Recoupling C and N cycles: a fertiliser template to synchronise N supply and limit C priming?

Despite their advantages in plant productivity, plant uptake from conventional nitrogen (N) fertilisers is around 50%. To test whether recoupling carbon (C) and N cycles can decrease losses and increase N use efficiency (NUE), four experiments were conducted. Two growth accelerator pot experiments in contrasting soils were conducted with N additions representing a range of molar C to N ratios (Cmol:Nmol; 0.5 to 11) formulated from urea (U) and waste-derived materials (sugarcane trash, Ct; pelletised beef manure, B; dairy digestate, D; insect larvae, L; protein meal from insect larvae, P; and anaerobic pond sludge, S) with and without a nitrification inhibitor (I). Two reaction vessel experiments were conducted for measuring N-transformation based on continuous on-line nitrous oxide (N2O) and carbon dioxide (CO¬2) monitoring, and soil analyses. Adverse impacts of these stoichiometric formulations on C priming were examined using natural abundance isotopic signatures. Relative to urea, higher N use efficiency (~2 to 3 times) was achieved with waste derived (i.e., D, S, B, and P) formulations with I, with and without C. Increased Cmol:Nmol ratio increased post-experiment N retention in the fertiliser layer and delayed N uptake by the plant. Carbon priming losses were least with formulation Cmol:Nmol from 3.1 to 6.2. Our results suggest waste-derived stoichiometrically tailored formulations with an inhibitor have potential to avoid soil C priming, deliver better NUE, decrease N leaching losses, and increase N stocks in the soil profile.