Maize root growth, Oxygen and N availability drive formation of N ₂ O hotspots in soil

Plant roots modify all major controls of denitrification in soils, particularly the availability of the main substrates (NO ₃ ⁻ and C _org ), soil moisture, soil O ₂ content, and root-associated microbial communities, and thus play an important role in N ₂ O formation. Direct in-situ measurements of N ₂ O concentrations in the rhizosphere are lacking, yet crucial to understanding how rhizosphere denitrification contributes to overall N ₂ O emissions from soil. We equipped rhizoboxes with O ₂ -sensitive planar optodes to simultaneously monitor root growth and rhizosphere/soil O ₂ concentrations. We measured soil surface N ₂ O fluxes and linked them to root growth, soil moisture, and root/soil O ₂ concentrations. Based on root growth and O ₂ concentrations, we identified regions of interest (ROI) and sampled small soil volumes, which were analyzed for C, N, abundance of microbial denitrifiers ( nirK, nirS ) and N ₂ O reducers ( nosZI, nosZII ), and soil N ₂ O concentrations. Plant roots determined depth gradients of nutrients and denitrification gene abundances in the soil of the rhizoboxes with higher resource availability (NO ₃ ^- , DOC) and lower soil moisture in the upper soil layers, which also had higher abundances of total bacteria, nirK and nosZII . We anticipate that these uppermost soil layers largely contributed to N ₂ O formation. For the first time we were able to show high in-situ N ₂ O concentrations with distinct depth profiles around roots, and O ₂ and N availability controlling N ₂ O production at the process scale.