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

Plant roots can modify all major controls of denitrification in soils, particularly the availability of the main substrates (NO3- and Corg), soil moisture, soil O2 content, and root-associated microbial communities, and thus play an important role in N2O formation. Direct in-situ measurements of N2O concentrations in the rhizosphere are lacking, yet are crucial to better understand how rhizosphere denitrification contributes to overall N2O emissions from soil. We equipped rhizoboxes with O2-sensitive planar optodes to simultaneously monitor root growth and rhizosphere/soil O2 concentrations. We measured soil surface N2O fluxes and linked them to root growth, soil moisture, and root/soil O2 concentrations. Based on root growth and O2 concentrations, we identified regions of interest (ROI) and sampled small soil volumes, which were analyzed for C and N content, and abundance of genes indicative of microbial denitrifiers (nirK, nirS) and N2O reducers (nosZI, nosZII), and soil N2O concentrations. Plant roots determined depth gradients of nutrients and denitrification gene abundances in the soil of the rhizoboxes with higher resource availability (NO3-, DOC) and lower soil moisture in the upper soil layers, which also had higher abundances of total bacteria, nirK and nosZII. These findings indicate that the uppermost soil layers largely contributed to N2O formation. Our study provides the first direct evidence of roots creating distinct O2 and N gradients controlling N2O production at the process scale leading to high in-situ N2O concentrations.