Promotion of AM utilization through reduced P fertilization 2. Field studies

The hypothesis of this study was that cumulative P fertilization decreases the contribution of arbuscular mycorrhiza (AM) to crop growth and nutrient uptake in Northern European field conditions. The modes of action of P fertilization were evaluated through effects on mycorrhization, crop dependence on AM, and AM fungal (AMF) community. Field studies were carried out within long-term experiments on soils with low and intermediate initial content of extractable P, where no P fertilization and 45 kg ha^–1 a^–1 P were applied for 20 years. AM effectiveness in terms of growth and nutrient uptake of flax, red clover and barley, percentage root length colonized by AMF, P response of flax, and spore densities and species composition of the AMF communities, were assessed. In the soil with low initial P supply, cumulative P fertilization decreased AM contribution to crop growth and nutrient uptake. The higher AM effectiveness in soil with no added P compensated the cumulative P fertilization (soil P_H2O 2.5 v. 9.5 mg kg^–1) for flax, but not completely for clover. In contrast, barley obtained no benefit from AM at harvest and only a slight benefit from cumulated P. In the soil with intermediate initial P supply, AM reduced growth of flax and barley, especially with no added P, and no response to AM was obtained on clover due to retarded mycorrhization. Cumulative P fertilization reduced yield losses of flax by AM (P_H2O 18.8 v. 5.4 mg kg^–1), because fertilization inhibited mycorrhization. In both soils, root colonization and spore density were decreased by cumulative P fertilization, but no changes in AMF species composition were observed.     

Is there a role for arbuscular mycorrhizal fungi in production agriculture?

This review presents the point of view that arbuscular mycorrhizal fungi (AMF) do not play a vital role in the nutrition and growth of plants in many production-orientated agricultural systems. Highly available soil P often limits AM colonisation and causes the C-costs to the host to outweigh any benefits from colonisation. Even when P availability is low and AM colonisation levels are high, as may occur in organic and biodynamic agricultural systems, AMF may not always contribute to plant growth for reasons not yet understood. AM fungal activity may also be greatly limited by soil fumigation, non-responsive plant varieties, or rotations based primarily on non-mycorrhizal crops or crops of low AM dependency. Thus, profitability may sometimes be enhanced by management practices, such as tillage and P-fertilisation, which limit AM colonisation. Manipulation of agricultural systems to favour AMF must occur only if there is clear evidence that AMF make a positive contribution to yield or are vital for maintenance of ecosystem health and sustainability. A crucial role for AMF in soil structural stability or in enhancing micronutrient concentrations in produce may be sufficient evidence and may eventually compel consideration of AMF responsiveness when breeding new crop varieties.     

Establishment and effectiveness of inoculated arbuscular mycorrhizal fungi in agricultural soils

Arbuscular mycorrhizal fungi (AMF) are promoted as biofertilizers for sustainable agriculture. So far, most researchers have investigated the effects of AMF on plant growth under highly controlled conditions with sterilized soil, soil substrates or soils with low available P or low inoculum potential. However, it is still poorly documented whether inoculated AMF can successfully establish in field soils with native AMF communities and enhance plant growth. We inoculated grassland microcosms planted with a grass–clover mixture (Lolium multiflorum and Trifolium pratense) with the arbuscular mycorrhizal fungus Rhizoglomus irregulare. The microcosms were filled with eight different unsterilized field soils that varied greatly in soil type and chemical characteristics and indigenous AMF communities. We tested whether inoculation with AMF enhanced plant biomass and R. irregulare abundance using a species specific qPCR. Inoculation increased the abundance of R. irregulare in all soils, irrespective of soil P availability, the initial abundance of R. irregulare or the abundance of native AM fungal communities. AMF inoculation had no effect on the grass but significantly enhanced clover yield in five out of eight field soils. The results demonstrate that AMF inoculation can be successful, even when soil P availability is high and native AMF communities are abundant.     

