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。植物死亡组织或其他生物制品中的有机氮只有在矿化后才能被植物吸收。不同的土壤有机质库具有不同的稳定性，因此富含活性有机质(SOM)的土壤中的N比稳定的SOM土壤中的N矿化更快。在每种情况下，有机质的主体化学都不同，这对N的化学和有效性有影响。在反应性SOM中，胺N很容易被矿化。在稳定的SOM中，存在芳香族化合物和相关的C化合物，芳香环结构中的N不太容易矿化。N周转模型将测得的N分配到不同稳定性的池中。目前，这些与直接测量离散土壤有机质池中的氮含量没有严格的联系。我们的提案开发了一种工具来实现这一点。我们已经证明，SOM在热分析系统中通过程序升温燃烧加热时，会以不同的气体形式损失N。最初，还原(基于氰化物的)气体是从脂肪族SOM的分解中释放出来的。然后，随着芳香化合物的分解，氮氧化物被释放出来。这种化学异质性反映了SOM中N的不同结构宿主，这是通过观察到的重量损失来衡量的。我们观察到这些宿主之间的碳同位素不均一性，提出了N同位素的自然丰度也不同的问题。我们的建议是建立一个全新的仪器组合，通过一个新的接口将现有的热分析和稳定的同位素仪器连接起来，该接口能够从热分析机中提取不同的N气体(它提供连续的重量损失记录，因此离散组分的比例)，并将它们提供给同位素比率质谱仪，以确定N同位素比率。然后，就有可能使用氮同位素指纹来跟踪氮素在肥料-土壤-植物系统中的周转。该系统将首次提供一种工具来研究土壤有机质中N的化学和自然/标记的同位素异质性，使N周转模型能够与来自农业土壤的可测量参数相关联。

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.