Nylon Mesh Bags Research Articles

Targeted Dispersal of the Aphid Pathogenic Fungus Erynia neoaphidis by the Aphid Predator Coccinella septempunctata

The potential of adult and larval C. septempunctata to vector the aphid-specific entomopathogenic fungus E. neoaphidis was assessed through a series of laboratory and field experiments. The ability of coccinellids to vector conidia from a colony of E. neoaphidis -infected pea aphids, Acyrthosiphon pisum, to a colony of uninfected A. pisum was demonstrated in a laboratory study. Adult coccinellids which had previously foraged on plants infested with different densities of sporulating cadavers (1, 5, 15, 30 cadavers per plant) initiated infection in a proportion of uninfected pea aphids (4, 0, 2 and 8%, respectively) when subsequently allowed to forage on A. pisum infested bean plants. Further laboratory studies demonstrated that fourth instar larvae and adult coccinellids artificially inoculated with conidia initiated infection in 11 and 13% of an A. pisum population in which they foraged, respectively. Furthermore, a proportion of A. pisum placed on bean plants which had previously been foraged on by inoculated larval and adult coccinellids also died from infection (3 and 10% of A. pisum, respectively). However, although coccinellid adults inoculated with conidia initiated infection in 19% of A. pisum, cereal aphids, S. avenae , exposed to the inoculated coccinellids did not become infected. A further laboratory study demonstrated that infection of A. pisum only occurred if inoculated coccinellids were transferred to A. pisum populations immediately post inoculation. However, a proportion of A. pisum placed on bean plants which had been foraged on by inoculated coccinellids transferred 0, 4 and 24 h post inoculation died from infection (9, 3 and 7%, respectively). A field study further demonstrated the potential of coccinellids to vector E. neoaphidis. Single spring sown field bean plants (Long Hoos Experimental Plots, IACRRothamsted Farm) were enclosed within nylon mesh bags and 25 adult A. pisum were added to each bag with one of the following treatments: no further addition (control), coccinellid adult (control), inoculated coccinellid adult, inoculated A. pisum or sporulating A. pisum cadavers. No aphids died of E. neoaphidis in the control treatments; 5, 16 and 33% of aphids were infected with E. neoaphidis on the other treatments, respectively.

Effect of Soil Solarization and Cover Crops on Populations of Selected Soilborne Plant Pathogens in Western Oregon.

Field experiments were conducted in silty-clay loam in Corvallis, OR during the summers of 1995 and 1996 to study the effects of green manure cover crops (Sudan grass, rape, and barley), soil solarization, soil fumigation, and combinations of those treatments on population densities of soil pathogens Verticillium dahliae, Phytophthora cinnamomi, Pratylenchus penetrans, and Agrobacterium rhizogenes. Nylon mesh bags containing soil infested with V. dahliae and Phytophthora cinnamomiwere buried 5, 10, 20, and 30 cm deep. Soil solarization was performed over a 54- to 59-day period using a 0.6-mil clear polyethylene film. Maximum soil temperatures recorded at depths of 5, 10, 20, and 30 cm were 53, 48, 39, and 34°C in solarized soil, respectively; these temperatures were 8 to 16°C higher than in corresponding nonsolarized plots. Soil samples were collected before, during, and after solarization to quantify pathogen populations at those four depths. Pot or field studies were conducted subsequent to treatments to determine the effects of treatments on susceptible plants. Soil solarization, cover crops plus solarization, or fumigation with metam sodium resulted in a significant decrease (P< 0.05) in density of P. cinnamomi populations at all four depths and reduced (P< 0.05) V. dahliae at 5 and 10 cm. In greenhouse assays of solarized soils, disease severity was reduced (P< 0.05) for Verticillium spp. on eggplant and Phytophthora spp. on snapdragons. Cover crops alone were not effective in reducing P. cinnamomi and V. dahliae populations. Agrobacterium spp. population densities declined within solarized plots and incidence of crown gall on 'Mazzard' cherry rootstock planted in solarized plots was reduced significantly. Population densities of Pratylenchus penetranswere reduced in the upper 30-cm soil profile by solarization.Solarization for an 8-week period during the warmest months of summer could provide an additional management alternative for several important soilborne pathogens in western Oregon.

