Showing papers in "Soil Biology & Biochemistry in 1981"

Carbon-nitrogen relationships during the humification of cellulose in soils containing different amounts of clay

Abstract: 14 C-labelled cellulose and 15 N-labelled (NH 4 ) 2 SO 4 were added to four soils with clay contents of 4, 11, 18 and 34%, respectively. Labelled cellulose was added to each soil in amounts corresponding to 1, 2 and 4 mg C g −1 soil, respectively, and labelled NH 4 + at the rate of 1 mg N per 25 mg labelled C. After the first month of incubation at temperatures of 10, 20 and 30°C, respectively, from 38 to 65% of the labelled C added in cellulose had disappeared from the soils as CO 2 , and from 60 to nearly 100% of the labelled N added as NH 4 + were incorporated into organic forms. The ratio of labelled C remaining in the soils to labelled N in organic forms was close to 25 after 10 days of incubation, decreasing to about 15 after 1 month and about 10 after 4 yr. The retention of total labelled C was largest in the soil with the highest content of clay where after 4 yr it was 25% of that added, compared to 12 in the soil with the lowest content of clay. The incorporation of labelled N in organic forms and its retention in these forms was not directly related to the content of clay in the soils, presumably because the two soils with the high content of clay had a relatively high content of available unlabelled soil-N which was used for synthesis of metabolic material. The proportionate retention of labelled C for a given soil was largely independent of the size of the amendments, whereas the proportionate amount of labelled N incorporated into organic forms increased in the clay-rich soils with increasing size of amendments. Presumably this is because the dilution with unlabelled soil-N was less with the large amendments. From 50 to 70% of the total labelled C remaining in the soils after the first month of incubation was acid hydrolyzable, as compared to 80–100% of the total remaining labelled organic N. This relationship held throughout the incubation and was independent of the size of the amendment and of the temperature of incubation. During the second, third and fourth year of incubation the half-life of labelled amino acid-N in the soils was longer than the half-life of labelled amino acid-C, presumably due to immobilization reactions. Some of the labelled organic N when mineralized was re-incorporated into organic compounds containing increasing proportions of native soil-C. whereas labelled C when mineralized as CO 2 disappeared from the soils. In general, native C and native organic N were less acid hydrolyzable and were accounted for less in amino acid form than labelled C and N. The amount of labelled amino acid-C, formed during decomposition of the labelled cellulose, and retained in the soil, was proportional to the clay content. This amount was about three times as large in the soil with the highest content of clay as in the soil with the lowest content. This difference between the soils was established during the first 10 days of incubation when biological activity was most intense, and it held throughout the 4 yr of incubation; proportionally it was independent of the amount of cellulose added and the temperature. In contrast, the labelled amino acid-N content was not directly related to the amount of clay in the soil, presumably because more unlabelled soil-N was available for synthesis of metabolic material in the two clay-rich soils than in those soils with less clay. The wider ratio between labelled amino acid-C and labelled amino acid-N in the two clay-rich soils as compared with those obtained with the soils with less clay indicates this. The effect of clay in increasing the content of organic matter in soil is possibly caused by newly synthesized matter, extracellular metabolites, as well as cellular material, forming biostable complexes and aggregates with clay. The higher the concentration of clay the more readily the interactions take place. The presence of clay may also increase the efficiency of using substrate for synthesis.

Microbial biomass formed from 14C, 15N-labelled plant material decomposing in soils in the field

