Reduction in N2O and NH3 emissions with combined use of dual inhibitors and biochar in a tea field soil in subtropical central China

Excessive nitrogen (N) fertilization in tea plantations often leads to substantial nitrous oxide (N 2 O) emissions, which exacerbate global warming, and to pronounced ammonia (NH 3 ) volatilization, which is closely associated with air pollution and aquatic eutrophication. Although N transformation inhibitors and biochar have shown promise in mitigating these gaseous losses, their combined effects and the underlying mechanisms in tea fields remain poorly understood. A 2-year field experiment was conducted in a subtropical hilly tea plantation to evaluate the individual and combined effects of dual inhibitors (the urease inhibitor N-( n -butyl) thiophosphoric triamide, NBPT, and the nitrification inhibitor 3,4-dimethylpyrazole phosphate, DMPP) and biochar (28 t ha −1 ) on N 2 O and NH 3 emissions. Four treatments were established: conventional N fertilization (CON), N fertilizer amended with dual inhibitors (NI), N fertilizer combined with both biochar and dual inhibitors (BNI), and a zero-N control (CK). The results showed that the CON treatment produced high cumulative gaseous emissions (N 2 O: 25.8 kg ha −1 ; NH 3 : 75.8 kg ha −1 ). The NI treatment reduced the N 2 O and NH 3 emission factors by 54.5% and 20.0%, respectively, while the BNI treatment achieved comparable mitigation efficiencies (49.8% for N 2 O and 20.2% for NH 3 ). Both treatments significantly suppressed the abundance of key N-cycling functional genes, including ammonia-oxidizing bacteria (AOB) and the nitrite reductase gene ( nirS ), with NI exerting a stronger inhibitory effect on AOB. Gaseous emissions originated predominantly from the tea rows rather than from the inter-row ridges. Structural equation modeling (SEM) and random forest (RF) analyses revealed that the mitigation effect was driven by shifts in soil N transformation dynamics and substrate availability. Specifically, NBPT significantly reduced short-term soil NH 4 + –N concentrations following fertilization, thereby decreasing substrate availability for NH 3 volatilization, whereas DMPP significantly suppressed the abundance of key N-cycling functional genes, particularly AOB and nirS , thereby inhibiting nitrification-driven N 2 O production. Additionally, the BNI treatment increased the tea yield by 6.7% and plant N uptake by 14.4%. In conclusion, applying dual inhibitors, either alone or in combination with biochar, effectively mitigates N 2 O and NH 3 emissions while maintaining tea productivity offering a practical strategy for environmentally sustainable tea cultivation. Highlights Conventional fertilization caused high N 2 O and NH 3 emissions from tea plantations. Applying dual inhibitors alone or with biochar reduced N 2 O and NH 3 emission factors by up to 50% and 20%. Combining inhibitors with biochar increased tea yield by 6.7% and N uptake by 14.4%. N emissions were cut by suppressing soil NO 3 − –N, altering NH 4 + –N transformation dynamics, and key microbial genes.