Decaying Plant Residues Research Articles

Pathogenicity of Trichoderma afroharzianum in Cereal Crops.

Species of the genus Trichoderma occur ubiquitously in soils, on plant roots and in decaying plant residues. Due to its competitiveness and mycoparasitic potential against other microorganisms, particular strains of Trichoderma spp. are used in agriculture as biocontrol agents against plant pathogens. However, Trichoderma afroharzianum has been recently reported as a pathogen causing ear rot disease on maize in Germany, France and Italy, leading to massive infections on maize cobs. This raised the question, whether and to what extent Trichoderma spp. can infect cereal crops other than maize and cause disease symptoms and yield losses. To address this question, two varieties of wheat, barley and sorghum were grown in the greenhouse and artificially inoculated with T. afroharzianum by both spray and point inoculation at the time of flowering. Disease severity was scored weekly, and thousand-kernel weight and colonization rate were determined after harvest. As early as 14 days after inoculation, the first visual symptoms appeared on wheat and barley as tan or brown discoloration of the base of a floret within the spikelets. After spray inoculation, clear discolorations of the entire ear were seen, while point inoculation only showed symptoms at the injection site and above. No visible symptoms were observed on sorghum millet. The colonization rate on wheat and barley grains was significantly increased compared to the control, while thousand-kernel weights (TKWs) were significantly reduced. No differences in colonization rate and TKW compared to the control were observed in sorghum. This is the first report of Trichoderma afroharzianum infecting wheat and barley, causing disease symptoms and significantly reducing thousand-kernel weights.

Radon-rich waters of the Tulinka aquifers, Novosibirsk, Russia

The first integrated isotope and chemical data on radon-rich waters of the Tulinka aquifers (Novosibirsk, West Siberia) are presented. The Tulinka radon waters are fresh, with neutral to moderately alkaline рН (7.3–7.7), SO4–HCO3 Na–Mg–Ca major-ion chemistry. The amount of total dissolved solids is 720–910 mg/L, Si content 6.41–9.02 mg/L. Eh is within +169.1 – +250.0 mV, corresponding to oxidized conditions. The concentration of dissolved oxygen is 2.86–7.37 mg/L. Radon activity (222Rn) varies from 173 to 276 Bq/L. The concentrations of U, Th, and Ra isotopes are 0.015–0.017 mg/L238U, 9.59∙10−7 to 1.58∙10−5232Th, and up to 4.93∙10−10226Ra; the 232Th/238U ratio is in the range 5.81∙10−5 to 9.42∙10−4. Total α activity is not higher than 891 mBq/L, and β activity is 80 mBq/L. The compositions of stable isotopes are −15.4 to −15.1‰ δ18O, −114.2 to −112.8‰ δD, and −13.9 to −9.9‰ δ13C. The isotope signatures and recharge patterns of the Tulinka waters are insensitive to seasonal effects, which suggests the slow water exchange. Substantial seasonal variations of the content of dissolved inorganic carbon (DIC) are detected. The range of δ13CDIC in the analyzed samples (from −14.3 to −9.0‰) corresponds to a mixed CO2 origin: diffusion of soil CO2 approaching the atmospheric δ13CCO2 values, and biogenic CO2 released by decaying plant residues. The measured isotope characteristics provide evidence that the waters recharge by infiltration from meteoric sources. The approximate radiocarbon age of DIC in the Tulinka waters, determined for the first time, is 2663 ± 144 years. However, this age may be underestimated due to groundwater mixing with the present-day surface waters.

Microflora for improving the Auricularia auricula spent mushroom substrate for Protaetia brevitarsis production

SummaryMushroom cultivation is a sustainable agricultural waste utilization method, but the lack of high-value utilization of the produced spent mushroom substrate (SMS) has hindered the development of mushroom cultivation-based circular agricultural systems. Conversion and utilization of SMS via Protaetia brevitarsis larvae (PBL) have proven to be a high-value AASMS utilization strategy. However, Auricularia auricula SMS (AASMS), which contains woodchips, is less palatable and digestible for PBL. To solve this problem, in this investigation, we screened out microflora (MF) for AASMS fermentation by comparing the fermentation performance as well as the effect on PBL feed intake, weight gain, and AASMS phytotoxic compound removal efficiency. In addition, by bacterial community analysis, the genera Luteimonas, Moheibacter, and Pseudoxanthomonas were predicted to be functional bacteria for AASMS fermentation and contribute to palatability and digestibility improvement.