Patterns in longevity of soil seedbanks in fire-prone communities of south-eastern Australia

Seed burial in nylon mesh bags over a 2-year period was used to examine seed longevity patterns in 12 shrub and two graminoid species in fire-prone habitats around Sydney, south-eastern Australia. Most species released a large fraction of their annual seed-crop in a dormant state and all species showed evidence for some form of persistent seedbank. However, regressions of seed persistence over time were in most cases poor predictors of seed decay (9 of 14 study species). Considerable variation in the degree and pattern of seed longevity was apparent in the study species. Three functional groupings of species are suggested. (1) Seed half-lives in the soil predicted to be greater than 2 years and evidence of imposed secondary dormancy (continuous, Kunzea spp. or seasonal, Grevillea caleyi). Only Kunzea capitata and G. caleyi showed significant seed decay in this group. (2) Seed half-lives in the soil predicted to be greater than 2 years and no evidence of secondary dormancy (nine species). Six species had high seed dormancy at release (only two of which showed significant seed decay). Three species had initial seed dormancy of 40–57%—two (Asterolasia elegans and Zieria involucrata) with significant decay only for the non-dormant seed fraction, and one (Comesperma ericinum) with significant decay of both the dormant and non-dormant seed fractions. (3) Two species (Darwinia biflora and Persoonia pinifolia) showed evidence of very short mean half-lives of seeds in the soil (0.4–1.0 years). The threatened species, D. biflora, had a rapid initial seed decay over 6 months followed by little decay for 18 months, and the half-life of seeds is likely to be a poor predictor of seed longevity. For P. pinifolia, maintenance of a soil seedbank is predicted to be dependent on continual inputs of seeds locally or dispersal of seeds from other sites.

Five year measurements of rock tablet weathering on a forested hillslope in a humid temperate region

In order to elucidate the effects of rock type and geomorphic setting on weathering rates, field experiments using microweight-loss techniques were carried out. Rock tablets for eight kinds of rock types: granite; granodiorite; gabbro; limestone; andesite; rhyolite; crystalline schist; and tuff, with a diameter of 3.5 cm and a thickness of about 1 cm, were enclosed in a nylon mesh bag and placed in the four corners of a soil–bedrock interface on a granodiorite hillslope of central Japan for over 5 years. The results show that: (1) the saturated grus layer is the main locus of solutional denudation; (2) the 18 and 11% weight losses of limestone and tuff under a saturated soil layer were clearly the largest of all the rock types examined; and (3) the weight loss is smaller for granite, gabbro, crystalline schist, and andesite, while it is larger for tuff and rhyolite at every location. These results suggest that weathering rates are influenced by both lithology and environmental conditions.

Decomposition of Medicago sativa Debris Incubated at Different Depths under Mediterranean Climate

Reports on the dynamics of carbon and nitrogen at different depths in soil profiles usually describe a higher mineralization of both at upper horizons. Nevertheless, under Mediterranean climate, upper soil horizons are strongly affected by drought, especially in summer, and may offer pedoclimatic conditions less favorable to microbial activity than deep soil layers. Therefore, decomposition could be slower in the upper soil layers. To test this hypothesis, mixtures of Medicago sativa ground plants and soil material were incubated in the field, at 5, 20, and 40 cm depth, in nylon mesh bags. Mineralization of carbon and nitrogen was studied for two years. At 5 cm, mineralization of both elements was lower, and no differences were found between 20 and 40 cm. After two years of field incubation, the remaining carbon (as a percentage of the initial content) was 27.95 % +/- 0.88 at 5 cm depth, 19.87% +/- 0.77 at 20 cm, and 18.78 % +/- 1.19 at 40 cm. Mineralization of nitrogen exceeded that of carbon. After two years of field incubation, the remaining nitrogen (as a percentage of the initial content) was 17.62% +/- 3.06 at 5 cm depth, 12.17% +/- 0.94 at 20 cm, and 11.26% +/- 0.99 at 40 cm. The biodegradation rate in upper layers was lower for all biochemical fractions (water-soluble compounds, lipids, polysaccharides, lignin). This contrasts with the usual findings on this topic, but is consistent with our previous results with forest litter samples. Mineralization clearly followed double-exponential kinetics, with a labile and a recalcitrant pool of both carbon and nitrogen. The labile pool of carbon accounted for about 50% of the total initial carbon, whereas that of nitrogen accounted for about 60%. No clear effect of depth on the proportion between the labile and the recalcitrant pool was observed, neither for carbon nor for nitrogen. It was not possible to identify both pools with the biochemical fractions, suggesting that these pools should be interpreted in physical terms (unprotected vs. protected ). No direct effect of depth on the global retention of N was detected.

