Microscale oxygen distribution to predict denitrification in structured soil

Different microbial species are capable of producing N₂O through multiple pathways, and these can coexist within short distances due to different microenvironmental conditions in the heterogeneous soil structure. Denitrification in soil occurs predominantly in microbial hotspots where denitrifiers use nitrate as an alternative electron acceptor. Soil water content has a profound influence on denitrification because it determines the diffusion lengths of oxygen through air- and water-filled pores, as well as the diffusion of denitrification products from the source in the soil to the atmosphere. Predicting N₂O emissions resulting from denitrification, however, is notoriously difficult without quantifying microscale hotspots.

In this experiment we evaluated results from an incubation experiment with undisturbed cores from two different soils having contrasting structures (cropland vs. meadow) at three different water contents. In addition to high-resolution gas chromatography, ¹⁵N-labeled nitrate solution allowed information on denitrification and its product ratios to be gained through IRMS measurements at selected time points. On the other hand, 7 needle-type optodes per core in combination with image analysis of images derived by X-ray tomography are used to quantify small scale diffusion distances and hotspots around POM. Last, a previous experiment, with the same but sieved soil and without particulate organic matter (POM), is used as a comparison to further investigate the influence local structure heterogeneity and POM on denitrification.

First results indicate that the reduction of diffusion pathways during sieving in the arable soil resulted in significantly lower emissions after sieving compared to the structured soil, while N₂O+N₂ emissions in the meadow soil were only slightly affected by sieving. For the first time, we were able to generate 3D images of O₂ saturation by combining image-derived diffusion length information with the measured O₂ concentrations. These allowed to explain high variabilities of N₂O+N₂ emissions from the field structured cores.