Showing papers in "Soil Biology & Biochemistry in 1989"

Ratios of microbial biomass carbon to total organic carbon in arable soils

Abstract: The ratio of microbial biomass carbon (C mic ) to total organic carbon (C org ) was measured in soils from long-term agricultural experiments located in the temperate climatic zone of Central Europe. One hundred and thirty-four plots located in 26 experimental sites were analysed. This survey of C mic -to-C org ratios indicated no universal equilibrium constant. In contrast, a comparative regression analysis of permanent monoculture plots (M) with continuous crop rotation plots (R) showed both to be highly significantly different at the P = 0.001 level: the regression line of continuous crop rotations shows a steeper slope, suggesting that a higher concentration of microbial carbon is characteristic of the crop rotation. The regressions accounted for 87% (M) and 94% (R) of the variances in the data, respectively. Mean % C mic in C org amounted to 2.3 for permanent monocultures and to 2.9 for continuous crop rotations, respectively, of the simplest, the mineral fertilizer management. The differences in C-to-C ratios between the two management systems is thought to be caused by the difference in cropping management rather than by soil texture, i.e. clay content. These C mic -to-C org ratios rose to 4 or 3.7% in both plots under monoculture and plots under rotation, when the plots had received organic fertilizer the year prior to sampling. This rise in C mic over C org is seen as a transient rather than an absolute phenomenon and is believed to be due to the easily available carbon fraction of the introduced organic materia. Linearity of the C mic -to-C org relationship seems to be expressed only up to 2.5% C org . Regressions of C mic -to-N t were as well highly significant ( P =0.001) and in addition, the comparative regression analysis between M and R plots.

Short-term partitioning of ammonium and nitrate between plants and microbes in an annual grassland

Abstract: We measured short-term (24 h) partitioning of 15NH4+ and 15NO3− into plants and microbes in a California annual grassland. Experiments were done in early spring at the peak of plant growth (February), and in late spring (April) when plant senescence had begun. Soil moisture decreased from 31 to 9% during this period. We injected either 15NH4+, 15NO3−, 15NH4+NO3− or NH415NO3− (≈ 2μg Ng−1 soil for each N-species) into the top 10cm of soil in cylinders which had been driven into the soil. After 24 h the soil and plants in the cylinders were harvested and we measured total N and 15N in inorganic forms, the microbial biomass (chloroform fumigation technique) and harvested plant material. Ammonium was the dominant source of N to both plants and microbes. In February, uptake rates of NH4+ were 81 and 426 mg N m−2 day−1 for plants and microbes, respectively, while in April the rates were 110 and 639 mg N m−2 day−1. Rates of NO3− uptake were 41 and 81 mgNm−2day−1 in February and 83 and 146mgNm−2 day−1 in April for plants and microbes, respectively. Thus, microbes took up substantially more NH4+ and NO3− than plants. On both sampling dates, NH4+ concentrations in the top 10cm of soil ranges from 600 to 800mg N m−2, andNO3− concentrations were <50 mg N m−2 High rates of microbial NO3−3 uptake may have resulted from the occurrence of microsites that were depleted in NH4+. Even though plants competed better for NO3− than for NH4+, microbial uptake was a major factor controlling NO3− availability to plants. The high rates of NH4+ and NO3− uptake by plants and microbes clearly demonstrate that the soil N pool bears little relationship to actual N availability.

Denitrification in north temperate forest soils: Spatial and temporal patterns at the landscape and seasonal scales

Abstract: Spatial and temporal patterns of denitrification were studied in nine forest soils of different texture and drainage classes in Michigan. Denitrification was measured using intact soil cores amended with acetylene. Sampling was done weekly during spring, and monthly at other times. To determine the factors limiting denitrification, selected cores were amended with either water or water plus nitrate. We found that activity was highest in spring and fall and was lowest during summer. Soils showed a characteristic relationship between denitrification rate and soil texture and drainage classes. Over 80% of the annual N loss to denitrification occurred during brief (3–6 week) periods of high activity in the spring and fall. Rates of denitrification during these periods exceeded 0.5kg N ha −1 d −1 in some soils. Estimates of annual N loss to denitrification ranged from −1 yr −1 in a well-drained sand soil to over 40kg N ha −1 yr −1 in a poorly-drained clay loam soil. Lack of available NO −1 1 was the primary factor limiting denitrification in summer, but available carbon was probably occasionally limiting, especially in the well-drained soils. Marked seasonal changes were observed in the variability and frequency distributions of denitrification rates. Denitrification rates showed lower variance in spring and fall compared to summer, regardless of the magnitude of the rates. These patterns may be related to seasonal changes in soil structure, carbon availability or aeration, which create a more uniform environment in the spring an fall. The characteristic relationship between denitrification rates and soil texture and drainage over two seasons and multiple sites suggests that geomorphologically-based models using these parameters may be feasible for producing landscape- and regional-scale estimates of denitrification.

