Ammonia and nitrous oxide emissions from a field Ultisol amended with tithonia green manure, urea, and biochar

Short-term mitigation of ammonia (NH₃) and nitrous oxide (N₂O) emissions by biochar soil amendments has been reported, but limited knowledge of the mechanisms, particularly those associated with long term changes, remain relatively unknown. In order to investigate potential mechanisms and residual effect of biochar on NH₃ and N₂O emission, a 3-year field trial was set up on an Ultisol in western Kenya with a three-replicate full factorial treatment structure. The factors investigated include the following: biochar (from eucalyptus wood, pyrolyzed at 550ºC, applied once before the start of the experiment at either 0 or 2.5t ha^-1); tithonia green manure applied at the start of each season at either 0, 2.5, or 5.0 t ha^-1; mineral nitrogen (N) (as urea applied each season at either 0 or 120 kg N ha^-1). NH₃ as well as N₂O emission and water-filled pore space (WFPS) were monitored throughout the 3 years. In the third year, soil mineral nitrogen (exchangeable NH₄⁺ and NO₃^–) contents were measured. Biochar reduced cumulative emissions of NH₃ and N₂O by 47 ± 5 and 22% ± 3, respectively, over the 3 years. Over the 3 years, the effect size of biochar was reduced by 53 and 59% for NH₃ and N₂O, respectively, indicating that the residual effect of biochar on NH₃ and N₂O persists at least up to 3 years under field conditions. Tithonia and urea additions increased both gas emissions by 13-68% compared to the control. Combination of the three amendments reduced cumulative NH₃ emissions by 18 ± 3%, but had no effect on cumulative N₂O. Our results show that biochar can influence emissions of NH₃ and N₂O longer than most previous studies have reported but is not explained by N dynamics. Other mechanisms such as direct interactions with oxidized biochar surfaces could be more likely to account for the residual effect of biochar on NH₃ and N₂O in agricultural soils.

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