The impact of wildfire on vesicular-arbuscular mycorrhizal fungi and their potential to influence the re-establishment of post-fire plant communities

Wildfires are a typical event in many Australian plant communities. Vesicular-arbuscular mycorrhizal (VAM) fungi are important for plant growth in many communities, especially on infertile soils, yet few studies have examined the impact of wildfire on the infectivity of VAM fungi. This study took the opportunity offered by a wildfire to compare the infectivity and abundance of spores of VAM fungi from: (i) pre-fire and post-fire sites, and (ii) post-fire burned and unburned sites. Pre-fire samples had been taken in May 1990 and mid-December 1990 as part of another study. A wildfire of moderate intensity burned the site in late December 1990. Post-fire samples were taken from burned and unburned areas immediately after the fire and 6 months after the fire. A bioassay was used to examine the infectivity of VAM fungi. The post-fire soil produced significantly less VAM infection than the pre-fire soil. However, no difference was observed between colonization of plant roots by VAM fungi in soil taken from post-fire burned and adjacent unburned plots. Soil samples taken 6 months after the fire produced significantly more VAM than corresponding soil samples taken one year earlier. Spore numbers were quantified be wet-sieving and decanting of 100-g, air-dried soil subsamples and microscopic examination. For the most abundant spore type, spore numbers were significantly lower immediately post-fire. However, no significant difference in spore numbers was observed between post-fire burned and unburned plots. Six months after the fire, spore numbers were the same as the corresponding samples taken 1 year earlier. All plants appearing in the burned site resprouted from underground organs. All post-fire plant species recorded to have mycorrhizal associations before the fire had the same associations after the fire, except for species of Conospermum (Proteaceae), which lacked internal vesicles in cortical cells in the post-fire samples.     

Effect of tillage and farming system upon VAM fungus populations and mycorrhizas and nutrient uptake of maize

Low-input agricultural systems that do not rely on fertilizers may be more dependent on vesicular-arbuscular mycorrhizal [VAM] fungi than conventionally managed systems. We studied populations of spores of VAM fungi, mycorrhiza formation and nutrient utilization of maize (Zea mays L.) grown in moldboard plowed, chisel-disked or no-tilled soil under conventional and low-input agricultural systems. Maize shoots and roots were collected at four growth stages. Soils under low-input management had higher VAM fungus spore populations than soils under conventional management. Spore populations and colonization of maize roots by VAM fungi were higher in no-tilled than in moldboard plowed or chisel-disked soil. The inoculum potential of soil collected in the autumn was greater for no-till and chisel-disked soils than for moldboard plowed soils and greater for low-input than conventionally farmed soil. The effects of tillage and farming system on N uptake and utilization varied with growth stage of the maize plants. The effect of farming system on P use efficiency was significant at the vegetative stages only, with higher efficiencies in plants under low-input management. The effect of tillage was consistent through all growth stages, with higher P use efficiencies in plants under moldboard plow and chisel-disk than under no-till. Plants grown in no-tilled soils had the highest shoot P concentrations throughout the experiment. This benefit of enhanced VAM fungus colonization, particularly in the low-input system in the absence of effective weed control and with likely lower soil temperatures, did not translate into enhanced growth and yield.     

Dynamics of the Leaf-Litter Arthropod Fauna Following Fire in a Neotropical Woodland Savanna

