Showing papers in "Soil Biology & Biochemistry in 1991"

Nitrogen release from the leaves of some tropical legumes as affected by their lignin and polyphenolic contents

Abstract: Leguminous plant materials used as mulches, green manures and cover crops are generally assumed to provide a readily-available source of N to crops. However, little is known about the chemical composition and N release patterns of the variety of legumes being used in tropical agroecosystems. N release patterns from the leaflets of 10 troplcal legumes and rice straw were determined in a laboratory experiment. Ground leaf material was allowed to decompose in an acid soil (pH 4.5) for 8 weeks and the soil was analyzed periodically for extractable NH4+-N and NO3∼, -N. N release in the soil plus plant material were compared to that of the soil without plant material added and related to the N, lignin and polyphenolic concentrations of the leaflets. Three patterns of net N mineralization emerged during the 8-weeks. One pattern exhibited by the control soil, rice straw and leaves of two of the leguminous plants was a low, positive net mineralization. Another pattern showed much higher rates of mineralization than the control soil and the third pattern showed initial net immobilization followed by low but positive net mineralization rates. The amount of N mineralized during the 8 weeks as compared to the control soil ranged from +46 to −20% of the N added in plant material. Net mineralization was not correlated to % N or % lignin in the leaf material but was found to be negatively correlated to the polyphenolic concentration, r = −0.63, or the polyphenolic-to-N ratio, r = −0.75. Mineralization in excess of the control soil was found only for materials with a polyphenolic-to-N ratio

Antifungal antibiotics produced by Trichoderma spp

Abstract: In recent years plant pathologists and commercial companies have shown considerable interest in the application of biological control agents. Trichoderma spp have received particular attention as agents for the biological control of fungai pathogens of plants and appear to be likeiy candidates for successful exploitation in the future. Despite extcnsivc research over the last 50 years on the capability of Trichoderma spp to reduce the incidence of disease caused by soil-borne plant pathogens, the mcchanisms by which disease control is achicvcd are not clearly understood. The mechanisms suggcstcd to bc involved in biocontrol by thcsc fungi arc antibiosis, lysis, competition. mycoparasitism and promotion of plant growth (Hcnis, 1984; Papavizas, 1985; Chet, 1987; Baker, 1988; Lynch, 1990) It seems rcasonablc to assume that successful antagonism may rely on a combination of these modes of action. In this rcvicw we document the ability of Tri~hodermu spp to produce antibiotics and discuss the nature and possible ecological relevance of the antibiotics produced by these fungi. Four reviews on Trichoderma have appeared in recent years (Evcleigh, 1985; Papavizas, 1985; Taylor, 1986; Chct, 1987) but none have specifically considered these topics.

Effects of heavy metals from past applications of sewage sludge on microbial biomass and organic matter accumulation in a sandy loam and silty loam U.K. soil

Abstract: Amounts of microbial biomass were measured in soils from two different U.K. field experiments, one on a sandy loam (15% clay) at Luddington (Wick series) and the other on a silty loam soil (21% clay) at Lee Valley (Hamble series), where sewage sludges, mainly enriched with single metals, were applied 22 yr ago. No single metal (Zn, Cu, Ni and Cd) at or below current EC permitted total soil metal concentrations, or limits, decreased the amounts of soil microbial biomass. However, Cu at about two and a half times permitted metal limits decreased the amounts of biomass by about 40% at both sites and caused an increased accumulation of organic C and total N of about 30% in the sandy loam and about 13% in the silty loam soil. Zinc, at about the same concentration, decreased the biomass by about 40% in the sundy loam and 30% in the silty loam soil while soil organic matter accumulation increased by only 9–14%. Cadmium, at about twice current EC limits did not affect the amount of biomass or soil organic matter in the silty loam soil. Similarly, neither were affected by Ni at 2–3 times current metal limits. The amount of microbial biomass C as a percentage of total soil organic C was much lower (

Carbon and nitrogen mineralization from two soils of contrasting texture and microaggregate stability : influence of sequential fumigation, drying and storage

Abstract: Two top soils (an Alfisol and a Vertisol) of contrasting cation exchange capacity, micro-porosityand microaggregate stability were sampled from the same climatic region and were incubated for 4 weeks with 14C-labelled plant material. Each soil was then subjected to combinations of two of the following treatments: (1) drying (40°C), remoistening and incubation for 10 days at 25°C, (2) fumigation with chloroform vapour and incubationand (3) storage (4°C) and incubation. The amounts of CO2 and 14CO2 evolved and inorganic N released during each incubation period were measured. Also, during drying and after remoistening of soils, concentrations of biomass C and 14C were monitored using a fumigation extraction technique. Biomass C and 14C decreased by 26–30% during desiccation and increased to 77–84% of untreated control soils during incubation after rewetting. The relative decline in biomass during soil drying was of similar magnitude for both soils and for 14C-labelled and total biomass C, indicating that factors other than soil properties had determined the extent of decline. The evolution of microbial populations exposed frequently to high temperatures and to extreme desiccation in the natural field environment has been proposed to explain the similar responses of biomass C to the imposed drying regime. Previous fumigation-incubation of soils to destroy the majority of the microbial biomass had little effect on the sizes of the C mineralization flushes obtained when the soils were subsequently dried, rewetted and incubated. The specific activities of the CO2-C flushes after drying were much lower than those from fumigated or stored control soils respectively. This was especially evident for CO2-C flushes from the well-aggregated Vertisol. From the magnitude of the flushes of CO2 and 4CO2 after the various combinations of treatmentsand from their specific activities, we have deduced that microbial cells killed by soil desiccation had made only a minor contribution to the C and N mineralization flushes after soil rewetting and incubation. The larger contribution had come from other sources, the relative importance of which appears to be influenced by soil characteristics, possibly cation exchange capacity and microporosity.