Visualizing the transfer of organic matter from decaying plant residues to soil mineral surfaces controlled by microorganisms

The interface between decaying plant residues and soil minerals represents an essential soil microenvironment at which soil organic matter forms. The high amount of microbial products and residues within this hot spot of microbial activity fosters the formation of mineral-associated organic matter. Besides classical quantitative analyses, our understanding of processes controlling soil organic matter formation greatly benefits from microscopic observations and measurements, which provide spatially resolved information at a meaningful scale for microbial processes and for the association between organic and mineral particles. We studied carbon and nitrogen transfer from fresh-plant residues to the mineral soil, through a litter decomposition experiment in an artificial soil mixture. Needles of Norway spruce (Picea abies L.) were placed in microbatch containers filled with an artificial soil mixture free of soil organic matter. Containers were buried in fresh organic layer material from a Norway spruce stand and incubated for 14 and 42 days. We applied nanoscale secondary ion mass spectroscopy (NanoSIMS) to investigate the spatial distribution of mineral and organic compounds at the needle vicinity and into the mineral soil (0–550 μm from the needle). After 14 days, we depicted the formation of mineral-associated organic matter in the surrounding of the decaying needles. After 42 days, we observed substantial colonization of the needles and the detritusphere by saprotrophic fungi. The fungal hyphae extended into the mineral matrix of the artificial soil acting as vectors for the transfer of litter-derived carbon and nitrogen into the bulk soil. This resulted in an increase of the area covered by organic matter in the detritusphere, with up to 10% of the total investigated area classified as organic matter closely associated with mineral surfaces. Our results provide evidence that the carbon and nitrogen derived from litter decomposition transformed by microorganisms is transferred as mineral-associated organic matter, heterogeneously distributed from the litter source, and still detected 550 μm away from the latter. The close association of newly formed soil organic matter and fine sized minerals suggests that the formation of mineral-associated OM and likely also microaggregates is directly driven by microbial activity in the vicinity of hot spots for plant carbon input (e.g. the detritusphere).

Trichoderma Afroharzianum Ear Rot–A New Disease on Maize in Europe

Trichoderma species are widespread filamentous fungi in soils, on plant roots and decaying plant residues. Due to their strong competitiveness and mycoparasitic activity against other fungi, particular strains of Trichoderma sp. are used in agriculture as biocontrol agents against plant pathogens. Commercial products based on strains of T. harzianum or T. afroharzianum have been applied to control Rhizoctonia spp., Fusarium spp. and Phytophthora spp. in various crops. In 2018, however, severe infections of Trichoderma on maize ears were recorded for the first time in a field in Southern Germany. Infected maize cobs were sampled, the fungus was isolated in pure culture and preliminary identified microscopically as Trichoderma harzianum. After silk channel inoculation in the greenhouse, ear rot disease of high severity was observed. In addition to fungal colonization, the dry matter content in cobs was significantly reduced compared to water inoculated cobs. In 2018 and 2019, a total of 13 T. harzianum isolates from maize cobs and maize stalks were isolated and tested, for pathogenicity on maize plants in the greenhouse, compared to several reference isolates. Four isolates proved to be highly aggressive, two biocontrol isolates, Trichodex (T39) and strain T12, induced slight infection and eleven isolates were non-pathogenic. After sequencing the translation elongation factor 1α (tef-1α), the four highly aggressive isolates were assigned to T. afroharzianum, while the commercial biocontrol isolates Trichodex (T39) and T12, as well as the other non-pathogenic strains belonged to T. harzianum, T. atroviride or T. tomentosum. This, to our knowledge, is the first report on Trichoderma sp. as a pathogen causing ear rot disease in maize in Europe with the potential to incite significant yield losses. We therefore propose to name this disease as ‚Trichoderma ear rot on maize‘.