Fire is an important agent of disturbance in tropical savannas, but relatively few studies have analyzed how soil-and-litter dwelling arthropods respond to fire disturbance despite the critical role these organisms play in nutrient cycling and other biogeochemical processes. Following the incursion of a fire into a woodland savanna ecological reserve in Central Brazil, we monitored the dynamics of litter-arthropod populations for nearly two years in one burned and one unburned area of the reserve. We also performed a reciprocal transplant experiment to determine the effects of fire and litter type on the dynamics of litter colonization by arthropods. Overall arthropod abundance, the abundance of individual taxa, the richness of taxonomic groups, and the species richness of individual taxa (Formiciade) were lower in the burned site. However, both the ordinal-level composition of the litter arthropod fauna and the species-level composition of the litter ant fauna were not dramatically different in the burned and unburned sites. There is evidence that seasonality of rainfall interacts with fire, as differences in arthropod abundance and diversity were more pronounced in the dry than in the wet season. For many taxa the differences in abundance between burned and unburned sites were maintained even when controlling for litter availability and quality. In contrast, differences in abundance for Collembola, Formicidae, and Thysanoptera were only detected in the unmanipulated samples, which had a lower amount of litter in the burned than in the unburned site throughout most of our study period. Together these results suggest that arthropod density declines in fire-disturbed areas as a result of direct mortality, diminished resources (i.e., reduced litter cover) and less favorable microclimate (i.e., increased litter desiccation due to reduction in tree cover). Although these effects were transitory, there is evidence that the increasingly prevalent fire return interval of only 1–2 years may jeopardize the long-term conservation of litter arthropod communities.     

Growth response ofTheobroma cacao L. seeldings to inoculation with vesicular-arbuscular mycorrhizal fungi

Summary The effects of vesicular-arbuscular mycorrhiza (VAM) on the growth and phosphorus uptake of cocoa seedlings (Theobroma cacao L.) grown for 100 days in polythene bags, were studied at five levels of phosphorus fertilization in both steamed and unsterile Bungor Series soil (a fine clayey, kaolinitic isohyperthermic Typic Paleudult). The cocoa seedlings responded well to phosphorus fertilization and mycorrhizal treatments. Plants inoculated with VAM fungi (Gigaspora spp.) gave the most vigorous growth and higher phosphorus in the leaf tissues in unsterile soil compared to plants grown in steamed soil. However, the mycorrhizal effect was significantly more pronounced (P<0.01) in plants grown in steamed than in unsterile soil. High levels of phosphorus application depressed mycorrhizal development. Phosphorus fertilizer applied at the rates of 250 and 500 ug g⁻¹ soil gave maximum root colonization and spore counts in both soil types used.     

Vesicular-arbuscular mycorrhizal effectiveness in an acid soil amended with fresh organic matter

A greenhouse investigation was undertaken to determine the influence of fresh organic matter on the formation and functioning of vesicular-arbuscular mycorrhizal symbiosis in Leucaena leucocephala grown in an acid aluminum-rich ultisol. In soil not amended with fresh organic matter or lime, plants failed to grow. Mycorrhizal infection level, mycorrhizal effectiveness measured in terms of pinnule P content of L. leucocephala leaves and dry matter yield of the legume increased with increase in fresh organic matter. Although VAM colonization level and dry matter yield of L. leucocephala were significantly higher if the test soil was limed (7.2 cmole OH^–) than if amended with fresh organic matter, the latter was as effective as lime in off-setting the detrimental effect of aluminum on mycorrhizal effectiveness. The lower mycorrhizal colonization level and the lower dry matter yield noted in the soil treated with fresh organic matter appears to be related to the inadequacy of Ca in the soil amended with fresh organic matter. These observations are supported by the low calcium status of soil and plant tissues in the absence of lime. It is concluded that while fresh organic matter, in appropriate amounts, could protect sensitive plants and VAM symbiosis against Al toxicity in acid soils, maximum mycorrhizal inoculation effects are not likely to be attained unless the soils are also amended with Ca.Contribution from Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No 3740.     

No short-term change in soil properties following four-fold litter addition in a Costa Rican rain forest

