Occurrence of arbuscular mycorrhizal fungi in a phosphorus-poor wetland and mycorrhizal response to phosphorus fertilization

The presence of arbuscular mycorrhizas in fens has received little attention, but because fen plants are often phosphorus limited, the plant-fungus interaction could be an important factor in plant competition for phosphorus. In this field study, we determined mycorrhizal colonization rates for 18 fen plant species. Also in the field, we examined the effect of four different forms of phosphorus on the percentage colonization for one fen plant species, Solidago patula. We found that in a species-rich, phosphorus-poor wetland both mycorrhizal and nonmycorrhizal species were common. Nine of ten dicotyledonous species examined formed arbuscular mycorrhizas, while all monocotyledonous species were at most very weakly mycorrhizal. A morphological explanation for this pattern is that the monocots in our study have more extensive aerenchyma, especially in coarse roots. Therefore, monocots are able to transport oxygen to their roots more effectively than dicots. In the organic wetland soil, additional oxygen in the rhizosphere promotes phosphorus mineralization and availability. Two of the monocot species (Typha latifolia and Carex lasiocarpa), which have been described previously as mycorrhizal in other wetland types, are surprisingly nonmycorrhizal in our phosphorus-poor study site, suggesting that a mycorrhizal association would not offer improved phosphorus nutrition to these species. In contrast, our field phosphorus addition decreased mycorrhizal colonization in S. patula, suggesting that one benefit to S. patula of the mycorrhizas is phosphorus uptake.     

Carbon flow from plant to arbuscular mycorrhizal fungi is reduced under phosphorus fertilization

Plant and Soil - Iodine (I) deficiency is distinct from other micronutrient deficiencies in human populations in having a high endemic prevalence both in well-developed and in developing countries....     

Seasonal changes of arbuscular mycorrhizal fungi as affected by tillage practices and fertilization : Hyphal density and mycorrhizal root colonization

The influence of tillage practices on native arbuscular mycorrhizal fungi (AMF) was studied in two, consecutive years in eastern Canada, in two 11 year-old long-term tillage-fertilizer experimental field soils, a sandy loam and a clay, growing corn in monoculture. The three tillage practices were: 1) conventional tillage (CT; fall plowing plus spring disking), reduced tillage (RT; spring disking) and no-till (NT). The corn crop received either inorganic (N and K) or organic (liquid dairy manure) fertilizers. Mycorrhizal hyphal density was estimated from soil samples obtained in early spring (before disking), at the 12–14 leaf stage, at silking, and at harvest. The percentage of corn root colonization by AMF at the 12–14 leaf stage, at silking and at harvest was also determined. The sandy loam was sampled over two consecutive seasons and the clay soil over one season.Densities of total and metabolically active soil hyphae, and mycorrhizal root colonization were significantly lower in CT soil than in RT and NT soil. Lowest soil hyphal densities were observed in early spring. The levels of intra- and extraradical fungal colonization always increased from spring to silking and decreased thereafter. Spring disking had only a small and transient negative effect on hyphal abundance in soil. Fertilization did not influence mycorrhizal colonization of corn or abundance of soil hyphae in the sandy loam soil, but in the clay soil metabolically active hyphae were more abundant with manure application than with mineral fertilization. In 1992, in both soils different tillage systems had same grain yield, however, in 1993, corn yield was higher in NT compared to CT system.     

Community analysis of arbuscular mycorrhizal fungi and bacteria in the maize mycorrhizosphere in a long-term fertilization trial

In this study, we investigated the impact of organic and mineral fertilizers on the community composition of arbuscular mycorrhizal (AM) fungi and bacteria in the mycorrhizosphere of maize in a field experiment established in 1956, in south-east Sweden. Roots and root-associated soil aggregates were sampled four times during the growing season in 2005, in control plots and in plots amended with calcium nitrate, ammonium sulphate, green manure, farmyard manure or sewage sludge. Fungi in roots were identified by cloning and sequencing, and bacteria in soil aggregates were analysed by terminal-restriction fragment length polymorphism, cloning and sequencing. The community composition of AM fungi and bacteria was significantly influenced by the different fertilizers. Changes in microbial community composition were mainly correlated with changes in pH induced by the fertilization regime. However, other factors, including phosphate and soil carbon content, also contributed significantly to these changes. Changes in bacterial community composition and a reduction in bacterial taxon richness throughout the growing season were also manifest. The results of this study highlight the importance and significant effects of the long-term application of different fertilizers on edaphic factors and specific groups of fungi and bacteria playing a key role in arable soils.     