Priming effects induced by glucose and decaying plant residues on SOM decomposition: A three-source 13C/14C partitioning study

Decomposition of soil organic matter (SOM) may either increase or decrease after fresh organic inputs, the phenomena which are termed as "priming effect". Crop residues and labile C additions can prime SOM decomposition, but it is not known how labile C inputs affect SOM in the presence of decaying plant residues varying in quality (e.g. from previous crops, a common situation in arable soils). We used a dual 13C/14C isotopic labelling to partition soil CO2 efflux and microbial biomass for three C sources: labile C (glucose), partly decomposed wheat residues (leaves and roots) and SOM. 14C-labelled glucose was added to the soil after 30 days of pre-incubation with 13C-labelled residues (separately leaves or roots). After glucose addition, the leaf residue decomposition rate declined by up to 65%, while roots remained unaffected. Despite the differences between residue decomposition rates, the quantity of primed SOM remained similar between leaf and root residue treatments after the addition of glucose. Glucose alone caused cumulative positive SOM priming of 193 μg C g−1 soil over 90 days, corresponding to 60% of SOM decomposition without addition. Addition of glucose to soil together with partly decomposed plant residues induced up to 45% higher SOM priming than single residues priming effect (∼250 μg C g−1). Remarkably, this priming effect induced by glucose and residues was only due to intensive SOM decomposition during the first 18 days. On the subsequent period (after 18 days of glucose), decline in SOM priming and increase in residue decomposition indicate a shift in microbial activity i.e. from active-to slow-growing microbes. Glucose addition strongly increased the proportion of microbial biomass from SOM but decreased the proportion from residue C, suggesting a preferential use of SOM over plant residues following glucose exhaustion. These results are consistent with the view that labile C inputs induce SOM priming and suggest for the first time, that labile C controls the intensity and decomposition rate of both SOM and decaying plant residues. Concluding, irrespective of the quality of partly decomposed residues, input of labile C (e.g. through rhizodeposition) has overall an additive effect in increasing decomposition of SOM. Such studies of interactions between pools and identification of three C sources were only possible by the application of an innovative dual 13C/14C labelling approach.

Nature and decomposition degree of cover crops influence pesticide sorption: Quantification and modelling

This study quantifies and models the influence of the type and the degree of decomposition of cover crops (CC) on three pesticides sorption: epoxiconazole (EPX), S-metolachlor (SMOC) and glyphosate (GLY). Residues of four cover crop species were incubated for 0, 6, 28 or 56d in controlled conditions. For each incubation time, adsorption of pesticides on CC residues was measured in batch experiments. Additionally, the biochemical and elemental composition (Van Soest fractionation, C:N, 13C NMR spectroscopy) of CC was characterized. Mineralization of CC residues was monitored at all incubation times using CO2 trapping. Results showed that the adsorption of pesticides differed significantly according to (i) the type of molecule, (ii) the type of CC, (iii) the degree of CC decomposition and the interaction CC×decomposition time. EPX and GLY were the most (Kd ranging from 188 to 267Lkg−1) and the least (Kd ranging from 18 to 28Lkg−1) sorbed pesticides respectively. With increasing decomposition of the CC residue, sorption increased by 1.6- to 4.7-fold according to the type of pesticide and cover crop. It was significantly correlated with the net cumulative mineralization (ρ>0.7) and other indicators of biochemical composition such as C:N ratio (ρ<−0.7), the Van Soest neutral detergent soluble fraction (ρ>0.5) and the alkyl/O-alkyl C ratio determined by NMR. An innovative model based on net cumulative mineralization of CC residues is proposed to describe the pesticide sorption and appears to be a promising approach to account for the effects of decaying plant residues on the environmental fate of pesticides.

