Nitrapyrin effectiveness in reducing nitrous oxide emissions decreases at low doses of urea in an Andosol

In the tropics, frequent nitrogen (N) fertilization of grazing areas can potentially increase nitrous oxide (N₂O) emissions. The application of nitrification inhibitors has been reported as an effective management practice for potentially reducing N loss from the soil-plant system and improving N use efficiency (NUE). The aim of this study was to determine the effect of the co-application of nitrapyrin (a nitrification inhibitor, NI) and urea in a tropical Andosol on the behavior of N and the emissions of N₂O from autotrophic and heterotrophic nitrification. A greenhouse experiment was performed using a soil (pH 5.9, organic matter content 78 g kg–¹, and N 5.6 g kg–¹) sown with Cynodon nlemfuensis at 60% water-filled pore space to quantify total N₂O emissions, N₂O derived from fertilizer, soil ammonium (NH₄⁺) and nitrate (NO₃–), and NUE. The study included treatments that received deionized water only (control, CK) and two doses of ¹⁵N-enriched urea (65 (UR) and 129 mg N kg–¹ (UD)) without or with 350 g nitrapyrin for each 100 kg N (UR + NI and UD + NI). No significant differences were observed in soil NH+ content between the UR and UR + NI treatments, probably because of soil mineralization and immobilization (influenced by high soil organic matter content). Nitrapyrin application failed to maintain a stable pool of labeled NO₃⁻ due to the additional NO⁻ produced by heterotrophic nitrification, which is not effectively inhibited by nitrapyrin. After 56 d, N₂O emissions in UR (0.51 ± 0.12 mg N₂O-N kg–¹) and UR + NI (0.45 ± 0.13 mg N₂O-N kg–¹) were not significantly different; by contrast, emissions were 36.3% lower in UD + NI than in UD. It was concluded that the soil organic N mineralization and heterotrophic nitrification are the main processes of NH₄⁺ and NO₃– production. Additionally, it was found that N₂O emissions were partially a consequence of the direct oxidation of the soil's organic N via heterotrophic nitrification coupled to denitrification. Finally, the results suggest that nitrapyrin would likely exert significant mitigation on N2O emissions only if a substantial N surplus exists in soils with high organic matter content.