We experimentally manipulated forest floor litter to investigate the influence of litter quality and quantity on soil properties over the short-term (weeks to months) in a wet tropical forest in NE Costa Rica. The study included old growth forest on high fertility soils, old growth forest on low fertility soils, and secondary forest on intermediate fertility soils. Forest floor litter was removed from a 16 m² area and added to an adjacent 4 m² area in March 2003, resulting in a one to four-fold increase in the annual litter input to the forest floor. We created three addition, three removal and three control plots per forest type. We measured treatment effects on variation in soil moisture, temperature, pH, and Bray-1 P (plant available) over a 5-month period that captured the dry-wet season transition. Litter manipulation had no effect on any of the soil properties measured during the 5-month study period. Significant variability through time and a similar temporal pattern across the three forest stands suggest that climatic variability is driving short-term patterns in these soil properties rather than seasonal inputs of litter. In general, soils were warmer, drier and more basic with higher available P during dry season months. Even in wet tropical forests, small variability in climate can play an important role in soil dynamics over periods of weeks to months. Although litter manipulation did not influence soil properties over the 5-month study period, a longer lag may exist between the timing of litter inputs and the influence of that litter on soil properties, especially plant available P.     

Effects of Nitrogen Addition on Litter Decomposition and CO2 Release: Considering Changes in Litter Quantity

This study aims to evaluate the impacts of changes in litter quantity under simulated N deposition on litter decomposition, CO₂ release, and soil C loss potential in a larch plantation in Northeast China. We conducted a laboratory incubation experiment using soil and litter collected from control and N addition (100 kg ha⁻¹ year⁻¹ for 10 years) plots. Different quantities of litter (0, 1, 2 and 4 g) were placed on 150 g soils collected from the same plots and incubated in microcosms for 270 days. We found that increased litter input strongly stimulated litter decomposition rate and CO₂ release in both control and N fertilization microcosms, though reduced soil microbial biomass C (MBC) and dissolved inorganic N (DIN) concentration. Carbon input (C loss from litter decomposition) and carbon output (the cumulative C loss due to respiration) elevated with increasing litter input in both control and N fertilization microcosms. However, soil C loss potentials (C output–C input) reduced by 62% in control microcosms and 111% in N fertilization microcosms when litter addition increased from 1 g to 4 g, respectively. Our results indicated that increased litter input had a potential to suppress soil organic C loss especially for N addition plots.     

Phosphorus cycling in a small watershed in the Brazilian Cerrado: impacts of frequent burning

Plant productivity in many tropical savannas is phosphorus limited. The biogeochemical cycling of P in these ecosystems, however, has not been well quantified. In the present study, we characterized P stocks and fluxes in a well-preserved small watershed in the Brazilian Cerrado. As the Cerrado is also a fire-dominated ecosystem, we measured the P stocks and fluxes in a cerrado stricto sensu plot with complete exclusion of fire for 26 years (unburned plot) and then tested some predictions about the impacts of fire impacts on P cycling in an experimental plot that was burned three times since 1992 (burned plot). The unburned area is an ecosystem with large soil stocks of total P (1,151 kg ha^?1 up to 50 cm depth), but the largest fraction is in an occluded form. Readily extractable P was found up to 3 m soil depth suggesting that deep soil is more important to the P cycle than has been recognized. The P stock in belowground biomass (0?C800 cm) was 9.9 kg ha^?1. Decomposition of fine litter released 0.97 kg P ha^?1 year^?1. Fluxes of P through bulk atmospheric deposition, throughfall and litter leachate were very low (0.008, 0.006 and 0.028 kg ha^?1 year^?1, respectively) as was stream export (0.001 kg ha^?1 year^?1). Immobilization of P by microbes during the rainy season seems to be an important mechanism of P conservation in this ecosystem. Fire significantly increased P flux in litter leachate to 0.11 kg ha^?1 year^?1, and added 1.2 kg ha^?1 of P in ash deposition after fire. We found an increase of P concentration in soil solution at 100 cm depth (from 0.03 ??g l^?1 in unburned plot to 0.3 ??g l^?1 in the burned plot). In surface soils (0?C10 cm) of the burned plot, fire decreased the concentrations of extractable organic-P fractions, but did not significantly increase inorganic-P fractions. The reduction of extractable soil organic P in the burned plot in topsoil and the increase of P in the soil solution at greater depths indicated a reduction of P availability and may increase P fixation in deep soils. Repeated fire events over the long term may result in significant net loss of available forms of phosphorus from this ecosystem.     