Abstract: The decomposition of 14C, 14N-labelled medic (Medicago littoralis) material and the net formation and decay of isotope-labelled biomass have been measured in four South Australian soils in the field over 4 yr. The field sites were in similar climatic zones but two sites received about twice as much rainfall as the others. The soils were calcareous and of similar pH, but differed in texture and organic matter content. The decomposition of the organic-14C and organic-15N residues were, for a given site, similar. Initially, the concentrations of labelled residues decreased rapidly, then very slowly. Decomposition rates in a heavy clay soil were significantly less than in the other soils during the first 16 weeks after incorporation of plant material, but thereafter, rates of decomposition in all soils were similar, despite differences in soil texture and climate. More than 50% of the medic-14C had disappeared from all soils after 4 weeks of decomposition and only 15–20% of the medic-14C remained as organic residues after 4 yr. Of the medic-15N 60–65% remained as organic residues after 32 weeks decomposition; the percentage decreased to 45–50% after 4 yr. The amounts of 14C, 14N-labelled biomass, formed from decomposing plant material, were maximal 4–8 weeks after incorporation of plant material into the soils. In samples taken at 8 weeks from the sandy Roseworthy soil, biomass-14C and -15N accounted for 14 and 22% respectively of the total organic-14C and -15N residues present. Thereafter in this soil, the concentrations of biomass-14C and -15N decreased, rapidly at first then more slowly. Nevertheless, throughout most of the decomposition the rates of decrease in the concentrations of biomass-14C and -15N exceeded those of the non-biomass, labelled organic residues. The proportions of 14C, 15N-labelled materials accounted for in the labelled biomass varied between soils. Soils of higher clay content generally retained higher proportions of residual organic-14C and -14N in the biomass, even though the net rates of decomposition of total labelled residues did not differ significantly between soils during most of the decomposition.

Distribution and recovery of nitrogen from legume residues decomposing in soils sown to wheat in the field

Abstract: Unground 15 N-labelled medic material ( Medicago littoralis ) was mixed with topsoils at 3 field sites in South Australia, allowed to decompose for about 8 months before sowing wheat, and then for a further 7 months until crop maturity. The site locations were chosen to permit comparisons of recoveries and distribution of 15 N in soils (organic N and inorganic N to 90 cm depth) and wheat (grain, straw and roots to 20 cm depth) in areas where rainfall (and wheat yields) differed greatly. Soils differed also in their texture and organic matter contents. Recoveries of applied 15 N in wheat plus soil were 93.1% from a sandy loam (Caliph) and 92.3% from a sandy soil (Roseworthy) despite differences in rainfall and extent of leaching of the 15 NO 3 − formed from the decomposing medic residues. From a heavy clay soil (Northfield), which received the highest rainfall, the 15 N recovery was 87.7%. The loss of 15 N at this site was not due to leaching, as judged by 15 NO 3 − distribution in the soil profile at seeding and crop maturity. Wheat plants took up only 10.9–17.3% of the 15 N added as legume material. Percentage uptakes of 15 N were not related to grain yields. The proportions of wheat N derived from decomposing medic residues were 9.2% at Caliph (input medic, N, 38 kg N ha −1 ), 10.5% at Roseworthy (input medic N, 57 kg N ha −1 ), and only 4.6% at Northfield (input medic N, 57 kg N ha −1 ). Most (51–70%) of the 15 N recovered in wheat was accounted for in the grain. Inorganic 15 N in the soil profiles was depleted during the cropping phase, and at wheat harvest represented from 0.6 to 3.1% only of 15 N inputs. The major 15 N pool was soil organic 15 N accounting for 71.9–77.7% of 15 N inputs. We conclude that, in the context of N supply from decomposing medic tissues to wheat crops, the main value of the legume is long-term, i.e. in maintaining soil organic N concentrations to ensure adequate delivery of N to future cereal crops. The N of the wheat was not uniformly labelled, root N being generally of the highest atom% enrichmensts, and straw N of the lowest. Nevertheless, at the Roseworthy site, the enrichments of wheat N were similar to those of NO 3 − N in the profile at seeding, indicating that the proportions of 14 N and 15 N in the inorganic N pool did not change appreciably during the cropping period. By assuming equilibrium at this site, we calculate that during 15 months decomposition the soil plus legume delivered about 189 kg N ha −1 , of which 93.2 kg ha −1 (49.3%) was taken up by the wheat, 37.2 kg ha −1 (19.7%) was immobilized or remained as fine root residues, and 17.3 kg ha −1 (9.2%) remained as inorganic N in the soil profile; 41.7 kg ha −1 (22.1%) was unaccounted for in the soil-plant system, and was probably lost via inorganic N. Thus about 6.5 kg inorganic N ha −1 was supplied by the soil plus medic residues per 100 kg dry matter ha −1 removed as wheat grain.

