**7.3.2. Weathering of apatite**

Weathering and leaching processes from millions of years ago led to the transfer of phos‐ phate to rivers and oceans where it was concentrated in shells, bones and marine organism that were deposited on the sea floor. Subsequent uplift and other geological movements led to these accumulations becoming dry land deposits [25],[78].

Generally, weathering of apatite occurs synergistically through biotic and abiotic processes and leads to the release of mineral phosphate. Inorganic phosphate cannot be assimilated by plants, but it can be converted to the bioavailable form orthophosphate (HPO4 2−, H2PO4 <sup>−</sup> ) by some species of phosphate-solubilizing fungi and bacteria. The main mechanism underlying the microbial phosphate solubilization is the secretion of organic acids that, by changing the soil pH and acting as chelators, may induce the dissolution of phosphorus from minerals and its release into the pore water of soils [79],[80]. The dissolution of apatite is described in **Section 3.4**.

Apatite represents an important source of inorganic P for natural ecosystems and may favor the establishment of microbial communities able to exploit it [79]. The microorganisms can cause the fixation or immobilization of phosphate, either by promoting the formation of inorganic precipitates or by the assimilation of phosphate into organic cell constituents on intracellular polyphosphate granules. Insoluble forms of inorganic phosphorus, e.g. calcium, aluminum and iron phosphates, may be solubilized through the microbial action. The mechanisms by which the microbes accomplish this solubilization vary [44]:


$$2\text{ FePO}\_4 + 3\text{ H}\_2\text{S} \rightarrow 2\text{ FeS} + 2\text{ H}\_3\text{PO}\_4 + \text{S}^0\tag{1}$$

The phosphate-solubilizing ability is a feature of many free-living and plant-symbiotic bacterial taxa, such as [79]:


Microbial rock weathering is common in all climate zones and usually acts very slowly [80].
