N isotope heterogeneity in soil organic matter: a new tool to characterise N cycles

氮素周转是植物-土壤系统的重要组成部分。死植物组织或其他生物产品中的有机氮在矿化之前不能被植物吸收。不同的土壤有机质库具有不同的稳定性，因此富含活性土壤有机质（SOM）的土壤中的N比稳定的土壤中的N矿化得更快。在每种情况下，有机物的整体化学性质不同，这对N的化学性质和可用性有影响。在反应性SOM中，胺N很容易用于矿化。在稳定的SOM中，芳香族和相关的C化合物存在，芳香环结构中的N不太容易矿化。N周转模型将测得的N分配到具有不同稳定性的池中。目前，这些都没有严格相关的离散SOM池的N含量的直接测量。我们的提案开发了一种实现这一目标的工具。我们已经表明，SOM失去N在不同的气体形式，因为它是由程序升温燃烧加热的热分析系统。最初，还原（氰化物基）气体从脂肪族SOM的分解中产生。氮氧化物，然后演变为芳香族化合物分解。这种化学异质性反映了不同的结构主机的N内SOM，这是从观察到的重量损失。我们观察到这些主机之间的C同位素的异质性，乞讨的问题，自然丰度的N同位素也各不相同。我们的建议是建立一个全新的组合的仪器，连接现有的热分析和稳定同位素仪器通过一个新的接口，能够采取不同的N气体从热分析机（这给出了一个连续的记录的重量损失，因此比例的离散组件），并将它们提交给同位素比质谱仪的N同位素比的测定。这样就有可能使用氮同位素指纹来跟踪肥料-土壤-植物系统中的氮周转。该系统将首次提供一种工具来调查土壤有机质中N的化学和天然/标记同位素异质性，使N周转模型与农业土壤的可测量参数相关。

Nitrogen turnover is an essential part of the plant-soil system. Organic N, within dead plant tissue or other biological products, is not available for plant uptake until it has been mineralised. Differing soil organic matter pools have different stabilities, and so N in soils rich in reactive soil organic matter (SOM) is much more rapidly mineralised than that in soils with stable SOM. In each case, the bulk chemistry of the organic matter differs, with implications for the chemistry and availability of N. In reactive SOM, amine N is readily available for mineralisation. In stable SOM, aromatic and related C compounds exist, and N within aromatic ring structures is much less readily mineralised. Models of N turnover allocate measured N to pools with differing stability. At present these are not rigorously related to direct measurements of the N content of discrete SOM pools. Our proposal develops a tool to achieve this. We have shown that SOM loses N in different gaseous forms as it is heated by temperature-programmed combustion in a thermal analysis system. Initially, reduced (cyanide-based) gases are evolved from the decomposition of aliphatic SOM. N oxides are then evolved as aromatic compounds break down. This chemical heterogeneity reflects differing structural hosts for N within SOM, which are measured from observed weight losses. We observe C isotope heterogeneity between these hosts, begging the question that natural abundances of N isotopes also vary. Our proposal is to build a completely new combination of instrumentation, linking existing thermal analysis and stable isotope instrumentation via a novel interface that is capable of taking different N gases from the thermal analysis machine (which gives a continuous record of weight loss, hence proportions of discrete components) and presenting them to the isotope ratio mass spectrometer for determination of N isotope ratios. It will then be possible to use N isotope fingerprints to track nitrogen turnover, in fertiliser-soil-plant systems. This system will provide for the first time a tool to investigate chemical and natural/labelled isotopic heterogeneity for N within soil organic matter, enabling N turnover models to be related to measurable parameters from agricultural soils.