Microcalorimetry and other methods to assess biomass and activity in soil

Abstract: The heat output from 12 soils from the north east of Scotland was measured using microcalorimetry. The results were compared with the biomass estimated by the amount of carbon mineralized after chloroform fumigation and by the rate of respiration in substrate-amended soil and also with the basal rate of respiration, the ATP content and the activities of amylase and dehydrogenase. Microcalorimetric values correlated well with the rate of respiration and to a lesser extent with the biomass, ATP content and amylase activity. It is suggested that microcalorimetry is a useful additional technique for assessing the overall biological activity of a soil.

Extractant ph and the release of phenolic compounds from soils, plant roots and leaf litter

Abstract: The influence of pH was examined, over the range from 6 to 14, on the amounts of p -hydroxybenzoic, vanillic, p -coumaric, ferulic and syringic acids, p -hydroxybenzaldehyde and vanillin, extracted from four soils and associated roots or leaf litter. Adjustment of pH was obtained by the addition of graded amounts of Ca(OH) 2 to water or by 2 m NaOH. The roots associated with three of the soils were from permanent pasture, perennial ryegrass and red clover, while the leaf litter associated with the fourth soil was from beech. The amounts of each phenolic compound extracted increased continuously with increasing pH, from a “threshold” value which varied between pH 7.5 and 10.5. The amounts extracted by water alone from the soil under permanent pasture, at pH 5.8, were equivalent to concentrations in the soil solution ranging from 1.4 μ m for p -hydroxybenzoic acid to m for ferulie acid. Amounts of up to 2000 times greater than these were extracted by 2 m NaOH. Similar effects of extractant pH were found with the other soils. Comparisons of the amounts of the phenolic compounds extracted from the soils, with the amounts extracted from the associated roots or leaf litter, suggested that substantial proportions of the soil phenolic compounds were either derived from organic residues more than 4 yr old or were the result of microbial synthesis.

Measurement of microbial sulfur in soil

Abstract: A technique was developed to estimate the amount of sulfur held by the microbial population in soil. This method involves lysing the microbial cells with chloroform and measuring the S released to 10 m m CaCl 2 or 0.1 m NaHCO 3 extracts. Calculations of biomass-S were made from the experimentally determined proportion of cell S released. To determine this proportion, two species each of bacteria and fungi, which were grown at three concentrations of S, were added to the soil and treated with CHCl 3 . The average values of 34.6% with CaCl 2 and 41.2% with NaHCO 3 were calculated for the proportion of microbial S extracted following CHCl 3 treatment. Biomass-S was found to represent approximately 2.3% of the total S in the soil.

Influence of soil inoculation with vesicular-arbuscular mycorrhiza and a phosphate-dissolving bacterium on plant growth and 32P-uptake

Abstract: The influence of inoculation of soil with a vesicular-arbuscular mycorrhizal fungus (Glomus fasciculatus) and a phosphate-dissolving bacterium (Bacillus circulans) on phosphate solubilization, growth of finger millet (Eleusine coracana) and phosphorus uptake from 32P-labelled tricalcium phosphate and superphosphate were studied. The mycorrhizal plants produced more dry matter and removed more 32P from the soil than non-mycorrhizal plants, but did not show increased 32P activity per unit plant mass. The 30 mm NH4F-HCl extractable 32P (available 32P) in soil, plant 32P activity and total P uptake were enhanced by soil inoculation with the bacterium. In the treatment receiving both inocula a synergistic effect was recorded with increased P uptake and dry matter production.

Competition for nodule formation between effective and ineffective strains of Rhizobium meliloti

Abstract: Ineffective mutants of four effective strains of Rhizobium meliloti were isolated and tested for their ability to compete with effective parents or antibiotic resistant mutants in the formation of nodules on Medicago sativa . In 5 out of the 6 cases studied, ineffective mutants were no different to effective strains of the same origin in their competitive ability. A difference in selection for infection by the host plant was observed between equally effective strains as well as between ineffective strains. Except for one pair of strains, the more-competitive effective strains (resistant or not to antibiotics) had the same origin as the more-competitive ineffective strains. For such strains the ability to compete with other strains to form nodules was a characteristic of each parent strain. Competitiveness was independent of effectiveness and had been retained during mutation.