Decomposition of woody legume nodules in two tree/grass associations under contrasting environmental conditions

Turnover of nitrogen-rich root nodules follows the pruning of legume trees, forming a potentially important yet little studied way of N release to the soil. The effects of soil moisture, herbivory by soil mesofauna and microbial decomposition on the disappearance rate of woody legume nodules was studied in two tree/grass forage production associations. Litter bags containing nodules of Erythrina variegata L. (Papilionoideae: Phaseoleae) were incubated for four weeks in grass-covered alleys between Gliricidia sepium (Jacq.) Walp. (Papilionoideae: Robinieae) hedgerows, established on a deep alluvial Oxisol under a humid tropical climate and on a shallow Vertisol under a subhumid tropical climate in Guadeloupe, French Antilles. Soil moisture was regulated by irrigating or covering small plots from natural rainfall. Fine nylon mesh bags were used to study the rate of microbial decomposition, and open-ended perforated cylinders were used to estimate nodule herbivory. The chemical traits, especially the lignin: nitrogen ratio, of E. variegata and G. sepium nodules were similar (lignin: N 1.70 and 1.55, respectively), and suggest that the results are probably also applicable to the G. sepium nodules in the associations. Both soil moisture and decomposing agent (microbes or mesofauna) had a significant effect on the nodule disappearance rate, but soil type did not have any apparent effect. The nodule half-life varied from three to seven days under different treatments. The N release rate from the nodules was high, with N half-life varying from three to five days. Herbivory accounted for ca. 10% of total mass and N loss from nodules during the four-week field incubation period, but its importance increased towards the end of the incubation, especially in Vertisol, after the most easily decomposable part of the nodules had decayed. After pruning, the nodule N is released to soil more rapidly than from mulch.

Influence of arbuscular mycorrhizal fungi on soil structure and aggregate stability of a vertisol

The influence of arbuscular mycorrhizal (AM) fungi on aggregate stability of a semi-arid Indian vertisol was studied in a pot experiment in which Sorghum bicolor (L.) was grown as test plant for 10 weeks. Pasteurized soil inoculated with AM fungi was studied with pasteurized and unpasteurized soils as references. A part of the soil in each pot was placed in nylon mesh bags to separate effects of roots and hyphae. The sorghum plants were planted outside the mesh bags which permitted AM hyphae to enter while excluding roots. Aggregate stability of the soil was determined by wet-sieving and turbidimetric measurements. Development of the AM fungi was quantified as colonized root length and external hyphal length. Soil exposed to growth of roots and hyphae (outside mesh bags) showed aggregates with larger geometric mean diameter (GMD) in pasteurized soil inoculated with AM fungi than in pasteurized uninoculated soil. There was no significant difference in GMD of the inoculated, pasteurized soil and the unpasteurized soil. No significant effects of inoculation or plant growth were found in pasteurized soil exposed to hyphal growth only (inside the mesh bags). However, the unpasteurized soil had significantly higher GMD than the pasteurized soil, irrespective of plants and inoculum. Turbidimetric measurements of soil exposed to roots and hyphae (outside mesh bags) showed the highest aggregate stability for the inoculated pasteurized soil. These results demonstrate that AM fungi contribute to the stabilization of soil aggregates in a vertisol, and that the effect is significant after only one growing season. The effect was associated with both AM hyphae and the stimulation of root growth by AM fungi. The contribution from plant roots and AM hyphae to aggregate stability of different size fractions is discussed.

Estimating Errors Associated with Extracting Phosphorus Using Iron Oxide and Resin Methods

AbstractDissolution of particulate P from adhering soil material may cause an error in estimating bioavailable P with Fe oxide‐impregnated filter paper (FeO) strips and anion exchange resin (AER) bags. This error was assessed with two clayey soil samples and agricultural runoff waters high in suspended clay. Iron oxide strips were shaken either unprotected in soil suspension or enclosed in nylon mesh bags. Resin in bags and loose AER beads were used in the AER extractions. Sodium chloride and HCl extractions were compared in the displacement of P retained by the AER. Aluminum and Si served as index elements for the presence of soil material. Overestimation of desorbable P by 2.5 to 2.7 mg kg−1 for each strip was measured, compared with about 30 mg kg1 of FeO strip‐extractable P in the soils. Enclosing the FeO strips in nylon mesh bags reduced soil attachment by about 60%. The error due to dissolution of particulate P was negligible with the AER method. The FeO‐strip procedure was problematic in the water sample study. The highest P yield occurred when FeO strips were protected with nylon mesh, followed by unprotected FeO strips, AER(HCl), AER(NaCl), and loose AER beads. Overnight shaking with an unprotected FeO strip extracted 23 to 65% more P, and four sequential extractions 140 to 196% more P, than the AER(NaCl) method. In analysis of turbid water samples, the risk of overestimating P bioavailability was much lower with the AER(NaCl) method than with the FeO‐strip method.

