Compost and urea combination: a 15N study to smarter nitrogen recovery in tropical maize

Context Lower nitrogen (N) use efficiency and direct N losses are adversely affecting global cereal production. Yet, the exact fate of applied N in the soil and the interactions that occur are largely unknown. Aims We aimed to quantify the fate, recovery and magnitude of losses from applied N and compare the effectiveness of compost application on N recovery and yield. We hypothesised that high N losses and low N use efficiency from tropical soils can be combatted by integrated application of compost and synthetic N. Methods A randomised complete block design with two N rates and two compost rates (0 and 15 Mg ha−1) with four replicates was used. Isotopically (15N) labelled urea was applied to tropical maize and the 15N recovery, grain yield and dry matter were analyzed. Key results The combination of compost and urea increased N recovery (16% reduction in N loss), and significantly increased maize yield by 50% and stover yield by 33% in applied urea 150N (urea at 150 kg N ha−1) + compost treatment relative to the 150N only treatment, demonstrating that compost-derived N considerably increased crop N acquisition compared to urea alone. Compost addition significantly increased both apparent N recovery and mineralised N by 31% and 62%, respectively. Up to 71% N harvested was obtained from non-fertiliser sources, with 80–90% recovered in the surface 40 cm of soil. Application of urea and compost demonstrated the considerable potential to increase N recovery and enhance crop productivity of cereals. Conclusion Proving the hypothesis, beyond the effect of mineral N alone, additional organic N provided by compost significantly contributed to crop productivity and N retention in the soil. Compost is thus characterised as an ecologically balanced alternative to synthetic fertiliser, with potential decreases in losses of N, while enhancing N recovery in the plant–soil system. Implications As compost is unlikely to be commercially feasible at the rates applied and difficult to handle as a bulk, further studies should explore the most cost-effective and practical rates of compost application combined with optimised urea rates.