Showing papers in "Soil Biology & Biochemistry in 1979"

Phases of denitrification following oxygen depletion in soil

Abstract: The short-term response of soil denitrification to reduced aeration was studied using the acetylene inhibition method for the assay of denitrification. Two distinct phases of denitrification rate were observed. An initial constant rate, termed phase I, was not decreased by chloramphenicol, was increased slightly or not at all by organic carbon amendment, and lasted for 1–3 h. Phase I was attributed to the activity of pre-existing denitrifying enzymes in the soil microflora. Following phase I the denitrification rate increased; chloramphenicol inhibited this increase. In soils without organic-C amendment a second linear phase, termed phase II, was attained after 4–8 h of anaerobic incubation. The linearity of this phase was attributed to the full derepression of denitrifying enzyme synthesis by the indigenous population and to the lack of significant growth of denitrifiers. Phase I rate was dependent on the initial or in situ aeration state of the soil sample; phase II was not. Therefore, phase I may be more directly related to field denitrification rates. Denitrification rate changes following water saturation of soils in aerobic atmospheres were also examined. Rates were greatly increased by wetting but only after a lag of several hours. Our interpretation is that following wetting of natural soils, anaerobic or partially anaerobic conditions are established by respiration and reduced O2 diffusion rate; this first eliminates O2 inhibition then derepresses the synthesis of denitrifying enzymes. Although denitrifying enzymes are apparently present even in relatively dry soils, their activity is low until O2 inhibition is eliminated. From this evidence we reason that most N is lost from soils during brief periods beginning a few hours after irrigation or a rainfall.

Calcium oxalate accumulation and soil weathering in mats of the hypogeous fungus Hysterangium crassum

Abstract: Fungal mats of Hysterangium crassum (Tul. and Tul.) Fischer occupied a mean of 9.6% of the upper 10 cm of soil developed under a 40–65 yr old stand of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in Oregon. This hypogeous basidiomycete exudes large amounts of oxalic acid, some of which precipitates with Ca in microscopic crystals of calcium oxalate, resulting in a mean CaC2O4 content of 82g m−2 for the entire soil. Soil oxalate concentration was significantly greater within fungal mats (P

A method for measuring adenosine triphosphate in soil

Abstract: A method was devised for the extraction and measurement of adenosine 5'-triphosphate (ATP) in soil that minimizes sorption of ATP on the soil colloids. Soil was ultrasonified for 1 min with a solution containing trichloracetic acid (0.5 m). disodium hydrogen orthophosphate (0.25 m) and paraquat dichloride (0.1 m). The ATP content of the filtered extract was determined without further treatment in a scintillation spectrometer by the firefly luciferin-luciferase system. Recovery of added ATP was greater using the extratant containing trichloracetic acid, orthophosphate and paraquat than with trichloracetic acid alone or with a sulphuric acid extradant. Recoveries of added ATP ranged from 45% to 84% in thirteen different soils; ATP contents from 0.64 to 9.03 μg g−1 soil.

Phosphorus and the formation of vesicular-arbuscular mycorrhizas

Abstract: Infection of ryegrass roots by vesicular-arbuscular mycorrhizal (v.a.m.) endophytes occurring in an unfertilized virgin soil of low P status was more sensitive to increasing P supply than that by endophytes occurring in an adjacent fertilized agricultural soil. P application to soil depressed mycorrhiza formation in subterranean clover by increasing plant P status and not by direct effects of soil P on v.a.m. endophytes. Localized placements of superphosphate by banding or topdressing did not affect the development of mycorrhizas of roots in the fertilized zones differently from those of roots not in fertilized zones. The amount of infection was not correlated with P concentrations within the plant at harvest. However, the extent of infection could be correlated with P concentrations of roots at early stages of penetration by the fungi. Additionally, the effects of P supply on frequency of penetration by hyphae and subsequent mycorrhiza development closely paralleled effects of P supply on soluble carbohydrate concentrations in roots.

Adenosine triphosphate and microbial biomass in soil.

