Biogeochemical cycling at the molecular level in intensive cropping systems

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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.