In vitro responses of Fusarium oxysporum f.sp.niveum to phenolic acids in decaying watermelon tissues

To establish the relationships among wilt disease and pathogens and the chemical environment during the allelochemical interactions of plant–microbe–soil, extracts from decaying watermelon tissues were added to media to investigate the effect of allelochemical decaying watermelon tissues on Fusarium oxysporum f.sp.niveum (FON) in vitro. Twelve types of phenolic acids from various decaying watermelon cultivars (Zaojia 82-24 and Jingxing 3) with different levels of resistance to FON were analysed using HPLC. The results demonstrated that there were fewer types and quantities of chemical components released from decaying susceptible watermelon tissues (Zaojia 84-24) than there were from decaying resistant watermelon tissues (Jingxing 3). More antifungal phenolic acids in decaying resistant watermelon tissues (with a ratio of 29.2:1 of antifungal phenolic acids to fungus-stimulation phenolic acids) were observed than in decaying susceptible tissues (with a ratio of 8.4:1 of antifungal phenolic acids to fungus-stimulation phenolic acids). Decaying resistant watermelon tissue extracts inhibited the growth and conidia germination and formation of FON, while decaying susceptible extracts stimulated growth and conidia germination and formation.Extracts from decaying resistant watermelon tissues stimulated virulence factors considerably less than did those from decaying susceptible tissues. We conclude that the decaying plant extracts from susceptible watermelon cultivars stimulated FON because of their higher content of fungus-stimulation phenolic acids, while the decaying resistant watermelon plants inhibited FON because of their higher content of antifungal phenolic acids. These results imply that phenolic acids containing decaying plant residues in soil would induce or inhibit pathogens in plant–microbe interactions.

Resource, biological community and soil functional stability dynamics at the soil–litter interface

The interface between decaying plant residues and soil is a focus for soil ecological processes because of resources from the residues diffusing into the soil, and microfauna that proliferate in the adjacent soil. Given that the recovery of soil function following disturbance depends on immigration, colonization and establishment of exotic organisms from adjacent un-disturbed habitats, and the availability of bio-available resources, we hypothesized that the soil–litter interface could contribute to soil functional stability. In laboratory pot trials, soil was separated into two parts by a mesh bag with the inner section amended, or not amended, with rice straw; an outer layer of unamended soil, adjacent to the litter (1.5 cm thick, either heated or not), provided a soil–litter interface. This enabled us to examine the dynamics of dissolved organic carbon (DOC), mineral nitrogen, microbial biomass carbon (MBC), nematode assemblages and functional stability during 35 days incubation. Either 1 mm or 5 μm meshes were used, which allowed nematodes to migrate (SR1) or not (SR5) through the mesh to the soil–litter interface; thus also enabling us to evaluate the role of nematodes in soil functional stability. Higher DOC and MBC but lower mineral nitrogen concentrations were found at the soil–litter interface. Heating increased the availability of soil resources such as mineral nitrogen and DOC, but decreased the MBC and total nematode abundance in the soil. The soil–litter interface was characterized by a higher abundance of nematodes, particularly microbivores, regardless of mesh aperture or disturbance. The difference in nematode abundance between SR1 and SR5 indicated that nematode propagation, due to resource diffusion and nematode migration through the mesh, contributed to the changing numbers of microbivorous nematodes depending on incubation time. The soil functional stability was calculated as a relative change in the functioning of short-term barley decomposition. Soil functional resistance, defined as the instantaneous effect of disturbance on decomposition measured on the first day, was highest in the SR5 treatment. However, soil functional resilience, defined as the recovery of soil function over the whole incubation period (35d), was highest in the SR1 treatment, which is most probably attributed to the functioning of microbivorous nematodes. Our results suggest that small-scale spatial heterogeneity, due to organic residue decomposition, can help maintain soil functions following disturbance.