Relationship of soil microbial biomass and activity with fertilization practice and crop yield of three ultisols

Abstract: The relationships of crop yield with soil factors such as microbial biomass (Cmic), basal respiration, microbial biomass-to-organic C (Cmic.:Corg) ratio, and metabolic quotient (qCO2) were investigated in three long-term field experiments in Alabama. On all three sites soybean (Glycine max L.) yield was significantly correlated with Cmic: (r = 0.77). On the Auburn and Brewton sites, there were positive correlations of Cmic with the yields of sorghum, rye and corn. Only in a few cases, soil nutrient contents (N, P. K) were significantly correlated with Cmic. The Cmk:Corg ratio increased with crop yield. The data suggest that part of the variability of Cmic and the Cmk:Corg ratio which to a great entent are determined by the climatic conditions at a certain site, may be explained with differences in crop yield. These may in turn be caused by mineral fertilization. No relationship was found between basal respiration and soybean yield. However, the qCO2 was negatively correlated with soybean yield (r = − 0.78) on all sites. This has important implications on agriculture: if more C is lost by respiration at less C input, more care must be taken to maintain organic C contents.

Field incorporation of straw and its effects on soil microbial biomass and soil inorganic n

Abstract: The fumigation-extraction method was used to measure changes in soil microbial biomass in a field experiment following incorporation of 10 t ha−1 wheat straw, with and without 100 kg N ha−1 (as NH4NO3). into a silty clay loam soil in autumn 1987. The first measurements were 7 days after straw incorporation and then periodically for ca 1 yr. The amount of biomass (measured as biomass C. N and ninhydrin-reactive N) roughly doubled (from initial levels of ca 340 kg C. 76 kg N and 17 kg ninhydrin-N ha−1) within 7 days of straw incorporation, remained constant for the next 27 days and then slowly declined. The increase in biomass was similar when N was incorporated with the straw, although maximal amounts were a little greater (by ca 30 kg N ha-) and attained a little later (14 days after incorporation). At the last sampling (363 days after incorporation) the biomass had declined to about half of its size shortly after straw addition; this was still ca 20% more than in the unamcnded soil. Initial soil inorganic N contents were small (< 10 kg N ha−1) so that the rapid increase in biomass in the soil amended with straw alone could not be explained by immobilization of inorganic N. Similarly, in the soil receiving straw and 100 kg ha−1 inorganic N. nearly all of this N was still present in inorganic form 7 days after straw incorporation, yet by this time the biomass had increased by ca 50kg N ha−1. In both treatments, most of the newly synthesized biomass probably cnmc from N already present in the straw. Although straw incorporation increased the size of the biomass, there were no significant differences in biomass C to N ratios, or in biomass C to ninhydrin-N ratios between treatments and at different times. The mean ratios were biomass C/N = 4.7; biomass C/ninhydrin-N = 21.4; biomass N/ninhydrin-N = 4.6. A feature of this work is the extraction of relatively large samples of soil for biomass measurements with minimal sample preparation; a procedure that gave reproducible results even with the very heterogeneous mixtures of soil and straw encountered.

Microbial biomass dynamics during the decomposition of glucose and maize in metal-contaminated and non-contaminated soils

Abstract: Metal-contaminated soils (produced by past long-term applications of contaminated sewagesludge) from the Woburn Market Garden Field Experiment were previously shown to contain only about half the amounts of microbial biomass as other soils from the experiment which received farmyard manure during the same period. In some cases, the amounts of biomass in the metal-contaminated soils were even smaller than in other soils from the experiment which received inorganic fertilizer throughout. It is possible that the metals were causing decreased efficiency of substrate utilization by the microbial biomass, leading, in turn, to a smaller microbial population. This was investigated in a laboratory experiment by adding 14C-labelled glucose and 14C-labelled maize shoots (maize) separately to a metal-contaminated and a non-contaminated soil from the field experiment. Microbial biomass C, ninhydrin-N, soil ATP content and CO2 evolution were measured during the next 50 days following glucose addition and 100 days following maize addition in both soils. The biomass formed following addition of glucose or maize was consistently smaller in the metal-contaminated soil throughout the incubations. Overall, about 15–32% less glucose-derived and 25–60% less maize-derived biomass was formed in the metal-contaminated soil. In contrast, more CO2-C was evolved from the metal-contaminated soil than from the non-contaminated soil. This suggests that the biomass in the metal-contaminated soil was less efficient in the utilisation of substrates for biomass synthesis. It is suggested that this may be a major reason lor the smaller biomass in the metal-contaminated Woburn soils.