Influence of macroclimate on soil microbial biomass

Abstract: Soils from 12 agricultural long-term experimental fields located in contrasting climatic regions were used to assess the influence of macroclimate on soil microbial biomass (Cmicr). Cmicr was measured using the substrate-induced respiration technique. When Cmicr was calculated on the basis of soil dry mass (mg Cmicr g−1 soil d.m.), only poor correlations with climatic variables were found. However, when Cmicr was calculated based on organic carbon (mg Cmicr g−1 Corg), close relationships with climatic variables were found. Especially, integrative climatic variables which reflected not only the temperature regime but also the moisture conditions, were found to be good predictors of the Cmicr-to-Corg ratio. The best predictor was the precipitation-evaporation quotient which accounted for 68% of the variance. With a stepwise, multiple-linear regression procedure, the clay content and pH of the soils were found to account for another 2 and 3% of the variance, respectively. Parts of the remaining variance could be accounted for by differences in fertilization, crops, tillage practices or residue returns. The Cmicr-to-Corg ratio of monocultures was significantly lower than that of crop rotations, and so was that of mineral fertilized compared to organically manured plots. The organic matter content of the soils studied were at or near the equilibrium level. An equilibrium function for the Cmicr-to-Corg ratio was calculated: y = 18.18+ 108.3-e−6.728x, where x = precipitation/ evaporation. Deviation from this equilibrium line would indicate that a certain soil is losing or accumulating organic matter.

Spatial and temporal effects on plant-microbial competition for inorganic nitrogen in a california annual grassland

Abstract: The changes in N-dynamics which occur after the start of autumn rains, following an extended summer drought, were examined in a California annual grassland. Competition for NH+4and NO−3 between plants and microbes, and the role of spatial compartmentalization. were studied using short-term ( < 24 h) experiments using 15N pool dilution and tracer techniques. Temporal dynamics of mineralization, nitrification and simultaneous plant and microbial uptake of NH4+ and NO−3 were assessed in intact soil microcosms periodically watered to simulate autumn rains. During the first week after initial soil wetting, both mineralization and immobilization rates increased; mineralization rate continued to increase during the next 6 weeks but immobilization rate remained constant. After 6 weeks of simulated wet-season, microbes consumed more of both NH+4 and no−3 than did plants in 8 h diurnal 15N tracer experiments. Though ambient NO−3 concentrations were low, nitrification accounted for about 13 of the N mineralized. Plants competed better for NO−3 than for NH4+. This suggests that nitrification benefitted plants by increasing accessible N. Spatial factors were critical in controlling N-dynamics. Microsites free of NH+4 were presumably responsible for the extensive microbial NO3− uptake. Of the activities measured in the top 9cm of the soil, the surface 5mm accounted for almost half of mineralization and plant NH4+-uptake, but only 11% of microbial NH4+-uptake.

Regulation of nitrate assimilation by ammonium in soils and in isolated soil microorganisms

Abstract: The regulation of NO−3 assimilation by NH+4 and amino acids was studied to better define the conditions under which raicrobial NO−3 assimilation could be expected to take place in nature. The assay system was microbial isolates from soil and slurries of two soils enhanced in NO−3 assimilating capacity by previous exposure to glucose for 18–24 h. The NO−3 assimilation rate was monitored by a NO−3 electrode and the inhibition determined by the change in rate following addition of NH+4 or amino acids. NH+4 inhibited (by 60%) NO−3 assimilation immediately (< 1 min) and at very low NH+4 concentrations (O.1 μg N g−1 soil). At higher NH+4 concentrations (10 μg N g− soil) a second state of inhibition (80%) occurred after 5 min. The speed of the first inhibition stage suggests that the site of inhibition is the transport of NO−3 into the cell. Asparagine was the only amino acid to act as rapidly and effectively as NH+4 in the soil, and thus the amino acids are unlikely to be important regulators of NO−3 assimilation in soils. Of the three soil isolates studied, Saccharomyces cerevisiae and Pseudomonas fluorescens responded to NH+4 and amino acids in a manner similar to the soil but NO−3 assimilation in the third organism, Azotobacter vinelandii, was generally insensitive to these N substrates in this short-term assay. Only high concentrations of NH+4 caused a partial inhibition of NO−3 assimilation in A. vinelandii. Due to the effectiveness of low NH+4 concentrations in inhibiting NO−3 assimilation in soils, microbial immobilization is not expected to be an important fate of NO−3 unless there are NH+4-free microsites in the soil.

