Short-circuiting the terrestrial phosphorus cycle: symbiotic control of organic phosphorus mineralisation and uptake

植物需要从土壤中吸收磷才能生长。要做到这一点，几十年来，人们一直认为植物只能从土壤中获取矿物(无机)形式的磷，事实上，这些矿物形式是大多数化肥的基础。土壤中的无机磷主要是通过微生物对有机形态的转化而产生的，有机形态通常构成土壤中主要的磷库。这种所谓的“矿化”过程也被认为主要是由自由生活的土壤微生物进行的。然而，我们最近在NERC资助的研究中的发现表明，与根部土壤真菌(称为外生菌根真菌)形成密切关系的树木可以从有机和无机形式中获取磷。这些发现提出了具有全球重要性的问题，挑战了我们对陆地磷循环的根深蒂固的理解：1.植物能否通过与根相关的共生外生菌根真菌直接获取有机形态的磷，即保持化学物质的完整，从而使传统的矿化途径变得短路？2.外生菌根真菌能否加速有机磷的矿化？3.当磷的需求增加时，例如由于大气中的氮污染，会发生什么？获取有机形态磷的潜力将使与外生菌根真菌形成联系的植物能够进入土壤中原本无法获取的养分池。这些获得磷的机制也可以解释为什么在根部形成不同类型联结的植物可以共存。这些发现还可能解释植物群落如何应对由于大气氮污染而导致的生态系统磷限制的增加。我们现在能够通过同位素标记的有机磷合成方面的最新进展来解决这些问题。因此，在这项提议中，我们将合成一套与生态相关的有机磷形式，在它们上面贴上放射性标签，使我们能够可视化和测量这些化学物质的移动和分解。我们将检验以下假设：i)外生菌根真菌直接获得有机形式并将这些营养转移到植物中，ii)外生菌根植物通过加速其矿化从有机形式中获取磷，以及iii)这些过程是在由于持续的氮输入而受到磷强烈限制的系统中被刺激的。我们的工作将对理解以外生菌根树为主的林地和森林中的生物地球化学循环，以及磷的生态位分配如何解释菌根类型的共存产生重大影响。

Plants need to take up phosphorus from soil to grow. To do this, it has been assumed for decades that plants can only access mineral (inorganic) forms of phosphorus from soil, and indeed these mineral forms are the basis of most fertilisers. Inorganic phosphorus is largely created in soil through microbial conversion of organic forms, which usually comprise the main pool of phosphorus in soils. This so-called 'mineralisation' process is also assumed to be largely undertaken by free-living soil microorganisms. However, our recent discoveries from NERC-funded research suggest that trees that form intimate relationships with soil fungi (called ectomycorrhizal fungi) on their roots can acquire phosphorus from both organic and inorganic forms. These findings raise questions of global importance that challenge our entrenched understanding of the terrestrial phosphorus cycle: 1. Can plants, via their symbiotic root-associated ectomycorrhizal fungi, acquire organic forms of phosphorus directly, i.e. keeping the chemical intact, thus 'short-circuiting' the conventional mineralisation pathway? 2. Can ectomycorrhizal fungi accelerate mineralisation of organic phosphorus? 3. What happens when the demand for phosphorus increases, for example because of nitrogen pollution from the atmosphere? The potential to acquire organic forms of phosphorus would give plants that form associations with ectomycorrhizal fungi access to otherwise inaccessible pools of nutrients in soil. These mechanisms of phosphorus acquisition may also provide explanations as to why plants that form different types of associations on their roots can coexist. The findings may also explain how plant communities may respond to increasing phosphorus limitation of ecosystems that is occurring as a consequence of atmospheric nitrogen pollution. We are now able to address these questions through recent developments in the synthesis of isotopically-labelled organic forms of phosphorus. In this proposal, we will therefore synthesise a suite of ecologically-relevant organic forms of phosphorus that have a radioactive tag attached to them to enable us to visualise and measure the movement and breakdown of these chemicals. We will test the hypotheses that i) ectomycorrhizal fungi acquire organic forms directly and transfer these nutrients to plants, ii) ectomycorrhizal plants acquire phosphorus from organic forms by accelerating their mineralisation, and iii) these processes are stimulated in systems that are strongly limited by phosphorus as a consequence of sustained inputs of nitrogen. Our work will have major impact on understanding biogeochemical cycles in woodlands and forests that are dominated by ectomycorrhizal trees, and how niche partitioning of phosphorus may explain coexistence of mycorrhizal types.