A dispersion and differential centrifugation technique for representatively sampling microorganisms from soil

Abstract: A multi-stage dispersion and differential centrifugation technique for repesentatively sampling non-filamentous microorganisms from soil was developed. The individual steps of the method aimed to disperse soil aggregates and dissociate microorganisms from soil particles. The released microorganisms were then separated by low-speed centrifugation. The dispersion and dissociation efficiency of the individual steps and the method as a whole were tested turbidometrically using a clay loam, a sandy loam and a peat soil. The evaluation indicated that individual steps rarely achieved more than 50% dispersion and. that the sandy loam was more readily dispersed than the other two soils and that the peat soil was particularly difficult to disperse. The multi-stage method was very efficient at dispersing all three soils. The efficiency of the method at sampling microorganisms was assessed by determining the biomass recovered in extracts and residues compared to the soil at the outset. Biomass was determined using direct microscopic cell counts, ATP, phospholipid, lipopolysaccharide, ergoslerol contents and viable counts. The method yielded extracts which were enriched with microorganisms as determined by direct microscopic counts, ATP, phospholipid and lipopolysaccharidc contents and residues enriched with the fungal biomarker, ergosterol. The sandy loam soil provided samples of microorganisms that were the least contaminated with soil material. Difficulties in separating either the smaller mineral particles or the less dense organic particles resulted in lower recoveries from the clay and peat soils. Density gradient ccntrifugution of the soil extracts indicated that a large number of the cells counted by direct microscopy in the extracts were associated with relatively dense soil material.

Soil microbial biomass C, N and ninhydrin-N in aerobic and anaerobic soils measured by the fumigation-extraction method

Abstract: The fumigation-extraction method was tested to see if it could be used to measure soil microbial biomass in waterlogged soil. Three Japanese paddy soils were incubated, aerobically or anaerobically (waterlogged), before being fumigated with CHCl3 and extracted by 0.5 m K2SO4. CHCl3-fumigation caused large increases in K2SO4-extractable C, N and ninhydrin-N in the aerobic soils, as observed previously. Fumigation caused smaller, but comparable, increases in waterlogged soils. The biomass, measured as K2SO4-extractablc C released by CHCl3. declined only slowly, if at all, during an 80 day aerobic incubation. However, under waterlogged conditions K2SO4-extractable C released by CHCl3 declined significantly: in 40 days by 11. 22 and 26% in the three soils. Aeration of waterlogged soils for up to 1 h before extraction did not change the amount of K2SO4-extractable C released by CHCl3. This suggests that O2 does not need to be rigorously excluded during the sampling or analysis of waterlogged soils. Microbial biomass C (BC) in aerobic soils can provisionally be calculated from the relationship BC = 2.64 EC. where EC is the organic C extracted by 0.5 m K2SO4 from fumigated soil, less than extracted from the non-fumigated control.

Microbial C, N and P in dry tropical forest soils: Effects of alternate land-uses and nutrient flux

Abstract: The effects of alternate land-uses (savanna, cropfield and mine spoil) on microbial C, N and P in dry tropical forest soil of India were studied. The mean microbial C, N and P, respectively, in the four major systems ranged from 250 to 609 μg C g −1 , 27 to 65 μg N g −1 and 12 to 26 μg P g −1 . The microbial biomass in these systems was characterized by a mean C:N:P ratio of 23:2:1. The microbial C, N and P were positively related to root biomass and total plant biomass (aboveground + root biomass). The derived ecosystems (savanna, cropland and mine spoil) have changed from the original forest ecosystems in terms of soil features and microbial biomass. The conversion of forest into other land-uses resulted in remarkable decline in the amounts of soil nutrients and microbial C, N and P. The microbial nutrients in this dry tropical environment are sensitive to land-use changes. The calculated flux of N and P through the microbial biomass ranged from 27 to 64 kg N ha −1 yr −1 and 13 to 26 kg P ha −1 yr −1 . Thus, in this dry tropical environment the microbial biomass appears to contribute substantially to the N and P requirements of higher plants.

Turnover of carbon through the microbial biomass in soils with different texture

Abstract: Ten soils, with the same type of clay and under the same management but having different clay contents, were amended with 14C-labelled glucose and incubated for 90 days to determine the influence of texture on the turnover of C through the microbial biomass. After 90 days, there was little difference among the soils in residual 14C. However, rates of CO2 evolution and mineralization indicated that during the first day of incubation, the rate of decomposition of substrate C was greater in soils with more clay. From 1 to 5 days the turnover of C through the biomass was relatively slower in these soils. Total biomass C (labelled + unlabelled C) contents reached maximal amounts after 1.25 days and were maintained at higher quantities in soils with greater amounts of clay until the end of the incubation. Biomass 14C was maintained at higher amounts in soils with more clay but only until 45 days. The proportion of C derived from the labelled glucose and incorporated in the biomass ranged from 59 to 73% after 1.25 days and from 16 to 20% after 90 days. The unlabelled biomass was significantly correlated with the amount of clay at all sampling dates and appeared to be qualitatively different than the labelled biomass in the heavy tcxturcd soils. After 1.25 days of incubation, the non-living 14C accounted for 12 to 23% of added to glucose 14C; after 90 days it ranged from 11 to 20%. The ratio of living to non-living 14C was consistently higher in soils with more clay. This was attributed to adsorption of non-living C by clay and to product utilization by a secondary population.

