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

氮(N)是控制农业和非管理生态系统净初级生产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，这对农业和环境的可持续性具有重要意义。尽管这种营养物质很重要，但我们对控制N循环过程的许多因素的理解仍然不完整，包括土壤有机质对N的保护（和释放）。长期以来，人们认识到钙（Ca）通过与土壤有机质的离子结合（即钙桥接）和从土壤溶液中沉淀的钙矿物对有机质的包封作用，对土壤有机氮起保护作用。这些物理保护机制通常归因于非生物过程，即钙与有机质发生反应，这是土壤条件变化的结果，例如影响钙在土壤溶液中溶解的干湿循环。然而，生物过程——特别是土壤微生物的作用——对钙矿物（如方解石）的沉淀的贡献，以及随之而来的对土壤有机氮的保护，在很大程度上仍未被探索，尽管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，微生物诱导方解石降水（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，是一种保护微生物细胞免于自我钙化的机制。此外，MICP通常与这些微生物胞外聚合物质（EPS）的增加有关，也是钙饱和环境中的一种保护机制。该计划探讨了MICP及其相关的EPS在土壤团聚体稳定中发挥重要作用的可能性，从而在加拿大草原典型的富钙土壤环境中保护土壤有机氮。前沿技术，如基于同步加速器的土壤微团聚体结构检查，以及负责团聚的微生物的分子鉴定，将用于阐明微观尺度上有助于宏观尺度上氮生物有效性的物理过程。最终，在我们能够成功地制定宏观战略来管理农业生态系统中的氮之前，我们需要了解微观过程。

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.