Biological denitrification, carried out by both bacteria and fungi, represents a major pathway for N₂O production in paddy soils due to active denitrification under fluctuating redox conditions from frequent flooding and drying cycles.
While previous research has established that warming generally accelerates microbial metabolism and enhances denitrification rates, the differential responses of bacterial versus fungal denitrifiers to rising temperatures remained poorly understood.
Bacterial vs fungal denitrification
Traditional methods for distinguishing microbial contributions, such as substrate-induced respiration with selective inhibition, face significant limitations including inefficient inhibition across different soil types and potential non-specific effects on non-target microbial groups.
This knowledge gap has hindered accurate predictions of how future climate warming will impact greenhouse gas emissions from rice-dominated agricultural systems, which are predominantly located in subtropical and tropical regions where warm, humid conditions support year-round cultivation.
A study published in Nitrogen Cycling by Di Wu’s team, Chinese Academy of Sciences, could change climate prediction models by enabling region-specific, microbe-targeted strategies to mitigate greenhouse gas emissions from agricultural ecosystems, which account for approximately 10-20% of global agricultural N₂O emissions.
Field study
The research team collected soil samples from 18 rice paddies across six Chinese provinces, spanning temperate, subtropical, and tropical monsoon climates. Soils were incubated under two thermal regimes: local average annual temperature and a +4°C warming treatment representing end-of-century high-emission projections.
Using the advanced N₂O site preference (SP) approach—which distinguishes bacterial from fungal denitrification based on intramolecular ¹⁵N positioning within the N₂O molecule—the researchers quantified microbial contributions.
Bacterial denitrification produces lower SP values (typically -7.5% to +3.5%), while fungal denitrification yields consistently higher SP values (around 33-37%). Quantitative PCR targeting denitrification-associated genes (nirS, nirK, FnirK, nosZ I, and nosZ II) revealed that bacterial denitrification rates were positively linked to nirS abundance, whereas fungal denitrification showed tight coupling to FnirK, nosZII, and nirS abundance.
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Structural equation modeling demonstrated that warming-induced increases in N₂O emissions were positively regulated by bacterial denitrification response ratios (path coefficient = 0.45, p < 0.001) and fungal denitrification response ratios (path coefficient = 0.33, p < 0.001), while revealing a significant competitive trade-off between bacterial and fungal pathways (path coefficient = -0.69, p < 0.001).
Regression analysis confirmed that the total denitrification response ratio rose linearly with bacterial denitrification response (r² = 0.47, p < 0.001) but showed no significant influence from fungal denitrification changes.
Agricultural implications
These findings demonstrate that future climate warming may pose a greater risk to nitrogen budgets and greenhouse gas reduction efforts in rice-dominated agricultural ecosystems by disproportionately enhancing bacterial contributions to N₂O emissions.
The research highlights the need for latitude-specific prediction models, as southern provinces like Hainan exhibited the highest increases in total denitrification (+23%) compared to northern regions like Heilongjiang (+4%).
However, the authors caution that as a short-term study, these results primarily capture acute physiological responses and may overestimate warming effects compared to long-term scenarios where microbial acclimation and community restructuring could moderate N₂O fluxes.
Future long-term field warming experiments with gradual temperature trajectories will be essential to validate these insights and develop effective mitigation strategies for sustainable rice production in a warming world.
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