Influence of sorghum residues and tillage on soil organic matter and soil microbial biomass in an australian vertisol

Abstract: Short- and medium-term changes in soil organic matter content following a change in soil management or land use are often difficult to measure because they occur slowly against a large background of soil organic matter which can have considerable spatial variability. Results from an experiment with grain sorghum (Sorghum bicolor L.) on a vertisol in sub-tropical Australia demonstrate the usefulness of two techniques for detecting trends in surface soil organic matter before they can be assessed by conventional methods. Firstly, using soil microbial biomass C as a sensitive indicator of changes in soil organic matter. Secondly, by using initial values of soil organic C or total N, measured before imposition of treatments, as a covariate in an analysis of variance. The combination of these techniques provided the most sensitive approach for detecting changes. The above-ground residues of sorghum (41 dry matter ha−1) were either retained or removed from plots that received conventional or zero tillage for 6 yr. Averaged over tillage treatments, soil organic C in the surface 0–10 cm layer was 8% greater in the residue-retained than in the residue-removed treatment, a difference equivalent to 16% of the C added as residues. The trend to increased soil total N was not significant. Residue retention caused larger percentage increases in microbial biomass C, measured by the chloroform fumigation-incubation method, than in total organic C and total N. The increase in biomass C was 12%, biomass N 23% and biomass P 45%, equivalent to 0.7% of the C, 7% of the N and 32% of the P added in residues. Residue retention decreased the biomass C-to-P ratio from 48 to 35, but these values were still much wider than those previously measured in U.K. soils. Residue retention increased respiration by about 45% (measured by CO2 evolution during a 30-day incubation) but had little effect on biomass C-to-N ratio or mineralization of N. Averaged over the two residue management treatments, soil organic C in the surface 10 cm layer was 7% greater under zero tillage than under conventional tillage. The corresponding increase in biomass C was 14–21%, but there were no differences in biomass N or biomass P. CO2 evolution and specific respiration by the biomass (μ g CO2-C evolved g−1 biomass C day−1) were less in zero-tilled than in conventionally tilled soils. The combined effects of residue retention and zero tillage caused increases of 15% in surface soil organic C, 18% in soil total N and 31% in biomass C.

Denitrification in north temperate forest soils: Relationships between denitrification and environmental factors at the landscape scale

Abstract: Relationships between annual dentrification N loss and soil physical and biological factors were investigated in nine north temperate forest soils of different texture and drainage classes. Soil texture was analyzed numerically by using percentage of sand as a variable and soil wetness was quantified by a continuous drainage index function. Correlations between soil texture, soil drainage, microbial biomass C and N content, the ratio of the flush of C mineralization to the flush of N mineralization following fumigation (CfNf), denitrification enzyme activity (DEA), the ratio DEA-to-biomass C, and annual denitrification N loss were quantified. We found that DEA and the DEA-to-biomass C ratio accounted for up to 96% of the variation in annual denitrification N loss. Percentage sand and soil wetness could also account for a large proportion of the annual variation in denitrification rates among sites (r2 = 0.86). Very low rates of denitrification in wet sandy soils were related to high CfNf values in these soils. The ratio DEA-to-biomass C is an indicator of the proportion of the soil microbial population that can denitrify. Differences in this ratio among the soils studied suggest that there is a selective advantage to organisms with the ability to denitrify. By analyzing annual denitification N loss rather than daily or hourly denitrification rates, and by evaluating landscape-scale factors, such as soil texture and drainage, rather than field-scale factors, such as soil moisture, we were able to establish stronger relationships between denitrification and environmental factors than have been achieved hitherto.

Priming effect of some organic additions to 14C-labelled soil.

Abstract: Small amounts of organic substances or finely-ground plant material were added to soil containing 14C-labelled organic matter. The rates of CO2 and 14CO2 production were measured with an i.r. spectrometer and a liquid scintillation counter, respectively. The extra 14CO2 (primed C) that evolved during decomposition most probably originated from conversions in the (microbial) biomass. We suggest that priming according to the original definition, viz., an acceleration of the decomposition of (stable) soil organic matter (humus), did not exist or was negligible in our experiments. Extra 14CO2 was produced after unlabelled glutamate was added to washed suspensions of 14C-labelled biomass in perlite. The amount of 14CO2 varied with the amount of biomass. The difference between the priming (extra 14CO2 production) caused by glucose (small and lasting only 2–3 days) and that caused by glutamate and aspartate (relatively large and lasting 30–40 days) was very pronounced. After decomposition, glucose, cellulose, wheat straw and sewage sludge left more carbon in the soil than was lost due to priming, so that there was a positive net effect. The two amino acids, glutamate and aspartate, however, had slightly negative net effects. The phenomena observed are probably the result of an accelerated turnover of biomass-carbon. The extra loss of 14C during the priming period was less than the original amount of 14C present in the biomass.

Absence of nitrogen fixation in clover grown on soil subject to long-term contamination with heavy metals is due to survival of only ineffective Rhizobium

Abstract: All strains of Rhizobium isolated from ineffective nodules on white clover (> 50 separate isolations) formed in metal-contaminated soils from a field experiment were demonstrated to be wholly ineffective in nitrogen fixation in plant infection tests on N-free nutrient agar. The plasmid profiles of these isolates were all very similar indicating a lack of genetic diversity in the population surviving in high concentrations of heavy metals. Isolates from comparable field plots of uncontaminated soil had a wide diversity of plasmid profiles. Inoculation of white clover at sowing with a large inoculum of effective Rhizobium leguminosarum bv. trifolii on metal contaminated soil resulted in effective N2-fixation. However, if the inoculated soils were held for 2 months in a moist condition before sowing with white clover, N2-fixation was not detected with inoculum doses of 107 cells pot−1 or less but was obvious where a very large inoculum of 1010 cells pot−1 was added. This indicates that white clover rhizobia are unable to survive (or at least unable to remain effective) in the presence of concentrations of heavy metals close to the current Commission of the European Communities guidelines for environmental protection. A rapid method of assessment of the toxic effects of pollutants on populations of Rhizobium spp is described.

