Soil Evaluation of Poly(isosorbide carbonate) Ammonolysis Products as a Nitrogen Fertilizer for Brassica rapa

The effectiveness of poly(isosorbide carbonate) (PIC)—a bio-based polycarbonate synthesized from isosorbide (ISB)—degradation products in complex soil environments remains unverified. In the present study, the agricultural potential of PIC ammonolysis products—comprising urea and ISB—as a nitrogen (N) fertilizer was evaluated in a soil system. A pot experiment was conducted using komatsuna (Brassica rapa var. perviridis) to compare PIC degradation products with commercial urea, a urea and ISB mixture, and a no-N control. Application of PIC degradation products significantly enhanced plant growth, yielding fresh weight and N uptake comparable to those associated with commercial urea. The calculated N recovery efficiency for the PIC degradation products was 50%, falling within the typical range (30–60%) for inorganic fertilizers. Contrary to previous in vitro study results, ISB exhibited no significant biostimulant effect in the soil environment, likely owing to rapid microbial degradation. These findings serve as a preliminary proof-of-concept that PIC degradation products are a highly effective and bioavailable N source. Overall, the results suggest that if scaled appropriately, PIC ammonolysis products could contribute to circular use pathways for this specific plastic. Nonetheless, further studies across a broader range of crops and soils are required to confirm the generalizability of these results.