# Phosphorus Dynamics in the Soil-Plant-Environment Relationship in Cropping Systems: A Review
## 5 Role of Soil Microbiota on P Processes  
Sustainable agriculture is the main strategy to fight the quick deterioration of the environmental quality by maintaining the ecosystem balance. Some restrictions can limit crop productivity, such as limited P in the soil, which frequently needs to be restored repeatedly in order to satisfy the demand of the plant. This is partly due to its quick loss when in its labile form, arising from the extraction of the element (P) by the crop (nutritional requirement). Furthermore, other processes include adsorption, precipitation, and microbial immobilization.  
The tight interaction of microorganisms with roots may provide a greater availability of nutrients for plant (Figure 2). Some microorganisms have the ability to promote plant growth and productivity and are recognized as plant-growth-promoting microorganisms (PGPM) [90]. Phosphate-solubilizing microorganisms (PSB), for example, constitute an important group of PGPM, since they are involved in a wide range of processes that affect P transformation, being integral components of the soil cycle of this nutrient [91].  
Bacteria are the predominant microorganisms that solubilize mineral phosphate in soil when compared to fungi and actinomycetes [92,93]. In general, solubilizer bacteria outnumber fungi from 2- to 150-fold [94,95]. On the other hand, most solubilizing microorganisms can solubilize calcium phosphate complexes and only some of them can solubilize aluminum or iron phosphates [96].  
As plants, microorganisms have developed several mechanisms to solubilize and mobilize P, including the release of P-solubilizing protons organic acid anions such as oxalate or citrate that solubilize inorganic P by chelating Al\({}^{3+}\) and basic cations (Ca\({}^{2+}\), Mg\({}^{2+}\)) bound to P, siderophores and polyphenols, and extracellular phosphatases that hydrolyze organic P [97,98,99].  
While microorganisms are able to produce both acid and alkaline phosphatases, plants can produce only acid ones [100; 101; 102].  
In addition, microorganisms directly affect the ability of plants to obtain P from the soil by increasing the extension of the root surface area through mycorrhizal associations or by promoting the growth of lateral roots and root-absorbing hairs through phytohormones [103].  
Microorganisms may affect P bioavailability due to the tight link with C and N cycles. Microbial organic matter mineralization generates a stoichiometric C:N:P relationship that must be considered in the application of fertilizers, either using N and P fertilizers combined with organic matrices, or maintaining crop residues with direct seeding or reduced tillage management [104]. Some studies [105] observed that applying N and P fertilizers increased rice straw mineralization by 25% and 10%, respectively. This is because N and P addition stimulates microbial activity, with higher CO\({}_{2}\) flow but a lower emission of methane. In addition, a combination of fertilization with N and P stimulated microbial activity, resulting in a higher production rate of extracellular enzymes, which accelerated organic matter decomposition. On the other hand, the high CO\({}_{2}\) emission under P deficiency depressed rice productivity in terms of biomass. Some studies [106] determined that the response of CO\({}_{2}\) to low P concentrations (<1.8 g kg\({}^{-1}\) in the leaf) was higher than with greater concentrations. This may be related to diverse P-recycling phases within the rice plant, which has multiple stems depending on phenology stage, and reflects the high P mobility and its redistribution among different organs or phonologic structures [107].  
Figure 2: Dynamics of some microbial agents that present a high activity rate in the worldwide agricultural systems.