Influence of edaphic and climatic factors on dynamics of root colonization and spore density of vesicular-arbuscular mycorrhizal fungi in Acacia farnesiana Willd. and A. planifrons W.et.A.

 A study was conducted to assess the dynamics of vesicular-arbuscular mycorrhizal (VAM) fungi associated with Acacia farnesiana and A. planifrons in moderately fertile alkaline soils. The intensity of root colonization by VAM fungi and the distribution of VAM fungal structures varied with host species over a period of time. The occurrence of vesicles with varied morphology in the mycorrhizal roots indicates infection by different VAM fungal species. This was further confirmed from the presence of spores belonging to different VAM fungal species in the rhizosphere soils. Root colonization and spore number ranged from 56% – 72% and 5 – 14 g^{–  1}soil in A. farnesiana and from 60% – 73% and 5 – 15 g^{–  1} soil in A. planifrons. Per cent root colonization and VAM spore number in the rhizosphere soil were inversely related to each other in both the Acacia species. However, patterns of the occurrence of VAM fungal structures were erratic. Spores of Acaulospora foveata, Gigaspora albida, Glomus fasciculatum, G. geosporum and Sclerocystis sinuosa were isolated from the rhizosphere of A. farnesiana whereas A. scrobiculata, G. pustulatum, G. fasciculatum, G. geosporum and G. microcarpum were isolated from that of A. planifrons. The response of VAM status to fluctuating edaphic factors varied with host species. In A. farnesiana though soil nitrogen (N) was positively correlated with root colonization, soil moisture, potassium and air temperature were negatively correlated to both root colonization and spore number. Per cent root colonization and spore number in A. planifrons were negatively related to each other. Further, in A. planifrons as the soil phosphorus and N were negatively correlated with the density of VAM fungal spores, the same edaphic factors along with soil moisture negatively influenced root colonization. Received: 16 May 1995 / Accepted: 7 February 1996     

Effect of salinity on mycorrhizal onion and tomato in soil with and without additional phosphate

Summary Vesicular-arbuscular mycorrhizal fungi (VAM) are known to increase plant growth in saline soils. Previous studies, however, have not distinguished whether this growth response is due to enhanced P uptake or a direct mechanism of increased plant salt tolerance by VAM. In a glasshouse experiment onions (Allium cepa L.) were grown in sterilized, low-P sandy loam soil amended with 0, 0.8, 1.6 mmol P kg^–1 soil with and without mycorrhizal inoculum. Pots were irrigated with saline waters having conductivities of 1.0, 2.8, 4.3, and 5.9 dS m^–1. Onion colonized withGlomus deserticola (Trappe, Bloss, and Menge) increased growth from 394% to 100% over non-inoculated control plants when soil P was low ( 0.2 mmol kg^–1 NaHCO₃-extractable P) at soil saturation extract salinities from 1.1 dS m^–1 to 8.8 dS m^–1. When 0.8 and 1.6 mM P was added no dry weight differences due to VAM were observed, however, K and P concentrations were higher in VAM plants in saline treatments.Glomus fasciculatum (Gerdeman and Trappe) andGlomus mosseae (Nicol. and Gerd.) isolates increased growth of VAM tomato 44% to 193% in non-sterilized, saline soil (10 dS m^–1 saturation extract) despite having little effect on growth in less saline conditions when soil P was low. Higher tomato water potentials, along with improved K nutrition by VAM in onion, indicate mechanisms other than increased P nutrition may be important for VAM plants growing under saline stress. These effects appear to be secondary to the effects of VAM on P uptake.     