Adsorption and binding of the transgenic plant proteins, human serum albumin, β-glucuronidase, and Cry3Bb1, on montmorillonite and kaolinite: Microbial utilization and enzymatic activity of free and clay-bound proteins

Human serum albumin (HSA), β-glucuronidase (GUS), and the Cry3Bb1 protein from Bacillus thuringiensis subsp. kumamotoensis are expressed by genetically-modified plants. Commercial samples of these proteins adsorbed and bound rapidly on the clay minerals, kaolinite (K) and montmorillonite (M). Adsorption increased as the concentration of protein increased and then reached a plateau. The greatest amount of adsorption and binding occurred with the Cry3Bb1 protein, of which there was no desorption: 6.7±0.21 μg adsorbed and bound μg−1 of M; 2.1±0.39 μg adsorbed and bound μg−1 of K. With GUS, 2.2±0.29 μg adsorbed and 1.7±0.21 μg bound μg−1 of M; 1.5±0.28 μg adsorbed and 1.0±0.03 μg bound μg−1 of K. HSA was adsorbed and bound the least: 1.2±0.04 μg adsorbed and 0.8±0.05 μg bound μg−1 of M; 0.4±0.05 μg adsorbed and 0.4±0.03 μg bound μg−1 of K. However, X-ray diffraction analyses indicated that only HSA intercalated M, and none of the proteins intercalated K, a nonswelling clay. When bound, the proteins were not utilized for growth by mixed cultures of soil microorganisms, whereas the cultures readily utilized the free (i.e., not adsorbed or bound) proteins as sources of carbon and energy. The enzymatic activity of GUS was significantly enhanced when bound on the clay minerals. These results indicated that recombinant proteins expressed by transgenic plants could persist and function in soil after release in root exudates and from decaying plant residues as the result of the protection provided against biodegradation by binding on clay minerals.

Detectability of Coleopteran-specific Cry3Bb1 Protein in Soil and Its Effect on Nontarget Surface and Below-Ground Arthropods

Corn engineered to produce the Cry3Bb1 protein from Bacillus thuringiensis (Bt ) kumamotoensis has provided unprecedented control for corn rootworm (Diabrotica spp.). However, the Bt protein may be released in soil by root exudates or decaying plant residues that may affect soil organisms. Field studies were conducted to determine the abundance of surface and below-ground nontarget arthropods in fields planted with Bt or non-Bt corn for the first year or planted over 3 consecutive yr. Results of these studies showed that there were no significant differences in numbers of surface and below-ground arthropods in soil planted with Bt and non-Bt corn at any of the studied locations. Enzyme-linked immunosorbent assay (ELISA) showed no detectable Cry3Bb1 protein in any of the soil samples collected in a field planted with a Bt corn hybrid and its non-Bt isogenic hybrid for the first year or planted over 3 consecutive yr near Manhattan, KS. However, a small amount of Cry3Bb1 protein (3.38–6.89 ng/g dry soil) was detected in the soil samples collected from an area near plants in a Bt corn field that was planted for the first year near Scandia, KS. These findings indicate that the Cry3Bb1 protein released from root exudates or decaying plant residues does not persist and is rapidly broken down in the soil. The rapid degradation of Cry3Bb1 in soil results in none or trace amounts of protein being detected by ELISA.

Solute recycling by crops and leaching in a drained arable soil

SummaryPreferential flow, as it bypasses the soil matrix, can greatly enhance the leaching of chemicals. When a soil is drained there is the risk that such short‐circuiting results in more or less direct passage of polluting chemicals from the soil to the groundwater. If the groundwater table is shallow the chemicals could be transferred back into the surface soil by hydraulic lift through roots and subsequent release by exudation or from decaying plant residues and again become exposed to leaching by preferential flow, thus strongly enhancing the chance of export via the drains.We investigated the leaching of bromide in a tile‐drained arable field over 2 years of crop rotation. The site was a former wetland, artificially drained a century ago for agriculture. Bromide was applied over 1.6 ha at a dosage of 10 g Br per m2 in August 1995 after the harvest of wheat. During the 2 years 18% of the applied bromide was exported via the drainage system, most of it in preferential flow events and more than half of it in a single winter storm 5 months after the application. Within 7 months 56% of the applied tracer was leached out of the main root zone into the groundwater. Subsequently the tracer re‐emerged in water taken up by sugar beet in the following season. The beet accumulated 50% of the initially applied bromide in their leaves and released it again after harvest when the leaves were left as green manure on the field. Our results show that this recycling of solutes to the topsoil can have an important influence on their leaching as the solutes are thus again exposed to preferential transport into drains in the course of preferential flow events.