Arbuscular mycorrhizae in wheat and field pea crops on a low P soil: increased Zn-uptake but no increase in P-uptake or yield

Few field studies have investigated the contribution of arbuscular mycorrhizal fungi (AMF) to agricultural systems. In this study, the role of AMF in nutrition and yield of dryland autumn-sown wheat and field pea was examined through a 2-year crop sequence experiment on a red loam (Kandosol) in SE Australia. The soil was P-deficient and had low levels of root pathogens. In Year 1, levels of AMF were increased by growing subterranean clover or Linola^TM and decreased by growing canola or through maintenance of bare fallow with herbicides or tillage. In Year 2, hosts of AMF (wheat and field pea) and non-mycorrhizal canola were grown with 0 P or 20 kg ha^–1 of P as superphosphate. Yields of all Year 2 crops were increased by P-fertiliser. Year 1 treatment led to 2–3 fold variation in colonisation by AMF at each P-level for Year 2 wheat and field pea. High colonisation did not correspond with greater crop growth, yield, or uptake of P, K, Ca, Cu or S in wheat or field pea. However, total crop Zn-uptake and grain Zn concentration were positively correlated with colonisation by AMF, due to enhanced Zn-uptake after anthesis. For wheat, high colonisation also corresponded with reduced Mn-uptake and lower grain Mn concentrations. In a glasshouse experiment using a second P-deficient Kandosol, inoculation of wheat with Glomus intraradices and Scutellospora calospora enhanced uptake of Zn and P when no P-fertiliser was applied. We conclude that high colonisation by AMF is unimportant for productivity of the major field crops grown on the Kandosol soils that occupy large areas of cropland in temperate SE Australia, even under P-limiting conditions. Investigation of the factors that control functioning of arbuscular mycorrhizae under field conditions, especially temperature, is required.     

The role of arbuscular mycorrhizas in improving plant zinc nutrition under low soil zinc concentrations: a review

Many of the world’s soils are zinc (Zn) deficient. Consequently, many crops experience reduced growth, yield and tissue Zn concentrations. Reduced concentrations of Zn in the edible portions of crops have important implications for human Zn nutrition; this is a cause of global concern. Most terrestrial plant species form arbuscular mycorrhizas (AM) with a relatively limited number of specialized soil fungi. Arbuscular mycorrhizal fungi (AMF) can take up nutrients, including Zn, and transfer them to the plant, thereby enhancing plant nutrition. Under high soil Zn concentrations the formation of AM can also ‘protect’ against the accumulation of Zn in plant tissues to high concentrations. Here, a short review focusing on the role of AM in enhancing plant Zn nutrition, principally under low soil Zn concentrations, is presented. Effects of Zn on the colonisation of roots by AMF, direct uptake of Zn by AMF, the role of AM in the Zn nutrition of field grown plants, and emerging aspects of Zn molecular physiology of AM, are explored. Emergent knowledge gaps are identified and discussed in the context of potential future research.     

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.     

Effects of long-term fertilization on AM fungal community structure and Glomalin-related soil protein in the Loess Plateau of China

Arbuscular mycorrhizal fungi (AMF) are crucial for ecosystem functioning, and thus have potential use for sustainable agriculture. In this study, we investigated the impact of organic and mineral fertilizers on the AMF community composition and content of Glomalin-related soil protein (GRSP) in a field experimental station which was established in 1979, in the Loess Plateau of China. Roots and soils were sampled three times during the growing period of winter wheat in 2008. The treatments including: N (inorganic N), NP (inorganic N and P), SNP (straw, inorganic N and P), M (farmyard manure), MNP (farmyard manure, inorganic N and P), and CK (no fertilization). AMF communities of root and soil samples were analyzed using PCR-DGGE, cloning and sequencing techniques; and GRSP content was determined by Bradford assay. Our results indicated that spore density, GRSP, and AMF community varied significantly in soils of long-term fertilization plots at three different wheat growing stages. The effects of wheat growing period on AMF community in roots were much more evident than fertilization regimes. However, the diversity of AMF was low in our study field. Up to five AMF phylotypes appeared in each sample, with the overwhelming dominance of a Glomus-like phylotype affiliated to G. mosseae. GRSP content was correlated positively with organic carbon, total phosphorus, available phosphorus, soil pH, and spore densities, but correlated negatively with soil C/N (P?<?0.05). The results of our study highlight that the richness of AMF in Loess Plateau agricultural region is low, and long-term fertilization, especially amendments with manure and straw, has beneficial effects on accumulation of soil organic carbon, spore density, GRSP content, and AMF diversity. Host phenology, edaphic factors (influenced by long-term fertilization), and habitats interacted to affect the AMF community and agoecosystem functioning. Additionally, soil moisture and pH make a greater contribution than other determined soil parameters to the AMF community dynamics in such a special semi-arid agroecosystem where crops rely greatly on rainfall.     