Longevity of crenate broomrape (Orobanche crenata) seed under soil and laboratory conditions

The effect of storage conditions on longevity of crenate broomrape seed was determined and then used to predict and simulate new infestations. Newly harvested broomrape seed mixed with soil and placed in nylon mesh bags was buried in soil 8 and 23 cm deep or kept at room temperature in the laboratory for 6 yr. In the field, seed exhibited a seasonal pattern in germination, showing consistent and poor germination from September to January and for the rest of the year, respectively. The two burial depths tested showed no significant differences in seed germination or survival. Seed stored under laboratory conditions germinated throughout the year. The negative exponential model (% germination =Aexp−-B-months of storage) described the time course of germination of crenate broomrape seed. Seed longevity decreased much quicker in the field than in the laboratory. For example, germination capacity decreased 66 and 95% and only 27 and 60% after 33 and 108 mo of soil burial or under laboratory conditions, respectively. According to these findings, the repeatedly reported infestations occurring in plots after many (up to 14) years of not growing broomrape-susceptible crops can not be attributed to the longevity of buried seed.

Winter survival of extraradical hyphae and spores of arbuscular mycorrhizal fungi in the field

The ability of the extraradical mycelium (EM) of arbuscular mycorrhizal (AM) fungi to colonize spring-sown seedlings in temperate regions requires that this EM survive winter conditions. We studied the production and winter survival of such an EM and its associated spores at its site of synthesis in a field soil. Pouch-like nylon mesh bags of sieved and pasteurized soil were buried in a field below where a maize ( Zea mays L.) crop was then grown. A mesh size of 45 μm prevented the entry of roots into the pouches. Extraradical hyphae and spores of AM fungi were formed in these pouches. The density of trypan-blue staining hyphae in the pouches increased from 16.3 to 60.0 m g −1 between planting and maize-harvest time, but it then fell to one-third of this value over the following winter. The density of fluorescein diacetate staining hyphae in the pouches were also highest at maize-harvest time, at 38.1 m g −1. In contrast, the density in soil of newly produced AM spores with cytoplasm was maximal earlier, reaching 28.8 g −1 during the grain-filling period; this density in soil had been halved by the time of maize harvest. At various times before and after winter, pouches were excavated from the field and used for growth-chamber bioassays. The extraradical component of the AM fungi retained over winter its capacity to colonize roots. Arbuscular colonization of sudangrass ( Sorghum sudanense Staph.) bioassay plants at 28 days ranged from 36.5% to 69.3% root length, and colonization of plants in bioassays after the winter was similar to that for bioassays before the winter. Disturbance of pouch soil after excavation reduced the P content of bioassay shoots, relative to pouches with undisturbed soil. We show here that extraradical structures of AM fungi can survive the winter in the soil at their site of synthesis. Reductions in soil densities of spores in autumn, and of hyphae over winter, did not affect the capacity of the extraradical mycorrhizal system to colonize plants swiftly in the spring, even after the elimination of any potential contribution from the part of the mycorrhizae inside the roots. However, disturbance of soil was a potent means of hindering the subsequent development by the plant of a functional mycorrhizal P-acquisition system.