Evidence that chitinase produced by Aeromonas caviae is involved in the biological control of soil-borne plant pathogens by this bacterium.

Abstract: A chitinolytic isolate of Aeromonas caviae was isolated from roots of healthy bean plants grown in soil artificially infested with Sclerotium rolfsii. Under greenhouse conditions, the bacterium controlled Rhizoctonia solani and Fusarium oxysporum f.sp. vasinfectum in cotton (78 and 57% disease reduction, respectively) and S. rolfsii in beans (60% disease reduction). Seed coating was the most effective application method for controlling R. solani in cotton. There was no evidence of inhibition of the fungal pathogens by A. caviae. A caviae partially lysed live mycelium of R. solani, S. rolfsii and F. oxysporum f.sp. vasinfectum when their mycelium served as a sole carbon source in liquid medium. A high chitinolytic activity was found when colloidal chitin was used as a sole carbon source, with an optimum pH between 6.0–7.0. No β-3-glucanase was produced by the bacterium. After partial purification of the enzyme by affinity adsorption to colloidal chitin, three bands appear in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE). One strong band with a molecular weight of ca. 80 kDa, and two weak bands with molecular weights of 48 and 59 kDa. Using the chromogenic substrate pNp-chitobiose, the partially purified chitinase from A. caviae was shown to act in an exo-splitting manner.

Is the dehydrogenase assay invalid as a method to estimate microbial activity in copper-contaminated soils?

Abstract: Soil dehydrogenase activity is commonly estimated from the conversion of triphenyltetrazolium chloride (TTC) to triphenylformazan (TPF). There are many reports of larger proportional decreases in dehydrogenase activity than in other indices of microbial activity in Cu-contaminated soils or in soils recently amended with Cu-contaminated sewage sludge. We describe four experiments which measured the effects of Cu on the dehydrogenase assay. In the first, soil microbial biomass, soil dehydrogenase activity and biomass specific dehydrogenase activity were compared in a soil contaminated with heavy metals, including Cu, and in an uncontaminated soil. The specific dehydrogenase activity of the biomass was about 35% less in the metal-contaminated soil than in the uncontaminated soil. In contrast, previous work showed that the biomass specific respiration rate was about 1.5 times faster in the metal-contaminated soil. In the second experiment, sewage sludges (uncontaminated, contaminated singly with Cu, Ni, Cd or Zn and a sludge containing the metals in combination) were added separately to an uncontaminated soil and incubalcd for 7 days at 25°C. Large, similar increases in biomass C, CO2 evolution and biomass specific respiration were measured within all sludge-amended soils at 1 and 7 days after sludge addition. Soil dehydrogenase activity increased similarly in the soils amended with sludges which contained no added Cu. Thus the specific dehydrogenase activity of the biomass was very similar in all these treatments. In contrast, soil dehydrogenase activity and specific dehydrogenase activity of the biomass in the soils amended with Cu-rich sludge or with the sludge containing all the metals, including Cu, were 2–3 times smaller than in the other treatments. In the third experiment, TPF was incubated with separate solutions (0–100 mg l−1) of heavy metals in the absence of soil. Absorbance of TPF was virtually unaffected at any concentration of Ni, Cd or Zn but declined almost to zero between 0 and 20 mg Cu l−1. If this abiological reaction occurred in soil it would be reported, incorrectly, as decreased dehydrogenase activity. This was tested in the fourth experiment by adding TPF to soils previously incubated with the various metal-rich sludges. Absorbance due to TPF was significantly decreased in the soils containing Cu-rich sludges, but was unaffected in the other treatments. It thus appears that the main reason why soils contaminated with Cu apparently have significantly decreased dehydrogenase activities is the abiological reaction between TPF and Cu. Previous research on the effects of Cu on soil dehydrogenase activity has not considered this phenomenon and much of it, therefore, is probably invalid.

Organic matter and microbial biomass in a vertisol after 20 yr of zero-tillage

Abstract: The effects of 20 yr of tillage practice, crop residue management and fertiliser (urea) N application on organic C, total N. microbial biomass anaerobic mineralisable N and pH at 0–25, 25–50 and 50–100 mm depths of a fine-textured (65% clay) vertisol were studied. The treatments, in a factorial combination, comprised of tillage (conventional tillage. CT vs zero-tillage, ZT). residue (retained, RR or burned, RB) and urea (0, 23 and 69 kg N ha −1 yr −1 ) applied at 40–50 mm depth. Wheat and barley were grown for 15 and 3yr, respectively. All soil properties showed a strong stratification with depth under ZT, RR treatments. Organic C, total N and microbial biomass N were highest and pH lowest in the 0–25 mm layer under ZT, RR and 69 kg N ha −1 yr −1 . In the 0–100 mm layer, similar trends were observed where residue was retained or fertiliser was applied but tillage had no effect on organic C and total N although higher microbial biomass was measured in soil under CT than ZT. Therefore, zero tillage, residue retention and fertiliser application results in stratification of soil properties, even in a verlisol.

