Brachiaria humidicola Cultivation Enhances Soil Nitrous Oxide Emissions from Tropical Grassland by Promoting the Denitrification Potential: A 15 N Tracing Study

Biological nitrification inhibition (BNI) in the tropical grass Brachiaria humidicola could reduce net nitrification rates and nitrous oxide (N 2 O) emissions in soil. To determine the effect on gross nitrogen (N) transformation processes and N 2 O emissions, an incubation experiment was carried out using 15 N tracing of soil samples collected following 2 years of cultivation with high-BNI Brachiaria and native non-BNI grass Eremochloa ophiuroide . Brachiaria enhanced the soil ammonium (NH 4 + ) supply by increasing gross mineralization of recalcitrant organic N and the net release of soil-adsorbed NH 4 + , while reducing the NH 4 + immobilization rate. Compared with Eremochloa , Brachiaria decreased soil gross nitrification by 37.5% and N 2 O production via autotrophic nitrification by 14.7%. In contrast, Brachiaria cultivation significantly increased soil N 2 O emissions from 90.42 μg N 2 O-N kg −1 under Eremochloa cultivation to 144.31 μg N 2 O-N kg −1 during the 16-day incubation ( p < 0.05). This was primarily due to a 59.6% increase in N 2 O production during denitrification via enhanced soil organic C, notably labile organic C, which exceeded the mitigated N 2 O production rate during nitrification. The contribution of denitrification to emitted N 2 O also increased from 9.7% under Eremochloa cultivation to 47.1% in the Brachiaria soil. These findings confirmed that Brachiaria reduces soil gross nitrification and N 2 O production via autotrophic nitrification while efficiently stimulating denitrification, thereby increasing soil N 2 O emissions.