Impact of animal manures on verticillium wilt, potato scab, and soil microbial populations

This study examined the effect of chicken, liquid swine, and solid cattle manures on verticillium wilt and potato scab incidence at two commercial farms near Alliston, Ontario. The manures (chicken, 66 t/ha; swine, 55 hLJha; cattle, 100 t/ha) were spread in the spring of 1996 and incorporated with a Rototiller to a depth of 15 cm. Survival of Verticillium dahliae microsclerotia (MS) was determined by burying nylon mesh bags containing MS in the plots. After 4 weeks, the MS were plated on agaz medium. Soil samples collected after incorporation of manures were brought back to the laboratory and used for concurrent laboratory experiments. Ten-gram amounts of soil were placed into test tubes, bags of MS buried in the soil,and survival measured under controlled conditions. Potatoes were planted in the field 1 month after amendment. Potatoes were planted in 1997 and 1998 without any further treatment. verticillium wilt incidence was determined by isolation of V.dahliae from petioles. Scab incidence was determined from visual estimation of lesion coverage on tubers. Changes in soil chemistry, in soil microbiology, and in populations of plant pathogenic nematodes were also determined. Chicken manure reduced scab and wilt incidence and nematodes to near zero levels at both sites. Swine manure reduced scab incidence at both sites but reduced wilt and nematode populations at one site only. Cattle manure application caused a small reduction in wilt at one site. Disease incidence in the second crop was similaz for all treatments except for swine manure at one site where a lower scab incidence was observed and chicken manure at both sites where wilt incidence remainded below that in untreated plots. Scab incidence in the third crop was reduced at one site treated with swine manure, but one of the chicken manure treated plots had a higher scab incidence than the untreated plot. The viability of MS was similar for identical treatments in the field and laboratory assay and predicted which treatments decreased verticillium wilt. Reductions in viability of MS were found to be manure and soil specific.

Pathology of conifer seed and timing of germination in high-elevation subalpine fir and Engelmann spruce forests of the southern interior of British Columbia

Unstratified seed of Engelmann spruce (Picea engelmannii Parry) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) in nylon mesh bags was placed on various natural and disturbed forest floor seed beds in the Engelmann Spruce - Subalpine Fir Zone in the southern interior of British Columbia in September 1995 and recovered just before snow melt in June 1996. Fifty-two and 86% of the viable spruce and fir seed, respectively, had germinated before snowmelt. Germination under snow may be an adaptation of these high-elevation species to short cool growing seasons. Seed viability at recovery was significantly lower on undisturbed forest floor seed beds (spruce, 13%; fir, 12%) than on exposed mineral soil (spruce, 57%; fir, 42%). Viability of seed placed on nurse logs was 38 and 23% for spruce and fir, respectively. Isolation from ungerminated seed yielded a Rhizoctonia sp., an as yet unidentified black mold at high frequencies, and several other seed pathogens at low frequency. Multiple linear regression of the frequency of isolation of Rhizoctonia and black mold on seed viability was highly significant for both tree species. Seed pathogens appear to cause a major loss of seed and seedlings in these forests, and this may explain both the common occurrence of regeneration on nurse logs and the requirement of mineral soil seed beds for adequate regeneration.

Comparison of two test systems for measuring plant phosphorus uptake via arbuscular mycorrhizal fungi

Plant phosphorus uptake via external hyphae of arbuscular mycorrhizal fungi has been measured using compartmented systems where a hyphal compartment is separated from a rooting compartment by a fine mesh. By labelling the soil within the hyphal compartment with a radioactive phosphorus (P) isotope, hyphal uptake of P into the plant can be traced. The objective of this growth chamber study was to test two hyphal compartments of different design with respect to their suitabilities for measurement of hyphal P uptake. One hyphal compartment was simply a nylon mesh bag filled with 32P-labelled soil. The labelled soil in the other hyphal compartment was completely surrounded by an 8–10 mm layer of unlabelled soil that served as a buffer zone. Mycorrhizal and non-mycorrhizal subterranean clover plants were grown in pots with a centrally positioned hyphal compartment. Uptake of radioactive P by non-mycorrhizal control plants was 25% of that by mycorrhizal plants with the mesh bag but only 3% when including the buffer zone. Based on this good control of non-mycorrhizal P uptake from within the hyphal compartment and its greater ease of handling once produced, we judged the hyphal compartment including a buffer zone to be superior to the mesh bag.

