions, and more sensitive in lower temperature with higher moisture conditions.There was a significant correlation (r² = 0.67, p < 0.05) between water-soluble carbon (WSC) and soil respiration rate. However, the correlation between soil respiration rate and microbial biomass carbon (MBC) was weak (r² = 0.27, p > 0.05). Although incubation temperature and moisture accounted for 40 and 29% (as r² × 100%), respectively, of variations in Q₁₀, soil water-soluble carbon content alone could have explained 79% of the variation, indicating that the availability of respiratory substrate, rather than the pool of soil microorganisms, played a crucial role in the response of the temperature coefficient to environmental factors. These results suggest that biotic factors should also be taken into consideration when using the Q₁₀ function to predict the response of soil respiration to global warming.     

Soil core incubation system for the field measurement of denitrification using acetylene-inhibition

Two systems for determining rates of denitrification, both based on the acetylene-inhibition technique, have been compared. One system involved in situ treatment of soil with C₂H₂ using an enclosure (500 × 150mm) placed over the soil surface followed by measurement of nitrous oxide (N₂O) emission. The other involved incubation, in the field, of soil cores with 5% (v/v) C₂H₂ in modified fruit preserving jars. Agreement between the two systems of measurement was close for well-drained soils over a wide range of rates of denitrification (0.005–1.27 kg N ha⁻¹ day⁻¹). Results obtained with poorly-drained soils having low air-filled porosities indicated that the incubation system with soil cores overcame the problems associated with applying the enclosure technique to soils of this type. Denitrification in the incubation system could be terminated by the addition of chloroform after an appropriate period (usually 24 h). The N₂O concentration in the air space of the jars then remained essentially constant for 14 days. This provided a second advantage in allowing the system to be used at sites remote from analytical facilities.     

Driving factors of temporal variation in agricultural soil respiration

Investigations of diurnal and seasonal variations in soil respiration support modeling of regional CO₂ budgets and therefore in estimating their potential contribution to greenhouse gases. This study quantifies temporal changes in soil respiration and their driving factors in grassland and arable soils located in Northern Germany. Field measurements at an arable site showed diurnal mean soil respiration rates between 67 and 99 mg CO₂ m^–2 h^–1 with a hysteresis effect following changes in mean soil temperatures. Field soil respiration peaked in April at 5767 mg CO₂ m^–2 day^–1, while values below 300 mg CO₂ m^–2 day^–1 were measured in wintertime. Laboratory incubations were carried out in dark open flow chambers at temperatures from 5°C to 40°C, with 5°C intervals, and soil moisture was controlled at 30%, 50%, and 70% of full water holding capacity. Respiration rates were higher in grassland soils than in arable soils when the incubating temperature exceeded 15°C. The respiration rate difference between them rose with increasing temperature. Monthly median values of incubated soil respiration rates ranged from 0 to 26.12 and 0 to 7.84 µg CO₂ g^–1 dry weight h^–1, respectively, in grassland and arable land. A shortage of available substrate leads to a temporal decline in soil respiration rates, as indicated by a decrease in dissolved organic carbon. Temporal Q₁₀ values decreased from about 4.0 to below 1.5 as temperatures increased in the field. Moreover, the results of our laboratory experiments confirmed that soil temperature is the main controlling factor for the Q₁₀ values. Within the temperature interval between 20°C and 30°C, Q₁₀ values were around 2 while the Q₁₀ values of arable soils were slightly lower compared to that of grassland soils. Thus, laboratory studies may underestimate temperature sensitivity of soil respiration, awareness for transforming laboratory data to field conditions must therefore be taken into account.     

Mycostasis in relation to the microbial nutrient sinks of five soils

[^l4C]exudation from fungal propagules on 5 soils over 4–24 h was studied in relation to mycostasis. [^l4C]exudation from sclerotia of Macrophomina phaseolina, chlamydospores of Thielaviopsis basicola, and conidia of Cochliobolus victoriae after 24 h on two sandy loam soils and a loam was generally greater than exudation on the two clay loam soils. Results were similar for conidia of Stemphylium sarcinaeforme but differences were not statistically significant. When natural soils were pulsed with [¹⁴C]glucose, ¹⁴CO₂ evolved by the soil microflora over 2–12 h showed a similar trend. [¹⁴C]exudation from M. phaseolina sclerotia and C. victoriae conidia incubated on soils was greater than that from propagules incubated aseptically on a bed of sand through which water percolated at a flow rate sufficient to inhibit germination. Propagules of C. victoriae, M. phaseolinia and T. basicola germinated greater on one or more of the coarse-textured soils than on fine-textured soils. Using γ-irradiated soils, more [^l4C]exudate was adsorbed by the clay loams than by the loam and sandy loam soils, suggesting that the adsorptive capacity of soils may be an important factor in controlling fungal utilization of soluble nutrients. Fungal germination in soil appears to be jointly influenced by two opposing tendencies: the ease with which germination occurs in response to exogenous nutrients and the amount of endogenous substrate lost or retained.     

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.     

Tracking litter-derived dissolved organic matter along a soil chronosequence using 14C imaging: Biodegradation,physico-chemical retention or preferential flow?