Plant-available P supply is not the main factor determining the benefit from arbuscular mycorrhiza to crop P nutrition and growth in contrasting cropping systems

Field studies have indicated that plant-available P supply is the main determinant of the performance of arbuscular mycorrhiza (AM) of crops. Direct evidence is, however, weak. Our aim was to test the hypothesis that plant-available P is the main factor determining the benefit from AM to plant P uptake and growth in contrasting cropping systems. We compared a conventional system with full and half fertilisation rates and a low-input system with and without composting of residues. After 15?years, plant P response functions, with and without AM, were determined in a bioassay. At equal plant availability of P, the benefits from AM were similar to those at the P status in the field: The benefits were greater in the low-input system than in the conventional system, irrespective of the fertilisation rate but clearest with composting. This shows that differences between the systems in the benefit from AM to a particular crop genotype are not mainly due to differences in plant-available P. The results suggest also that differences in P pools, in growth-limiting factors such as N supply and in phytotoxicity to AMF hyphae play roles in determining the benefit. It is not likely that differences in AMF communities are important.     

Arbuscular mycorrhizal fungi benefit from 7 years of free air CO2 enrichment in well-fertilized grass and legume monocultures

Rising atmospheric carbon dioxide partial pressure (pCO₂) and nitrogen (N) deposition are important components of global environmental change. In the Swiss free air carbon dioxide enrichment (FACE) experiment, the effect of altered atmospheric pCO₂ (35 vs. 60 Pa) and the influence of two different N‐fertilization regimes (14 vs. 56 g N m^?2 a^?1) on root colonization by arbuscular mycorrhizal fungi (AMF) and other fungi (non‐AMF) of Lolium perenne and Trifolium repens were studied. Plants were grown in permanent monoculture plots, and fumigated during the growth period for 7 years. At elevated pCO₂ AMF and non‐AMF root colonization was generally increased in both plant species, with significant effects on colonization intensity and on hyphal and non‐AMF colonization. The CO₂ effect on arbuscules was marginally significant (P=0.076). Moreover, the number of small AMF spores (≤100 μm) in the soils of monocultures (at low‐N fertilization) of both plant species was significantly increased, whereas that of large spores (>100 μm) was increased only in L. perenne plots. N fertilization resulted in a significant decrease of root colonization in L. perenne, including the AMF parameters, hyphae, arbuscules, vesicles and intensity, but not in T. repens. This phenomenon was probably caused by different C‐sink limitations of grass and legume. Lacking effects of CO₂ fumigation on intraradical AMF structures (under high‐N fertilization) and no response to N fertilization of arbuscules, vesicles and colonization intensity suggest that the function of AMF in T. repens was non‐nutritional. In L. perenne, however, AM symbiosis may have amended N nutrition, because all root colonization parameters were significantly increased under low‐N fertilization, whereas under high‐N fertilization only vesicle colonization was increased. Commonly observed P‐nutritional benefits from AMF appeared to be absent under the phosphorus‐rich soil conditions of our field experiment. We hypothesize that in well‐fertilized agricultural ecosystems, grasses benefit from improved N nutrition and legumes benefit from increased protection against pathogens and/or herbivores. This is different from what is expected in nutritionally limited plant communities.     

