## 4 Discussion
### Crop Productivity
Under excessive phosphorus conditions, P can suppress the absorption of cationic micronutrients, particularly zinc (Zrn), but also copper (Cu), iron (Fe), and manganese (Mn). Excessive P use can also reduce CO\({}_{2}\) fixation and starch synthesis [40], and this nutrient imbalance may explain thedecrease in morphological attributes and crop productivity at the highest residual dose (240 kg ha\({}^{-1}\) P\({}_{2}\)O\({}_{5}\)). However, under P deficiency, plants accumulate more sugars as a form of potential chemical energy. At the same time, the lack of ATP limits biosynthesis processes, decreasing RNA, starch, and lipid synthesis, leading to protein deficiency and the increased accumulation of soluble nitrogen compounds, which hinders vegetative development. P also regulates photosynthesis and carbohydrate metabolism, mitigating plant growth limitations [9]. This suggests that the soybean crop in this study did not experience P deficiency, as uniform productivity was observed across residual P\({}_{2}\)O\({}_{5}\) treatments. In another study evaluating the effect of broadcast phosphorus doses (40, 80, 120, and 160 kg ha\({}^{-1}\) of P\({}_{2}\)O\({}_{5}\)) on soybean development and yield, no significant differences were observed among treatments regarding grain yield [41]. However, these results are justified by unfavorable climatic conditions during the experiment. The bacterium _A. brasilense_ has a modest biological nitrogen fixation (BNF) capacity compared to _Bradyrhizobium_. However, _Azospirillum_'s primary microbial function is promoting plant growth through phytohormone production, particularly in root systems, enhancing nodulation and BNF performed by _Bradyrhizobium_. Additionally, it increases root volume, expanding the soil contact surface for nutrient uptake. Thus, soybean plants co-inoculated with _Bradyrhizobium_ on seeds and _Azospirillum_ in the soil (residual effect from previous grass crops in rotation) exhibited earlier and increased nodulation, leading to an average yield increase of 16% [42; 43]. In a study conducted by [44], it was also reported that co-inoculation with _Bradyrhizobium_ sp. and _Azospirillum_ in soybeans promoted an 11% increase in root dry mass, a 5.4% increase in the number of nodules, a 10.6% increase in nodule dry mass, and a 3.2% increase in grain yield compared to conventional inoculation with _Bradyrhizobium_ sp. However, the results obtained in the present study, which showed no significant differences in productivity between inoculated and non-inoculated areas, can possibly be explained by the favorable chemical conditions of the soil. These conditions are a result of the management practices employed in the area, such as the adoption of no-tillage systems and the rotation of different plant species. These practices also contribute to the formation of biopores, which are subsequently reused by successive crops, reducing the plant's energy expenditure for root development [45]. Additionally, they allow for deeper root growth, leading to a greater volume of explored soil, which ensures proper nutrition and production of the crop. Given the above, it can be inferred that the lack of significant differences between treatments may be attributed to the high quantity and quality of organic material in the area. This results from a consolidated no-tillage system under an integrated crop-livestock production management, where the chemical and biological balance of the soil is stabilized, thus minimizing the effects of the treatments. Since up to 80% of the total phosphorus (P) in soils originates from plant residues (straw) and microbial biomass, which, upon decomposition by soil microbiota, converts organic P into mineral P through mineralization [46], this process is likely the reason why sorghum productivity did not show significant differences in response to residual P\({}_{2}\)O\({}_{5}\) doses in the present study (Table 7).