Diurnal migration and responses to simulated rainfall in desert soil microarthropods and nematodes

Abstract: Diurnal patterns of microarthropod abundance in surface leaf litter were related to its moisture content. Leaf litter moisture was nearly 7% by weight at 0800h but fell to less than 1% by mid-day. Oribatid and tydeid mites moved into litter in the early morning and back into the soil before mid-day. There were no significant differences in numbers of nematodes in litter or soil and 78–98% of the nematodes were anhydrobiotic (coiled) in soil and litter at all times sampled. Following simulated rainfall there were fewer microarthropods in litter at mid-day in the absence of marked decreases in soil and litter moisture content. During drying, there were gradual reductions in numbers and species diversity of litter microarthropods. Nematode numbers did not change as litter dried. Anhydrobiotic nematodes in the soil increased from 14% on day 1 to 85% on day 4. Between 24 and 36 h after simulated rainfall, the proportion of anhydrobiotic litter nematodes increased from 35 to 80%,. Within 1 h after simulated rainfall, there were marked increases in numbers and diversity of microarthropods in surface litter. No collembolans were extracted from dry litter controls but the wet litter was dominated by isotomid, sminthurid and onychiurid collembolans. There were increases in numbers and diversity of oribatid, tydeid and gamasid mites in the wet surface litter within l h after wetting compared to controls.

Soil respiration in a tropical grassland

Abstract: Soil respiration throughout an annual cycle was measured at three different stands in a tropical grassland situated at Kurukshetra at 29°58' N lat. and 76°51' E long. Rates of CO2 evolution were measured by alkali absorption using 13 cm dia × 23 cm aluminium cylinders inserted 10 cm into the ground. Both movable and permanently-fixed cylinders were used. The CO2 evolution rates for the three stands were: Stand I (dominated by Sesbania bispinosa) 49–358 mg CO2 m−2 h−1; Stand II (mixed grasses) 55–378 mg CO2m−2 h−1; and Stand III (dominated by Desmostachya bipinnata) 55–448 mg CO2 m−2 h−1. A positive significant relation existed between rate of CO2 evolution and soil water content (r = 0.59−0.740), and between soil respiration and temperature (r = 0.58−0.69). A statistical model developed on the basis of the relationship between CO2 evolution rates and certain abiotic environmental factors showed 69% comparability between the calculated and observed values of soil respiration. The contribution of root and root-associated microorganisms to total soil respiration was estimated at 42% using the relationship between root biomass and CO2 output from movable cylinders.

Microbial biomass and activity in soils amended with glucose

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

Bacterial growth on proteins in the presence of clay minerals

Abstract: The growth of microorganisms on gelatin, bovine serum albumin and lysozyme in the presence of the clay minerals, montmorillonite and kaolinite was studied. The growth of microorganisms was diauxic, protein hydrolysis taking place in the second phase of growth. Depending on the protein-to-clay ratio, the second phase of growth could be divided into one of three types. At high protein-to-clay ratios, growth was not affected by the clay; at intermediate protein-to-clay ratios, growth rate but not final yield was reduced; and at low protein-to-clay ratios, the protein was unavailable for hydrolysis. The protein-to-clay ratios differentiating the three growth types were not well correlated with change in the affinity of adsorption of the protein to the clay and were different for different organisms. A model that explains the results is proposed. The main assumption of this model is that protein can bind strongly to clay and is either available or unavailable to proteases. The quantities of protein bound in these two modes are defined by the clay mineral, the protein and the protease.

