Lauric acid controls nitrogen release and inhibits nitrification in wastewater-enriched compost

Compost is a sustainable soil amendment but typically lacks sufficient nitrogen (N) to function as a fertiliser. In this study, compost granules were enriched with NH₄NO₃-rich wastewater, increasing total N content from 1.28 % to 10 %. To regulate N release and potentially reduce environmental losses, lauric acid (LA) was applied as a coating at 5 %, 10 %, and 15 % (w/w). A 42-day glasshouse experiment with kikuyu grass evaluated N availability through shoot biomass, shoot N content, soil mineral N concentrations, and microbial responses. The modified compost (CGWW) produced the highest shoot N content (59.3 mg/pot) and biomass (3.75 g/pot). LA-coated compost treatments reduced shoot N content by 22-30 % and biomass by up to 17 % compared to CGWW, indicating lower plant N uptake due to delayed N release. Similar soil NH₄⁺-N and NO₃^--N levels at harvest suggest more organic N remained locked up in CGWWLA, likely due to microbial immobilisation. LA application suppressed ammonia-oxidising bacteria (Nitrosomonadaceae), with the most significant reduction observed in the 15 % LA-only treatment (log₂-fold change = -50.8). This microbial inhibition aligns with reduced shoot N content and supports the role of LA as a nitrification inhibitor. However, the 5 % LA coating was equally effective as higher rates, highlighting cost-efficiency. The findings highlight LA's potential in delaying N release and inhibiting ammonia oxidation, making it an effective agent for reducing N losses in fertilisers.