Abstract: Two methods for measuring adenosine 5'-triphosphate (ATP) in soil were compared, one based on extraction with NaHCO3-CHCl3 and thel other on extraction by a trichloracetic acid-phosphate-paraquat reagent. Recoveries of added ATP were greater with the NaHCO3-CHCl3 reagent but the extraction of “native” soil ATP by NaHCO3-CHCl3 was only about a third of that by TCA-phosphate-paraquat. Microbial biomass C and ATP were measured in 8 contrasting English soils, using the fumigation method to measure biomass C and the TCA-phosphate-paraquat method to measure ATP. Except in one acid woodland soil, the ratio (ATP content of the soil)/(biomass C content of the soil) was relatively constant, with a mean of 7.3 mg ATP g−1 biomass C for the different soils. This value is very similar to that obtained earlier in a range of 11 grassland and arable soils from Australia. Taking the English and Australian grassland and arable soils together, there is a close (r = 0.975) linear relationship between ATP and microbial biomass C that holds over a wide range of soils and climates. From this relationship, the soil biomass contains 7.25 mg ATP g−1 biomass C, equivalent to an ATP-to-C ratio of 138, or to 6.04 μmoles ATP g−1 dry biomass. The acid woodland soil (pH 3.9) contained much less biomass C, as measured by the fumigation method, than would have been expected from this relationship. This, and other evidence, suggests that the fumigation method for measuring microbial biomass C breaks down in strongly acid soils. The ATP content of the biomass did not depend on the P status of the soil, as indicated by NaHCO3-extractable P.

Adenosine triphosphate content of the soil microbial biomass

Abstract: In a heterogeneous group of 11 soils there was a linear relationship (r = 0.98) between ATP content and biomass C content, as measured by the fumigation technique. Biomass can be calculated from the ATP content using the relationship Biomass C in soil = 120 (ATP content of soil). When a soil was fumigated and then incubated for 10 days, both biomass C and ATP fell to about one-fifth their initial values. Both biomass C and ATP increased in a soil incubated for 10 days with glucose, the relative increase in ATP being slightly greater than that in biomass.

Effects of phosphorus addition and energy supply on acid phosphatase production and activity in soils

Abstract: To find out how acid phosphatase activity and production in some Alberta soils may be related to soil properties and past fertilizer history, soils of varying organic matter content, extractable P and P fertilization history were assayed for acid phosphatase using p -nitrophenyl phosphate as substrate. The effect of solution P concentration during the phosphatase assay was examined. The effect of P on the production of new phosphatase was examined in soils incubated with an added energy supply or orthophosphate. Phosphatase activity was influenced by P fertilization practices during the 5 yr before sampling. In a Black Chernozemic soil (Malmo SiCL) with a high organic matter content and high initial phosphatase activity, P fertilization at 27 or 54 kg P ha −1 y −1 for 5 yr reduced phosphatase activity by about 20%. However, in a Grey Luvisolic soil (Cooking Lake L) with low organic matter and initial phosphatase, P fertilization at 54 kg P ha −1 y −1 for 5 yr tended to increase activity, probably by increasing plant root growth and organic matter additions. Assay solutions containing orthophosphate at 0.55 mM reduced activity by 25% and 47% in a Malmo SiCL and Maleb L (Orthic Brown Chernozem) soil respectively. Further increases of phosphate concentration to 5.5 mM reduced phosphatase activity by 50% and 76% in the Malmo and Maleb L soils respectively. Phosphatase activity was increased up to 6-fold by incubation of soil with glucose and NH 4 NO 3 . Addition of P to produce an added C: added P ratio of 20:1 completely prevented synthesis of phosphatase by proliferating organisms and had a slight inhibitory effect on phosphatase already present. Similarly, addition of P without C in a 6-week incubation had only a small effect on phosphatase activity and maintained P concentrations in the assay solutions slightly below 0.55 mM. It was concluded that the effect of phosphate on soil phosphatase operates more through its effect on phosphatase synthesis than on activity of existing phosphatase.

Fungal biomass and nitrogen in decomposing scots pine needle litter

Abstract: The development of fungal biomass and increase of amounts of N was studied in decomposing pine needle litter for about 3 yr. After a relatively rapid increase of the amount of mycelium the fungal biomass became rather constant after about 2 yr. The absolute amount of N in the needles increased between the 4th and the 16th months and this increase was correlated to the increase of fungal biomass in the needles.

