Maize root growth, Oxygen and N availability drive formation of N2O hotspots in soil

Summary 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.