Cultivar variation in the effect of chlorsulfuron in depressing the uptake of copper in wheat

The application of herbicides has induced symptoms of nutrient deficiencies under some circumstances. This glasshouse study examined the effect of chlorsulfuron on the uptake and utilization of copper (Cu) in four cultivars of wheat plants (Triticum aestivum L. cvs. Kulin, Cranbrook, Gamenya and Bodallin) on a Cu-responsive soil. Application of chlorsulfuron depressed the concentration of Cu in wheat plants receiving either inadequate or adequate Cu. In plants with inadequate Cu supply, chlorsulfuron increased the severity of Cu deficiency. Shoot weight was markedly decreased by chlorsulfuron at all levels of Cu, through decreasing the number of tillers and the elongation of leaves. This decreased growth of shoots occurred prior to the effect on Cu concentration in tissues. The retranslocation of Cu in old tissues over time was unaffected by chlorsulfuron. In all wheat cultivars, the decreased growth of shoots were correlated with the concentration of Cu in the youngest fully emerged leaf blade with critical levels of 1.6−1.7 at day 25 and 0.9−1.0 μg g⁻¹ d. wt. at day 60. The application of chlorsulfuron tended to increase the critical level at day 25 but not at day 60. In addition, Kulin seems to be most, and Cranbrook least, sensitive to chlorsulfuron. This sensitivity was associated with the sensitivity of the cultivars to Cu deficiency. It is suggested that chlorsulfuron application induces Cu deficiency in wheat plants mainly due to effects on the uptake of Cu. This revised version was published online in June 2006 with corrections to the Cover Date.     

Glycine-Glomus-Bradyrhizobium Symbiosis : X. Relationships between Leaf Gas Exchange and Plant and Soil Water Status in Nodulated, Mycorrhizal Soybean under Drought Stress

Soybean (Glycine max [L.] Merr.) plants were colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe (VAM plants) or fertilized with KH₂PO₄ (nonVAM plants) and grown for 50 days under controlled conditions. Plants were harvested over a 4-day period during which the soil was permitted to dry slowly. The harvest was terminated when leaf gas exchange was no longer measurable due to drought stress. Significantly different effects in shoot water content, but not in shoot water potential, were found in VAM and nonVAM plants in response to drought stress. Leaf conductances of the two treatments showed similar response patterns to changes in soil water and shoot water potential but were significantly different in magnitude and trend relative to shoot water content. The relationships between transpiration, CO₂ exchange and water-use efficiency (WUE) were the same in VAM and nonVAM plants in response to decreasing soil water and shoot water potential. As a function of shoot water content, however, WUE showed different response patterns in VAM and nonVAM plants.     

Cessation of Burning Dries Soils Long Term in a Tallgrass Prairie

Soil moisture is a critical variable in grassland function, yet how fire regimes influence ecohydrology is poorly understood. By altering productivity, species composition, and litter accumulation, fire can indirectly increase or decrease soil water depletion on a range of time scales and depths in the soil profile. To better understand how fire influences soil moisture in grasslands, we analyzed 28 years of soil moisture data from two watersheds in a central North American grassland which differ in their long-term fire frequency. Across 28 years, cessation of prescribed burning initially led to wetter soils, likely as litter accumulated and both transpiration and evaporation were suppressed. Long-term, cessation of burning led to soils drying more, especially at depths greater than 75 cm. The long-term drying of deep soils is consistent with the increase in woody species in the infrequently burned grassland as woody species likely have a greater reliance on soil water from deeper soil layers compared to co-occurring herbaceous species. Despite the ecohydrological changes associated with the cessation of prescribed burning, watersheds with different burn regimes responded similarly to short-term variation in climate variation. In both watersheds, low precipitation and high temperatures led to drier soils with greater responses in soil moisture to climate variation later in the season than earlier. There is no current evidence that the cessation of burning in this ecosystem will qualitatively alter how evapotranspiration responds to climate variation, but the use of deeper soil water by woody plants has the potential for greater transpiration during dry times. In all, modeling the depth-specific responses of soil moisture and associated ecosystem processes to changes in burn regimes will likely require including responses of plant community composition over short and long time scales.     

