Background and Objective

Denitrification is an anaerobic microbial process of successive reduction of nitrate (NO₃^-) and nitrite (NO₂^-) to molecular nitrogen (N₂) through the following reaction steps: NO₃^- → NO₂^- → NO → N₂O → N₂. A significant fraction of nitrous oxide (N₂O) produced in this process is not further reduced to N₂ and constitutes the main emission source of this greenhouse gas from agricultural soils. Hence, our understanding and ability to quantify soil denitrification is crucial for mitigating nitrogen fertiliser loss as well as for reducing N₂O emission. Robust denitrification data suitable to validate N₂ fluxes in denitrification models are scarce due to previous methodical limitations and the extreme spatio-temporal heterogeneity of denitrification. The coordinated DFG research unit „Denitrification in Agricultural Soils: Integrated Control and Modelling at Various Scales (DASIM)” (https://www.thuenen.de/de/ak/aktuelles-und-service/detail-aktuelles/news/detail/News/neue-dfg-forschergruppe-beschaeftigt-sich-mit-der-denitrifikation-in-agrarboeden/) investigates the denitrification process chain in agricultural soils using advanced analytical and molecular biologic methods as well as field studies and various modelling approaches. The aim is to investigate activity and regulation of denitrification in unpreceded spatial and temporal resolution and to use results to develop mathematical models from the micro-scale to the field scale and to improve existing simulation approaches. The anaerobic soil volume fraction, a major control of denitrification, is known to depend on the spatial distributions of gas diffusivity and respiration. However, estimating the anaerobic soil volume fraction has been hampered in the past by the difficulty to quantify its controls. Today, new and improved methods are available to fill those gaps.

The objectives of our subproject P6 are to:

• quantify N₂, N₂O and NO fluxes and the contribution of different N-transformation processes under variable denitrification control factors (organic C, mineral N, water potential, pore volume) as a basis to validate existing denitrification models.
• provide data sets comprising as much as possible detail on activity and regulation (rates and types of N₂ and N₂O producing processes as well as the physical, chemical and microbiological control factors) as a basis to calibrate and validate new DASIM models.
• determine the minimum averaging volume for N₂ and N₂O fluxes from arable soils.
• determine the anaerobic soil volume fraction of DASIM soils under varying conditions.
• visualize organic matter distribution in selected soil samples at the µ-scale.
• test the feasibility to localize denitrification activity in selected soil samples at the µ-scale.

Approach

Our objectives will be tackled in four parts:

1. We will use new and improved stable isotope approaches to provide denitrification data sets comprising as much as possible detail on activity and regulation as a basis to validate existing and calibrate new denitrification models that are applied and/or developed by other DASIM subprojects.
2. Incubations with increasing soil volumes under standardized conditions will be conducted to determine the minimum elementary volume of denitrification as a basis for upscaling.
3. In cooperation with other subprojects we will employ spatial gas diffusivity measurement and modeling as well as denitrification measurements at defined O₂ status to obtain independent estimates of the anaerobic soil volume fraction. These approaches will be cross-validated and the most promising one will be used to calibrate the anaerobic soil volume fraction in the new denitrification models.
4. We will test to which extent the measurement of the µ-scale distribution of denitrification and its controls is feasible using nano-scale secondary ion mass spectrometry (NanoSIMS) and whether it is a suitable tool to further investigate µ-scale heterogeneity in phase 2 of DASIM. This test includes the measurement of organic matter distribution within selected soil aggregates and a new ¹⁵N tracer approach to locate nitrate consumption in aggregates.

Our Research Questions

The open questions addressed within the entire DASIM RU are:

• How is the activity of denitrifiers and their community structure controlled at the micro-scale?
• How is denitrification affected and controlled by other simultaneously occurring N cycling processes?
• What controls the development of “hot spots” (i.e. specialised habitats) and “hot moments” (i.e. temporary hot spots) for denitrification activity?
• Do fundamental relationships exist between controls of denitrification at various scales (from micro- to meso- and plot-scale)?
• Is it possible to predict denitrification of a given soil in response to atmospheric boundary conditions based on measurable structural and biochemical properties?

The questions addressed in our subproject P6 are:

• How are N₂, N₂O and NO fluxes and their interaction with different N-transformation processes affected by physical, chemical and microbiological control factors?
• What is the size of the minimum averaging volume for N₂ and N₂O fluxes from arable soils?
• How can we quantify the anaerobic soil volume fraction and to which extent is the measurement is this quantity suitable to improve denitrification models?
• How is soil organic matter distributed at the micro-scale and to which extent is the measurement is this quantity suitable to improve denitrification models
• Can we localise denitrification activity at the micro-scale using nano-scale secondary ion mass spectrometry (NanoSIMS)?

Results