From single fractions to undisturbed soil: Role of soil organic matter in denitrification

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Organic matter (OM) is an important fuel for denitrification since organic compounds donate electrons to oxidized nitrogen and thus control N2O und N2 emissions from soils. Currently only little information is available about the effects of functional OM fractions (dissolved, particulate, and mineral-associated OM) on denitrification regarding to their quantity, quality, and spatiotemporal availability. Results of Phase I reveal that all OM fractions, even the relatively stable mineral-associated OM, quantitatively contribute to denitrification. Particulate plant material loses its ability to promote denitrification once water-soluble substances are removed by leaching. Dissolved OM thus seems to be of central importance, considering that this fraction may originate from several sources (e.g., release during litter decomposition, desorption from mineral surfaces). During Phase II we aim at an improved mechanistic understanding of these processes, delivering quality indicators describing the contribution of OM in denitrification models. We are planning a litter decomposition experiment in combination with standardized incubations in order to examine the gas production (CO2, N2O, N2) based on the temporal changing quality of plant residues and dissolved OM. Incubation experiments will also access the contribution of mineral-associated OM on denitrification, specifically addressing the question of C-limitation due to the presence of reactive mineral surfaces. Finally, we will test by lysimeter experiments with undisturbed soil samples if the indicators of OM quality allow to predict denitrification on the pedon scale.