Influence of fertilizer and straw baling on soil organic matter in a thin black chernozem in western Canada

Abstract: Heavy crop residues usually present disposal problems for producers. A 30 yr study carried out on a thin Black Chernozem at Indian Head. Saskatchewan, Canada was used to study how fertilization and straw removal influenced soil organic matter of a fallow-wheat-wheat (Triticum aestivum L.) rotation. When straw was incorporated, fertilizer increased soil organic N (P < 0.01) and tended to increase soil organic C (P < 0.14) in the lop 15cm. When straw was removed there was a tendency for soil organic N to be depleted (5.7% decrease, significant at P < 0.20); but surprisingly, soil organic C was not affected. C and N balance sheets, developed using plausible assumptions, were used to estimate what should have happened to soil organic matter. The estimates compared favourably with measurements, except for C lost when straw was removed. We estimated that there should have been a 9% loss in organic C in the latter case. compared to when straw was not removed. We hypothesized that. in situ, C from roots contribute more to maintenance of soil organic matter than docs straw.

Volatile loss of nitrogen during decomposition of legume green manure

Abstract: Significant amounts of volatile ammonia (NH 3 ) may be lost from agricultural ecosystems While NH 3 volatilization from fertilizers has been well-documented, corresponding losses from crop residues, particularly legume green manures, have not been adequately quantified Ammonia losses from decomposing lentil ( Lens culinaris Medik) green manure were measured under controlled conditions by applying residue to soil inside sealed chambers, establishing air flow and periodically measuring accumulated NH 3 loss using acid traps Three consecutive experiments were conducted to determine the effect of residue placement, air flow rate and green manure composition, respectively The first experiment, using a relatively slow flow rate (007 chamber displacements min −1 ), demonstrated significant volatilization of NH 3 (5% of applied N after 56 days) from green manure placed on or suspended above the soil Incorporating the green manure into soil almost eliminated NH 3 losses Drying and rewetting the residues after the initial 28 days had only a small stimulatory effect on subsequent volatile losses A second experiment indicated that maximum volatilization could be achieved at air flow rates of 03 chamber displacements min −1 or higher A third experiment, using an optimum flow rate (05 displacements min −1 ), demonstrated significantly higher volatile N losses from field-grown lentil material (14% over 14 days) than from hydroponically cultured lentil material (8% over 14 days) This difference was attributed, in part, to higher soluble N concentrations in the former residue Ammonia volatilization consistently demonstrated similar temporal patterns: a rapid initial flush, apparently from the ammonification of labile N followed by an indefinite period of slow volatilization, probably from the mineralization of more recalcitrant N fractions The volatile loss of labile N from decomposing green manure may appreciably diminish its fertility benefit and represent an important contribution to atmospheric N concentrations

Relationships between fungal and bacterial substrate-induced respiration, biomass and plant residue decomposition

Abstract: Residues of six plant species were incubated in the field and analyzed for decomposition rates, fungal, bacterial and total substrate-induced respiration (SIR), total fungal and bacterial biomass and changes in residue composition during 161 days. Plant residues included crimson clover ( Trifolium incarnatum L.), hairy vetch ( Vicia villosa Roth), crabgrass [ Digitaria sanguinalis , (L.) Scop.], winter rye ( Secale cereale L.), grain sorghum ( Sorghum bicolor L. Moench) and chestnut oak ( Quercus prinus L.) leaves. Plant residues were incubated in litterbags placed on the soil surface in no-tillage ( T. incarnatum, V. villosa, S. bicolor, S. cereale ), old-field ( D. sanguinalis ) or hardwood forest ( Q. prinus ) plots at the Horseshoe Bend Experimental Area in Athens, Ga and collected periodically for analyses. Decomposition rate constants ( k ) were greatest for V. villosa followed by T. incarnatum, D. sanguinalis, S. cereale, S. bicolor and Q. prinus . Net N loss generally followed the pattern of dry matter loss. Net N gain was observed after 100 days of decay for those residues with high initial C:N ratios. Initial N concentration was exponentially related with the annual decay rale constant ( r 2 = 0.93) for all species; however, on an individual species basis, lignin content was best correlated to dry matter weight loss. Total SIR was greatest on T. incarnatum and V. villosa followed by D. sanguinalis, S. bicolor, S. cereale and Q. prinus . Across all sample dates, residue carbon-to-nitrogen ratio was the best predictor of total SIR. Measurements of potential fungal and bacterial activity by SIR as well as biomass-C estimates by direct counts indicated that fungi were the dominant decomposers of these surface residues. For most residues, lignin content through time exerted the greatest influence on fungal SIR and fungal biomass-C ( r = −0.56 to −0.93). Total SIR, fungal SIR and total fungal biomass tended to decrease through time as residues decomposed. Total SIR, on any given sample date, was significantly correlated with residue dry weight remaining and annual decay rate constants were exponentially related to overall mean values of total SIR for all residues excluding S. cereale ( r 2 = 0.99). Residue SIR rates as a measure of the potentially active microbial biomass reflected the resource qualities of the plant residues investigated here and were positively correlated to their decomposition rates.

