The mercury-nitrogen nexus: Trade-off between nitrous oxide emissions and mercury methylation in rice paddy systems

Greenhouse gas (GHG) emissions from rice paddy fields and methylmercury (MeHg) accumulation in rice grains represent two critical, yet traditionally separately managed, environmental concerns in agricultural ecosystems. However, the biogeochemical interactions between mercury (Hg) contamination and soil nitrogen (N) cycling, particularly their coupled influence on nitrous oxide (N₂O) emissions, remain poorly understood. In this study, we conducted pot experiments utilizing paddy soils from three distinct locations with varying historical Hg gradients (low background, naturally elevated, and heavily contaminated) to investigate soil N₂O fluxes and N transformation processes under exogenous Hg^II spiking scenarios at the soil-water interface. Our results demonstrated that Hg contamination significantly promoted cumulative N₂O emissions across different geological gradients, converting a net N₂O sink into a source in rice paddies with low background Hg. Notably, the mechanisms driving these emissions were regulated by water induced alternating redox and sites conditions. Furthermore, a negative correlation between cumulative N₂O emissions and the Hg methylation potential (MeHg/THg%) was observed and supported by partial least-squares path modeling (PLS-PM), indicating that N₂O acts as a consistent metabolic inhibitor of MeHg production across contamination gradients. These findings highlight a critical biogeochemical trade-off: mitigation strategies focusing singularly on minimizing N₂O emissions may inadvertently amplify the risk of MeHg accumulation in rice. This study not only provides more understandings of the Hg-N interplay in rice paddy systems but also contributes knowledge to the Hg-microbe-GHG nexus.