Fig. 5.

Phosphorus cycling in three different climatic zones. For each climatic zone, arid (a), subhumid (b), and humid (c), the size of boxes represents the relative size of the pool and the size of arrows represents the relative importance of the respective flux. Pool colors correspond to δ¹⁸O_P values. We assumed four process signals: first, weathering of primary apatite with a δ¹⁸O_P of 10‰; second, equilibration with water by pyrophosphatase in soil microbes (18–25‰); third, mineralization of organic P by hydrolyzing enzymes (7–17‰, depending on the enzyme), and fourth release of plant P (23–35‰) (Supplementary Tables 4 and 5). At arid sites, δ¹⁸O_P of soil NaOH-Po could not be measured due to small concentrations