Biotic and abiotic processes of nitrogen immobilization in the soil-residue interface

The interface between decaying plant residues and soil is a hotspot for microbial immobilization of soil inorganic N. Recent studies on forest and grassland soils have demonstrated that rapid abiotic immobilization of inorganic N is also induced by the presence of plant residues. We, therefore, examined (1) how N immobilization varies with distance from the soil-residue interface and (2) whether abiotic immobilization occurs in agricultural soils. Spatiotemporal changes of N immobilization in the soil-residue interface were evaluated using a box that enabled soil to be sampled in 2 mm increments from a 4 mm-thick residue compartment (RC). The RC was filled with paddy soil containing ground plant residue (rice bran, rice straw or beech leaves) uniformly at a rate of 50 g dry matter kg −1. Soil in the surrounding compartments contained no residue. After aerobic incubation for 5, 15 and 30 days at 25 °C, soils in each compartment were analyzed. After 5 days, significant depletion of inorganic N occurred throughout a volume of soil extending at least 10 mm from the RC in all residue treatments, suggesting extensive diffusion of inorganic N towards the RC. The depletion within 10 mm of the RC amounted to 5.0, 4.3 and 3.4 mg for rice bran, rice straw and beech leaf treatment, respectively. On the other hand, microbial N had increased significantly in the RC of the rice bran and rice straw treatments (11 mg and 5.5 mg, respectively) and insignificantly in the RC of the beech leaf treatment (0.06 mg). This increase amounted to 221% (rice bran), 129% (rice straw) and 1.7% (beech leaves) of the decrease in inorganic N within 10 mm of each RC. Thereafter the rate of N mineralization exceeded that of immobilization, and inorganic N levels had recovered almost to their original level by 15 days (rice bran) and 30 days (rice straw and beech leaves). These results suggested the predominance of biotic immobilization in soil near rice bran and rice straw and of abiotic immobilization in soil near beech leaves. No significant increase in both microbial and soluble organic N in the vicinity of beech leaves after incubation for 5 days further suggested that the abiotic process was responsible for the transformation of inorganic N into the insoluble organic N.

Mathematical modelling of residue allelopathy: the effects of intrinsic and extrinsic factors

The mechanistic model of residue allelopathy that was constructed before only simulates allelopathic phenomena caused by decaying plant residues and does not include factors controlling expression of such phenomena. As both intrinsic and extrinsic factors are affecting the potential of residue allelopathy, the model has been developed further to include these factors. The concept of a constant for residue-allelopathic potential, functions for effects of temperature, water, and their combination on allelochemical concentrations were added to the model. As a result, the improved residue model is able to examine the responses to these phenomena under a wide variety of conditions. Several sets of published experimental data were compared with prediction from the model, and a good agreement has been achieved. Its significance in helping better understanding of residue allelopathy is discussed.

A substrate-induced respiration (SIR) method for measurement of fungal and bacterial biomass on plant residues

The substrate-induced respiration (SIR) method was modified and adapted to measure fungal, bacterial and total microbial contributions to glucose-induced respiration and the potentially active microbial biomass on decaying plant residues of differing composition. Decomposing residues from natural and agricultural ecosystems were chopped and sieved to include the > 1 mm fraction for routine SIR analyses on a continuous flow-through respiration system. SIR procedures were optimized for sample size (0.5–1.0g dry wt), glucose concentration (80mg g −1), antibiotic concentrations (16mg streptomycin g −1; 80 mg cycloheximide g −1), total solution volume (5 ml), antibiotic preincubation conditions (12 h at 4 C), and total assay time following glucose addition (2–3 h). Analyses of antibiotic selectivities for target populations were made from agar plate culture experiments with mixed residue-microbial populations under in vitro and in situ exposure to antibiotics. The results support those concentrations optimized by SIR and emphasize the importance of independent analysis of antibiotic selectivity. Measures of fungal, bacterial and total SIR (μg CO 2-C g −1 dry residue h −1) were linearly correlated ( P<0.001) with biovolume-derived estimates of total ( r 2 = 0.91) and FDA-active ( r 2 = 0.93) fungal biomass, bacterial biomass ( r 2 = 0.87) and total microbial biomass ( r 2 = 0.91), respectively. Fungal to bacterial SIR ratios (1.6:1 to 2.5:1) emphasize the dominance of fungi in the early stages of plant residue decay (⩽ 3 weeks). Plant residue SIR rates were 38–600 times greater than those of soils. Biomass specific SIR rates from plant residues (72 ng CO 2 -Ch −1 μg −1 biomass-C) were 5– 6 times higher than those reported from soil SIR. These findings suggest both a larger microbial biomass as well as a much higher proportion of physiologically active microorganisms on plant residues as compared to soils.

