Award details

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

Reference	BB/K001051/1
Principal Investigator / Supervisor	Dr Laura Cardenas
Co-Investigators / Co-Supervisors	Professor Graham Matthews
Institution	Rothamsted Research
Department	Sustainable Soils and Grassland Systems
Funding type	Research
Value (£)	373,529
Status	Completed
Type	Research Grant
Start date	18/03/2013
End date	17/03/2016
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

The outputs of this research will impact on society and the economy by influencing: Scientific community: This research will inform scientists working in similar areas of research by providing understanding of the processes and N pools utilisation originating gaseous emissions from agricultural soils. It will provide tools for expanding the knowledge and understanding of soil processes by the use of a combination of experimental techniques and models at the soil core scale. This project will directly impact our national collaborators, Dr. Catherine Watson from AFBI in Northern Ireland by providing data for ring testing of the gaseous emissions and isotope data and our international collaborators Prof. Christoph Müller from University Giessen in Germany and Reinhard Well from the Johann Heinrich von Thünen Institute in Germany on the application of their models and isotopomers data to aid interpretation of the results. It will also provide the opportunity to continue to improve the Pore-Cor model with the University of Plymouth and to gain understanding of the effect of the proximity of pools to microbes on emissions. Current national (Prof. G.P. Matthews in Plymouth) and international collaborations (with Prof. A. Vallejo from the Polytechnic University of Madrid, Dr. M. Alfaro from INIA in Chile and Prof. Reinhard Well in Germany) will be maintained and new ones developed (Dr. C. Watson from AFBI and Prof. C.Müller in Germany) and during this project to raise awareness of this project beyond Rothamsted Research. This will be done in combination with Laura's role within the ISPG and the Defra greenhouse gas platform within which she has an important role (Defra projects AC0112, AC0114, AC0116, web site: http://www.ghgplatform.org.uk/ As a multidisciplinary study, this project is an opportunity for the various expertises to influence the outcome to the best of their knowledge. The project team will benefit in the combined use of experiments and models increasingtheir skills in mechanistic studies. Experiments will be presented at the institute's yearly tour of experiments to colleagues from Rothamsted Research. Attendance to two International conferences will help disseminate results from the project as well as generate interest for future work. The Nitrogen Workshop which gathers researchers mostly from Europe on nitrogen. The EGU, which covers a wider range of subject areas and although it is an European meeting, it is also attended by researchers from outside Europe. Policy community: The results from the effect of water filled pore space and applied C:N ratios studies will suggest mitigation strategies for greenhouse gases that can be further tested at the field scale. Recommendations will be provided in the peer reviewed publications. Laura Cardenas is part of the greenhouse gas platform that Defra has funded, and is regularly invited to give oral presentations to various audiences; including policy people, farmers and industry in general and these will provide opportunities to communicate the findings of this project. Wider community: A web page on the institute's web site will provide information on the project and its results. A presentation in the school's science week at North Wyke will be carried out in the second year of the project.

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/K001566/1 Elucidating the importance of the pools of nitrate in soils on denitrification

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