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

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.