Decomposition of atmospheric hydrogen by soil microorganisms and soil enzymes

Abstract: The decomposition of atmospheric hydrogen in different types of soil was measured. The decomposition of H 2 was apparently a first-order reaction. H 2 decomposition activity was proportional to the amount of soil with maximum activities at soil water contents of approx. 6–11% (w/w). The activity was lower under anaerobic conditions, but was constant between 1–20% O 2 . It was destroyed by autoclaving and was partially inactivated by fumigation with NH 3 , CHC1 3 or acetone, by u.v. irradiation and by treatment with NaCN or NaN 3 , indicating that biological processes in the soil were responsible for the observed H 2 decomposition. Treatment of soil with toluene or CHCl 3 caused only a partial inactivation. Incubation of soil in the presence of streptomycin or actidione reduced H 2 decomposition by less than 50%, whereas CO consumption was abolished. The H 2 decomposition rates showed H 2 saturation curves with apparent Michaelis-Menten kinetics. Cooperative effects were not observed. V max was reached at approx. 200 μl1 −1 . The K m values for H 2 were in the range of 30μl 1 −1 , but increased to higher values, when the soil had been pretreated with high H 2 mixing ratios. Apparently, the observed H 2 decomposition by soil is not only due to the activity of viable microorganisms, but soil enzymes as well.

Changes in microbial biomass and activity in soils amended with phenolic acids

Abstract: The phenolic acids p -hydroxybenzoic, ferulic, caffeic and vanillic acid, were added to soil of the Countesswells series that had been fallow or carried crops of potatoes, peas or barley for two consecutive years. Changes in phenolic acid concentration, the soil biomass, the respiration rate, and soil amylase activity were measured over 28 days. All the phenolic acids were sorbed by the soils which was generally in the order caffeic > ferulic = vanillic > hydroxybenzoic acid. The phenolic acids stimulated soil respiration and increased the biomass as determined by the substrate-induced respiration method. but the fumigation method of biomass assessment gave anomalous results. The soil amylase activity was initially increased by phenolic acid amendments but soon decreased, and after 7 days was less than in non-amended soil although activity had increased again after 28 days. The rates of respiration and the total phenolic acid concentrations were similar to unamended controls after 28 days. The immediate respiration response, measured 1–6 h after amendment, indicated that caffeic acid gave the largest initial response of the phenolic tested, this being 55–72% of that given by glucose. Soil from the potato plot showed the highest immediate response to the phenolic acid amendments measured as a proportion of the respiration response to glucose. The findings suggest that some crops stimulate the growth of phenolic-acid degrading organisms.

Nitrate reduction to ammonium and organic nitrogen in an estuarine sediment

Abstract: Nitrate reduction to NH 4 + and incorporation into organic matter was investigated in sediment-water systems composed of a 2.4-cm layer of estuarine sediment covered by 2 cm of water. Between 15 and 28% of the 15 N-labelled NO 3 − added to the sediment or floodwater of the systems was recovered as NH 4 + and organic N. The results indicated that NO 3 − was reduced to NH 4 + by a dissimilatory mechanism. A separate experiment examined the influence of redox potential (+300, 0 and −200 mV) on NO 3 − reduction in sediment suspensions maintained at pH 7.5. Conversion of NO 3 − to NH 4 + and organic N increased markedly with decreasing redox potential. The results suggested that although the reaction accounted for 35–42% of the NO 3 − reduced under intensely-reduced conditions (−200 mV), the significance of the reaction in nature was dependent upon NO 3 − movement into zones sufficiently reduced for reduction of NO 3 − to NH 4 + instead of denitrification. Under conditions in which NO 3 − moved downward through a sediment-water column into a reduced sediment zone approximately 15% of the NO 3 − was reduced to NH 4 + and recycled in the sediment.

Decomposition of lipids in soils: Free and esterified fatty acids, alcohols and ketones

Abstract: Because lipids are important to the properties of soils and the composition of sedimentary organic matter, the rates of decomposition of selected lipids in soils were determined. The rates of decomposition of total lipid extracts from plant materials and of carbohydrates showed marked differences that were dependent on the biological activity of the soils. In biologically-active soils, such as a rendzina or a brunic luvisol, pure lipids were readily degraded, regardless of their functional groups and chain length. An exception was montanic acid ( n C-28), which was refractory in all soils studied. In acid soils, such as a glossic luvisol or a dystric histosol, both chain length and functional groups affected the rate of decomposition of lipids, e.g. after 4 weeks at 28 C, the ratio of decomposition of oleic acid to, stearic acid was as high as 5. The same effect was found when 3-octadecanone (C-18 ketone) was compared to myristone (C-27 ketone). In most soils, the rate of decomposition of free and esterified stearic acid was similar, implying preliminary enzymatic hydrolysis of both the waxes and the fats.