Integration of crop rotation and arbuscular mycorrhiza (AM) inoculum application for enhancing AM activity to improve phosphorus nutrition and yield of upland rice (Oryza sativa L.)

Upland rice (Oryza sativa L.) is a major crop of Eastern India grown during the wet season (June/July to September/October). Aerobic soils of the upland rice system, which are acidic and inherently phosphorus (P) limiting, support native arbuscular mycorrhizal (AM) activity. Attempts were made to improve P nutrition of upland rice by exploiting this natural situation through different crop rotations and application of AM fungal (AMF) inoculum. The effect of a 2-year crop rotation of maize (Zea mays L.) followed by horse gram (Dolichos biflorus L.) in the first year and upland rice in the second year on native AM activity was compared to three existing systems, with and without application of a soil–root-based inoculum. Integration of AM fungal inoculation with the maize–horse gram rotation had synergistic/additive effects in terms of AMF colonization (+22.7 to +42.7%), plant P acquisition (+11.2 to +23.7%), and grain yield of rice variety Vandana (+25.7 to +34.3%).     

Arbuscular mycorrhizal fungi colonization and phosphorus nutrition in organic field pea and lentil

Phosphorus (P) can be low in soil under low input organic management; however, beneficial crop plant associations with arbuscular mycorrhizal fungi (AMF) are known to promote crop nutrition and increase phosphorus uptake. Thus, management strategies that promote AMF associations are particularly desirable for low-input cropping systems. The objectives of this study were to determine the impact of seeding rate on AMF colonization and the impact of AMF colonization on P concentration and uptake by organically grown field pea and lentil. Field experiments examined the impact of three seeding rates of field pea and lentil on P uptake and crop yield. Phosphorus accumulation was examined further in a controlled growth chamber experiment, in which field pea was sown at rates corresponding to those used in the field and harvested at 10-day intervals until 50 days after emergence. In the field, the level of AMF colonization of roots remained at 80% for field pea, while colonization of lentil increased with increasing seeding rates from 77% to 88%. The level of AMF colonization of field pea achieved in the growth chamber after 50 days was 80% for the two highest seeding rates and 60% for the low seeding rate. The rate at which AMF colonization occurred did not vary between treatments. Ultimately, AMF colonization level did not affect P accumulation. In contrast to several previous studies, both field and growth chamber experiments revealed that AMF colonization was not reduced at higher seeding rates. These results suggest that organic farmers may increase seeding rates without adversely affecting P nutrition.     

Glomus intraradices dominates arbuscular mycorrhizal communities in a heavy textured agricultural soil

Arbuscular mycorrhizal fungal (AMF) spore communities were surveyed in a long-term field fertilization experiment in Switzerland, where different amounts of phosphorus (P) were applied to soil. Plots receiving no P as well as plots systematically fertilized in excess to plant needs for 31 years were used to test the hypothesis that application of P fertilizer changes the composition and diversity of AMF communities. AMF spores were isolated from the field soil, identified, and counted so as to quantify the effect of P fertilization on AMF spore density, composition, and diversity. Trap cultures were established from field soil with four host plants (sunflower, leek, maize, and Crotalaria grahamiana), and the spore communities were then analyzed in substrate samples from the pots. Altogether, nine AMF species were detected in the soil. No evidence has been acquired for effect of P fertilization on spore density, composition, and diversity of AMF in both the field soil and in trap cultures. On the other hand, we observed strong effect of crop plant species on spore densities in the soil, the values being lowest under rapeseed and highest under Phacelia tanacetifolia covercrop. The identity of plant species in trap pots also significantly affected composition and diversity of associated AMF communities, probably due to preferential establishment of symbiosis between certain plant and AMF species. AMF spore communities under mycorrhizal host plants (wheat and Phacelia in the fields and four host plant species in trap pots) were dominated by a single AMF species, Glomus intraradices. This resulted in exceptionally low AMF spore diversity that seems to be linked to high clay content of the soil.Electronic supplementary material Supplementary material is available for this article at and accessible for authorised users.     

