Isotopic Signatures and Fluxes of N2O Emitted from Soybean Plants and Soil During the Main Growth Period of Soybeans

Soil microorganisms have long been recognized as primary producers of biogenic N2O in terrestrial ecosystems. Terrestrial plants can contribute to N2O emissions by transporting N2O produced in soils, and there is also evidence that plants may serve as direct producers of N2O. However, to date, direct evidence for N2O production by plants remains limited. To exclude N2O emissions resulting from soil-to-plant transport, this study conducted incubation experiments using cut soybean branches and leaves (cSBF) and intact soil cores under an N2O-free air background. The natural isotopic signatures (δ15N and δ18O) and fluxes of N2O produced by cSBF and soil were compared across different soybean growth stages over two growing seasons. The observed δ15N and δ18O values of N2O from soil ranged from −26.7‰ to −5.3‰ and −24.1‰ to 22.8‰, respectively. In contrast, the values for N2O produced from cSBF ranged from −4.7‰ to 33.1‰ and from 23.7‰ to 88.8‰, respectively. Notably, N2O emitted from plants exhibited significantly higher δ15N and δ18O values than soil-derived N2O (p < 0.05). These findings indicate that the pathways and mechanisms of N2O production and emission in soybean plants differ from those mediated by soil microorganisms and nitrogen transport processes. Additionally, a significantly higher amount of N2O emission was observed during early growth stages compared to late growth stages (p < 0.01), suggesting that plant N2O production may be associated with elevated water content and oxygen-limited conditions within plant cells. In addition to the N2O uptake by plants observed in some literature, the positive relationship between δ15N values and N2O fluxes suggests that N2O could be consumed in plant cells (p < 0.01), with a high consumption rate often associated with a high production rate. The results of this study provide compelling evidence that plants may represent an overlooked source of N2O in terrestrial ecosystems.