Earthworms as vectors of viable propagules of mycorrhizal fungi

Abstract: Casts of 13 earthworm species with differing ecological strategies were collected from more than 60 sites and examined for the presence of propagules of vesicular arbuscular mycorrhizal (VAM) fungi. Intact spores of VAM fungi were found in all but one collection. VAM root fragments were also present in some cast samples. The diversity of VAM spore types found in casts was similar to that in adjacent, uningested soils. However, the relative concentration of spores (No. g−1 dry wt) in casts was higher than in the general soil for the two earthworm species (Pontoscolex corethrurus and Diplotrema heteropora) for which this was assessed. There was a strong seasonality in the number of spores recovered from casts of D heteropora and this reflected seasonal patterns of spore production in soil. Glasshouse experiments demonstrated that spores recovered from casts of P. corethrurus and D. heternpora maintained viability and initiated mycorrhizal infection on Sorghum bicolor. Similarly, some VAM root fragments found in casts were also able to infect S. bicolor. Spores of ectomycorrhizal fungi and fragments of ectomycorrhizal roots were observed at low frequencies in some cast samples from sites dominated by sclerophyllous vegetation. The infectivity of these propagules was not determined. The ubiquity and ecological significance of short distance transport of VAM propagules by earthworms is discussed in relation to the ecological strategics of earthworms.

Influence of fragmentation and bioturbation on the decomposition of 14C-labelled beech leaf litter

Abstract: The effects of mechanical fragmentation (< 1 mm), the formation of faecal pellets by the millipede Giomeris marginata (Villers) and of bioturbation by two earthworm species [Lumbricus castaneus (Savigny) and Octulasion lacteum (Orley)] on the decomposition of 14C-labelled beech leaf litter (4 month old) and total CO2-C production of soil and litter was studied over a period of 168 days. Three phases of the effect of faecal pellet formation by G. marginata on C mineralization of the litter could be distinguished: during the first 14 days C mineralization rates were reduced (up to 0.42 of control); from day 15 to day 77 litter decomposition was increased (up to 1.72 of control); during the last 70 days C mineralization rates were reduced again (up to 0.82 of control). The results are discussed in relation to the effect of the gut passage of the litter on litter components. L. castaneus and O. lacteum fed only on fragmented beech leaf litter. Cumulative C mineralization of intact beech leaf litter was not affected by the presence of either earthworm species. Ingestion of Gtomeris faecal pellets and incorporation in the soil by L. custaneus and O. lacteum caused a strong increase in C mineralization of the pellets during the first 14 days. After this initial phase the effect depended on earthworm species. The effect of the litter-dwelling earthworm species L. castaneus was restricted to the initial flush in C mineralization, whereas ingestion and incorporation of faecal pellets in the soil by the soil-dwelling earthworm species O. lacteum caused a significant reduction in cumulative C mineralization (-39%). Mean C mineralization rates of faecal pellets of G. marginata and of mechanically-fragmented litter ingested by O. lacteum were almost identical to the mean rates of CO2-C production by the systems. The stabilization of organic matter in the mineral soil horizon caused by the burrowing activity of soil-dwelling earthworms is considered lo be a key process for the formation of woodland mull soils

Relationships between denitrification, microbial biomass and indigenous soil properties

Abstract: The relationships between denitrification, soil microbial biomass and selected soil properties were investigated in 13 soils which varied in physical and chemical properties. It was hypothesized that estimates of biomass C may be directly related to the background (unamended) denitrification rates. Denitrification potential (with C and NO−3 amendments) was also compared to biomass C and the indigenous soil properties. The background and potential denitrification rates were measured in these soils during 75 h of anacrobiosis. Background denitrification was highly correlated with biomass C. organic C content and moisture content at field capacity (−33 kPa). Soil organic C was also highly correlated with microbial biomass C. Nitrate was not limiting for most soils as in all but two soils the residual NO−3 content following incubation exceeded 10mg N kg−1. Hence. NO−3 concentration was not strongly correlated with the background denitrification rate. The denitrification potential was 5 times greater than background denitrification. In our study, C was the factor limiting denitrification. Further, microbial biomass appeared to be a sensitive indicator of both soil C content and background denitrification. There was no significant relationship between any of the other soil chemical and physical properties measured and the denitrification potential.

Microbial activity measured in soils stored under different temperature and humidity conditions

Abstract: Long-term experiments (1, 2 and 20 months) were made to measure the effect of storage on soil microbial activities. Five soils differing in texture and with different C-content: sandy 1.1%, loamy 1.4%, forest mineral layers 5.1%, peaty 11.9% and forest organic layers 34.1% were stored moist at 4, −18 or −140°C or air-dry at 21°C. Indicators for microbiul activity included, ATP, heat output and fluorescein diacctate hydrolysis (FDA), showed different effects. The ATP-content in most cases decreased markedly, except in peaty soil and in forest organic layers stored for 20 months at −18, −140 or 21°C. The heat output was enhanced extremely in peaty soil (except at 4°C) and slightly in forest organic layers stored at 21 or −140°C for 20 months. In the other soils storage caused a marked reduction of heat output. FDA-hydrolysis did not change during 2 months in most cases, except in peaty soil, forest mineral layers (−140 and 21°C) and forest organic layers (21°C) where it increased significantly during storage. In most cases, air drying caused a significant decrease in the quantity of these indicators of microbial activity. The ditferences between the effects on samples stored at 4, −18 or −140°C were slight and in most cases insignificant. The adenylate energy charge (AEC) measured after 20 months of storage was markedly lower in samples of sandy soil and soils derived from forest stored air-dry, while in loamy soil and in peaty soil the results were less clear. The highest AEC values were obtained in loamy soil and forest organic layers stored at −18°C. The signature indicators muramie acid, glucosamine and crgosterol increased after 20 months storage except for crgosterol in sandy soil, loamy soil, forest mineral layers and forest organic layers (21°C), where it decreased rapidly.