Effect of lead on soil respiration and dehydrogenase activity

Abstract: The effect of added Pb on the respiration and dehydrogenase activity of two sandy soils, a clay soil and a peat soil, (all with different physico-chemical properties), was studied. A concentration of 375 μg Pb· g− inhibited the respiration of the sandy soil by ca. 15%, 1500 μg Pb· g−ca. 50%. In the clay soil 1500 μg Pb· g− caused a 15% reduction in respiration. The inhibition of respiration in the sandy soil was still ca. 30% 40 months after the addition of Pb. Respiration of the peat soil was not affected by even 7500 μg Pb· g−. Dehydrogenase activity was affected by Pb in a similar way to soil respiration. In the sandy soil a considerable reduction occurred, while in the clay and peat soils dehydrogenase activity was not reduced. It was concluded, that a relationship exists between the inhibitory effects of Pb and the buffering capacity of the soil as expressed by its cation-exchange capacity. Because of these different effects of the same Pb concentration on the various soil types, no single value for the permitted concentration of lead pollution in soil could be established.

Effects of collembolan grazing on fungal colonization of leaf litter

Abstract: Using simple laboratory systems, an attempt was made to assess the importance of selective collembolan (Onychiurus subtenuis Folsom) grazing on the colonization of L layer leaf litter from a cool temperate deciduous forest by two common litter fungi (a Sterile dark form and a Basidiomycete). The results indicate that relatively low selective grazing rates could have potentially important effects on the growth and competitive colonizing ability of individual fungi. The relation of such a laboratory study to actual field events is only feasible when there is a thorough knowledge of the organisms being used. Preamble In common with numerous other forest systems, detailed data have been accumulated over the last decade on various groups of organisms active in the litter layers of a cool temperate deciduous forest (Kananaskis, Alberta, Canada) where the major tree species is Populus tremuloides Michx. Detailed studies on fungi in the litter layers of this forest (Visser & Parkinson, 1975 a, b) indicate that the time of spring thaw is one of considerable and rapid change in the populations of these organisms. This change was particularly well seen when concurrent (April) replicate samples of L layer leaf litter were taken from the melting edge of snow patches and from areas where complete snow melt had occurred and fungal biomass and species complement were determined. The data obtained from this study are summarized in Table 1.

Interactions between bacteria and ectomycorrhizal fungi

Abstract: Interactions between eight ectomycorrhizal fungi and eight bacteria were tested on five laboratory media and in the rhizoplane of Pinus radiata . Depression of growth of the fungi by the bacteria in laboratory media was dependent on the medium and bore little relation to effects in the rhizoplane. In the rhizoplane, different bacteria could depress, have no effect or even stimulate growth of mycorrhizal fungi. Competition and antagonism are suggested as mechanisms for depression of the fungi. Some bacteria gave protection against the depressive effects of other bacteria. Considerable differences occurred between ectomycorrhizal fungi in their colonization of the rhizoplane in the absence of bacteria and also in their presence. The common mycorrhizal fungi Rhizopogon luteolus and Thelephora terrestris generally colonized roots well but the strain of Pisolithus tinctorius studied colonized poorly. Direct microscopy showed the percentage cover of the root by microorganisms was usually only 10–20%. It is proposed that interactions of ectomycorrhizal fungi with soil organisms are important in determining the successful introduction and persistence of inoculated ectomycorrhizal fungi. Fungi should be selected for compatibility with a wide range of soil microflora as well as efficiency in plant stimulation.