Organic and mineral fertilization, respectively, increase and decrease the development of external mycelium of arbuscular mycorrhizal fungi in a long-term field experiment

Effects of long-term mineral fertilization and manuring on the biomass of arbuscular mycorrhizal fungi (AMF) were studied in a field experiment. Mineral fertilization reduced the growth of AMF, as estimated using both measurements of hyphal length and the signature fatty acid 16:1ω5, whereas manuring alone increased the growth of AMF. The results of AMF root colonization followed the same pattern as AMF hyphal length in soil samples, but not AMF spore densities, which increased with increasing mineral and organic fertilization. AMF spore counts and concentration of 16:1ω5 in soil did not correlate positively, suggesting that a significant portion of spores found in soil samples was dead. AMF hyphal length was not correlated with whole cell fatty acid (WCFA) 18:2ω6,9 levels, a biomarker of saprotrophic fungi, indicating that visual measurements of the AMF mycelium were not distorted by erroneous involvement of hyphae of saprotrophs. Our observations indicate that the measurement of WCFAs in soil is a useful research tool for providing information in the characterization of soil microflora.     

Fenpropimorph and fenhexamid impact phosphorus translocation by arbuscular mycorrhizal fungi

Fenpropimorph and fenhexamid are sterol biosynthesis inhibitor (SBI) molecules widely used to control diseases in agriculture. Both molecules, at increasing concentrations, have been shown to impact on the non-target arbuscular mycorrhizal (AM) fungi. Root colonization, spore production and mycelium architecture, including the branched absorbing structures which are thought to be involved in phosphorus (P) uptake, were affected. In the present study, we investigated the capacity of Glomus sp. MUCL 43204 to take up, transfer and translocate labelled P to Medicago truncatula in the presence of these SBI molecules. We used a strict in vitro cultivation system associating an autotrophic plant of M. truncatula with the AM fungus. In addition, the effects of both SBI molecules on the proportion of hyphae with alkaline phosphatases (ALP), succinate dehydrogenase (SDH) activity and on the expression of the mycorrhiza-specific plant phosphate transporter MtPT4 gene were examined. We demonstrated that the two SBI molecules impacted the AM fungus. This was particularly evidenced for fenpropimorph. A decrease in P transport and ALP and SDH activities associated with the extraradical mycelium and MtPT4 expression level was noted. These three factors were closely related to the development of the AM fungus, suggesting a direct impact not only on the AM fungal growth but also on the physiology and metabolic activities of the AM fungus. These results further emphasized the interest on the autotrophic in vitro culture system as an alternative to pot experiments to investigate the mechanisms behind the impact of disease control molecules on the non-target AM fungal symbionts.     

Effects of using different host plants and long-term fertilization systems on population sizes of infective arbuscular mycorrhizal fungi

The effect of cultivation of mycorrhizal and non-mycorrhizal plants and mineral fertilization on the arbuscular mycorrhizal fungal (AMF) community structure of maize (Zea mays L.) plants was studied. Soil samples were collected from two field experiments treated for 5 years with three fertilization systems (Control – no fertilization; Mineral – NPK fertilization; and Organic – Farmyard manure fertilization). Soil samples containing soil and root fragments of rapeseed (Brassica napus L., non-mycorrhizal plant) and wheat (Triticum aestivum L., mycorrhizal plant) collected from the field plots were used as native microbial inoculum sources to maize plants. Maize plants were sown in pots containing these inoculum sources for four months under glasshouse conditions. Colonization of wheat roots by AMF, AMF community structure, AMF diversity (Shannon’s index), AMF dominance (Simpson’s index) and growth of maize were investigated. Sixteen AMF species were identified from rhizosphere soil samples as different species of genera Acaulospora, Claroideoglomus, Dentiscutata, Funneliformis, Gigaspora, Quatunica, Racocetra, and Rhizoglomus. Maize plants grown in manure-fertilized soils had a distinct AMF community structure from plants either fertilized with mineral NPK-fertilizer or non-fertilized. The results also showed that inoculum from non-mycorrhizal plants combined with mineral fertilization decreased AMF diversity (Shannon’s index), AMF dominance (Simpson’s index) and growth of maize. Our findings suggest that non-mycorrhizal plants, such as B. napus, can negatively affect the presence and the effects of soil inoculation on maize growth. Also, our results highlight the importance of considering the long-term effect of rapeseed cultivation system on the reduction of population sizes of infective AMF, and its effect on succeeding annual crops.     