Effect of ectomycorrhizae on the growth and uptake and transport of 15 N-labeled compounds by Pinus tabulaeformis seedlings under water-stressed conditions

Seedlings of Chinese pine (Pinus tabulaeformis Carr.) were grown in a growth chamber using a sterilized soil/sand/vermiculite mixture, and were inoculated with the ectomycorrhizal fungus Cenococcum graniforme (Sow.) Ferd. and Winge. The function of external mycelium of ectomycorrhiza for uptake and transport of N from 15N-labeled NH4 + and NO3 – for plant nutrition was evaluated under well-watered conditions and water stress. The pots comprised two-compartments, whereby the penetration of roots was prevented by a nylon mesh bag which the mycorrhizal hyphae were allowed to pass in order to colonize the rest of the pot. 15N-labeled NH4 + or NO3 – was applied to the area of the pot to which the root had no access. At harvest, the 15NH4 + concentration in plant tissues was significantly promoted by the formation of mycorrhizae both under well-watered conditions and water stress. The 15NO3 – concentration was reduced by water stress and increased by mycorrhizal formation. The enhancement of 15NO3 – uptake caused by mycorrhizal formation was more evident under water stress than under well-watered conditions. The external mycelia of the ectomycorrhizae took up and transported NH4 + and NO3 – from the soil to the plant, thereby improving plant nutrition and growth, in addition to helping the plants to avoid the effects of water stress.

Soil nutrient input effects on seed longevity: a burial experiment with fen‐meadow species

1. Effects of high nutrient input on the longevity and viability of buried seed are examined. Seeds of 17 fen‐meadow species were buried in nylon mesh bags at four sites in the Netherlands and one site in Great Britain in plots to which N, P, K fertilizers are applied. Prior to burial germination tests were conducted on the seeds of each species. This paper describes the results of the viability tests on the seeds that were exhumed after one and 2 years of burial. 2. The percentage of seeds that germinated after 1 year of burial was significantly lower than the pre‐burial percentage for the majority of the species. After 2 years of burial the germination percentage further decreased. A few species, such as the Carex species, did, however, show an increase in germination percentage indicating that the burial conditions allowed dormancy controls to be broken. 3. Differences in the edaphic conditions between the sites appeared to affect germination percentages after 1 year of burial. A difference in germination response between sites was observed for Carex acutiformis, Filipendula ulmaria and Lychnis flos‐cuculi. 4. A significantly higher germination percentage was found at the Great Britain site for F. ulmaria in the phosphate treatment compared with the potassium treatment and the control after 1 year of burial. In contrast to many literature assessments no significant effects of fertilizer application was found after 2 years. 5. For all sites, except one in the Netherlands, the total number of seeds that germinated was lower in 1996 than in 1995.

A Novel Technique to Determine Platelet Deposition on an Experimental Nylon Membrane Using Lvsem and Tem

Abstract Porous nylon fabric is used as a filter in blood devices. This study was undertaken to evaluate the interaction between an experimental nylon filter and the cellular elements which pass through it. Our investigation was mainly centered on determining the deposition of cellular material on the nylon filter using Low Voltage Scanning Electron Microscopy (LVSEM) and Transmission Electron Microscopy (TEM). The filter was prepared for LVSEM by cutting a 2.5 cm2 of the nylon filter material with a razor and placing it in a nylon mesh bag which was stapled shut. The bag was rinsed three times for ten minutes each in fresh Sorensen's phosphate buffer. A graded alcohol series of 25%, 35%, 50%, 70%, 80%, and 90% was used to dehydrate the filter for ten minutes each. This was followed by dehydration twice for 20 minutes in fresh 100% ETOH. The filter was then Critical Point Dried (CPD) in the Tousimis AUTOSAMDRI-814.

Phytochrome-mediatedAmaranthusgermination I: effect of seed burial and germination temperature

This research was conducted to evaluate the light requirement for redroot and smooth pigweed germination in soil, and how this requirement is affected by germination temperature and seasonal periodicity in seed dormancy. Seed enclosed in nylon mesh bags was buried in the field in December 1993 and 1994 and was recovered throughout the spring and summer of the following year, respectively. Germination was highest with red light or at 30 C. The requirement for red light was more pronounced at 20 vs. 30 C. The saturating fluence of red light was as low as 3 μmol m−2in buried seed and 1,000 μmol m−2in unburied control seed, depending on germination temperature. The effect of light and germination temperature pigweed germination also changed throughout the growing season. Our results indicate that light may be a requirement for germination in only the most dormant weed seed in the soil seedbank.