Plant life span and response to inoculation with vesicular-arbuscular mycorrhizal fungi

Summary This study sought to investigate the relationships among residual soil phosphorus (P) pools, plant life span, successional persistence and responsiveness to vesicular-arbuscular mycorrhizal (VAM) infection. Plants of five species which varied in life span, persistence, and VAM responsiveness were grown in nutrientpoor soils in a glasshouse for 8 weeks and given weekly feedings with either high- or low-P solutions. There was little effect of plant life span or VAM status on changes in residual available (1 M KCl extractable) P. In contrast, there were strong correlations between VAM responsiveness and changes in the exchangeable P pool (1 M NH₄C₂H₃O₂ extractable). Plants with greater VAM responsiveness and greater persistence through succession were able to reduce this potentially available P pool by as much as 50% in 8 weeks. In contrast, plants with poor or negative responsiveness to mycorrhizal infection and little successional persistence exhibited little control over potentially available P pools. These data confirm other studies which demonstrate that VA mycorrhizae access insoluble forms of P, thereby controlling potential supply rate over the longer term. Such control over soil P pools may contribute to successional persistence via either inhibition or tolerance mechanisms, and should be considered in any comprehensive theory of the mechanisms underlying succession.     

Survival of and wheat-root colonization by alginate encapsulated Herbaspirillum spp

The survival ofHerbaspirillum spp. cells added directly or encapsulated in alginate beads and colonization of wheat roots was evaluated in soil microcosms. Cells entrapped in alginate in the presence of JNFb-broth and introduced into unplanted non-sterile clay loamy and sandy soils survived better than cells added directly to the same soils after 50 d incubation. On amendment by JNFb broth and/or skim milk the entrapped cells survived better than those prepared in water. Encapsulated cells survived better in a heavier textured soil (clay-loamy) than in a lighter (sandy) soil. Wheat plants growing in microcosms inoculated with various bead types from day 0 to day 30 exhibited high levels of histosphere colonization, nitrogenase activity (in situ) measured by acetylene reduction assay, plant dry mass and total N content but no symptoms of mottled stripe disease were observed. Comparable results of growth criteria and nitrogenase activity, but relatively lower bacterial populations, were obtained with wheat grown for 45 d after the inoculant had been introduced into the soil with different bead types.     

Influence of vesicular-arbuscular mycorrhiza and straw mulch on growth of barley

Summary The effects of vesicular-arbuscular mycorrhiza (VAM) and of mulching on growth of barley were investigated in a factorial experiment. Plants were grown in cylinders buried in a field in soil with moderate amounts of available phosphate. VAM infection, dry weight and P uptake were determined at harvest after 10 and 161/2 weeks growth.VAM infection was reduced in the upper soil layer by straw mulch, possibly through a reduction in temperature. By the second harvest VAM increased growth by 56% in the non-mulched plots through increased P uptake but VAM did not increase growth in the mulched plots. Mulch increased growth by 85% in the non-mycorrhizal plots, and 28% in the mycorrhizal plots.     

Preinoculation of lettuce and onion with VA mycorrhizal fungi reduces deleterious effects of soil salinity

The hypothesis that inoculation of transplants with vesicular-arbuscular mycorrhizal (VAM) fungi before planting into saline soils alleviates salt effects on growth and yield was tested on lettuce (Lactuca sativa L.) and onion (Allium cepa L.). A second hypothesis was that fungi isolated from saline soil are more effective in counteracting salt effects than those from nonsaline soil. VAM fungi from high- and low-salt soils were trap-cultured, their propagules quantified and adjusted to a like number, and added to a pasteurized soil mix in which seedlings were grown for 3–4 weeks. Once the seedlings were colonized by VAM fungi, they were transplanted into salinized (NaCl) soil. Preinoculated lettuce transplants grown for 11 weeks in the saline soils had greater shoot mass compared with nonVAM plants at all salt levels [2 (control), 4, 8 and 12 dS m^–1] tested. Leaves of VAM lettuce at the highest salt level were significantly greener (more chlorophyll) than those of the nonVAM lettuce. NonVAM onions were stunted due to P deficiency in the soil, but inoculation with VAM fungi alleviated P deficiency and salinity effects; VAM onions were significantly larger at all salt levels than nonVAM onions. In a separate experiment, addition of P to salinized soil reduced the salt stress effect on nonVAM onions but to a lesser extent than by VAM inoculation. VAM fungi from the saline soil were not more effective in reducing growth inhibition by salt than those from the nonsaline site. Colonization of roots and length of soil hyphae produced by the VAM fungi decreased with increasing soil salt concentration. Results indicate that preinoculation of transplants with VAM fungi can help alleviate deleterious effects of saline soils on crop yield.     

