Moderate effects of distance to air-filled macropores on denitrification potentials in soils

Denitrification is a major source of the greenhouse gas N₂O. As a result of spatial heterogeneity of organic carbon, oxygen and nitrate, denitrification is observed even under relatively dry conditions. However, it is unclear whether denitrification potentials of microbial communities exhibit spatial patterns relative to variations in distance to soil pores facilitating oxygen exchange and nutrient transfer. Thus, we determined genetic and process-level denitrification potentials in two contrasting soils, a cropland and a grassland, with respect to the distance to air-filled pores. An X-ray computed tomography aided sampling strategy was applied for precise sampling of soil material. Process-level and genetic denitrification potentials in both soils were spatially variable, and similar with respect to distance to macropores. In the cropland soil, a minor increase of process-level potentials with distance to pores was observed and related to changes in NO₃⁻ rather than oxygen availability. Genetic denitrification potentials after the short-term incubations revealed a certain robustness of the local community. Thus, distance to macropores has a minor impact on denitrification potentials relative to the observed spatial variability. Our findings support the notion that the impact of macropore induced changes of the environmental conditions in soil does not overrule the high spatial variability due to other controlling factors, so that the rather minor proportion of spatial heterogeneity of functional genes and activity potentials related to macropore distances in soil need not be considered explicitly in modelling denitrification.