丛枝菌根真菌群落对白三叶草生长的影响

不同施肥处理影响AMF(Arbuscular mycorrhizal fungi)群体结构,然而不同AMF群体结构对植物的生长以及养分吸收的影响尚未见报道,试验利用盆栽实验研究了7种不同来源的丛枝菌根真菌(AMF)群落对白三叶草生长和N、P、K以及微量元素Cu、Zn、Mn的吸收的影响。7种AMF群落分离自长期定位施肥试验地,分别为NPK、OM、CK、1/2OM、NP、NK和PK。每年施肥量是300kg N/hm2,135kg P2O5/hm2,300kg K2O/hm2。有机肥处理的N、P、K养分量与试验地NPK处理含量相同,原料以粉碎的麦秆为主,加上适量的大豆饼和棉仁饼,有机肥经堆制发酵后施用。试验土壤采用封丘试验地土壤,经灭菌处理。试验结果表明,接种不同AMF群落均能促进三叶草的生长,对养分吸收则表现不同。分离自CK试验地的AMF群落对三叶草侵染率显著低于其它6种AMF群落。分离自1/2OM和OM试验地的AMF群落较分离自NPK、CK、NP和NK的AMF群落显著促进了三叶草对P的吸收;各种AMF群落都促进了对N和K的吸收;分离自OM、CK、1/2OM、NP、NK试验地的降低了三叶草植株N含量;分离自NPK试验地的AMF群落提高了三叶草植物K含量;对于Cu、Zn、Mn元素的吸收,不同处理存在较大的差异。AMF群落对三叶草生长以及养分吸收贡献不同,这与不同施肥管理下不同AMF群落的优势种属的侵染率、养分转化以及菌丝发育及分布有关。     

Short-term temporal variation in sporulation dynamics of arbuscular mycorrhizal (AM) fungi and physico-chemical edaphic properties of wheat rhizosphere

In this study, we investigated the pattern of short-term temporal variation in the arbuscular mycorrhizal (AM) fungi and physico-chemical edaphic properties of some wheat growing areas of the Bundelkhand region, Central India. Rhizospheric soil samples were collected every month from December 2007 to May 2008 from four wheat growing sites around Jhansi (Bundelkhand region). AM fungal root colonization, sporulation and physico-chemical edaphic properties during this period were determined and compared to evaluate the dynamics of response of wheat towards the AMF along crop maturation. Maximum AMF root colonization recorded was 54.3% indicating that AMF, particularly in low phosphorus (P) soils, can be important even in case of less responsive crop like wheat. In the two out of four sites studied, the AMF spore density increased with the increase in soil temperature. Absence of this type of pattern in remaining two sites indicated that site-specific environmental and agricultural conditions may affect the degree of wheat response to AMF. It also suggested that AMF communities inhabiting agroecosystems may exhibit considerable temporal sporulation patterns. The maximum AMF colonization was observed during February–March 2008, whereas maximum AMF sporulation was noticed during March–April 2008. Statistically significant negative correlation of AMF spore density with pH, organic carbon (OC) and available P was observed in the one of the sites studied. Overall assessment of the data indicated that season and location significantly affected the interaction of AM fungi with winter wheat necessitating the further need to understand the ecology of AMF populations with reference to specific host species under different micro-climatic conditions of Bundelkhand region.     

