A two years study on the combined effects of biochar and inhibitors on ammonia volatilization in an intensively managed rice field

China is the world’s largest emitter of gaseous ammonia (NH3), a compound that poses severe risks to human and ecosystem health. Adding biochar and inhibitors to soils has been suggested as a method to increase carbon sequestration and reduce nitrous oxide emissions, however, the effects of the amendments on NH3 emissions are poorly understood. We conducted a field experiment to evaluate the effect of applying biochar combined with a urease inhibitor (hydroquinone, HQ) and a nitrification inhibitor (dicyandiamide, DCD) on NH3 emissions, rice yields, and N use efficiency (NUE) during two growth seasons. Four replicates of seven treatments comprising no urea fertilizer (control), urea (N), biochar (B), urea + biochar (NB), NB + urease inhibitor (NBUI), NB + nitrification inhibitor (NBNI), and NB + urease and nitrification inhibitors (double inhibitor) (NBDI), were arranged in a randomized complete block design. Wheat straw biochar was applied once, in June 2014. Biochar in the NB treatment increased NH3 losses by 14.1% in the first rice season (P < 0.05), primarily due to increased pH and concentrations of NH4+-N in the floodwater, and decreased NH3 losses in the second rice growth season by 6.8%, probably due to its high adsorption capacity for NH4+ and increased nitrification. The combined application of biochar and DCD increased NH3 losses by 47.0% and 17.2% in 2014 and 2015, respectively. The application of biochar + double inhibitors was shown to have no effect on NH3 losses in the first rice growth season, but in the second year, NH3 losses were reduced by 19.8% (P < 0.05). The combination of biochar and HQ decreased NH3 losses by 10.5–23.4% during the two growth seasons (P < 0.05) and the addition of biochar either alone (NB), or in combination with HQ (NBUI) or double inhibitors (NBDI), increased rice yield by 7.4–16.5% and NUE from 29.4% in the N treatment (N) to 42.5%. The combined application of biochar and DCD did not have an effect on rice yield and NUE in the first year, but increased yield and NUE in the second year (P < 0.05). Overall, our results suggest that the combination of biochar and HQ or the combined application of urease and nitrification inhibitors to soil enriched with biochar at least one year previously could be an effective practice for reducing NH3 emissions and increasing rice yields.