How available is phosphorus in soils?
Phosphorus is often in short supply
Phosphorus is an essential nutrient limiting ecosystem processes in a wide range of natural and managed land systems. Biological processes which are affected by phosphorus include plant growth and the response of ecosystem productivity to increasing carbon dioxide concentration which has far reaching consequence for food and fiber production, climate mitigation, and the functioning of the Earth system.
Soil phosphorus availability varies in time and space
The annual phosphorus requirement of plants is generally minor compared to the amount of phosphorus in the soil: e.g. 0.04 g m-2 yr-1 compared to 14 g m-2. The wide-spread limitation of plant productivity by phosphorus originates partly from the uneven spatial distribution of soil phosphorus which is highly variable in space. More importantly, limitations originate from physical and chemical sorption of phosphorus in soils which renders most of the various inorganic and organic soil phosphorus forms unavailable for direct biological uptake.
The availability of soil phosphorus for organisms is highly uncertain - in particular on time scales relevant for the current human perturbation of the carbon cycle: decades and longer. On the one hand, a complex interplay of biological, chemical and physical processes affects the transformation of phosphorus from one form to another. On the other hand, human activities affect the availability of phosphorus directly and indirectly via the resulting perturbation of the Earth system. As a consequence, large uncertainties remain regarding soil phosphorus constraints on carbon dioxide fixation and ecosystem carbon storage for present and future conditions as well as on agricultural production.
Will phosphorus become more scarce?
Current proxies for soil phosphorus availability rely on measurements of contemporary soil phosphorus stocks, in particular on readily bio-available forms not taking into account the availability of other soil phosphorus forms with turnover times of years and longer. Large-scale estimates of soil phosphorus have been based on the extrapolation of limited site data. Still they form the basis of various assessments of soil phosphorus availability for future land carbon uptake, forest and agricultural production (ref1, ref2, ref3). This in particular worrisome as only tiny fraction of soil phosphorus can make a huge difference in the land carbon balance: to sustain current carbon sequestration by the terrestrial biosphere, each year an amount corresponding to only 0.005% of total soil phosphorus is needed. As a consequence, there is a substantial risk for misinformed societal & political decisions when rely on such assessments.
Predicting soil phosphorus availability
The first process-based models of soil phosphorus dynamics which were able to predict changing soil phosphorus bio-availability on a global scale emerged more than a decade ago. Since then, little progress has been made in modelling inorganic soil phosphorus dynamics despite the identification of deficiencies in existing approaches. More progress has been made in characterizing and predicting organic phosphorus dynamics. Then years later, our capabilities to predict phosphorus availability under changing conditions are still low. To overcome these issues, we are using a suite of statistical and process-based models in combination with data from soil phosphorus fractionation and isotopic labeling experiments in order to generate data-constrained maps of soil phosphorus stocks, of biological phosphorus mineralisation, and tools to predict temporal dynamics of biological and abiotic processes.