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

点击翻译按钮获取中文摘要

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.