Elucidating the importance of the pools of nitrate in soils on denitrification

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The existence of different pools of NO3- in soils has been suggested through laboratory experiments using a denitrification incubation system, DENIS, at Rothamsted Research, North Wyke. This was observed from isotopomer (location of the 15N at the central, alpha or peripheral, beta N of the N2O molecule at natural abundance levels) data of the N2O emitted after application of C and N to soil cores. The data suggested when combined with a Rayleigh-type model that there were two pools of NO3- for N2O production represented by existing (soil NO3-) and fresh NO3- added as fertiliser. A third NO3- pool, from the nitrification of applied NH4+, can also be a source of N2O via denitrification and also from nitrification. Nitrification and denitrification may occur simultaneously in soil but there is often uncertainty associated with which process dominates in a particular soil under specific conditions. This study aims to determine the factors controlling the utilization of different NO3- pools in nitrification/denitrification, namely i) existing NO3-, ii) newly formed NO3- and iii) added NO3-. Isotopomers of N2O will be used with the Rayleigh-type model to assess the utilization of both existing and added NO3- to soil at two different ratios of C:N in the added amendment. Triple isotope labelling (15N-NH4+, 15N-NO3-, 15NH415NO3) will be applied combined with a model (Müller model) to quantify N transformation rates. This technique will provide an estimate of the source of N2O from nitrified NH4+ and NO3- added. A further model - the dual porous Pore-Cor model will be used to predict the location of hot spots of biological activity at the 10 to 100 um scale in relation to the critical percolation path where the transport of added nutrients (N and C) occurs. This project will provide a better understanding of the relative importance of different sources of N2O in a grassland soil and possible switch between NO3- pools to help develop mitigation strategies for N2O.

This work will benefit the scientific community in providing further understanding of the processes originating the gaseous emissions from soils by means of peer reviewed publications in high impact journals.
A combination of continuous direct measurements of evolved gases from soils with stable isotope techniques and models will aid in establishing the reliability of commonly used techniques for this kind of studies at the laboratory scale. This will be disseminated by peer reviewed publications.
The results from the soil moisture and C:N ratios studies will help develop mitigation strategies that can be further tested at the field scale. Recommendations will be provided in the peer reviewed publications.
This study will provide data imput for models in order to provide estimate of fluxes in various environmental conditions. The data will be supplied to modellers for use in their models (Prof. Christoph Müller from University Giessen in Germany, Reinhard Well from the Johann Heinrich von Thünen Institute in Germany and Prof. Matthews from Plymouth University).
A web page in the institute's web site will provide information on the project and its results. The results will inform Defra by suggesting potential mitigation strategies.
Attendance to an International conference (Nitrogen Workshop) will help divulgate results from the project as well as generate interest for future work.
A workshop will be organised in the last year of the project to gather scientists and policy people interested in this subject area, including experimentalists from field and lab scale studies, as well as modellers.
Communication from school children to summer/MSc students will help to divulgate at the academic scale to help motivate new scientists to work in related areas.