Combined application of the natural abundance stable isotope and 15N tracing methods to quantify denitrification in agricultural soils in field studies

量化农田土壤中的反硝化作用对于预测微生物对氮肥的消耗和相关的一氧化二氮(N2O)排放是至关重要的，但由于直接测量氮素(N2)排放的困难以及可能共存的多种微生物N转化，仍然具有挑战性。对排放的N2O(d15N、d18O和SP=线性N2O分子中15N的位置偏好)进行的同位异构体分析可能有助于区分产生途径和量化N2O还原为N2。然而，这种解释往往是模棱两可的，因为：(1)关于同位素分馏系数的大部分基本知识来自实验室实验；(2)关于将方法转移到现场条件的系统研究仍然缺乏。这项建议的目的是将实验室研究和实地研究结合起来，以获得关于在实验室条件下开发的方法是否有可能转移到实地规模的信息。其总体目标是开发一种对农业土壤中整个反硝化过程进行原位量化的方法。最终开发的方法将完全基于自然丰度稳定同位素方法，即N2O的同位素分析和土壤氮化合物的同位素分析相结合。为了准确地确定与反硝化作用有关的同位素分馏系数和在野外条件下有效的可能伴随过程，提出了详细的受控条件下的微观实验，并以15N示踪为参考方法。与N2O还原相关的分馏系数将通过从与N2O同位素分析平行获得的15N示踪结果直接确定反硝化产物比率(N2O/(N2+N2O))来验证现场条件。此外，将使用15N示踪剂模型来估计可能同时发生的N转化过程(硝化、氨化、固定化和氨氧化)的程度及其对排放的N2O的同位素值的影响。同时开展的土壤氮化合物自然丰度研究将旨在确定这些过程的同位素分馏特征。此外，将首次对土壤亚硝酸盐的自然丰度同位素特征进行分析，以便更好地评估这一关键化合物在氮循环中的过程动力学。作为该项目的最终结果，将开发一个复杂的同位素模型，该模型将提供一种方法来量化农田土壤在田间条件下的整个反硝化过程。从长远来看，该模型可以与N2O排放的基于过程的模拟相结合，以帮助更好地估计N2O的减排贡献。

Quantifying denitrification in arable soils is crucial in predicting the microbial consumption of nitrogen fertilizers and the associated nitrous oxide (N2O) emissions, but still challenging due to difficulties in direct measurements of dinitrogen (N2) emissions and due to possible co-existence of numerous microbial N-transformations. Isotopomer analyses of emitted N2O (d15N, d18O and SP=Site Preference of 15N within the linear N2O molecule) may help to distinguish production pathways and to quantify N2O reduction to N2. However, such interpretations are often ambiguous because: (i) the majority of basic knowledge about isotopic fractionation factors originate from laboratory experiments and (ii) systematic studies on method transfer into field conditions are still lacking. This proposal aims at combining laboratory and field studies to obtain information on the possible transferability of the methods developed under laboratory conditions to the field scale. The general aim is to develop a method for in situ quantification of the entire denitrification process in agricultural soils. The finally developed method will be solely based on the natural abundance stable isotope approach, i.e., combined isotopomer analyses of N2O and isotopic analyses of soil N-compounds. To precisely determine the isotopic fractionation factors associated with denitrification and possible accompanying processes valid for field conditions, detailed microcosm experiments under controlled conditions and 15N-tracing as a reference method are proposed. The fractionation factors associated with N2O reduction will be validated for field conditions through direct determination of the product ratio (N2O/(N2+N2O) of denitrification from 15N tracing results obtained parallel to N2O isotopomer analyses. Moreover, a 15N-tracer model will be used to estimate the extent of the possibly co-occurring N-transformation processes (nitrification, ammonification, immobilisation and ammonia oxidation) and their impact on isotopic values of emitted N2O. Natural abundance studies of soil N-compounds carried out in parallel, will aim to determine the isotopic fractionation characteristic for these processes. Additionally, the analysis of natural abundance isotopic signatures of soil nitrite will be conducted for the first time in order to better assess the process dynamics of this crucial compound in N-cycling. As an ultimate result of this project, a complex isotopic model will be developed, which should provide a method to quantify the whole denitrification process in agricultural soils under field conditions. In perspective, this model can be combined with the process-based modelling of N2O emissions to help in better estimation of the N2O reduction contribution.