The cycling of dissolved organic matter (DOM) in soils is controversial. While DOM is believed to be a C source for soil microorganisms, DOM sorption to the mineral phase is regarded as a key stabilization mechanism of soil organic matter (SOM). In this study, we added ¹⁴C-labelled DOM derived from Leucanthemopsis alpina to undisturbed soil columns of a chronosequence ranging from initial unweathered soils of a glacier forefield to alpine soils with thick organic layers. We traced the ¹⁴C label in mineralized and leached DOM and quantified the spatial distribution of DO¹⁴C retained in soils using a new autoradiographic technique. Leaching of DO¹⁴C through the 10 cm-long soil columns amounted up to 28% of the added DO¹⁴C in the initial soils, but to less than 5% in the developed soils. Biodegradation hardly contributed to the removal of litter-DO¹⁴C as only 2–9% were mineralized, with the highest rates in mature soils. In line with the mass balance of ¹⁴C fluxes, measured ¹⁴C activities in soils indicated that the major part of litter DO¹⁴C was retained in soils (>80% on average). Autoradiographic images showed an effective retention of almost all DO¹⁴C in the upper 3 cm of the soil columns. In the deeper soil, the ¹⁴C label was concentrated along soil pores and textural discontinuities with similarly high ¹⁴C activities than in the uppermost soil. These findings indicate DOM transport via preferential flow, although this was quantitatively less important than DOM retention in soils. The leaching of DO¹⁴C correlated negatively with oxalate-extractable Al, Fe, and Mn. In conjunction with the rapidity of DO¹⁴C immobilization, this strongly suggests that sorptive retention DOM was the dominating pathway of litter-derived DOM in topsoils, thereby contributing to SOM stabilization.     

Microbial activity in soils frozen to below −39 °C

Recent research on life in extreme environments has shown that some microorganisms metabolize at extremely low temperatures in Arctic and Antarctic ice and permafrost. Here, we present kinetic data on CO₂ and ¹⁴CO₂ release from intact and ¹⁴C-glucose amended tundra soils (Barrow, Alaska) incubated for up to a year at 0 to −39°C. The rate of CO₂ production declined exponentially with temperature but it remained positive and measurable, e.g. 2-7 ng CO₂-C cm⁻³ soil d⁻¹, at −39 °C. The variation of CO₂ release rate (v) was adequately explained by the double exponential dependence on temperature (T) and unfrozen water content (W) (r²>0.98): v=A exp(λT+kW) and where A, λ and k are constants. The rate of ¹⁴CO₂ release from added glucose declined more steeply with cooling as compared with the release of total CO₂, indicating that (a) there could be some abiotic component in the measured flux of CO₂ or (b) endogenous respiration is more cold-resistant than substrate-induced respiration. The respiration activity was completely eliminated by soil sterilization (1 h, 121 °C), stimulated by the addition of oxidizable substrate (glucose, yeast extract), and reduced by the addition of acetate, which inhibits microbial processes in acidic soils (pH 3-5). The tundra soil from Barrow displayed higher below-zero activity than boreal soils from West Siberia and Sweden. The permafrost soils (20-30 cm) were more active than the samples from seasonally frozen topsoil (0-10 cm, Barrow). Finding measurable respiration to −39 °C is significant for determining, understanding, and predicting current and future CO₂ emission to the atmosphere and for understanding the low temperature limits of microbial activity on the Earth and on other planets.     

Mineralisation dans le sol de materiaux microbiens marques au carbone 14 et a l'azote 15: Quantification de l'azote de la biomasse microbienne

The mineralization of microbial material of different C-to-N ratios (5.2, 7.9, 10.2, 12.7) was followed in fumigated soil. The microbial materials used were from Aspergillus flavus cultures, grown in liquid media and labelled with [¹⁴C]glucose and (¹⁵NN4)3804. Three contrasting soils were used and the microbial materials incubated with the fumigated soils for 28 days at 28°C.The evolution of the added organic microbial C was fast: 80% of the [¹⁴C]CO₂ produced during the whole 28 days incubation was evolved in the first week. Microbial C mineralization was mainly related to soil type; the C-to-N ratio had small effect on the ratio (mineralized microbial carbon-to-added microbial carbon). Calculation of the K_c- coefficient (the fraction of the added microbial C mineralized in 7 days) shows that K_c values lie between 0.38 and 0.43 in the 3 soils.Organic N in the added microbial material also breaks down quickly: between 60 and 100% of the organic nitrogen mineralized was evolved during the first week of incubation. Mineralization kinetics are related to soil type and to the C-to-N ratio of the microbial material.The proportion of N mineralized in 7 days was lower in an acid soil than in near neutral soils and lower with high C-to-N ratio material than with low C-to-N ratio material. The ratio (mineralized microbial N-to-added microbial N) depends on soil type and is negatively correlated with the C-to-N ratio of the microbial material. The K_N value (the fraction of the added microbial N mineralized in 7 days) lies between 0.22 and 0.47 for the three soils and four materials investigated. The added microbial material induced a priming effect on soil native N: materials with C-to-N ratios of 10.2 and 12.7 produced negative priming effects whereas materials with C-to-N ratios of 5.2 and 7.9 sometimes produced a positive priming action.From the relationship between the C-to-N ratio of the added material and the (mineralized microbial C-to-mineralized microbial N) ratio, the soil native microbial biomass was estimated using the fiush-C-to-flush-N ratio. Biomass nitrogen was then calculated from the formula biomass-N = biomassC/(biomass C-to-N ratio). Calculated in this way, 2–4% of the total nitrogen in the three soils was in microbial biomass.