Poly-3-hydroxybutyrate (P3HB), a biopolymer synthesized by soil bacteria, has emerged as a promising tool for sustainable agriculture, offering dual benefits as a carbon reservoir and an eco-friendly biotechnological product. However, its impact on soil nutrient dynamics and plant nutrient uptake remains underexplored. This study evaluated the effects of P3HB biodegradation on soil properties and maize (Zea mays) growth in a pot experiment with five P3HB application rates (0-10 % w/w), in both planted and unplanted soils. Key analyses included soil pH, enzyme activity, microbial biomass carbon (MBC), nutrient contents in soil and plant biomass, and residual P3HB (a rarely addressed aspect in previous research). The addition of P3HB influenced soil biota in both planted and unplanted soils, showing consistent trends across application rates. P3HB reduced soil pH (from 7.4 to 7.1 at 1 % and 6.4 at 10 % P3HB in unplanted soil) and increased total carbon (by approximately 100 % in unplanted and 65 % in planted soils at 10 % P3HB). In unplanted soils, P3HB degraded more quickly, but enzyme activities of β-glucosidase and phosphatase decreased by 20 % and 15 %, respectively. Conversely, arylsulphatase and urease activities increased by 80 % and 200 %, respectively, in both soil variants in both variants. Microbial biomass carbon increased by 500 % in unplanted soils compared to the unamended control, while planted soils showed a 10 % increase. Available nutrients (K and P) were higher in unplanted soils compared to planted soils. In planted soils, competition for nutrients (N, P, K) among maize plants, the rhizobiome, and P3HB-degrading microbes led to reduced above-ground biomass at higher P3HB application rates (from 5.6 g to 0.5 g per plant at 1 % P3HB). Statistical analysis (Eta-squared values and ANOVA) revealed that P3HB dose primarily influenced soil physico-chemical properties and plant parameters, whereas maize planting had a smaller impact, affecting only pH and MBC. P3HB biodegradation improved soil properties, particularly by increasing MBC and total carbon. However, application rates of 1 % and above caused slight acidification, increased nutrient competition, and reduced nutrient availability, ultimately hindering maize growth. These results underscore the trade-offs between improving soil quality and maintaining crop productivity, highlighting the importance of optimizing P3HB application rates in agricultural systems. This study provides critical insights into the dual effects of biodegradable plastics like P3HB, emphasizing their potential as microbial carbon storage polymers while cautioning against excessive use in crop production.