Increasing soil aeration reduces mitigation efficacy of dicyandiamide when targeted at ruminant urine-derived N₂O emissions

Soil aeration effects on the efficacy of dicyandiamide (DCD) at reducing reactive nitrous oxide (N₂O) fluxes from ruminant urine patches have not been assessed. We tested the null hypothesis that increasing soil aeration and dissolved organic carbon (DOC) levels, following ruminant urine deposition, would reduce DCD efficacy, as determined from measurements of N₂O fluxes. Soil aeration was controlled (-1 or -10 kPa, the former being wetter and less aerated) within a factorial experiment that also included DCD and ruminant urine (700 kg N ha⁻¹) treatments, destructively sampled over 40 days. Increased soil aeration corresponded with a decrease in DCD efficacy. However, contrary to the null hypothesis, increasing soil DOC concentrations following urine application did not correspond with greater DCD degradation rates. In fact, at -1 kPa applying urine increased the DCD half-life from 23.5 (±3.0) to 53.3 (±7.5) days, while at -10 kPa the DCD half-life was not affected by urine application (14.9 [±2.9] and 16.7 [±2.7] days, with or without urine, respectively). The efficacy of DCD at mitigating urine-induced cumulative N₂O fluxes was 95% at -1 kPa and 57% at -10 kPa. This study confirms that the degradation of DCD is slower, and thus its efficacy is better, in wet soils compared with well-aerated soils