Biodegradation of synthetic organo-metallic complexes of iron and aluminium with selected metal to carbon ratios

Abstract: Synthetic organo-ferric and organo-aluminous complexes with various metal :C ratios were prepared and incubated for 44 days in an Eutrochrept A1 horizon. Citric acid was used as a model of a natural soil acid. Fungal melanins synthesized by Epicoccum purpurescens were separated in fulvic acid-like and humic acid-like polymers and used as models for natural transformed soil organic matter. Under certain conditions, the biodegradation of such metal-organic complexes was slower than that of the free forms of the corresponding organic compounds. The intensity of this protective effect depended mainly on the metal to complexing functional groups molar ratio (M/CG) of the complexes and was also related to their aqueous solubility. For some organics, the protective effect was observed at ratios exceeding the saturation of their complexing sites, and sometimes needed a metal content exceeding more than twice their complexing capacity. In such cases, the formation of a flocculated metallic hydroxide trapping and wrapping the organic molecules is suggested to account better for the observed protective effect than chemical binding per se. As the solubility of the “humic” acids decreased faster than that of the “fulvic” ones when their metallic charge increased, the biodegradability of the former decreased more rapidly than that of the latter by metal complexing. For the same reasons, citric acid was more protected by complexing Al than Fe. The protective effect of both aluminium and iron against the biodegradation of all types of organic matter was high and of a comparable intensity at high M/CG molar ratios. These data are interpreted in terms of pedological processes.

Production of antibiotics by Pseudomonas cepacia as an agent for biological control of soilborne plant pathogens

Abstract: Antibiotics produced by Pseudomonas cepacia strain RB425 obtained from a lettuce root and active as a biological control agent against several soilborne plant pathogens, were studied to determine the mechanism of disease suppression by this strain. Cultures of RB425 were extracted with chloroform or ethyl acetate and the antibiotics isolated in the pure or semi-pure form by silica gel column chromatography, gel filtration and high performance liquid chromatography. The active fractions were detected by bioautography on TLC plates using Rhizoctonia solani and Venicillium dahliae as sensitive indicators of antibiotic activity following each fractionation. On the basis of u.V., i.r., NMR and mass spectra, the antibiotics were shown to be similar or identical to pyrrolnitrin and two kinds of pseudane, 2-(2-heptenyl)-3-methyl-4-quinolinol and 2-(2-nonenyl)-3-methyl-4-quinoIinol. The antibiotics showed high activity against several species of fungal pathogens including Pyricularia oryzae as well as R. solani and V. dahliae, but relatively less antibacterial activity. These substances may be involved in disease suppression by P. cepacia RB425.

Immobilization of enzymes on clays and soils

Abstract: Various enzymes such as glucose oxidase, β- d -glucosidase, acid phosphatase. tyrosinase and laccases were immobilized on clays and soils activated with 3-aminopropyltriethoxysilane and glutaraldehyde. After immobilization the enzymes retained a large amount of their original activities. The retention of laccase activity increased with the increase of clay content in soils, whereas the activity of β- d -glucosidase, tyrosinase and acid phosphatase decreased. The immobilized enzymes showed varying degrees of resistance to proteolysis and storage at high temperatures. After addition to soil suspensions, soluble laccase and glucose oxidase were rapidly inactivated (100 and 85% loss of activity in 15 days for laccase and glucose oxidase, respectively) whereas after immobilization, these enzymes were extraordinarily stable (12 and 25% loss of activity in 15 days for laccase and glucose oxidase, respectively). The possibility of incorporating the stabilized enzymes into soil for the improvement of numerous desired biochemical processes is discussed.

Influence of a fungal polysaccharide, scleroglucan, on clay microstructures

Abstract: The microorganization of complexes formed between clay minerals (Ca-kaolinite, Ca-montmorillonite) and a fungal polysaccharide (scleroglucan) was studied with water content and apparent volume measurements. X-ray diffractometry and scanning electron microscopy. Scleroglucan did not appear to intercalate montmorillonite. The fabric of the complexes was a card-house structure for koalinite and a three-dimensional network of quasicrystals for montmorillonite, both with enhanced porosities. The present results indicate that the strong water-stabilizing effect of fungal polysaccharides occurs without major microstructural rearrangements but could be related to the formation of stable organo-mineral networks.

