Mitigating nitrate leaching using dicyandiamide and acidified ammonium sulfate solid digestate in Western Australian sandy soil

Context Food waste contributes significantly to greenhouse gas (GHG) emissions, particularly CO2 and N2O from agricultural production processes via landfill and incineration disposal. Aims This study evaluated the potential of the nitrification inhibitor dicyandiamide (DCD) to reduce nitrate leaching and N2O emissions from applied acidified ammonium sulfate solid digestate (ASSD), a stabilised by-product of food waste anaerobic digestion, compared to conventional urea ammonium nitrate (UAN) fertiliser. Methods Two soil incubations and a leaching trial were conducted using a sandy soil from southwestern Australia, testing five treatments (control, UAN, UAN + DCD, ASSD, and ASSD + DCD) at 140 mg N kg−1 soil and 35 mg DCD kg−1. Key results Application of DCD was associated with inhibited nitrification and lower nitrate-nitrogen leaching by 75.54% (ASSD; 104.27 to 25.51 μg) and 34.11% (UAN; 469.09 to 309.07 μg). The DCD also lowered N2O emissions from UAN-treated soil by 44.64% (0.56 to 0.31 μg N2O g−1 soil) but had no significant impact (P > 0.05) with ASSD. While CO2 emissions were unaffected by DCD, ASSD showed reduced emissions compared to the control during days 2–14, suggesting possible microbial suppression due to acidity or negative nitrogen priming. Conclusion Under the sandy-soil laboratory conditions tested, DCD may improve nitrogen retention for ASSD, primarily through reduced nitrate-nitrogen leaching, although N2O mitigation was observed for UAN only. Implications These findings support nitrification inhibitors as a tool to reduce nitrate leaching risk in sandy soils, but GHG outcomes may be fertiliser-form-specific. Future research should explore soil amendments to buffer acidification and evaluate performance across a wider range of soils and field conditions. Including a mineral ammonium sulfate control, a DCD-only control and multiple DCD rates would further strengthen fertiliser-form comparisons.