Soil mycorrhizal and nematode diversity vary in response to bioenergy crop identity and fertilization

The mandate by the Energy Independence and Security Act of 2007 to increase renewable fuel production in the USA has resulted in extensive research into the sustainability of perennial bioenergy crops such as switchgrass (Panicum virgatum) and miscanthus (Miscanthus× giganteus). Perennial grassland crops have been shown to support greater aboveground biodiversity and ecosystem function than annual crops. However, management considerations, such as what crop to plant or whether to use fertilizer, may alter belowground diversity and ecosystem functioning associated with these grasslands as well. In this study, we compared crop type (switchgrass or miscanthus) and nitrogen fertilization effects on arbuscular mycorrhizal fungal (AMF) and soil nematode abundance, activity, and diversity in a long‐term experiment. We quantified AMF root colonization, AMF extra‐radical hyphal length, soil glomalin concentrations, AMF richness and diversity, plant‐parasitic nematode abundance, and nematode family richness and diversity in each treatment. Mycorrhizal activity and diversity were higher with switchgrass than with miscanthus, leading to higher potential soil carbon contributions via increased hyphal growth and glomalin production. Plant‐parasitic nematode (PPN) abundance was 2.3 ×  higher in miscanthus plots compared to switchgrass, mostly due to increases in dagger nematodes (Xiphinema). The higher PPN abundance in miscanthus may be a consequence of lower AMF in this species, as AMF can provide protection against PPN through a variety of mechanisms. Nitrogen fertilization had minor negative effects on AMF and nematode diversity associated with these crops. Overall, we found that crop type and fertilizer application associated with perennial bioenergy cropping systems can have detectable effects on the diversity and composition of soil communities, which may have important consequences for the ecosystem services provided by these systems.     

Molecular diversity and distribution of indigenous arbuscular mycorrhizal communities colonizing roots of two different winter cover crops in response to their root proliferation

A clear understanding of how crop root proliferation affects the distribution of the spore abundance of arbuscular mycorrhizal fungi (AMF) and the composition of AMF communities in agricultural fields is imperative to identify the potential roles of AMF in winter cover crop rotational systems. Toward this goal, we conducted a field trial using wheat (Triticum aestivum L.) or red clover (Trifolium pratense L.) grown during the winter season. We conducted a molecular analysis to compare the diversity and distribution of AMF communities in roots and spore abundance in soil cropped with wheat and red clover. The AMF spore abundance, AMF root colonization, and abundance of root length were investigated at three different distances from winter crops (0 cm, 7.5 cm, and 15 cm), and differences in these variables were found between the two crops. The distribution of specific AMF communities and variables responded to the two winter cover crops. The majority of Glomerales phylotypes were common to the roots of both winter cover crops, but Gigaspora phylotypes in Gigasporales were found only in red clover roots. These results also demonstrated that the diversity of the AMF colonizing the roots did not significantly change with the three distances from the crop within each rotation but was strongly influenced by the host crop identity. The distribution of specific AMF phylotypes responded to the presence of wheat and red clover roots, indicating that the host crop identity was much more important than the proliferation of crop roots in determining the diversity of the AMF communities.     

Creation of a non-mycorrhizal control for a bioassay of AM effectiveness

and application times 2 weeks before sowing, at sowing and 1 week after sowing were investigated. Various Finnish field soils with their indigenous AMF communities were used. The main test plant species was oil-seed flax (Linum usitatissimum). In a comparison of sampling time, barley (Hordeum vulgare) was also used and phytotoxicity was studied additionally on red clover (Trifolium pratense), barley and pea (Pisum sativum) mutants. Sampling in the spring after the thaw resulted in the highest infectivity and AM response and the clearest differences between soils with varying AM potential. No evidence of temporal variation in benomyl effectiveness on mycorrhiza was found. The dose of benomyl sufficient to create a control with suppressed mycorrhization was 20 mg per kg soil at target moisture incorporated in the soil. Plant growth reduction in irradiated soil was observed with benomyl application 1 week after sowing only with flax and red clover. The most effective application time for benomyl was immediately before sowing.     