In situ studies of nitrogen mineralization and uptake in forest soils; some comments on methodology

Abstract: Aspects of the methodology of and interpretation of results from, in situ studies of N-mineralization are discussed with reference to data collected from 17 eucalypt forests in south-eastern Australia during a 5-year period. Results suggest that: (i) it is possible to maintain moisture of soils contained within corers at levels not significantly different from those of the surrounding soil; (ii) inorganic-N is not produced linearly over time under field conditions, nor should we expect it to be; (iii) mineralization rates are affected by all in situ methods. In each of the forests examined, the average rate of net N-mineralization decreased as the period of containment increased; (iv) shorter periods of containment (e.g. 1–2 weeks) reduce artifacts due to containment and are therefore preferable to longer periods (e.g. 4–8 weeks); and (v) longer periods of containment cannot substitute for increased replication of sampling. Further, when annual or seasonal rates of N-mineralization and uptake are to be calculated, it is desirable to increase replication of the bulk soil sampling for the first and last sampling dates.

Microsite variations in soil chemistry and nitrogen mineralization in a beech-maple forest

Abstract: Though whole forest stand and ecosystem estimates of soil nitrogen dynamics are common, few studies have included measurements of within-stand variations in N mineralization and nitrification among microsites. We evaluated soil chemical status, net mineralization and nitrification potential during Autumn 1986 in A- and B-horizon soils taken from near the bases of Fagus grandifolia (American beech), Acer saccharum (sugar maple), and Fraxinus americana (white ash) trees and from sites > 2 m from trees using buried bag exposures. Fagus sites had significantly lower pH in both horizons than did other sites. Soils near Acer trees had higher organic C and NH4+ concentrations and Fraxinus soils had significantly higher PO43− availability. Moisture and NO3− concentrations were similar among microsites. Using stepwise discriminant analysis, we constructed functions which differentiated among site types at P < 0.01 on the basis of these chemical measures. A-horizon N mineralization rates were generally 50% greater near Acer trees than away from trees, with Fagus and Fraxinus sites intermediate. B-horizon mineralization rates were highest under Fraxinus, but only 2–36% of A-horizon rates. Relative nitrification was approached 100% in all site types despite soil pH < 4. Calculation of N mineralization on an area basis with and without accounting for the differences among microsite types indicates that whole-ecosystem estimates of N mineralization generated by sampling schemes which do not explicitly include unique microsites around tree bases may be biased low by 8–20%.

Identification and quantification of IAA and IBA in Azospirillum brasilense-inoculated maize roots

Abstract: Inoculation of maize seedlings with 107 colony forming units (cfu) of Azospirillum plant−1 significantly increased the root surface area 2 weeks after sowing as compared to non-inoculated plants. Indole-3-acetic acid (IAA) was identified by gas liquid chromotography (GLC) and gas chromatography-mass spectrometry (GC-MS) in Azospirillum tryptophan-free culture medium. The amount of IAA in the medium was calculated from the gas chromatogram by isotope dilution analysis and found to be 32–40 ng ml−1. Roots of Azospirillum-inoculated maize seedlings were found to have higher amounts of both free and bound IAA as compared to control. The amount of free IAA significantly increased in the inoculated roots 2 weeks after sowing. IAA and indole-3-butyric acid (IBA) were identified by GC-MS only in the roots of Azospirillum-inoculated seedlings 2 weeks after sowing. The significance of the higher amounts of IAA following inoculation with the bacteria is discussed.

Molybdenum limitation of asymbiotic nitrogen fixation in forests of Pacific Northwest America

Abstract: Nitrogenase activity as measured by acetylene reduction and nitrogen fixation as measured by 15N2 uptake is widespread in forest litter, decaying wood and the lichen Lobaria in Pacific Northwest forests In all cases the activity of nitrogenase is enhanced by addition of plant nutrient solutions and this is shown to be a specific molybdenum effect A wide range of forest species have litter which supports nitrogen fixation during decay and the majority show molybdenum stimulation The effect is most prominent in the acid soils situated between the Coast Range and the Cascade Range and extends from Oregon through Washington to British Columbia Nitrogenase is increased quantitatively by Mo additions and the effect, though best seen in the laboratory, is also measurable in the field The results are discussed on the basis of geographical distribution of the effect, plant species and the likely role of asymbiotic nitrogen fixation in contributing to long-term nutrient cycling

The nitrogen cycle in the broadbalk wheat experiment: A model for the turnover of nitrogen through the soil microbial biomass

Abstract: A simple four-compartment model is presented for the turnover of N through the soil microbial biomass, based on measurements made in the Broadbalk Continuous Wheat Experiment at Rothamsted. The model comprises an input compartment (containing the N in roots and other plant debris entering the soil plus the inorganic N immobilized during the decomposition of this debris), feeding two organic compartments, the microbial biomass compartment and the humus compartment, which in turn feed the soil inorganic N compartment. The model was fitted to data from the plot receiving 192 kg N ha−1 yr−1 by iterative adjustment of the five independent parameters it contains. The value thus modelled for the turnover time of N in the soil microbial biomass is 1.52yr, the corresponding annual flux through the biomass being 125 kg N ha−1 yr−1. Of this flux, 59 kg comes from the input compartment and 66 kg from the humus compartment.

