Microscale controls on soil organic matter stabilization and impacts on nitrogen cycling in Canadian prairie agroecosystems

氮（N）是控制农业生态系统和非管理生态系统净初级生产力的关键生态系统养分。在土壤有机质的微生物分解过程中，生物可利用的无机氮被释放到环境中，对农业和环境的可持续性产生影响。尽管这种营养素的重要性，我们的理解的许多因素控制氮循环过程，包括保护（和释放）的土壤有机质的氮仍然是不完整的。长期以来，人们已经认识到钙（Ca）通过与土壤有机质的离子缔合（即，钙桥）和有机质包裹在钙矿物沉淀从土壤溶液中。这些物理保护机制通常归因于非生物过程，其中Ca与有机物质反应，作为改变土壤条件的结果，例如影响土壤溶液中Ca溶解的润湿和干燥循环。然而，生物过程（特别是土壤微生物的作用）对方解石等钙矿物沉淀的贡献以及随之而来的对土壤有机氮的保护作用，在很大程度上还没有得到探索。虽然微生物在方解石沉淀中的作用已经在一系列水生和陆地环境中进行了研究，但微生物诱导方解石沉淀（MICP）对土壤有机氮物理保护的意义和贡献尚未得到解决。据推测，MICP发生在富钙环境中作为一种机制，以保护微生物细胞从自我钙化。此外，MICP通常与这些相同的微生物的胞外聚合物（EPS）的增强生产有关，也作为Ca饱和环境中的保护机制。拟议的方案探讨了可能性，MICP连同相关的EPS生产在土壤团聚体的稳定，从而在典型的加拿大草原富钙土壤环境中的土壤有机氮保护发挥了重要作用。尖端技术，如同步加速器为基础的土壤微团聚体结构的检查，连同负责聚集的微生物的分子鉴定，将被用来阐明在微观尺度的物理过程，有助于在宏观尺度上的N生物利用度。最终，我们需要了解微尺度的过程之前，我们可以成功地制定宏观战略来管理农业生态系统中的N。

Nitrogen (N) is a key ecosystem nutrient controlling net primary production in both agricultural and unmanaged ecosystems. During microbial decomposition of soil organic matter, bioavailable inorganic N is released to the environment, with implications for agricultural and environmental sustainability. Despite the importance of this nutrient, our understanding of the many factors controlling N-cycling processes, including the protection (and release) of N from soil organic matter remains incomplete. It has long been recognized that calcium (Ca) plays a role in the protection of soil organic N through ionic associations with soil organic matter (i.e., Ca bridging) and encapsulation of organic matter in Ca minerals precipitated from the soil solution. These physical protection mechanisms typically are attributed to abiotic processes, whereby Ca reacts with organic matter as a consequence of changing soil conditions, such as the wetting and drying cycles that influence Ca solubilization in the soil solution. What remains largely unexplored, however, is the contribution of biotic processes—specifically the action of soil microorganisms—to the precipitation of Ca minerals such as calcite, and the consequent protection of soil organic N. Although the role of microbes in the precipitation of calcite has been investigated in a range of aquatic and terrestrial environments, the significance and contribution of microbially-induced precipitation of calcite (MICP) to the physical protection of soil organic N has not been addressed. It has been hypothesized that MICP occurs in Ca-rich environments as a mechanism to protect microbial cells from self-calcification. Moreover, MICP is often associated with the enhanced production of extracellular polymeric substances (EPS) by these same microbes, also as a protection mechanism in Ca-saturated environments. The proposed program explores the possibility that MICP together with the associated production of EPS plays a significant role in soil aggregate stabilization and thus soil organic N protection in Ca-rich soil environments typical of the Canadian prairies. Cutting edge techniques, such as synchrotron-based examinations of soil microaggregate structures, together with molecular identification of the microorganisms responsible for aggregation, will be used to shed light on physical processes at the microscale that contribute to N bioavailability at the macroscale. Ultimately, we need to understand microscale processes before we can successfully develop macroscale strategies to manage N in agroecosystems.