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

在农业生态系统和无管理生态系统中，氮(N)是控制净初级生产力的关键生态养分。在微生物分解土壤有机质的过程中，生物可利用的无机氮释放到环境中，这对农业和环境的可持续性具有重要意义。尽管这种养分很重要，但我们对控制氮素循环过程的许多因素，包括土壤有机质对氮素的保护(和释放)的了解仍然不完整。长期以来，人们一直认为钙通过与土壤有机质的离子缔合(即钙桥联)和将有机质包裹在土壤溶液中的钙矿物中而发挥保护土壤有机氮的作用。这些物理保护机制通常归因于非生物过程，即由于土壤条件的变化，如影响土壤溶液中钙的增溶的湿循环和干燥循环，钙与有机质发生反应。然而，生物过程--特别是土壤微生物的作用--对方解石等钙矿物的沉淀以及土壤有机氮的保护作用在很大程度上还没有被探索。尽管在一系列水生和陆地环境中已经研究了微生物在方解石沉淀中的作用，但微生物诱导的方解石沉淀对土壤有机氮的物理保护的意义和贡献还没有被讨论。有人假设，MICP存在于富钙环境中，是一种保护微生物细胞免受自身钙化的机制。此外，MICP通常与这些相同的微生物增加胞外聚合物(EPS)的产生有关，也是钙饱和环境中的一种保护机制。该计划探讨了在典型的加拿大草原富钙土壤环境中，MICP和EPS的相关产物在土壤团聚体稳定和土壤有机氮保护方面发挥重要作用的可能性。尖端技术，如基于同步加速器的土壤微团聚体结构检查，以及负责聚集的微生物的分子鉴定，将用于阐明在微观尺度上有助于在宏观尺度上促进N的生物有效性的物理过程。最终，我们需要了解微观过程，然后才能成功地制定宏观战略来管理农业生态系统中的氮。

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.