Phosphorus Dynamics in Stressed Soil Systems: Is There a Chemical and Biological Compensating Effect?

Here, we hypothesized the occurrence of a compensatory relationship between the application of P and different microbial communities in the soil, specifically in relation to the chemical and biological effects in the soil–plant–microorganisms’ interaction. We aimed to evaluate the plant–microbiota responses in plants grown in soils hosting distinct microbial communities and rates of P availability. Two experiments were carried out in a greenhouse. The first experiment evaluated four manipulated soil microbiome compositions, four P rates, and two plant species. Manipulated soil systems were obtained by the following: (i) autoclaving soil for 1 h at 121 °C (AS); (ii) inoculating AS with soil suspension dilution (AS + 10⁻³); (iii) heating natural soil at 80 °C for 1 h (NH80); or (iv) using natural soil (NS) without manipulation. The P rates added were 0, 20, 40, and 60 mg kg⁻¹, and the two plant species tested were grass (brachiaria) and leguminous (crotalaria). Inorganic labile P (P_AER), microbial P (P_MIC), acid phosphatase activity (AP_ASE), and shoot P uptake (P_UPT) were assessed for each system. Brachiaria presented a compensatory effect for P_UPT, whereby the addition of P under conditions of low microbial community enhanced P absorption capacity from the soil. However, in a system characterized by low P input, the increase in the soil biodiversity was insufficient to enhance brachiaria P_UPT. Likewise, crotalaria showed a higher P_UPT under high P application and low microbial community. The second experiment used three manipulated microbiome compositions: AS + 10⁻³; NH80; and NS and three P rates added: 0, 20, and 40 mg kg⁻¹. In addition, two treatments were set: without and with mycorrhiza inoculation. Brachiaria showed an increase in the P_UPT under low microbial communities (AS + 10⁻³; NH80) with P addition (20 and 40 mg kg⁻¹ of P), but no mycorrhization was observed. In the undisturbed microbial community (NS), under no P input (0 mg kg⁻¹ of P), brachiaria showed low mycorrhization and low P_UPT. Finally, NS and the recommended P input (40 mg kg⁻¹ of P) represented a balance between chemical and biological fertility, promoting the equilibrium between mycorrhization and P_UPT.