Influence of different initial soil moisture contents on the distribution and population dynamics of introduced Rhizobium leguminosarum biovar Trifolii

Abstract: Data on bacterial distribution in soil were obtained with a method of washing and thoroughly shaking of the soil. Bacterial cells attached to or enclosed in different size groups of soil particles or aggregates were separated and enumerated on plates containing selective media. Soil portions of a loamy sand and a silt loam with different initial moisture contents were inoculated with Rhizobium leguminosarum biovar trifolii. Results of this experiment indicated that the initial moisture content influenced the distribution of the inoculated rhizobial cells. Differences in distribution were still found after prolonged incubation periods, suggesting a lack of transport and migration of the rhizobial cells. It was shown that rhizobial cells survived better in soils with a lower, than in soils with a higher initial moisture content. Rhizobial cells attached to or enclosed in soil particles or aggregates larger than approx. 50μm had a more favourable microhabitat than unattached cells or cells attached to smaller particles.

The nitrogen cycle in the broadbalk wheat experiment: 15N-labelled fertilizer residues in the soil and in the soil microbial biomass

Abstract: A single pulse of 15 N-labelled fertilizer was applied in spring as NH 4 NO 3 to each of four plots on the Broadbalk Continuous Wheat Experiment. Wheat has been grown on this experiment for more than 140 yr. The labelled N was given at the customary rates for the four plots, nominally 48, 96, 144 and 192 kg N ha −1 yr −1 . In subsequent years the plots reverted to unlabelled N, again given at the customary rates for the main Broadbalk experiment. Soils receiving inorganic fertilizer contained more biomass than soil from the corresponding plot that has never received inorganic N, but there was little difference in the microbial biomass N content of soils that had had 48, 96, 144 or 192 kg N ha −1 for many years. There were no consistent changes during a 4-yr period in total microbial biomass N, which averaged 190 kg N ha −1 in the plot receiving 192kg fertilizer N ha −1 yr −1 . Of the labelled fertilizer applied, 3–8% was present in the soil microbial biomass at the first harvest after application of labelled fertilizer. Expressed as a percentage of the total labelled N remaining in the soil at the first harvest, 19–27% was present in the microbial biomass. In the plot receiving 192 kg N ha −1 yr −1 , labelled biomass N declined from 5.69 kg ha −1 at the first harvest, to 4.50 at the second, to 3.35 at the third and to 2.35 at the fourth. In a subsidiary experiment, more N was retained in the soil at harvest when the fertilizer was added in the ammonium form than as nitrate; 32.6 and 19.5 kg ha −1 , respectively, for additions of 147 kg fertilizer N ha −1 . However, of the labelled N retained in the soil, 34% was present in the microbial biomass, whether the labelled N had originally been added in the ammonium form or in the nitrate form.

Direct extraction of microbial biomass nitrogen from forest and grassland soils of california

Abstract: The direct extraction method for estimating microbial biomass-N, which involves chloroform fumigation and extraction without an incubation step, affords the opportunity to analyze soils for which the incubation step is problematic. We tested the direct extraction method on freshly sampled grassland and forest soils of California, which experience seasonal wetting and drying cycles and labile substrate inputs. Time-course profiles of extractable-N during fumigation varied among soils, with the forest samples achieving peak N-flush after only 1-day fumigation, while the grassland soils required 5–7 days. When dry grassland soil was wetted to different moisture contents before fumigation, direct extraction efficiency was positively related to soil moisture. Direct extraction efficiency appeared to vary seasonally in the forest soils. Comparisons of N-flushes from direct extraction and fumigation-incubation methods are equivocal because of differential microbial immobilization during incubation. Agreement between the two methods was best when a variable k N was used to convert the fumigation-incubation data to microbial biomass-N. Although the direct extraction procedure offers some promise for evaluating a labile fraction of the microbial biomass-N, which is difficult to evaluate in these soils by any current method, uncertainties remain regarding duration of fumigation, consistency of water content and conversion to biomass-N.

Methodology for the quantification of root and rhizosphere nitrogen dynamics by exposure of shoots to 15N-labelled ammonia

Abstract: Investigations of carbon dynamics indicate that a significant proportion of plant assimilates may be released into the rhizosphere by root exudation processes. The corresponding deposition of nitrogen, however, has not been adequately quantified, largely because of methodological limitations. A series of investigations was therefore conducted under controlled conditions to devise a method for labelling roots with 15 N in situ to facilitate measurement of N fluxes from the root. In the proposed methodology, wheat plants with the rooting medium (hydroponic solutions or soil) sealed from the atmosphere were periodically exposed to an atmosphere containing 15 NH 3 for a duration of approx. 6h. Analysis of shoot tissues from the wheat plants indicated significant assimilation of NH 3 -N by shoots. Up to 30% of the N present in the shoot tissues was derived from atmospheric ammonia as determined by 15 N analysis. The amount of 15 N assimilated was directly related to the rate of ammoniation, whether that rate was adjusted by varying the ammoniation frequency or the amount of 15 NH 3 introduced into the atmosphere. Separate analysis of plant parts (senescent leaves, green leaves, heads and stems) indicated reasonable uniformity of 15 N assimilation among aboveground parts. The results, furthermore, demonstrated effective translocation of foliarly absorbed 15 N into the rooting system. While appreciably lower than that in shoot tissues, 15 N enrichment values for the roots were high enough to permit ready tracing of root N dynamics. On the assumption that the 15 N enrichment of root exudate N was the same as that of root N. the distribution of plant N among plant and soil fractions was estimated for wheat grown in soil columns to illustrate the potential application of the proposed method. According to these estimates, close to 50% of N assimilated by wheat after 58 days of growth was present belowground. Of this belowground N, approximately half appeared to have been released into the rhizosphere soil. These estimates, while requiring further confirmation, illustrate the potential use of the proposed methodology for quantification of rhizosphere N dynamics.

