Role of Nitric Acid in Chemical Weathering and Pedogenic Thresholds

点击翻译按钮获取中文摘要

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.