Arbuscular mycorrhizal fungal diversity and potential association networks among African tropical forest trees.

Tropical forests represent one of the most diverse and productive ecosystems on Earth. High productivity is sustained by efficient and rapid cycling of nutrients, which is in large part made possible by symbiotic associations between plants and mycorrhizal fungi. In these associations, an individual plant typically associates simultaneously with multiple fungi and the fungi associate with multiple plants, creating complex networks among fungi and plants. However, there are few studies that have investigated mycorrhizal fungal composition and diversity in tropical forest trees, particularly in Africa, or that assessed the structure of the network of associations among fungi and trees. In this study, we collected root and soil samples from Ise Forest Reserve (Southwest Nigeria) and used a metabarcoding approach to identify the dominant arbuscular mycorrhizal (AM) fungal taxa in the soil and associating with ten co-occurring tree species to assess variation in AM communities. Network analysis was used to elucidate the architecture of the network of associations between fungi and tree species. A total of 194 Operational Taxonomic Units (OTUs) belonging to six AM fungal families were identified, with 68% of all OTUs belonging to Glomeraceae. While AM fungal diversity did not differ among tree species, AM fungal community composition did. Network analyses showed that the network of associations was not significantly nested and showed a relatively low level of specialization (H2 = 0.43) and modularity (M = 0.44). We conclude that, although there were some differences in AM fungal community composition, the studied tree species associate with a large number of AM fungi. Similarly, most AM fungi had great host breadth and were detected in most tree species, thereby potentially working as interaction network hubs.

Spatial variability and environmental drivers of cassava-arbuscular mycorrhiza fungi (AMF) associations across Southern Nigeria.

Cassava, forming starch-rich, tuberous roots, is an important staple crop in smallholder farming systems in sub-Saharan Africa. Its relatively good tolerance to drought and nutrient-poor soils may be partly attributed to the crop's association with arbuscular mycorrhiza fungi (AMF). Yet insights into AMF-community composition and richness of cassava, and knowledge of its environmental drivers are still limited. Here, we sampled 60 cassava fields across three major cassava-growing agro-ecological zones in Nigeria and used a DNA meta-barcoding approach to quantify large-scale spatial variation and evaluate the effects of soil characteristics and common agricultural practices on AMF community composition, richness and Shannon diversity. We identified 515 AMF operational taxonomic units (OTUs), dominated by Glomus, with large variation across agro-ecological zones, and with soil pH explaining most of the variation in AMF community composition. High levels of soil available phosphorus reduced OTU richness without affecting Shannon diversity. Long fallow periods (> 5 years) reduced AMF richness compared with short fallows, whereas both zero tillage and tractor tillage reduced AMF diversity compared with hoe tillage. This study reveals that the symbiotic relationship between cassava and AMF is strongly influenced by soil characteristics and agricultural management and that it is possible to adjust cassava cultivation practices to modify AMF diversity and community structure.

Improved genotypes and fertilizers, not fallow duration, increase cassava yields without compromising arbuscular mycorrhizal fungus richness or diversity.

Arbuscular mycorrhizal fungi (AMF) are ubiquitous in agroecosystems, but their role in mediating agricultural yield remains contested. Field experiments testing effects of realistic agronomic practices of intensification on AM fungus composition and yields are scarce, especially in the low-input systems of sub-Saharan Africa. A large, full-factorial field experiment was conducted in South-Kivu (DR Congo), testing effects of fallow duration (6 vs. 12 months), genotype (landrace vs. improved), and fertilizer management (control vs. five combinations omitting N, P, K, and/or secondary macro- and micronutrients) on yields of cassava, an important staple crop strongly colonized by AMF. Furthermore, we used DNA-metabarcoding to evaluate effects of these agronomic practices on the AM fungal communities on the roots. The shorter fallow duration strongly increased diversity and richness of AMF, but this did not correspond with increased yields. Cassava yield was mainly determined by genotype, being largest for the improved genotype, which coincided with a significantly higher sum of AM fungal sequences. Effects of fertilizer or genotype on community composition were minor to absent. We found no evidence that increased AMF richness and diversity enhanced cassava yields. In contrast, the use of the improved genotype and mineral fertilizers strongly benefitted yields, without compromising richness or diversity of AMF. Cassava-AMF associations in this work appear robust to fertilizer amendments and modern genotype improvement.