Cycling of nitrogen by surface-casting earthworms in a pasture ecosystem

Abstract: Production of surface casts and the removal of plant litter from the soil surface by earth-worms had similar seasonal variations, with maximum values in May and minimum values occurring in July and August. Seasonal variations in the total nitrogen (TN) and oxidizable carbon contents of casts were closely related to variations in litter production. The C:N ratio of casts (10.7) was consistently smaller than that of underlying soil material (15.0 and 14.2 for the 0–5 and 18–22 cm depths, respectively), which is probably due to the mineralization of plant-derived organic material during passage through earthworms and utilization of low C:N ratio litter. Seasonal changes in the amounts of inorganic N forms in casts showed a build-up of NH4+N in the cooler winter months (July and August), attaining a maximum of 112 μg.g−1. with a decrease in autumn (April and May) and early spring (September and October), reaching a minimum of 54 μg.g−1. The opposite trend existed for seasonal variations in the NO3−-N content of the casts. Because only minor fluctuations in the amounts of N forms were obtained for underlying soil material during the casting period, the more dramatic changes observed in the casts could not be explained by soil variations. Seasonal variations in urease enzyme activity, associated with fluctuations in organic matter content, were more important than the effect of temperature on enzyme activity in accounting for seasonal variations in the NH4+-N content of casts. It was calculated that 73% of the TN content of litter removed from the surface by earthworms was accumulated in casts, indicating both the importance of earthworms in incorporating litter N with soil material and the inefficiency of N digestion by earthworms.

Germination and growth of Glomus caledonius spores: The effects of inhibitors and nutrients

Abstract: Spores of the vesicular-arbuscular mycorrhizal endophyte Glomus caledonius (Nicol. and Gerd.) Trappe and Gerdemann readily germinated in the absence of soil or host roots. Hyphal growth was improved by incorporation of nutrients into the medium and by the addition of pieces of cotyledon from boiled seeds, but was still dependent on the spore reserves. The response of ungerminated and pregerminated spores to inhibitors of protein and nucleic acid synthesis (cycloheximide, actinomycin D, proflavine hemisulphate, 5-fluorouracil and ethidium bromide) resembled those of saprophytic fungi more than other obligate biotrophs.

Mycorrhizal fungi of Pinus radiata in New Zealand.

Abstract: Mycorrhizal fungi of a timber species, Pinus radiata D. Don, exotic to New Zealand were isolated. Two isolation methods were used, and root washing without surface sterilization was found to produce a higher proportion of mycorrhizal fungi. Mycorrhizal roots were plated out on Hagem medium and from 7000 pieces 32% yielded Rhizopogon spp; 3% gave rise to Amanita muscaria (L. ex Fr.) S.F. Gray, Hebeloma crustuliniforme (Bull. ex St. Am.) Quel., Suillus spp, Thelephora terrestris Ehrh. ex Fr., and unidentified basidiomycetes; 14% yielded dark sterile mycelia; 19% were sporing Fungi Imperfecti presumed to be contaminants; and 32% remained free of fungal outgrowth. Some of the fungi commonly occurring in P. radiata stands, such as Laccaria laccata (Scop. ex Fr.) Berk. & Br. and Scleroderma verrucosum Vaill. ex Pers. and generally supposed to be mycorrhizal associates of this tree species, were not isolated even though they grow well on the media used for isolation. Mycorrhizas were synthesized on P. radiata seedlings by 16 basidiomycete fungi but under the test conditions Boletus piperatus (Bull. ex Fr.) O. Kuntze and Inocyhe sp., commonly assumed to be mycorrhiza formers on this tree species, did not form mycorrhizas. The results indicate Rhizopogon spp are a major component of mycorrhiza formers isolated from P. radiata in New Zealand.

Seasonal fluctuations of active fungal biomass in horizons of a podzolized pine-forest soil in central Sweden

Abstract: Metabolically-active fungal biomass determined by fluorescein diacetate (FDA) staining, was studied during a 27-month period in three horizons of a podzolized pine-forest soil. Recurrent biomass peaks were registered in autumn and early spring. Biomass increase was also noted during the winter with soil temperatures below 0 deg C. Only a minor fraction (2.4-4.3%) of the total fungal biomass was active. The FDA-active biomass was equally distributed between the organic (5 cm) and mineral (15 cm) soil horizons, and varied between 0.5 and 2.4 g d.w. m-2. The amount of FDA-active biomass was correlated with soil moisture content.

