The distribution of particulate organic matter in the heterogeneous soil matrix - Balancing between aerobic respiration and denitrification

Abstract

Denitrification, a key process in soil nitrogen cycling, occurs predominantly within microbial hotspots, such as those around particulate organic matter (POM), where denitrifiers use nitrate as an alternative electron acceptor. For accurate prediction of dinitrogen ( N 2 ) and nitrous oxide ( N 2 O ) emissions from denitrification, a precise quantification of these microscale hotspots is required. The distribution of POM is of crucial importance in this context, as the local oxygen ( O 2 ) balance is governed not only by its high O 2 demand but also by the local O 2 availability. Employing a unique combination of X-ray CT imaging, microscale O 2 measurements, and 15 N labeling, we were able to quantify hotspots of aerobic respiration and denitrification. We analyzed greenhouse gas (GHG) fluxes, soil oxygen supply, and the distribution of POM in intact soil samples from grassland and cropland under different moisture conditions. Our findings reveal that both proximal and distal POM, identified through X-ray CT imaging, contribute to GHG emissions. The distal POM, i.e. POM at distant locations to air-filled pores, emerged as a primary driver of denitrification within structured soils of both land uses. Thus, the higher denitrification rates in the grassland could be attributed to the higher content of distal POM. Conversely, despite possessing compacted areas that could favor denitrification, the cropland had only small amounts of distal POM to stimulate denitrification in it. This underlines the complex interaction between soil structural heterogeneity, organic carbon supply, and microbial hotspot formation and thus contributes to a better understanding of soil-related GHG emissions. In summary, our study provides a holistic understanding of soil-borne greenhouse gas emissions and emphasizes the need to refine predictive models for soil denitrification and N 2 O emissions by incorporating the microscale distribution of POM. • Denitrification occurs in hotspots at the microscale and is thus difficult to predict. • We combined X-ray CT imaging, microscale O 2 sensors, and 15 N labeling to map hotspots. • The position of the POM separates enhanced aerobic respiration and denitrification. • Distal POM (to air-filled pores) as driver of denitrification (N 2 O + N 2 ) in soils • Including POM distribution in models will enhance accuracy for soil GHG predictions.

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