Sulfur transformations in relation to carbon and nitrogen in incubated soils

Abstract: Changes in biomass-S in relationship to biomass-C and N were evaluated, and the transformation of 35S-labelled SO42− 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 49–77, respectively Microbially-incorporated S was found concentrated within the biomass or partially transformed into soil organic matter Fractionation of soils after incubation, by a 01 m NaOH-01 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 (

Priming effect of small glucose additions to 14C-labelled soil

Abstract: Soil was freed of its organic matter by heating it to 400°C. Plants were grown in a 14CO2 atmosphere and from them a labelled “soil organic matter” (humus) was prepared by composting the plant material for more than 3 yr in the modified soil under laboratory conditions. The influence of small additions of unlabelled glucose on the decomposition of the labelled soil organic matter was studied. Shortly after the addition of glucose there was a small extra evolution of 14CO2, which lasted about 1 day. It is claimed that the extra evolution of 14CO2 was caused by conversion of labelled material in the living biomass and was not due to a real priming action, i.e. an accelerated decomposition of humic substances or dead cellular material.

Surface phosphatase activity of Sitka spruce mycorrhizas from a serpentine site

Abstract: The p -nitrophenyl phosphatase activity of mycorrhizal roots of Sitka spruce from a phosphorus deficient site was inversely proportional to the concentrations of 0.5 n acetic acid-extractable phosphorus in the decaying organic matter from which the roots were removed. This is interpreted as the derepression of phosphatase synthesis under conditions of inorganic P deficiency and is discussed in relation to the possible roles of mycorrhizal surface phosphatase activity.

Metabolic capabilities of forest soil microbial populations with reduced species diversity

Abstract: Microbial populations of forest soil were modified by dilution and filtration of soil to reduce species diversity. Modified populations were allowed to increase their numbers fully after inoculation into soil sterilized by irradiation. Population modification was also attempted by starvation during prolonged incubations. Fully colonized soil was suspended and inoculated into sterile soil and respiration was followed. Metabolic capabilities of populations, altered by soil dilution and filtration, were considerably diminished compared to full soil populations. Starvation did not change the metabolic capabilities of the soil population.

Methods to evaluate pesticide damage to the biomass of the soil microflora

Abstract: Respiratory methods to estimate the amount of C in the soil microbial biomass and the relative contributions of prokaryotes and eukaryotes in the biomass were used to evaluate the influence of pesticides on the soil microflora. Experiments were conducted with 5 and 50 micrograms per gram of three fungicides, captan, thiram and verdesan. At 5 micrograms per gram they caused significant decreases (40%) in the biomass; the organomercury fungicide verdesan also caused a shift from fungal to bacterial dominance. Within 8 days, biomass in captan- and thiram-amended soils had recovered to that of controls. Although the fungal to bacterial balance was restored in verdesan-amended soils, biomass recovery was not complete. At 50 micrograms per gram the fungicides caused long-term decreases in the biomass and altered the relative proportions of the bacterial and fungal populations. Verdesan had the greatest effect on soil microbial biomass and competition.

Mycorrhizal fungi of Pseudotsuga menziesii in the south island of New Zealand

Abstract: Attempts were made to isolate mycorrhizal fungi of a timber species, Pseudotsuga menziesii (Mirb.) Franco which is exotic to New ZeaLond. Over 2000 pieces of mycorrhizal roots were plated out on Hagem medium and modified Melin-Norkrans medium; 14% of these yielded Rhizopogon vinicolor A. H. Smith, 6% gave rise to Amanita muscaria (L. ex Fr.) S.F. Gray, and 7% to unidentified basidiomycetes, 21% yielded dark sterile mycelia, 13% were sporing Fungi Imperfecti presumed to be contaminants, and 39% remained free of fungal growth. Eleven basidomycetous fungi were tested for their mycorrhiza-forming ability on P. menziesii seedlings. Under the test conditions R. vinicolor, Hebeloma crustuliniforme (Bull, ex St. Am) Quel., A. muscaria, Scleroderma bovista Fr., Laccaria laccata (Scop, ex Fr.) Berk. & Br., Inocybe corydalina (?), I. jurana (?), and I. maculata (?) formed mycorrhizas.