Organic carbon and nitrogen mineralization under Mediterranean climatic conditions: The effects of incubation depth

In a soil profile, temperature and humidity regimes change with depth. Under Mediterranean conditions, upper horizons are more affected by water deficits and drying-rewetting cycles than deep horizons. Our aim was to study how carbon and nitrogen mineralization are affected by depth, and special attention is paid to separating the effects of pedoclimate fromthe effects of other constraints like amount and quality of organic matter. To this end, mixtures of plant + soil material were exposed by incorporation in the field, at depths of 5, 20 and 40 cm, in nylon mesh bags. Mineralization of C and N was studied for 2 y. For all types of plant material studied ( Eucalyptus globulus, Quercus ilex and Pinus halepensis), mineralization of both carbon and nitrogen was lower at 5 cm. No differences were between 20 and 40 cm. This result, probably as a result of the higher drying of the upper-most horizons, contrasts with the usual findings on this topic. The amounts of both C and N mineralized were lower than expected, probably because plant materials were finely ground, allowing stabilization in the mineral matrix of soil. With the possible exception of Pinus, depth affected the rate of mineralization, not the relation between C and N. It is concluded that, at least under Mediterranean conditions, the pedoclimate in deep layers is more favourable to microbial activity than in upper layers, in which drought is a strong limiting factor. Reduced oxygen availability in the subsoil layers did not inhibit decomposition and mineralization to the same extent as did desiccation in the surface layer. The higher mineralization of C and N usually found in upper horizons may be attributed to the higher amount and quality of organic matter in these horizons, rather than to pedoclimatic constraints.

Decomposition of Nothofagus fallen woody debris in forests of Tierra del Fuego, Argentina

We report the decay constants of fallen fine (<1 cm in diameter) and coarse ($1 cm) woody debris of the southern beeches lenga (Nothofagus pumilio(Poepp. et Endl.) Krasser), nire (Nothofagus antarctica(Forst.) Oerst.), and guindo (Nothofagus betuloides(Mirb.) Oerst.) in Tierra del Fuego, Argentina. The decomposition of small branches (<1 cm) was assessed with the nylon mesh bag technique. A tethered-branch method was used for the decomposition of branches 1-16 cm in diameter of nire and guindo incubated in the field for a period of approximately 4 years. For large lenga branches (8-17 cm) and boles (19-60 cm), we used a chronosequence method, using a 3.5- to 55-year series of natural field incubations. Small lenga branches decayed faster (k =0 .47 ·year -1 ) than those of nire (k =0 .23 ·year -1 ) and guindo (k =0 .17 ·year -1 ). For branches 1 cm, decomposition rates of the species were not significantly different, but diminished with diameter increase. The rate of change of k versus diameter was significantly greater for lenga. In the full range of lenga wood diameters studied, k decreased exponentially with diameter (k =0 .553 ·exp(-0.177·diameter)) attaining a nearly constant value (k =0 .010 ·year -1 ) between 30 and 60 cm. Residence time increased from 2 to 100 years for small branches to large boles, respectively. There was no lag time for decomposition initiation. The decay constants for large lenga branches and boles appeared similar to or slightly greater than those of cold temperate conifers and less than those of hardwoods of the cold and warm temperate zones of the Northern Hemisphere. Resume : Cet article traite des constantes de decomposition des debris ligneux tombes au sol, fins (<1 cm de diametre) et grossiers ($1 cm), des lenga (Nothofagus pumilio(Poepp. et Endl.) Krasser), nire (Nothofagus antarctica(Forst.) Oerst.) et guindo (Nothofagus betuloides(Mirb.) Oerst.) a la Terre de Feu, en Argentine. La decomposition des petites branches

Microhabitat-independent regional differences in survival of unfed Ixodes scapularis nymphs (Acari:Ixodidae) in Connecticut.

The effects of habitat and microclimate on survival of unfed nymphal black-legged ticks, Ixodes scapularis Say (approximately I. damnini Spielman, Clifford, Piesman & Corwin), were studied under natural conditions in southcentral and northwestern Connecticut. At both coastal and inland locations, survival of 3 groups of 20 wild-caught questing nymphs placed in nylon mesh bags was monitored in each of 3 different habitats (field, forest canopy, and forest/field edge) during summer 1995. Simultaneously, soil temperature, ground-level air temperature, and relative humidity were measured continuously within each habitat at both sites. The number of ticks surviving in each habitat was monitored weekly. Average daily survival rates of nymphs were related inversely to soil temperature but were not related to air temperature or humidity. Overall, nymphal ticks at the inland site survived significantly longer than those at the coastal site; however, no significant differences in mortality rates were found among habitats. These results suggest that inland environmental conditions are suitable for lengthy survival of unfed nymphal I. scapularis in regions where this tick is not yet abundant.