Nitrous oxide emission from soils at low moisture contents

Abstract: The production of nitrous oxide by soils was studied over short periods at a range of moisture contents up to field capacity with a highly-sensitive gas Chromatographic method. Nitrous oxide (N 2 O) was emitted from all soils studied at all soil moisture contents, which ranged from air dry to field capacity. The rate of emission increased with increasing moisture content and with increasing temperature up to 37°C. The evolution of N 2 O was not due to displacement of soil air during wetting. It was inhibited by HgCl 2 and toluene, and was prevented by formaldehyde and autoclaving. Thus it appeared to be due to microbiological processes. The results of experiments with nitrification and denitrification inhibitors suggest that a considerable part of the N 2 O was produced by the oxidation of ammonia. Production by denitrification of nitrate cannot be ruled out. The relative importance of these two mechanisms probably depends on the moisture and oxygen content of the soil. It is concluded that the microbial production of N 2 O is continuous in soil at all moisture contents. The process at low moisture contents constitutes an important component in the cycle which maintains the N 2 O concentration in the atmosphere.

Effect of surface-casting earthworms on the transport of phosphorus and nitrogen in surface runoff from pasture

Abstract: Treatment of a soil under permanent pasture with carbaryl (a broad spectrum carbamate biocide) resulted in a 2-fold increase in the volume of surface runoff. This was attributed to a 3-fold reduction in infiltration rate as a result of litter accumulation at the soil surface in the absence of surface-casting earthworm activity. The amounts of dissolved inorganic P (DIP), NH+4-N, and NO−3-N in surface runoff from pasture treated with carbaryl (1.18, 9.53 and 4.25 kg ha−1 yr−1, respectively) were appreciably greater than those from untreated pasture (0.31, 1.63 and 0.52 kg ha−1 yr−1). This was attributed to the large amounts of DIP, NH+4-N, and NO−13-N released from decomposing litter. Following incubation at 4°C for 18 days the release of DIP, NH+4-N and NO−3-N from litter was 160, 1600 and 950 μg g−1, respectively. Losses of particulate P and sediment in surface runoff were lower in the absence (0.31 and 290 kg ha−1 yr−1, respectively) than in the presence (0.56 and 1120 kg ha− yr−1) of surface casts, pointing to the importance of surface casts as a source of sediment. Surface casts accounted for 45 and 75%, respectively, of the annual loading of particulate P and sediment in surface runoff. Nevertheless, the total loss in surface runoff of P and N forms was increased substantially when the production of earthworm casts was eliminated

Nitrification and autotrophic nitrifying bacteria in acid tea soils

Abstract: Pure cultures of ammonium-oxidizing, autotrophic, nitrifying bacteria were isolated from acid soils (pH range, 4.0–4.5) from tea estates in Sri Lanka (8 soils) and Bangladesh (4 soils). All the Bangladesh nitrifiers were Nitrosospira spp but the Sri Lanka isolates were identified as Nitrosolobus spp Nitrosospira spp and one species of Nitrosovibrio. Nitrite-oxidizing nitrifiers were detected in several of the soils but pure cultures were not isolated. Evidence was obtained that Nitrosospira caused nitrification in situ in an acid soil (pH 4.1). Indigenous nitrate was first eliminated from the soil by denitrification. The soil was then incubated aerobically and the nitrate formed was estimated as N2O by gas chromatography after denitrification.

Digestion of lignin by termites

Abstract: The termite Nasutitermes exitiosus (Hill) was fed natural and synthetic [14C]labelled lignins and related compounds, and the respired CO2 collected. All the compounds were partially degraded to CO2, from an average of 7% of the added 14C for ring-labelled phenate to 63% for methoxyl-labelled maize lignin and 64% for ring-labelled ferulic acid, during periods of 6–69 days. The breakdown commenced immediately and was linear until the food was consumed. Thereafter a slow release continued for some time. Termite bodies assayed at the end of the experiments contained only a small proportion of the added radioactivity. When live termites were separated from their faeces, it became apparent that most of the decomposition of lignin had taken place in the termites and not externally in the voided faeces. An estimate of respiration rates of N. exitiosus indicates a significant release of C as respired CO2, and this species of termite would appear to return a considerable proportion of the C in litter lignin directly to the atmosphere.