Partner turnover and changes in ectomycorrhizal fungal communities during the early life stages of European beech (Fagus sylvatica L.).

The first life stages of a tree are subject to strong environmental stresses and competition, limiting their chances of survival. Establishing a mutualistic relationship with mycorrhizal fungi during early life stages may increase growth and survival rates of trees, but how mycorrhizal communities assemble during these stages remains unclear. Here, we studied variation in the ectomycorrhizal (EcM) fungal communities in the soil and roots of Fagus sylvatica seedlings and saplings. Fungal DNA was extracted from the soil and seedling and sapling roots collected in 156 plots across the beech-dominated Sonian forest (Belgium) and community composition was determined through metabarcoding. EcM fungal community composition significantly differed between soil, seedlings and saplings. Russula, Amanita and Inocybe were most abundant in soil, while Lactarius and Scleroderma were more abundant in seedling and sapling roots and Xerocomellus and Laccaria were most abundant in sapling roots. Our results provide evidence of partner turnover in EcM fungal community composition with increasing age in the early life stages of F. sylvatica.

The Effect of Surrounding Vegetation on the Mycorrhizal Fungal Communities of the Temperate Tree Crataegus monogyna Jacq.

About 90% of all land plants form mycorrhiza to facilitate the acquisition of essential nutrients such as phosphorus, nitrogen, and sometimes carbon. Based on the morphology of the interaction and the identity of the interacting plants and fungi, four major mycorrhizal types have been distinguished: arbuscular mycorrhiza (AM), ectomycorrhizal (EcM), ericoid mycorrhiza, and orchid mycorrhiza. Although most plants are assumed to form only one type of mycorrhiza, some species simultaneously form associations with two mycorrhizal types within a single root system. However, the dual-mycorrhizal status of many species is under discussion and in some plant species the simultaneous association with two mycorrhizal types varies in space or time or depends on the ecological context. Here, we assessed the mycorrhizal communities associating with common hawthorn (Crataegus monogyna), a small tree that commonly associates with AM fungi, and investigated the potential factors that underlie variation in mycorrhizal community composition. Histological staining of C. monogyna roots showed the presence of a Hartig net and hyphal sheaths in and around the roots, demonstrating the capacity of C. monogyna to form EcM. Meta-barcoding of soil and root samples of C. monogyna collected in AM-dominated grassland vegetation and in mixed AM + EcM forest vegetation showed a much higher number of EcM sequences and OTUs in root and soil samples from mixed AM + EcM vegetation than in samples from pure AM vegetation. We conclude that C. monogyna is able to form both AM and EcM, but that the extent to which it does depends on the environmental context, i.e., the mycorrhizal type of the surrounding vegetation.

Mycorrhizal Switching and the Role of Fungal Abundance in Seed Germination in a Fully Mycoheterotrophic Orchid, Gastrodia confusoides.