Approach to the nature of volatile compounds that dominate the ecological niche of basidiomycetous ground fungi in the edaphosphere of grassland

The mycelia of 12 basidiomycetous ground fungi were subjected to conditions which are normally characteristic of edaphosphere soil substrates in the vicinity of a natural grass cover. Excess CO2 as produced by soil substrates and living plants exhibited stimulatory or inhibitory effects on mycelial development, but did not alter the mycelial growth figures beyond the range of 30 to 50%. Allelopathic exhalations of sound ryegrass sods, free from decaying plant residues, appeared to contribute to the reduction of the mycelial dominance factor down to 10%. In 11 of 12 test fungi allelopathic volatiles, released from decaying grass residues, considerably impaired mycelial development, with 50% of the mycelia being completely inhibited. The fungitoxic effect of the decaying grass residues was thereat determined by the microbial populations that had colonized the plant material. At low concentrations, however, the volatile inhibitors showed a slightly stimulatory component. It is concluded that in the vicinity of a natural grass cover the growth of fungal mycelia is moderately influenced by CO2 of soil and plant origin, more by allelopathic exhalations of the intact plant and, to the greatest extent, by allelopathic volatiles produced and/or set free by certain microbial populations on decaying plant residues. Their degree of inhibition changes with the concentration. Among the exhalations investigated, a number of ecologically compatible soil fumigants may be found. Die Myzelien von 12 zu den Basidiomyceten gehörenden bodenbewohnenden Pilzen wurden Bedingungen unterworfen, wie sie normalerweise für die Erdschichten der Edaphosphäre in der Nähe einer natürlichen Grasnarbe typisch sind. Das von Erdsubstraten und lebenden Pflanzen erzeugte CO2 hatte stimulierende wie auch hemmende Wirkungen auf die Myzelentwicklung, es konnte aber die Kennwerte des Myzelwachstums nicht über den Bereich von 30 bis 50% hinausgehend verändern. Allelopathische Exhalationen von gesunden Raigrasrasen in einem Stadium frei von faulenden Pflanzenteilen schienen dazu beizutragen, den Dominanzfaktor des Myzels bis auf 10% zu senken. Bei 11 von 12 Testpilzen beeinträchtigten flüchtige allelopathische Substanzen von verrottenden Grasresten die Myzelentwicklung wesentlich, wobei 50% der Myzelien vollständig gehemmt wurden. Der Grad der Fungitoxizität der verrottenden Grasreste wurde dabei von der Art der mikrobiellen Populationen bestimmt, die das Pflanzenmaterial abbauten. In geringen Konzentrationen zeigten die flüchtigen Hemmstoffe jedoch auch eine stimulierende Komponente. Es wird gefolgert, daß in der Nähe einer natürlichen Grasnarbe das Wachstum der Pilzmyzelien in begrenztem Maß von CO2 aus Boden und Pflanze, stärker von den allelopathischen Exhalationen der intakten Pflanze und im größten Ausmaß von flüchtigen allelopathischen Substanzen aus verrottendem Pflanzenmaterial beeinflußt wird, die von gewissen mikrobiellen Populationen produziert und/oder freigesetzt werden. Der Grad ihrer Hemmwirkung ändert sich mit der Konzentration. Unter den untersuchten Exhalationen mag sich eine Reihe ökologisch kompatibler Bodenentseuchungsmittel befinden.