Adsorption and binding of protein on "clean" (homoionic) and "dirty" (coated with Fe oxyhydroxides) montmorillonite, illite and kaolinite

Abstract: The adsorption and binding of two proteins, catalase (CA) and β-lactoglobulin (LG) on clay minerals (montmorillonite, illite and kaolinite) homoionic to Ca2+ (ldclean” clays) or coated with two types of polymeric oxyhydroxides of Fe(III) (“dirty” clays) were studied. Equilibrium adsorption isotherms were generally of type L (Langmuir). The shapes of the adsorption isotherms for the Ca-clays and the corresponding dirty clays were similar, except for CA adsorbed on Ca-montmorillonite coated with polymeric Fe(OH)3 (prepared from Fe(NO3)3 at pH 2.2). CA and LG intercalated some montmorillonite systems. The shapes of the binding isotherms of CA and LG were similar to those of the equilibrium adsorption isotherms, and the retention of relatively large amounts of protein after washing reflected a strong affinity of these proteins for the clays. Adsorption and binding of proteins occurred in all systems, regardless whether the pHb of the clay suspensions was above or below the isoelectric point of the proteins. The higher external surface area of the dirty clays than of the clean clays did not significantly increase the amounts of CA and LG adsorbed, indicating that the proteins did not penetrate the spaces between the spheres or rods of the polyhydroxides of Fe on the surface of the clays, i.r. Spectra of the protein-clay complexes showed some shifts in the vibration bands of the CO and N—H groups of peptide bonds, suggesting that slight perturbations in protein conformation occurred as a consequence of their adsorption on the clays.

Properties of worm casts and surface soils under various plant covers in the humid tropics

Abstract: Worm casts and the corresponding surface soils were collected in an Ultisol at Onne (south-eastern Nigeria) under a cover of Pueraria phaseoloides and on Alfisols at Ibadan (south-western Nigeria) under Leucaena leucocephala , under Treculia africana and in a secondary forest. The biological and physico-chemical properties of these samples were assessed. Worm casts composed of finer soil particles had higher pH values, higher mineral element concentrations and more water-stable aggregates than the corresponding soils. Their higher humic acid (HA) fulvic acid (FA) contents, and HA: FA ratio, suggested that worm casting activity accelerates the humification process. Numbers of denitrifying microorganisms or cowpea rhizobia were the same in both casts and soils from each site, but worm casts contained more nitrifiers, had a larger biomass-C content and a higher percent of stress-labile N than the corresponding soils. Enzyme activities were also higher in casts than they were in the corresponding topsoils. Granular casts had similar properties as turret-shaped casts collected at the same site. In general, no microbiological and enzymatic differences were found among samples of casts or soils from different sites, but soil and the corresponding casts collected under T. africana were relatively low in organic-C, total N, mineral N, fulvic and humic acids, biomass-C content, nitrifiers and acid phosphatase activity.

Decomposition and chemical composition of cereal straw

Abstract: The influence of environment, management practices and cultivar on the rate of cereal straw decomposition was assessed in laboratory and field studies. Decomposition was measured using the reduction in dry wt as an indicator of decomposition. The rate of straw decomposition decreased with reductions in water potential at potentials below − 0.1 MPa. The decomposition rate at−0.01 MPa was lower than at −0.1 MPa. The rate of straw decomposition increased with increasing temperature over the range measured (5–35°C). Buried straw decomposed more rapidly than surface straw in all treatments except at −0.01 and −5.0 MPa. The influence of three stubble management practices on the rate of decomposition of straw of two wheat cultivars, Suneca and Kite, was assessed in field experiments lasting 2 yr. The rate of decomposition was greatest following the partial incorporation and burial of wheat residues. The retention of residues on the soil surface retarded decomposition. The straw of wheat and barley cultivars tested in the laboratory varied significantly in respect of the rate of decomposition. The straw of barley cultivars decomposed more rapidly than straw of the wheat cultivars tested. In the field studies there was no significant difference in the rate of decomposition of straw of the two cultivars. The percentage of hot water-soluble components, hemicellulose and cellulose decreased, while the proportion of lignin increased as the straw decomposed. Total N increased in buried straw, but decreased in the straw retained on the soil surface. There was no relationship between the amount of a chemical component and the rate of straw decomposition other than the higher percentage of hot water-soluble components found initially in barley straw.