Organic fertilization changes the response of mycelium of arbuscular mycorrhizal fungi and their sporulation to mineral NPK supply

The synergetic effect of organic (cow manure) and mineral fertilization on the development arbuscular mycorrhizal (AM) fungi was demonstateded. The length of AM mycelium and sporulation were used as sensitive markers of the physiological state of soil AM fungal population. In manured treatments, both parameters increased in proportion with increasing mineral fertilization. In unmanured soil, the opposite trend was observed for the length of AM hyphae, which decreased with increasing mineral fertilization. Correlation analysis showed the dependence of length of AM hyphae and sporulation on soil available phosphorus. The correlation was negative in soil with no mineral fertilization and positive in soil supplied with luxury doses of mineral fertilizer.     

Effect of long-term nitrogen fertilization on mycorrhizal fungi associated with a dominant grass in a semiarid grassland

We studied the diversity of arbuscular mycorrhizal fungi (AMF) in semiarid grassland and the effect of long-term nitrogen (N) fertilization on this fungal community. Root samples of Bouteloua gracilis were collected at the Sevilleta National Wildlife Refuge (New Mexico, USA) from control and N-amended plots that have been fertilized since 1995. Small subunit rDNA was amplified using AMF specific primers NS31 and AM1. The diversity of AMF was low in comparison with other ecosystems, only seven operational taxonomic units (OTU) were found in B. gracilis and all belong to the genus Glomus. The dominant OTU was closely related to the ubiquitous G. intraradices/G. fasciculatum group. N-amended plots showed a reduction in the abundance of the dominant OTU and an increase in AMF diversity. The greater AMF diversity in roots from N-amended plots may have been the result of displacement of the dominant OTU, which facilitated detection of uncommon AMF. The long-term implications of AMF responses to N enrichment for plant carbon allocation and plant community structure remain unclear.     

Variable responses of old-field perennials to arbuscular mycorrhizal fungi and phosphorus source

If arbuscular mycorrhizal fungi (AMF) promote phosphorus partitioning of plant hosts, they could provide one mechanism for the maintenance of plant community diversity. We investigated whether AMF improved the ability of old field perennials to grow on a range of phosphorus sources and whether AMF facilitated differential performance of plant species on different phosphorus sources (phosphorus niche partitioning). We manipulated form of phosphorus (control versus different inorganic and organic sources) and AM fungal species (control versus four individual AMF species or an AMF community) for five old field perennials grown in a greenhouse in individual culture. Based on biomass after four months of growth, we found no evidence for phosphorus niche partitioning. Rather, we found that effects of AMF varied from parasitic to mutualistic depending on plant species, AMF species, and phosphorus source (significant Plant × Fungus × Phosphorus interaction). Our results suggest that the degree of AMF benefit to a plant host depends not only on AMF species, plant species, and soil phosphorus availability (as has also been found in other work), but can also depend on the form of soil phosphorus. Thus, the position of any AMF species along the mutualism to parasitism continuum may be a complex function of local conditions, and this has implications for understanding plant competitive balance in the field.     