Soil temperature, mycorrhizal infection and nodulation of Medicago truncatula and Trifolium subterraneum

Abstract: Establishment of vesicular-arbuscular mycorrhizal fungi in plant roots involves a pre-infection phase of propagule germination, hyphal growth and appressorium formation, followed by growth of the fungus within the root. The effect of soil temperature on the pre-infection stage was examined by counting the numbers of fungal “entry-points” on the main roots of Medicago truncatula and Trifolium subterraneum, grown at soil temperatures of 12°, 16°, 20° and 25°C for periods up to 12 days. Increased root temperature was positively associated with increased numbers of “entry-points”. This effect was more marked between 12° and 16°C than at higher temperatures, as shown by comparing plants at the same stage of development (emergence of spade leaf) and by calculating the results as entry points per cm root. The first root nodules appeared sooner at higher temperatures (20° and 25°), but subsequent development of nodules (measured as nodule number and aggregate volume of nodules per plant, up to 21 days) was best at 16°C for both host Rhizobium combinations in non-sterile and autoclaved soil. There was no evidence that competition between mycorrhizal fungi and Rhizobium for infection sites occurred. A method of obtaining numbers of infective propagules of vesicular-arbuscular mycorrhizal fungi in soil is described.

Effects of lead on the soil bacterial microflora

Abstract: The sensitivity of soil bacteria towards Pb was investigated. Soil suspensions from fourteen different soil types with a high or low Pb content were plated out on soil extract agar containing various concentrations of PbCl2. In agar with a high Pb content, higher bacterial counts were found with suspensions from Pb-containing soils than with those of soils with a low Pb content. In the Pb-containing soils, proportionally more gram-negative rods were present while coryneform bacteria decreased. In an additional experiment, in which Pb was added to a sandy soil, more Pb-tolerant bacterial strains were found 3 years later than in the same soil without Pb. When pure cultures of the bacteria isolated from the soils were tested in liquid media for Pb tolerance, a higher proportion of tolerant strains was found in Pb-containing soils. Among strains of gram-negative bacteria isolated from these soils a higher proportion of tolerant strains was found than in corneform bacteria. It was concluded that as a consequence of Pb pollution of soil a selection of Pb-tolerant bacteria may take place.

Fluorescent antibody technique to identify Azospirillum brasilense associated with roots of grasses

Abstract: Bacteria associated with roots of grasses from Florida, Ecuador and Venezuela were isolated and their N 2 -fixing ability was demonstrated by C 2 H 2 reduction assay. The bacterial isolates have been classified as Azospirillum brasilense (formerly Spirillum lipoferum ). These N 2 -fixing isolates have been compared with several Brazilian strains. Fluorescent antibody (FA) techniques were used to assist identifying isolates of N 2 -fixing bacteria from grass roots. Tests with antisera prepared against four strains of Azospirillum were used to define serological groups. Antigen-antibody specificity was demonstrated using both Azotobacter and Azospirillum antisera against known species of other soil microorganisms and numerous unidentified soil bacteria. Several applications of the FA technique are suggested to identify N 2 -fixing bacteria associated with grass roots.

Some problems in assessing the fluorescein diacetate-active fungal biomass in the soil

Abstract: A technique for assessing the metabolically-active fungal biomass by means of fluorescein diacetate (FDA) staining was recently described (Soderstrom, 1977). In this note some practical details in the application of this technique to studies of FDA-active fungal biomass in the soil are discussed.

An analytical model of the decomposition of soil organic matter

Abstract: An intermediate resolution model of the decomposition of soil organic matter is developed from a comprehensive study of published experimental work. The many organic and inorganic forms of soil N, P, and K are mathematically treated, together with the various transformations between forms. Most of the transformations are moderated by microbes, and the dynamics of the microorganisms are explicitly represented. A simulation is made of a general heterotrophic population using organic C and N for energy, as well as nitrifiers which oxidize nitrogenous compounds chemotrophically. Such explicit treatment of microbe dynamics permits among other things the study of microbe immobilization of important plant nutrients. In addition to the simulation of biological aspects of decomposition, the model treats the physicochemical processes of precipitation, fertilizer and native mineral inputs, leaching loss, sorption of organic and inorganic ions on soil colloids, condensation between organic N and aromatic compounds, and exchange reactions. Model parameters are dealt with in detail in order to base them as firmly as possible on experimental information. The process rates include functional dependence on soil temperature and moisture. For clarity of presentation, the model is divided into four submodels, one each for N, P, and K, and the C energy substrate. The total model is coupled with a previous plant growth model, and thereby simulates complete element cycles within the plant-soil system.

