Biogeochemical cycling at the molecular level in intensive cropping systems

土壤健康退化仍然是对农业生态系统可持续性的持续威胁。土壤有机质（SOM）是调节土壤健康和土壤生态系统服务功能的最重要因素之一。尽管SOM很重要，但人们对其中的碳(C)和氮(N)仍然知之甚少，而且在分子水平上基本上没有表征。了解SOM的分子组成、稳定性和周转对于预测其对作物管理的动态响应以及开发更可持续的农业实践至关重要。要做到这一点，我们必须更好地了解调节碳和氮转化的机制，以及对土壤生态系统服务的影响。新兴的分子技术已经从根本上改变了我们对SOM的理解——从以前长期坚持的概念，即SOM主要是植物输入、它们的化学作用和大分子腐殖质物质的重新合成的功能——到新兴的概念，即稳定的SOM形成是由微生物衍生的分解产物和生物分子通过有机矿物和有机金属氧化物相互作用稳定的结果。除了对SOM的理解取得这些进展外，很少有研究在“集约种植系统”的分子水平上评估SOM动态；相反，研究主要集中在非管理系统上。可以说，理解集约管理作物系统中的SOM更为重要，不仅因为土壤健康退化的风险更高，而且随着全球人口的增长，管理系统可能会扩大——特别是集约蔬菜作物生产。至关重要的是保持/提高这些土壤的生产力，以可持续地养活不断增长的人口。这就有必要进行研究，以便更好地了解集约化种植系统中碳和氮化合物的储量和流量；因此，我的研究计划的目的。在这里，我的目标是A)量化长期集约种植对SOM分子组成和周转的影响；B)评价土壤有机氮动态变化及其对作物氮素利用的贡献；C)研究SOM和N2O排放之间的联系，作为生态系统服务功能的一个指标。我提出了一种新方法的组合：1)化合物特异性同位素分析（13C和15N）和基于同步加速器的技术来表征有机物的周转和分子组成；2)稳定同位素富集连接土壤和植物养分动态；3)利用空腔环降光谱和同位素特征（N2O- 15n）将C和N底物与微生物对N（和N2O通量）的处理联系起来。三名研究生和五名本科生将在这个创新的环境中接受培训。我们的研究结果将对加拿大可持续种植系统的发展产生重大影响。

Soil health degradation remains a constant threat to agroecosystem sustainability. One of the most important factors regulating soil health – and the functioning of soil ecosystem services – is soil organic matter (SOM). Despite its importance, SOM and the carbon (C) and nitrogen (N) therein remain poorly understood and largely uncharacterized at the molecular level. Understanding the molecular composition, stability, and turnover of SOM is critical to both predicting its dynamics in response to crop management and to developing more sustainable agricultural practices. To do this, we must better understand the mechanisms that regulate C and N transformations, and the implications for soil ecosystem services. Emerging molecular techniques have radically changed our understanding of SOM – from the former long-held conceptualization that SOM was mostly a function of plant inputs, their chemistry, and the de novo synthesis of macromolecular humic substances – to the emerging conceptualization that stable SOM formation is resultant from microbial-derived products of decomposition and the stabilization of biomolecules via organo-mineral and organo-metal oxide interactions. Apart from these advancements in understanding of SOM, very little research has evaluated SOM dynamics at the molecular level in ‘intensive cropping systems'; rather, research has largely focused on unmanaged systems. It is arguably more important to understand SOM in intensively managed cropping systems, not only because soil health degradation is at a higher risk, but with a growing global population, managed systems are likely to expand - particularly intensive vegetable crop production. It is crucial that the productivity of these soils are maintained/enhanced to sustainably feed a growing population. This warrants research into better understanding the stocks and flows of C and N compounds in intensive cropping systems; hence the purpose of my research program. Here, I aim to A) quantify the influence of long-term intensive cropping practices on the molecular composition and turnover of SOM; B) evaluate the changes in soil organic N dynamics and link its contribution to crop N use; C) investigate the link between SOM and N2O emissions, as an indicator of ecosystem service functioning. I propose a combination of novel approaches: 1) compound-specific isotope analyses (of 13C and 15N) and synchrotron-based techniques to characterize the turnover and molecular composition of organic matter; 2) stable isotope enrichments to link soil and plant nutrient dynamics; and 3) linking C and N substrates to microbial processing of N (and N2O fluxes) using cavity ring down spectroscopy and isotopomer signatures (N2O-15N). Three graduate and five undergraduate students will be trained in this innovative environment. Our findings will have a significant impact on the development of sustainable cropping systems in Canada.