Increasing phosphorus removal in willow and poplar vegetation filters using arbuscular mycorrhizal fungi

Fast growing woody species are increasingly used in vegetation filters for wastewater treatment. Their efficiency in phosphorus (P) removal notably depends on plant uptake and storage in aboveground tissues. In this study, Populus NM5 (P. nigra × P. maximowiczii), Salix miyabeana (SX64) and Salix viminalis (5027) were planted in pots to evaluate the influence of colonization by arbuscular mycorrhizal fungi (AMF) Glomus intraradices on P uptake using two different P concentrations in irrigation water. Based on analysis of the foliar and woody components, our results show that the two treatments (inoculation with G. intaradices and P-irrigation) interact differently with total P content. Foliar P content is principally enhanced by the P-irrigation concentration, whereas the mycorrhizal colonization increases stem P content. In the presence of G. intraradices, both S. miyabeana and S. viminalis showed a 33% increase in stem P content. The latter finding is mainly due to an increase in biomass production, without modification of the P concentration, indicating that AMF associations affect P use efficiency. Thus, using arbuscular mycorrhizal fungi for phytoremediation strategies may increase biomass productivity and hence improve pollutant uptake.     

Impact of weed control on arbuscular mycorrhizal fungi in a tropical agroecosystem: a long-term experiment

Cover crop species represent an affordable and effective weed control method in agroecosystems; nonetheless, the effect of its use on arbuscular mycorrhizal fungi (AMF) has been scantily studied. The goal of this study was to determine root colonization levels and AMF species richness in the rhizosphere of maize plants and weed species growing under different cover crop and weed control regimes in a long-term experiment. The treatment levels used were (1) cover of Mucuna deeringian (Muc), (2) "mulch" of Leucaena leucocephala (Leu), (3) "mulch" of Lysiloma latisiliquum (Lys), (4) herbicide (Her), (5) manual weeding (CD), (6) no weeding (SD), and (7) no maize and no weeding (B). A total of 18 species of AMF belonging to eight genera (Acaulospora, Ambispora, Claroideoglomus, Funneliformis, Glomus, Rhizophagus, Sclerocystis, and Scutellospora) were identified from trap cultures. Muc and Lys treatments had a positive impact on AMF species richness (11 and seven species, respectively), while Leu and B treatments on the other hand gave the lowest richness values (six species each). AMF colonization levels in roots of maize and weeds differed significantly between treatment levels. Overall, the use of cover crop species had a positive impact on AMF species richness as well as on the percentage of root colonized by AMF. These findings have important implications for the management of traditional agroecosystems and show that the use of cover crop species for weed control can result in a more diverse AMF community which should potentially increase crop production in the long run.     

Impact of long-term conventional and organic farming on the diversity of arbuscular mycorrhizal fungi

Previous work has shown considerably enhanced soil fertility in agroecosystems managed by organic farming as compared to conventional farming. Arbuscular mycorrhizal fungi (AMF) play a crucial role in nutrient acquisition and soil fertility. The objective of this study was to investigate the diversity of AMF in the context of a long-term study in which replicated field plots, at a single site in Central Europe, had been cultivated for 22 years according to two organic and two conventional farming systems. In the 23rd year, the field plots, carrying an 18-month-old grass-clover stand, were examined in two ways with respect to AMF diversity. Firstly, AMF spores were isolated and morphologically identified from soil samples. The study revealed that the AMF spore abundance and species diversity was significantly higher in the organic than in the conventional systems. Furthermore, the AMF community differed in the conventional and organic systems: Glomus species were similarly abundant in all systems but spores of Acaulospora and Scutellospora species were more abundant in the organic systems. Secondly, the soils were used to establish AMF-trap cultures using a consortium of Plantago lanceolata, Trifolium pratense and Lolium perenne as host plants. The AMF spore community developing in the trap cultures differed: after 12 months, two species of the Acaulosporaceae (A. paulinae and A. longula) were consistently found to account for a large part of the spore community in the trap cultures from the organic systems but were found rarely in the ones from the conventional systems. The findings show that some AMF species present in natural ecosystems are maintained under organic farming but severely depressed under conventional farming, indicating a potentially severe loss of ecosystem function under conventional farming.     

