The mycorrhizal hyphosphere: a key driver of biogeochemical cycles?

自工业革命以来，大气中的二氧化碳（CO2）水平一直在稳步上升，这主要是人类活动的结果。二氧化碳含量的增加导致更多的热量被太阳吸收，从而导致全球变暖或“温室效应”。这反过来又对地球上的生命造成严重后果，造成生物多样性丧失、冰川融化、森林火灾和致命的热浪。二氧化碳含量增加的主要原因之一是燃烧化石燃料，将储存了几个世纪的碳迅速释放回大气中。为了减少化石燃料的使用，我们可以种植能源作物。这类作物的大规模生产将极大地改变农业格局。能源（或“生物质”）作物是“碳中性的”；当燃烧发电时，它们释放回大气的二氧化碳量与它们固定的量相同。因此，没有“额外”的二氧化碳被释放到大气中。能源作物包括柳树和杨树等树种。这些植物的根与天然存在于土壤中的菌根真菌形成共生关系，菌根一词的字面意思是“真菌的根”。在7种不同类型的菌根结合力中，最重要的两种类型是在树种上形成的外生菌根结合力（ECM）和主要在草本物种上形成的丛枝菌根结合力（AM）。虽然大多数植物只形成一种类型的联系，但柳树和杨树都是不寻常的，因为它们可以形成外生菌根或丛枝菌根联系。与菌根真菌的关联可以通过促进植物生长、增强营养捕获和抑制疾病而对植物产生直接益处。作为回报，菌根真菌从寄主植物获得碳供应，帮助其生长并在根外产生广泛的菌丝体。由于生长的根的存在以及从根释放到土壤中的化合物，植物直接影响其根周围的土壤（称为“根际”）。这些化合物大多是通过一种被称为“渗出”的被动过程从根部流失的，它们是低分子量的，包括氨基酸、单糖和有机酸。这些化合物是微生物群落的理想底物；因此，与散装土壤（即不含根的土壤）相比，根际的微生物数量总是更高。菌根真菌在根中的定植改变了从根中释放化合物的数量和质量，但尚不清楚真菌在多大程度上对释放的化合物的数量和类型做出了贡献。此外，在菌根植物中，真菌的外部阶段（而不是根）是与土壤接触的主要区域，因此从菌丝中渗出的化合物也可能影响“菌丝圈”中的微生物活动。有一些证据表明，ECM真菌可以释放简单的化合物，但关于AM真菌的信息却很少，尽管它们被认为可以通过从菌丝中释放化合物来改善土壤结构。因此，在本研究中，我们将研究ECM和AM真菌对周围环境的影响程度。我们将通过使用碳同位素14C（这是一种放射性同位素，但非常敏感，我们可以从中获得14C从植物流向真菌的图像）和13C（一种稳定的、无害的C同位素）来确定有多少碳从植物流向真菌。我们还将通过化学分析和使用生物传感器微生物来确定从真菌中渗出的化合物类型，生物传感器微生物可以使用一种称为nanoSIMS的新方法（通过光遗漏）报告释放的化合物和沿着菌丝释放的位点。菌丝渗出对土壤反硝化这一关键过程的影响将被量化，这一过程会导致N的损失，也会引起环境和经济方面的关注。

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.