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

这项研究的目的是确定在硝化和反硝化过程中控制不同氮库利用的因素，即现有的N3-、新形成的N3-和添加的N3-。通过使用北威克Rothamsted Research的Denis反硝化培养系统进行的一系列实验室培养实验，表明土壤中存在不同的NO3-库。这是从排放的N2O的同位素特征数据中观察到的。预计在土壤岩心施N和C后，由于微生物倾向于使用较轻的同位素，导致Delta15N Bulk-N2O的增加，留下一个富集池。在施N和C后，这种增加持续了几天，但随后出现了下降。这种行为的变化，结合模型的使用，表明了NO3的来源可能发生了变化--在添加NO3的情况下--和本地土壤库之间。第三个NO3-池来自外加NH4+的硝化作用，也可以通过反硝化作用和硝化作用产生N2O。土壤中的硝化作用和反硝化作用可能同时发生，但在特定条件下，在特定的土壤中，哪个过程占主导地位，往往存在不确定性。为了制定N2O排放的缓解策略，有必要了解土壤中产生N2O的来源和过程，并了解这些过程的控制因素，以具体针对这些来源。本研究旨在确定控制硝化/反硝化过程中不同NO_3~-库利用的因素，即：(1)已有的NO_3~-；(2)新形成的NO_3~-；土壤水分(充满水的孔隙空间)和施C/N比将是研究的控制因素，为了达到总体目标，将使用实验室培养系统(DENIS)直接测量施N和C后N2O、N_2、NO、N_2O的释放气体和N_2O的同分异构体。当施加标记的氮源时，还将测量15N-N2O。N2O的同位异构体描述了自然丰度下15N在N2O分子上的位置，并有助于确认工艺来源。这些数据将被用在一个模型中，以评估现有的(土壤NO3-)和添加的NO3-在两个不同的C：N比例下对土壤的反硝化利用。同位素标记技术将与一个能够确定N同时复杂转化速率的模型(Müler模型)相结合，允许量化新形成的微生物生物量中的硝化、矿化、固定化和再矿化反应速率。这项技术将提供对硝化NH4+和添加的NO3-的N2O来源的估计。另一个模型-双孔(大孔和微孔)孔-COR模型将被用来预测10-100um尺度上的生物活性热点相对于发生营养物质(N和C)运输的临界渗流路径的位置。通过直接气体测量(NO，N2O和N2)和同位素技术的结合，通过标记和使用稳定同位素，将有可能区分参与反硝化过程的不同NO3-池。该项目将实现模型的改进和从三个数学模型中获得的见解；最后将为土壤管理提供减少N2O排放的建议。

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.