The mycorrhizal hyphosphere: a key driver of biogeochemical cycles?

Atmospheric carbon dioxide (CO2) levels have been rising steadily since the Industrial Revolution mainly as a consequence of human activity. Increased CO2 levels result in more heat absorbed from the sun thus contributing to global warming or 'the greenhouse effect'. This in turn has serious consequences for life on earth with loss of biodiversity, melting glaciers, forest fires and fatal heat waves. One of the major causes of increased CO2 levels is the burning of fossil fuels rapidly releasing carbon that has been stored for centuries back into the atmosphere. In order to cut our use of fossil fuels we can grow crops for energy. The large-scale production of such crops will dramatically alter the agricultural landscape. Energy (or 'biomass') crops are 'carbon neutral'; when burned to generate electricity they only release the same amount of CO2 back into the atmosphere as they fixed. Thus no 'extra' CO2 is released into the atmosphere. Energy crops include tree species such as Willow and Poplar. The roots of these plant species form symbiotic associations with mycorrhizal fungi naturally present in the soil and the term mycorrhizal literally means 'fungus-root'. Of the seven different types of mycorrhizal associations the two most important types are the ectomycorrhizal (ECM) which forms on tree species, and the arbuscular mycorrhizal association (AM), which forms mainly with herbaceous species. While most plants only form one type of association, both Willow and Poplar are unusual in that they can form both ectomycorrhizal or arbuscular mycorrhizal associations. Associations with mycorrhizal fungi can have direct benefits to the plant through increased growth, enhanced nutrient capture and disease suppression. In return, the mycorrhizal fungus obtains a supply of carbon from the host plant which helps it grow and produce an extensive mycelium external to the root. Plants influence the soil immediately surrounding their roots (called the 'rhizosphere') due to the presence of the growing root and as a result of compounds released from the root into the soil. Most of these compounds lost from the root through a passive process called 'exudation', are of low molecular weight and include amino acids, simple sugars and organic acids. These compounds are an ideal substrate for the microbial community; hence microbial populations are always higher in the rhizosphere compared to the bulk soil (i.e. soil not containing roots). Colonisation of roots by mycorrhizal fungi modifies the quantity and quality of compounds released from the roots but it is unknown to what extent the fungus actually contribute to the amount and types of compounds released. Moreover, in mycorrhizal plants, the external phase of the fungus (rather than the root) is the main zone of contact with the soil thus exudation of compounds from the hyphae may also influence microbial activities in the 'hyphosphere'. There is some evidence that ECM fungi can exude simple compounds but much less information on AM fungi although they are believed to improve soil structure by release of compounds from their hyphae. Thus, in this study we will examine to what extent ECM and AM fungi influence their surrounding environment. We will determine how much carbon flows to the fungus from the plant by using the carbon isotopes 14C (which is a radioactive isotope but very sensitive and from which we can obtain images of 14C flow through the plant to the fungus) and 13C (a stable hence non harmful isotope of C). We will also determine what types of compounds are exuded from the fungi by chemical analysis and by using biosensor microorganisms which can report back (via light omission) on the compounds released and the sites of release along the hyphae using a new approach called nanoSIMS. The influence of this hyphal exudation on a key soil process, denitrification, which results in a loss of N and which is also of environmental as well as economic concern will be quantified.

Mycorrhizal symbiosis is known to play a key role in plant nutrient capture and ecosystem sustainability, but current intensive agricultural practises tend to reduce the formation of this symbiosis. However, with the increasing diversification of arable land into biomass crop production mycorrhizal associations, and in particular mycorrhizal hyphae, will likely become the key interface between plants and soil, profoundly impacting upon soil biodiversity and biogeochemistry. However, little is known about the external hyphal phase of this important group of soil fungi. In particular, we need to know the nature of the mycorrhizal 'hyphosphere': an important mycorrhizal fungal mediated micro-environment in the soil with properties distinct from the overall rhizosphere, and if this hyphosphere differs between mycorrhizal associations (i.e. ecto- (ECM) & arbuscular mycorrhizal (AM)) and among different fungi forming these associations. To prevent confounding effects of different host plants, we will use Poplar as a model plant because it can form both ECM and AM. Investigating the nature and functional significance of the hyphosphere is therefore the aim of this proposal which will advance our understanding of plant-microbe interactions in soil and pave the way to their effective manipulation. We propose a multidisciplinary approach to address our aim involving the Universities of York (Hodge: AM, root physiology & isotopic expertise), Aberdeen (Killham, Baggs, Prosser: rhizobacterial, isotopic & molecular ecology expertise) and Warwick (Bending: ECM & isotopic expertise).We will determine the quantity and quality of carbon compounds released by the mycorrhizal fungi, the spatial-temporal dynamics of this C flux along the hyphae and its impact upon a key soil nitrogen flux: denitrification. This will be achieved by using newly developed stable isotope techniques (13C-SIP, NanoSIMS, 14C-phosphorimaging), analytical approaches (NMR, HPLC-MS) and rhizobacterial biosensors.