Mycorrhizal associations are essential for orchid germination and seedling establishment, and thus may constrain the distribution and abundance of orchids under natural conditions. Previous studies have shown that germination and seedling establishment in several orchids often decline with increasing distance from adult plants, resulting in non-random spatial patterns of seedling establishment. In contrast, individuals of the fully mycoheterotrophic orchid Gastrodia confusoides often tend to have random aboveground spatial patterns of distribution within bamboo forests. Since G. confusoides is parasitic on litter-decaying fungi, its random spatial patterns of distribution may be due to highly scattered patterns of litter-decaying fungi within bamboo forests. To test this hypothesis, we first identified the main mycorrhizal fungi associating with developing seeds and adult plants at a bamboo forest site in Taiwan using Miseq high-throughput DNA sequencing. Next, we combined seed germination experiments with quantitative PCR (qPCR) analyses to investigate to what extent the abundance of mycorrhizal fungi affected spatial patterns of seed germination. Our results show that seed germination and subsequent growth to an adult stage in G. confusoides required a distinct switch in mycorrhizal partners, in which protocorms associated with a single Mycena OTU, while adults mainly associated with an OTU from the genus Gymnopus. A strong, positive relationship was observed between germination and Mycena abundance in the litter, but not between germination and Gymnopus abundance. Fungal abundance was not significantly related to the distance from the adult plants, and consequently germination was also not significantly related to the distance from adult plants. Our results provide the first evidence that the abundance of litter-decaying fungi varies randomly within the bamboo forest and independently from G. confusoides adults.

Local abiotic conditions are more important than landscape context for structuring arbuscular mycorrhizal fungal communities in the roots of a forest herb.

Due to human influence, large tracts of natural vegetation have been cleared and replaced by other types of land use, resulting in highly fragmented landscapes consisting of small fragments of well-conserved habitat scattered within a matrix of intensively managed land. Changes in land use and associated fragmentation have important consequences for biodiversity in the remaining fragments. Most studies so far have investigated the impact of land use change on macro-organisms, but little is known about how landscape fragmentation affects microbial communities. Here, we studied how changes in land use and abiotic conditions affected the arbuscular mycorrhizal fungal (AMF) communities in the roots of the forest herb Stachys sylvatica. Root samples were collected from 40 populations occurring in fragmented forest patches of varying age and size embedded within an agricultural landscape. Our results showed that forest age and isolation did not affect AMF diversity or community composition, suggesting that AMF disperse easily throughout the landscape and that AMF communities reassemble fast in recently established forest patches. On the other hand, AMF richness increased with increasing forest area, indicating that small forest sizes limit AMF richness. Additionally, AMF richness increased with increasing soil pH and decreased with soil nitrate content, while AMF community composition was affected plant-available phosphorus. Overall, these results show that landscape context is less important than local abiotic conditions for structuring AMF communities. However, the significant area effect indicates that further reductions in forest area will lead to impoverished AMF communities, potentially affecting long-term plant fitness and community structure.

Forest edge effects on the mycorrhizal communities of the dual-mycorrhizal tree species Alnus glutinosa (L.) Gaertn.

Forest conversion into agricultural land has resulted in a continuous decline in forest cover and in a reduced size and increased edge-to-core ratio of the remaining fragments. Forest edges are more directly exposed to sunlight, wind and pollutants and the resulting changes in habitat quality might have a large impact on plant and animal communities. Few studies, however, have focused on forest edge effects on mycorrhizal fungus communities. Here, we used high-throughput sequencing to study how communities of arbuscular mycorrhizal (AMF) and ectomycorrhizal fungi (EcMF), present in both the roots of the dual mycorrhizal tree Alnus glutinosa and in the soil, changed with increasing distance from the forest edge within fragmented forests embedded in an intensively managed agricultural matrix. Overall, we found 158 AMF OTUs and 275 EcMF OTUs. Soil moisture content increased with increasing distance from the forest edge, whereas soil nitrate concentration increased with increasing distance in south-facing and decreased in north-facing edges. Distance to the forest edge had a significant effect on EcMF community composition that largely overlapped with the observed changes in soil variables, especially soil moisture content. Apart from this distance effect, there were also clear effects of edge orientation on mycorrhizal diversity and community composition. While AMF OTU richness was higher at south- than at north-facing edges, the opposite pattern was found for EcMF. Community composition of both mycorrhiza types also differed significantly between south- and north-facing edges. We conclude that altered environmental conditions at forest edges cause significant changes in mycorrhizal communities, which could subsequently affect ecosystem functioning.