Award details

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

Reference	BB/K001566/1
Principal Investigator / Supervisor	Professor Graham Matthews
Co-Investigators / Co-Supervisors
Institution	University of Plymouth
Department	Sch of Geog Earth & Environ Sciences
Funding type	Research
Value (£)	9,602
Status	Completed
Type	Research Grant
Start date	28/02/2013
End date	27/02/2016
Duration	36 months

Abstract

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.

Summary

The aim of this study is to determine the factors controlling the utilisation of different pools of N in nitrification and denitrification, namely existing NO3-, newly formed NO3- and added NO3-. The existence of different pools of NO3- in soils has been suggested through a series of laboratory incubation experiments using a denitrification incubation system, DENIS, at Rothamsted Research, North Wyke. This was observed from data on the isotopic signature of the emitted N2O. It was expected that after application of N and C to soil cores, an increase in the delta15Nbulk-N2O would result due to the preference of the microbes to use the lighter isotope, leaving an enriched pool behind. This increase occurred for several days after N and C application but then a decrease was observed. This change in behavior combined with the use of a model suggested the possible change in the source of NO3- between the added NO3- and the native soil pool. 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. In order to develop mitigation strategies for N2O emissions, it is necessary to gain knowledge of the sources and processes that produce N2O in soils, and to understand the controlling factors of these processes to specifically target these sources. 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-. Soil moisture (water filled pore space) and the ratios of applied C:N will be the controlling factors studied and in order to achieve the overall aim, direct measurements of evolved gases N2O, N2, NO, isotopomers of N2O after application of N and C will be carried out by using the laboratory incubation system (DENIS). Also 15N-N2O will be measured when applying a labeled N source. Isotopomers of N2O describe the location of the 15N on the N2O molecule at natural abundance and are useful for confirmation of the process source. This data will be used in a model to assess the utilization for denitrification of both existing (soil NO3-) and added NO3- to soil at two different ratios of C:N in the added amendment. Isotope labelling techniques will be applied combined with a model (Müller model) capable of determining rates of simultaneous complex transformations of N, allowing the quantification of reaction rates of nitrification, mineralisation, immobilisation and re-mineralisation in newly formed microbial biomass. This technique will provide an estimate of the source of N2O from nitrified NH4+ and NO3- added. A further model - the dual porous (macro and micropores) 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 nutrients (N and C) occurs. Through a combination of direct gaseous measurements (NO, N2O and N2) and isotopic techniques, via labelling and use of stable isotopes, it will be possible to differentiate between the different pools of NO3- involved in the process of denitrification. The project will achieve model refinements and insights gained from three mathematical models; and finally will provide recommendations for soil management to reduce N2O emissions.

Impact Summary

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.

Committee	Research Committee B (Plants, microbes, food & sustainability)
Research Topics	Soil Science
Research Priority	X – Research Priority information not available
Research Initiative	X - not in an Initiative
Funding Scheme	X – not Funded via a specific Funding Scheme

Associated awards:

BB/K001051/1 Elucidating the importance of the pools of nitrate in soils on denitrification

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