Background and Objective

The massive increase in reactive nitrogen (N) use in agricultural soils has led to a cascade of detrimental environmental consequences including emission of the greenhouse gas nitrous oxide (N₂O) via biological processes including denitrification and nitrification. In addition, there is great uncertainty about the fate of applied N as we suspect a large proportion might be lost as dinitrogen (N₂) through denitrification. We certainly need to improve current techniques to quantify N₂ losses from agricultural soils to better understand the N cycle in various agricultural systems which is needed for optimizing N management.

The Yangtze River Delta Region is one of the most densely populated and economically developed areas in China and being one of the heaviest N loading regions in the world. The research team led by Prof. Xiaoyuan Yan at the Institute of Soil Science at CAS conducted intensive monitoring of N fluxes at watershed scale in the delta region of Yangtze River. By budget calculation it was found that more than half of the N input was permanently removed from terrestrial ecosystems, likely through N₂ production, either through denitrification and anaerobic oxidation of ammonium. However, such budget calculation is an indirect method and the results are imprecise, resulting from the cumulated uncertainties of the various N fluxes, each with their respective uncertainty.

In this project, we study soils from three common Chinese agricultural management systems (paddy rice, fruit and vegetable fields) with new robust methods to i) precisely quantify direct N₂ and N₂O fluxes under different land-use, ii) distinguish the production pathways of N₂O for estimating the processes governing N removal, iii) and to understand driving forces of N₂O production and reduction processes.

Approach

The research team at the Thünen Institute investigates N cycling in the biosphere and processes governing N₂O turnover, with special emphasis in using stable isotope tools to study N transformations in soils and aquifers. A major focus has been the development of methods to measure denitrification at lab and field scales. We enhanced the N₂O isotopomer approach to determine N₂O producing processes in agricultural soils to use it also for calculating N₂O reduction during denitrification.

In this project, we will: a) develop robotized continuous flow soil incubation system to study direct N₂ and other N gas fluxes, b) conduct laboratory experiments to validate the N₂O isotopomer approach that is used to determine N₂O production processes; c) apply the N₂O isotopomer approach also to determine the N₂O/(N₂+N₂O ) product ratio; and d) cooperate with CAS work packages to characterize the tempo-spatial variation of denitrification, N₂O emission, N₂O/(N₂O+N₂) product ratio and their key controlling factors under field conditions. A fully automated robotized soil incubation system will be established at both, Thünen Institute and Chinese Academy of Sciences (CAS, China), to set up parallel incubation trials for direct N₂ measurements and for the calibration of N₂O isotopomer approaches. 

Our Research Questions

The goals of our work package are::

• Determine direct N₂, N₂O and NO fluxes from soils of the project sites
• Calibrate and validate the N₂O isotopomer approach for the experimental field sites
• Apportion N₂O fluxes to the processes of nitrification, fungal denitrification and bacterial denitrification.
• Quantify N₂ fluxes and denitrification product ratio at the field scale during phases of expected relevant flux events
• Elucidate control factors of above processes
• Determine potential denitrification rates (the maximum rates of N₂O+N₂ production under fully anoxic conditions without limitation by nitrate availability) of contrasting soils collected from the Yangtze Delta Region
• Provide data on denitrification gaseous product stoichiometry that improves our process understanding and allows modelling of the important process of N₂O reduction as affected by environmental variables.