Microbial carbon turnover in beech forest soils at different stages of acidification.

Abstract: Microbial carbon turnover was investigated in six beech forest soils (0–10 cm). The soils were sampled in the Gottinger Wald area (Germany) and had developed under identical environments (climate, topography, vegetation) but from different parent materials. The pH-H2O of the soils was in the range between 4.8 and 8.3. The content of soil organic C was positively related to factors indicating soil acidification and decreased from 96.6 mg C g−1 dry wt in the soil with the highest amount of exchangeable Ca to 31.3mg C g−1 dry wt in the soil with the lowest amount of exchangeable Ca. The size of the microbial C pool varied between 0.70 and 1.52 mg C g−1 soil, the metabolic quotient qCO2 varied between 1.4 and 2.4 mg CO2-C g−1 biomass C h−1. Exchangeable Ca was positively correlated to the biomass of the micronora and to the microbial incorporation of C from freshly fallen litter. In contrast, exchangeable Ca was negatively correlated to the Cbiomass:Corg ratio, to C mineralization rate, to qCO2 and to the specific death rate (qD). It was concluded that the small but metabolically-active microflora in the more acid soils has a reduced ability to incorporate C from freshly fallen litter into the subsurface C cycle and may thus partly be responsible for the low content of organic C in these soils.

Mycelial responses of the soil fungus, Mortierella isabellina, to grazing by Onychiurus armatus (Collembola)

Abstract: Hyphal morphology and extracellular enzyme production of the fungus Mortierella isabellina were studied during grazing by a soil Collembola, Onychiurus armatus . Grazing induced switching from a “normal” hyphal mode, with appressed growth and sporulating hyphae, to fan shaped sectors of fast growing and nonsporulating mycelium which developed extensive aerial mycelium. Specific protease and α-amylase activities were several times higher in grazed cultures where switching occurred compared with plates without switching. Increased protease activity was localized to switched sectors within the mycelium. On the basis of inhibitor studies, the protease of “normal” mycelium was classified as a metalloprotease. Switching induced activity of an additional serine protease. Anion-exchange chromatography showed that the two proteases had similar charges and, according to SDS-gclatine-PAGE. the molecular weight of the serine protease was 80.000. Switching to a fast growing hyphal mode could be one explanation for compensatory growth of grazed fungi.

Nitrogen transfer from Phaseolus bean to intercropped maize measured using 15N-enrichment and 15N-isotope dilution methods

Abstract: Transfer of N from Phaseolus bean to intercropped maize was studied in glasshouse experiments using 15 N-foliar feeding and 15 N-isotope dilution methods. Nodulated and non-nodulating Phaseolus genotypes were included in separate treatments to help distinguish between benefits due to transfer of fixed N and competition for N in the growth medium. When intercropped with bean (bliarly fed with ( 15 NH 4 ) 2 S0 4 , maize was enriched with 15 N, showing that N had been transferred. The amounts of N transferred were small, and always 2 -fixing beans. There was a decrease in shoot-N in maize intercropped with N 2 -fixing bean compared to maize intercropped with the non-nodulating beans. Non-nodulating bean transferred comparable amounts of N to intercropped maize plants although their total N content was less than a quarter of that in the N 2 -fixing beans. For the isotope dilution experiments, 15 N-fertilizcr was incorporated into a soil-based compost together with sucrose to stabilise the 15 N-cnrichmcnt of available N. When plants grew vigorously no transfer of N from bean to maize was detected by isotope dilution, and again shoot N of maize intercropped with N 2 -fixing beans was less than that of maize with non-nodulating beans. In a further experiment, growth of maize and bean plants was reduced by severe insect attack and up to 15% (between 9 and 15mgN pot −1 ) of the N in N 2 -fixing beans was estimated by isotope dilution to have been transferred. Small (5–10 mg N pot −1 ) but significant increases in total N yield were found in the maize intercropped with N 2 -fixing bean compared to maize intercropped with non-nodulaling bean. In this experiment treatments with or without vesicular -arbuscular mycorrhiza were established but showed no significant differences in N-transfer from uninoculatcd plants. As transfer of N from the beans to intercropped cereals showed such little benefit under conditions of severe N limilation. our results indicate that many careful field experiments arc required before we can conclude that N-transfcr from Phaseolus to intercropped cereals is significant in agriculture.

