Role of Nitric Acid in Chemical Weathering and Pedogenic Thresholds

土壤矿物质的分解有助于土壤的形成，并影响当地乃至全球范围的养分供应和碳动态。硝酸可能是一个重要的，但在很大程度上被忽视的化学风化剂，有助于土壤矿物质的分解。硝酸是土壤中微生物在富氮条件下产生的，在生态系统中自然发生，也是人类活动的结果。该项目将研究硝酸在地球上广泛分布的玄武岩土壤矿物化学风化中的作用。了解硝酸在何处、何时以及在多大程度上增强了风化作用，可以提高对大气二氧化碳通量、土壤肥力和养分可持续性以及森林和农业系统中碳储存潜力的理解。 这项工作的更广泛影响将包括对本科生进行研究培训，并吸引高中生参与，重点是增加来自农村高中的代表性不足的学生人数，这些学生为STEM领域的“大学做好了准备”。风化率取决于许多因素，包括矿物学、表面性质、水流和植被。 碳酸是许多环境中化学风化的主要驱动因素，但一个未解决的问题是硝酸促进风化的贡献和影响。硝酸促进的风化可能是人为污染物、氮肥的添加或通过固氮生物的自然氮富集的结果。 因此，相对于单独的碳酸驱动风化，硝酸风化可能会改变碳、二氧化硅和岩石来源的营养物质的通量。 在这项研究中，研究人员将通过研究俄勒冈州玄武岩生物固氮的长期历史中自然发展的富氮和贫氮森林生态系统，评估硝酸对化学风化和成土作用的长期影响。他们还将使用田间直接氮肥试验来确定加速氮循环和硝酸驱动风化的短期影响。通过研究87 Sr/86 Sr比值、土壤水化学和通量，沿着土壤性质和矿物学，他们将检验以下假设：短期N富集和硝酸盐淋溶主要从土壤交换复合物中置换阳离子，而长期N富集加速风化并改变土壤矿物学。 这项评估将扩展到整个生物群落，还检查了几十年的N施肥实验在哥斯达黎加和夏威夷，跨越土壤发育的早期和晚期阶段的风化效应。研究人员将使用实验室柱玄武岩风化实验直接比较硝酸与碳酸和有机酸的影响，这将更精确地区分与每种风化机制相关的独特化学指纹。 通过确定硝酸何时、何地以及如何增强风化作用，本研究的结果将有助于了解在氮含量日益丰富的世界中，受管理的系统和自然系统的敏感性和弹性。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The breakdown of soil minerals contributes to the creation of soil and affects nutrient supply and carbon dynamics at local to global scales. Nitric acid may be an important but largely overlooked agent of chemical weathering that contributes to the breakdown of soil minerals. Nitric acid is produced in soils by microbes under nitrogen-rich conditions, which occur both naturally in ecosystems and as a result of human activities. This project will investigate the role of nitric acid in chemical weathering of soil minerals that originate from basaltic rocks, which are widespread on Earth. Knowing where, when, and how much nitric acid enhances weathering can improve understanding of atmospheric carbon dioxide fluxes, soil fertility and nutrient sustainability, and carbon storage potential in both forests and agricultural systems. The broader impacts of this work will include training of undergraduate students in research, and engaging high school students with a focus on increasing the number of underrepresented students from rural high schools who are “College Ready” for STEM fields. Weathering rates depend on many factors, including mineralogy, surface properties, water flow, and vegetation. Carbonic acid is a primary driver of chemical weathering in many settings, but an unsolved problem is the contribution and effects of nitric acid-promoted weathering. Nitric acid-promoted weathering may occur as a result of either anthropogenic pollutants, addition of nitrogen (N) fertilizers, or via natural N enrichment through N-fixing organisms. Nitric acid weathering may therefore change fluxes of carbon, silica, and rock-derived nutrients relative to carbonic acid-driven weathering alone. In this research investigators will evaluate the long-term effects of nitric acid on chemical weathering and pedogenesis by studying N-rich and N-poor forest ecosystems that have developed naturally from a long history of biological N fixation on basalt in Oregon. They will also determine the short-term effects of accelerated N cycling and nitric acid-driven weathering using direct N fertilization experiments in the field. By examining 87Sr/86Sr ratios, soil water chemistry and fluxes, along with soil properties and mineralogy, they will test the hypothesis that short-term N enrichment and nitrate leaching primarily displaces cations from the soil exchange complex, while long-term N enrichment accelerates weathering and alters soil mineralogy. This assessment will be extended across biomes by also examining weathering effects of decades-long N fertilization experiments in Costa Rica and Hawaii, spanning both early and late stages of soil development. Researchers will directly compare the effects of nitric acid to carbonic and organic acids using laboratory column basalt weathering experiments that will more precisely distinguish the unique chemical fingerprints associated with each weathering mechanism. By identifying when, where, and how nitric acid enhances weathering, the results of this study will contribute to understanding the sensitivity and resiliency of both managed and natural systems in an increasingly N-rich world.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.