Humic substances and phosphatase activities in plant tissues

Abstract: Humic acid and its fractions obtained by water- or acid-boiling inhibited phosphatase activity per se in beet, carrot and potato discs, pea roots and epicotyls and wheat roots, coleoptiles and leaves. In wheat roots the order of effectiveness of various humic acid fractions in inhibiting phosphatase activity was acid-boiled soluble > water-boiled soluble > acid-boiled insoluble > water-boiled insoluble > original humic acid. A number of synthetic humic acids were also effective inhibitors of enzyme activity but phenolic acids had no effect. The degree of inhibition was not related to C, H or N contents or to the total ash, carboxyl or phenolic contents of the humic acid samples. Magnesium ions enhanced phosphatase activity and decreased the inhibition of phosphatase activity produced by the humic acid fractions. Humic acid fractions did not affect the maximum temperature for enzyme activity or its pH optimum and had little effect on the Michaelis constant. They did, however, reduce the maximum velocity of the enzyme reaction thus producing a non-competitive inhibition of enzyme activity. It is suggested that the humic acid fractions inhibit phosphatase activity by combining with the enzyme, but not at the most active site of enzyme activity.

Phenoxy herbicide degradation in soils: Quantitative studies of 2,4-D- and MCPA-degrading microbial populations

Abstract: Microbial populations able to degrade 2,4-D (2,4-dichlorophenoxyacetate) and MCPA (4-chloro-2-methylphenoxyacetate) were enumerated by means of a most probable number (MPN) procedure in eight Natal soils not previously treated with these herbicides. Estimated 2,4-D-degrading populations ranged from 1.26 to 245.2 and MCPA-degrading populations from 0.34 to 1377 g−1 dry soil; in seven of the soils the populations of these organisms were less than 40 and 30 g−1, respectively. Such counts indicate that for the successful isolation of 2,4-D- or MCPA-degrading microorganisms from soil, at least 1 g dry weight of soil should be used for enrichment cultures. The 2,4-D-degrading organisms occurred among the aerobic soil bacteria detectable by plate count, at frequencies of only 1 in 30 × 103 to 1 in 36 × 106 and the MCPA-degrading organisms at frequencies of 1 in 5 × 103 to 1 in 133 × 106; the ease with which the herbicide-degrading organisms can be isolated from enriched soil cultures treated with 2,4-D or MCPA is evidence of their massive preferential proliferation in response to the herbicides. Log 2,4-D- and MCPA-degrading populations did not differ significantly in four soil samples, but in the others either the 2,4-D- or the MCPA-degrading population was dominant. The longer persistence of MCPA compared with that of 2,4-D could therefore not be ascribed to quantitative differences in the populations of MCPA- and 2,4-D-degrading soil microorganisms. No relationship was evident between the soil populations of 2,4-D- or MCPA-degrading microorganisms and aerobic soil bacteria, and variations of the three populations among the soil samples were not associated in any obvious way with the soil physical and chemical characteristics, except perhaps an association of the highest counts of herbicide-degrading organisms with a sugar cane soil of sandy texture and high C: N ratio.

Transfer of legume nitrogen to associated grass

Abstract: In improved hill-peat a rapid increase in rhizosphere soil bacteria occurs following clover defoliation. The increase in bacterial-cell N is equivalent to 19% of the N released from the clover roots in the first 5 days after cutting. Subsequent ammonification of the immobilized N is dependent on the resistance of the microbial fraction to further degradation. In three fertilized hill soils examined, [ 15 N]labelled bacterial N was mineralized quite rapidly, some 20% of the bacterial N being taken up by grass over 17 days. Under glasshouse conditions the rate of mineralization is different in the three soils examined, the peat providing a more favourable environment than brown earth for bacterial N transfer.

Effect of organic matter on the survival of Phytophthora cinnamomi rands in soil

Abstract: Soil organic matter collected from beneath an unburnt stand of Eucalyptus marginata was added in increasing amounts to lateritic soil. Phytophthora cinnamomi incubated in soils containing 50% or more organic matter was extensively lysed, and many of the sporangia produced were abortive. With increasing organic matter there is an increase in nutrient concentrations and in the microbial population and it is suggested that these factors are the basis of the antagonism.