Interactions between earthworms and their soil environment

Abstract: Laboratory experiments were used to study the effect of food quantity and quality on the biomass of earthworms, and the influence of earthworms on plant growth and infiltration of water into soil. Earthworms with the most food gained weight faster than those with little or no supplementary food. The latter also failed to become reproductively mature. Earthworms lost weight on a nitrogenpoor diet, but this was not rectified by supplementing such food with inorganic nitrogen added to the soil 2 weeks before the worms. Ryegrass grown in soil in which earthworms (Allolobophora trapezoides) had been kept grew more slowly than in soil which had no previous worm activity, perhaps indicating that earthworms had converted relatively-available organic N into less available forms. Microscolex dubius gave the fastest infiltration rates of water into soil when clover mulch was present. With Eisenia foetida there was little effect of worm density on infiltration rates; the highest density significantly increased infiltration but only when clover hay had been mixed in the soil. The surface casting behaviour of the two earthworm species varied with the placing of the food offered.

Plant availability of phosphorus in dead herbage ingested by surface-casting earthworms

Abstract: A combined field and glasshouse technique was developed to evaluate the plant-availability of phosphorus from two sources. The first source was ryegrass which had been ingested by earthworms and excreted in surface casts, and the second was untreated ryegrass. Surface casts labelled with 32P were obtained in the field from earthworms fed with 32P-labelled ryegrass. In a glasshouse study, labelled casts or labelled herbage were placed on the surface of soil in which ryegrass was growing. Ryegrass recovered proportionately more 32P from cast material than from dead herbage. The results suggest that earthworms increase the short-term plant availability of P derived from plant litter by two to three fold. The effect of surface-casting earthworms on the rate of cycling of P in soils of different P status is discussed.

Response of Rhizobium strains to acid and aluminium stress

Abstract: Synchronous-culture, enrichment and isolation trials were done to determine effects of simulated soil acidity stress on growth of Rhizobium sp. (cowpea group), and to test whether tolerance of a strain is stable. In synchronous cultures, acidity and Al reduced the frequency of cell division. Non-dividing cells did not die, but those near division snowed evidence of heightened sensitivity to Al 3+ and H + . Differences in tolerance within single-strain populations were evidently not genetically determined. Prolonged culture under stress (72 generations) failed to enrich a strain in putative tolerant variants. And from six strains of different tolerance, isolates from single colonies that had grown on stress medium were no more tolerant than isolates from colonies on non-stress medium. Tolerance is a consistent and stable strain property.

Microbial aggregation of sand in a maritime dune succession

Abstract: Aggregation of sand by microorganisms was assessed in a dune succession ranging from unstable foredunes to stable fixed dunes. The microorganisms isolated from aggregates were fungi, bacteria, actinomycetes and algae, including cyanobacteria. Aggregation increased as the dunes became stabilized and the higher plant succession developed. The number and weight of aggregates were interrelated and most aggregates were in the > 1 2 mm ranges. The smaller aggregates were colonized by fewer fungal species than the larger ones. The general aspects of microbial aggregation in sand dunes are discussed.

Field inoculation of Medicago with V-A mycorrhiza and Rhizobium in phosphate-fixing agricultural soil

Abstract: Field inoculation of Medicago sativa with its symbiotic partners Rhizobium meliloti and the endomycorrhizal fungus Glomus mosseae was assayed under standard agricultural conditions in untreated arable phosphate-fixing soil. Glomus mosseae was successfully introduced and efficiently stimulated plant growth, N incorporation and P uptake. In contrast with a previous filed experiment, Rhizobium meliloti was also effective when inoculated alone. The dual inoculation of Rhizobium + Glomus more than doubled yield compared to an uninoculated control.