Arbuscular mycorrhizal fungi can induce the production of phytochemicals in sweet basil irrespective of phosphorus nutrition

The potential of three arbuscular mycorrhizal fungi (AMF) to enhance the production of antioxidants (rosmarinic and caffeic acids, RA and CA) was investigated in sweet basil (Ocimum basilicum). After adjusting phosphorus (P) nutrition so that P concentrations and yield were matched in AM and non-mycorrhizal (NM) plants we demonstrated that Glomus caledonium increased RA and CA production in the shoots. Glomus mosseae also increased shoot CA concentration in basil under similar conditions. Although higher P amendments to NM plants increased RA and CA concentrations, there was higher production of RA and CA in the shoots of AM plants, which was not solely due to better P nutrition. Therefore, AMF potentially represent an alternative way of promoting growth of this important medicinal herb, as natural ways of growing such crops are currently highly sought after in the herbal industry.     

Community assembly, species richness and nestedness of arbuscular mycorrhizal fungi in agricultural soils

Understanding how communities assemble is a central goal of ecology. This is particularly relevant for communities of arbuscular mycorrhizal fungi (AMF), because the community composition of these beneficial plant symbionts influences important ecosystem processes. Moreover, AMF may be used as sensitive indicators of ecological soil quality if they respond to environmental variation in a predictable way. Here, we use a molecular profiling technique (T-RFLP of 25S rRNA gene fragments) to test which factors determine AM fungal community composition in 40 agricultural soils in the Netherlands. In particular, we test whether species richness, dominance structure and community nestedness are influenced by management type (in pairs of organically and conventionally farmed fields), and we examine the contribution of crop species (maize vs. potato), soil type (sand vs. clay-textured soils) and habitat (plant root vs. bulk soil) on AMF community characteristics. AMF richness varied from 1 to 11 taxa per field. Communities from species-poor fields were found to be subsets of those in richer fields, indicating nestedness and a progressive 'loss' from the species pool. AMF taxa richness and occurrence in soil and plant roots were highly correlated, and richness was related to management intensity (phosphate availability and grass-cropping history together explained 32% and 50% of richness in roots and soils). Soil type together with soil chemical parameters explained only 17% of variance in AMF community structure. We synthesize these results by discussing the potential contribution of a 'bottleneck effect' on AMF communities through increased stochastic effects under environmental stress.     

Mycotrophy of crops in rotation and soil amendment with peat influence the abundance and effectiveness of indigenous arbuscular mycorrhizal fungi in field soil

Mycotrophy of previous crops has been shown to have an impact on arbuscular mycorrhizal fungi (AMF), and the growth and productivity of succeeding crops. We studied the impact of 3 years of cultivation of eight crops with different degrees of mycotrophy, including mycorrhizal (strawberry, rye, timothy, onion, caraway) and non-mycorrhizal (turnip rape, buckwheat, fiddleneck) hosts, as well as the impact of peat amendment, on the effectiveness, amount and diversity of indigenous AMF. A field experiment having a split-plot design with peat amendment as the main plot, crop cultivation as a sub-plot and three replications, was carried out on silt clay mineral soil in 1999–2001. A well-humified dark peat was applied immediately before establishment of the field experiment. Each year, the relative mycorrhizal effectiveness of soil collected in September, in terms of shoot dry weight (RME_DW), was determined in a bioassay. In the 3rd year of the experiment, AMF spores were also extracted and identified from the field soil. Expressed as the mean of 3 years of cropping in unamended soil, the mycorrhizal crops strawberry and caraway maintained RME_DW most effectively, while the values were lower in the non-host crops buckwheat, turnip rape and fiddleneck. In addition, the numbers of AM spores detected in soil were considerably greater during 3 years of strawberry cultivation. In soil under caraway, there were high numbers of AM spores compared to the other crops. In soil amended with peat, the situation was in some cases opposite of that of unamended soil; RME_DW was highest in rye and onion and lowest in strawberry and caraway. The reasons behind the negative impact of peat on mycorrhizal effectiveness in strawberry soil may be due to the microbiological properties of peat. The importance of including mycotrophic species in crop rotations for maintaining high soil quality and for increasing yields of subsequent crops is discussed.