Endogenous Plant Nitrous Oxide Formation: Evidence, Mechanisms and Ecosystem Implications.

Nitrous oxide (N2O) is a potent greenhouse gas and a major ozone-depleting substance of the nitrogen cycle. While global N2O budgets focus on microbial soil sources, evidence suggests that living plants can also contribute to N2O emissions. However, the origin of plant-associated N2O fluxes is contested: measured emissions may arise from endogenous plant metabolism or merely from transport of soil-derived N2O. In this review, we clarify these pathways and synthesize current findings from laboratory and field studies. Experimental evidence from 15N-tracer and axenic-culture studies supports N2O formation linked to nitrate (NO3-) and nitrite (NO2-) in photosynthetic organisms, although mechanistic resolution varies among taxa. In angiosperms, proposed plastid/chloroplast and hypoxia-associated mitochondrial routes are linked to NO3-/NO2- metabolism and NO formation, but the terminal NO-to-N2O step remains unresolved and differs from the flavodiiron-dependent mechanism demonstrated in algae. We discuss key environmental and physiological controls (substrate availability, light, O2 status, reductant supply) on these processes and highlight methodological challenges in source attribution (chamber artefacts, destructive sampling, isotope analysis). Our analysis suggests that plant-associated N2O fluxes are real, but highly variable and context-dependent. Rather than assuming generic "plant emission factors", we emphasize source-specific approaches that distinguish endogenous formation from soil-derived transport and microbial production. This framework can improve N2O budgets and guide mitigation by indicating whether management should target soil N availability and microbial processes or physiological conditions favouring plant-associated N2O formation.