Hydrolysis of fluorescein diacetate in sphagnum peat container media for predicting suppressiveness to damping-off caused by Pythium ultimum

Abstract: Microbial activity, based on the rate of hydrolysis of fluorescein diacetate (PDA), accurately predicted supprcssivcness to Pythium damping-off of cucumber in media containing light-colored. relatively nondecomposed Canadian sphagnum peat. Suppressiveness was directly related to the quantity of FDA hydrolysed. Organic matter in container media adsorbed fluorescein. Adsorption was directly related to peat decomposition level. In media prepared with dark, decomposed sphagnum peats, that are H3-H4 or higher on the von Post scale, adsorption of fluorescein was strong enough to interfere with the assay. Since such decomposed peats were consistently conducive, the procedure did not yield false readings even here.

Spatial variability of phosphatase, urease, protease, organic carbon and total nitrogen in soil

Abstract: The spatial variability or urcase, phosphatase, casein-hydrolysing activities, organic C and total N was evaluated in a 5 yr old grass-legume association. Twenty four soil samples from the superficial layer (0–20 cm) were collected from a 15 x 40 m meadow. Samples were air-dried, sievedand then stored at room temperature for I yr before being analysed. The enzymatic activities found after this time period are considered to be mainly due to enzymes protected and stabilized by their associaton or entrapment with clay minerals and humic molecules. Four different procedures of statistical analysis were carried out: Analysis of Variance, statistical comparison of Coefficients of Variation. Principal Components Analysis and Proximities Analysis. With euch procedure, urcase activity was the most variable parameter and total N the least variable parameter. Phosphatase and casein-hydrolysing activities showed a similar variability. Organic C was the least variable property when a comparison among the coefficients of variation of the means was carried out. hut it was the most variable property when using the analysis of variance test. These contrasting results can be explained by the fact that organic C analytical variability is so low as to make its effective spatial variability very important. Phosphalase was correlated with protcasc and total N (P Newman Keuls multiple range test and Principal Components analysis gave complex patterns of between sample differences.

Geovariability and biodegradability of the water-extractable organic material in an agricultural soil

Abstract: An analysis of the quantity, biodegradability. three-dimensional distribution and u.v. absorption at 254 nm of the water-extractable organic carbon (WEOC) from an agricultural loess soil was made. In addition, the total organic carbon (TOC) in the samples was quantified. The following results were obtained. The average concentration (9.3μg g−1 DW) of the WEOC was fairly constant to a depth of 50cm. which was approximately the boundary between the B and C horizons. Below 50cm it again remained constant at significantly lower amounts (6.5 μg g−1 DW) down to 100cm. The WEOC consisted of two fractions: one that was not biodegradable with an average concentration of 3.9μg g−1 DW and one that was 85% degradable under the conditions used. This was independent of depth. The WEOC in the Ap horizon absorbed more u.v. radiation than the WEOC from the deeper horizons. This was especially the case for the refractory material. An analysis of the data with semivariograms indicated that the WEOC concentrations in the upper 50 cm were random while the ones below this depth were isotropic, being dependent on the direction of tillage. This may have been the result of preferential flow, resulting from root channels. Another possibility is organic solute enrichment below the crop rows due to the water movement into the plants. TOC decreased sharply below 25 cm depth and tended to decline further with depth. The amount of the WEOC relative to TOC increased with depth. There was no tight statistical covariation between WEOC and TOC.

Wheat yield depression associated with conservation tillage caused by root pathogens in the soil not phytotoxins from the straw

Abstract: Wheat planted directly into soil mulched with straw of a previous wheat crop (mulch or conservation tillage) typically grows and yields poorly relative to that planted into a prepared seedbed with straw residue burned or buried (clean tillage). This injurious effect associated with straw mulches has been greatest in the higher-rainfall wheat-growing areas, or in wet years in normally dry areas. Researchers have focused for the past 30 yr on putative phytotoxins thought to be liberated during microbial colonization or breakdown of the straw on or near the soil surface when wet. The results of experiments reported herein indicate that the causal microorganisms are in the soil and not the straw as would be required if phytotoxic straw decomposition products were important. The injury in these experiments resulted from at least three root diseases, all favored by the lack of crop rotation. The three root diseases were take-all caused by Gueumannomyces graminis var. trici, Rhizoctonia root rot caused mainly by Rhizoctonia solani AG8, and Pythium root rot caused by several Pythium spp. The effect of straw on, or mulched into, the soil surface possibly amounts to no more than helping to keep the top 10–15 cm of soil, the zone occupied by the root pathogens, more ideally moist for their activity. The results suggest that conservation tillage is feasible for wheat in the higher rainfall areas when used in combination with a break from wheat.

On the relationship between specific respiration activity and microbial biomass in soils

Abstract: We demonstrate a steep negative hyperbolic relationship between the specific respiration activity of soil microorganisms (expressed as production or CO 2 -C per unit microbial biomass and unit time) and their microbial biomass. Activity drops by two orders of magnitude when microbial biomass increases from very low values of 7μg Cg −1 dry soil to about 0.4 mg and then remains nearly constant irrespective of biomass. The same relationship was found to be valid for original data from fallow, field, meadow and forest soils at a locality in Southern Bohemia and also a set of “world” data collected from the literature. The existence of a similar relationship in aquatic bacteria as well as in laboratory cultures of fungi and bacteria is pointed out. The statistical significance of the relationship is evaluated from the “spurious” correlations point of view. Four hypotheses for the possible cause of this dependence are outlined. Inhibition of the specific respiration activity by the CO, produced metabolically seems probable.