Fungal spore diversity of arbuscular mycorrhizal fungi associated with spring wheat: effects of tillage

We investigated the influence of tilling, N fertilization and crop stage on arbuscular mycorrhizae (AM) fungal species diversity in a wheat monoculture in the Pampa region of Argentina. Glomalean spores were isolated by wet sieving and decanting from conventionally tilled and nontilled soils cropped with wheat with or without N fertilization, at three phenological stages of the crop (tilling, flowering and grain filling) and fallow. Morphological characterization yielded at least 24 AM fungi taxa in the field samples, belonging to six genera of AMF: Acaulospora Archaeospora, Entrophospora, Gigaspora, Glomus and Scutellospora. Tilling and fertilization treatments did not result in decreased spore biodiversity. Wheat phenology influenced AM communities, with highest spore biodiversity during grain filling.     

The influence of Rhizobium and arbuscular mycorrhizal fungi on nitrogen and phosphorus accumulation by Vicia faba

BACKGROUND AND AIMS: The aim of this study was to investigate the effects of the interactions between the microbial symbionts, Rhizobium and arbuscular mycorrhizal fungi (AMF) on N and P accumulation by broad bean (Vicia faba) and how increased N and P content influence biomass production, leaf area and net photosynthetic rate. METHODS: A multi-factorial experiment consisting of four different legume-microbial symbiotic associations and two nitrogen treatments was used to investigate the influence of the different microbial symbiotic associations on P accumulation, total N accumulation, biomass, leaf area and net photosynthesis in broad bean grown under low P conditions. KEY RESULTS: AMF promoted biomass production and photosynthetic rates by increasing the ratio of P to N accumulation. An increase in P was consistently associated with an increase in N accumulation and N productivity, expressed in terms of biomass and leaf area. Photosynthetic N use efficiency, irrespective of the inorganic source of N (e.g. NO3- or N2), was enhanced by increased P supply due to AMF. The presence of Rhizobium resulted in a significant decline in AMF colonization levels irrespective of N supply. Without Rhizobium, AMF colonization levels were higher in low N treatments. Presence or absence of AMF did not have a significant effect on nodule mass but high N with or without AMF led to a significant decline in nodule biomass. Plants with the Rhizobium and AMF symbiotic associations had higher photosynthetic rates per unit leaf area. CONCLUSIONS: The results indicated that the synergistic or additive interactions among the components of the tripartite symbiotic association (Rhizobium-AMF-broad bean) increased plant productivity.     

Effects of long-term NP-fertilization on abundance and diversity of arbuscular mycorrhizal fungi under a maize cropping system

Diversity of arbuscular mycorrhizal fungi (AMF) in 27-year long-term NP-fertilization plots under a maize cropping system in Thailand was studied through spore morphological characterization. The plots received 0–0, 60–60, 120–120 and 180–180 kg N-P₂O₅ ha^–1 year^–1 as ammonium sulfate and triple superphosphate. The plots were sampled monthly for one year, the AMF spores were counted and morphotyped, and taxa were identified after morphotyping and monospecific pot culture. Spore number g^–1 soil, relative spore abundance and Shannon-Wiener indexes were calculated. Sixteen putative taxa were recorded from the field of which nine sporulated on maize roots in pot culture. The long-term fertilization caused decreases in AMF total spore numbers and variation in species diversity depended on sampling time. Effects of fertilization on spore number and also relative spore abundance varied with species and sampling time. Among the nine species sporulating under maize, only Acaulospora sp.1 showed no change (P > 0.003 after Bonferroni correction) in spore number with fertilization in the field; and was therefore classified as an AMF species insensitive to fertilization. Spores of Entrophospora schenckii, Glomus mosseae, Glomus sp.1, Glomus geosporum-like and Scutellospora fulgida, though they decreased in absolute numbers in response to fertilization, showed no change (P > 0.003 after Bonferroni correction) in relative abundance; these species were classified as AMF species slightly sensitive to fertilization. Three unidentified species of Glomus, though they decreased in absolute numbers in response to fertilization, showed decreases (P < 0.003 after Bonferroni correction) in relative abundance; these species were classified as AMF species highly sensitive to fertilization.