GROWTH OF LITTLE BLUESTEM (SCHIZACHYRIUM SCOPARIUM) (POACEAE) IN FUMIGATED AND NONFUMIGATED SOILS UNDER VARIOUS INORGANIC NUTRIENT CONDITIONS

Little bluestem plants (Schizachyrium scoparium (Michx.) Nash) were grown in fumigated and nonfumigated soil under manipulated levels of three inorganic nutrients: nitrogen, phosphorus, or bases (Ca + Mg). Plants grown in nonfumigated soil had significantly (P < 0.05) higher tissue levels of inorganic nutrients (Cu, Zn, Al, S, Mg, Mn, Ca, and P), smaller shoots, less total biomass, fewer flowering plants but more VAM fungal colonization than plants grown in fumigated soil that were essentially nonmycorrhizal (colonization vs. 1.2 ± 4.9%, for plants grown in nonfumigated and fumigated soil, respectively). Levels of phosphorus (14–33 μg/g) available (Bray No. 1) in the soil prior to manipulation, which are adequate for little bluestem, likely resulted in the development of an ineffectual mycorrhizal association, which in turn, caused the depressed growth of plants in nonfumigated soil. Among plants grown in nonfumigated soil, there was significant variation in VAM fungal colonization and sporulation owing to nutrient treatment. Nitrogen treatment and deionized water control had significantly lower levels of colonization than phosphorus and base treatments. However, plants in the nitrogen and base treatments had significantly more spores/100 cc of rhizosphere soil than plants grown in the deionized water control.     

Arbuscular mycorrhizal colonization of semi-aquatic grasses along a wide hydrologic gradient

The effect of soil flooding on arbuscular-mycorrhizal (AM) fungal colonization of wetland plants was investigated using Panicum hemitomon and Leersia hexandra , two semi-aquatic grasses (Graminaceae) that grow along a wide hydrologic gradient in Carolina bay wetlands of the southeastern US coastal plain. Three related investigations were conducted along the dry-to-wet gradient in these wetlands; a field survey of AM fungal root colonization in eight wetlands, monthly monitoring of colonization patterns in P. hemitomon over a growing season, and an inoculum potential bioassay of soils collected along the gradient. The field survey showed that AM fungal colonization was strongly negatively correlated with water depth, but colonization was present in most root samples. The monthly assessment indicated that AM fungal colonization was lowest in plots that were consistently wet but rose as some plots underwent seasonal drying. The inoculum potential assay of dry, intermediate, and wet soils performed under both dry and saturated conditions showed that soils that were wet for >1 yr had the same ability to form mycorrhizas in bait plants as those that had remained dry. These findings suggested that the lower degree of colonization in wet areas observed in the field survey was because of the presence of surface water rather than low numbers of mycorrhizal propagules in the soil. Overall, the results of these investigations show that flooding is partially but not totally inhibitory to AM fungal colonization of wetland grasses.     

Mycorrhizal status of the endangered species Astragalus applegatei Peck as determined from a soil bioassay

 The mycorrhizal status of Astragalus applegatei Peck is reported for the first time on plants from a greenhouse soil bioassay. Seedlings were grown in a potting mix inoculated with soil collected near A. applegatei plants in nature. Plants were also grown in non-inoculated potting mix. Only plants from the native soil inoculation survived. Abundant colonization of VAM fungi was found in all 15 plants analyzed from the native soil treatment, and chlamydospores produced by Glomus spp. were observed. Mycorrhizal colonization was estimated to be 23% of total fine root length after 6 weeks and 53% after 14 weeks. Our results provide ecologically important information for conservation and restoration efforts underway to recover populations of this endangered species. Accepted: 22 June 1998