Plant growth-promoting rhizobacteria: effects on growth and nitrogen fixation of lentil (Lens esculenta Moench) and pea (Pisum sativum L.)

Abstract: The potential of a number of putative plant growth-promoting rhizobacterial strains to enhance the growth and nitrogen fixation of western Canadian cultivars of lentil ( Lens esculenta Moench) and pea ( Pisum sativum L.) was assessed in field and laboratory studies. Nine strains were tested on a single cultivar of lentil (Eston) and pea (Trapper) in the field. None of the strains had any effect on growth of pea in the field, but in lentil inoculated with one or more of the rhizobacterial strains, there were significant increases in emergence, vigor, nodulation, C 2 H 2 reduction activity and root weight. Further laboratory studies conducted with the two best strains, G2-8 and G11-32 and lentil cv. Eston used four in vitro cultivation systems to determine the optimal conditions for plant growth enhancement. There was variation among experiments and cultivation systems but growth stimulation similar to that in the field was observed in trials with a sand column system. The best plant growth-promoting strain for Eston under in vitro conditions, G2-8, was then tested with lentil cv. Laird and plants inoculated with G2-8 had higher root and shoot dry weights and greater acetylene reduction than control plants in pot and sand column systems. Leonard jar and growth pouch systems were not satisfactory methods for observing growth enhancement of lentil by these bacterial strains, probably due to the slow growth rate of plants and watering method in the former system and the large seed size and short assay time in the latter. Results from these experiments suggest that strains such as G2-8 and G11-32 may be of value as adjunct inoculants for lentil, but effects are dependent on the cultivar and cultivation system used.

Inorganic N incorporation by coniferous forest floor material

Abstract: We examined the control of incorporation of N into material from a coniferous forest floor in the Sierra Nevada of California. Forest floor material was slurried in solutions containing 15N and the incorporation of N was measured in short-term (< 7 h) experiments. Ammonium incorporation was rapid (≈10 μ g g−1 h−1), but was limited by NH4+ at concentrations found in the soil solution. Methionine sulfoximine (MSX), a glutamine synthetase inhibitor, reduced NH4+ incorporation, also indicating that NH4+ supply to microorganisms was limiting. The half-saturation constants for NH4+ incorporation (Kt) were much greater than published values for fungi, suggesting that diffusion into organic material limited NH4+incorporation. Approx. 20% of NH4+incorporation occurred by abiotic processes. Nitrate incorporation averaged 6% of NH4+ incorporation and was not inducible. Uptake rates by fungal strands and root-fungus associations were similar to those of the bulk material, suggesting that these structures are not a dominant sink for N in this system.

Pathogen-suppression: A case study in biological suppression of Gaeumannomyces graminis var. Tritici in soil

Abstract: L'expression d'une maladie chez une plante depend des interactions entre le pathogene, l'hote et l'environnement. La suppression biologique des pathogenes dans les sols est exposee dans cette revue bibliographique, qui traite de la nature, de l'induction, du mecanisme et des relations avec l'environnement de ce phenomene. On porte une attention particuliere a la suppression biologique de Gaeumannomyces graminis var. tritici, champignon responsable du pietin-echaudage

Importance of soil water content when estimating soil microbial C, N and P by the fumigation-extraction methods

Abstract: The influence of soil water content on the estimation of microbial C, N and P by the fumigation-extraction (FE) method and microbial C and N by the fumigation-incubation (FI) method was investigated using a range of air-dry soils. The estimates of microbial C were compared with those obtained by the substrate-induced respiration (SIR) method. Soils were fumigated overnight with CHCl3 when either air-dry, or rewetted to 50% w/w water content immediately before fumigation. The presence of water during fumigation greatly increased the C and N extracted by 0.5 M K2SO4 compared with soils fumigated while air-dry. Overnight rewetting of non-fumigated (control) soils decreased extractable-C but the effect on extractable-N was variable. Rewetting before fumigation also increased CO2-C production and net N-mineralization (during subsequent incubation) compared to soils fumigated while air dry. However, because of high variability the increases were often not significant. The flushes of extractable-C and N (the difference between fumigated and non-fumigated soils) were calculated in three ways. Comparison with the biomass C estimated on air-dry soils by the SIR method suggests the most appropriate way to estimate the flush is: flush = (extractable-C from wetted, fumigated soil) — (extractable-C from air-dry, non-fumigated soil). Estimates of microbial C varied by up to 5-fold depending on how the flush was calculated. The release of inorganic P (P1) by fumigation of air-dry soil was generally increased by rewetting. Releases were greatest from rewetted soil fumigated with CHCl3 vapour, lower when using liquid CHCl3 and usually lowest when air-dry soil was fumigated with CHCl3 vapour. On two soils, gradually air-dried in the laboratory, the estimates of microbial C by the FE method were affected by rewetting (to 50% w/w water content) once the soils had dried below 20% w/w water content, and the rewetting effects were highly significant at