Postdoctoral Fellowship: EAR-PF: Linking soil nitrogen enrichment to mineral weathering and associated organic matter persistence

Fitch, Amelia A, South Royalton VT

Investigators

Abstract

Soils store a huge amount of organic matter - more than the atmosphere or land combined! Soil organic matter is mostly made up of carbon, and if it becomes available to microbes living in the soil, they will “eat” this organic matter and respire carbon dioxide (CO2). As the climate crisis continues to alter local temperatures and precipitation, we want to understand how the SOM will stay in the ground instead of being released to the atmosphere as carbon dioxide. One mechanism that helps carbon stay in the soil, which we call SOM persistence, are positive and negative charges between SOM particles and minerals, which come from rocks broken into tiny pieces. The process of breaking rocks down into minerals that help protect SOM from microbes is called weathering. In soils where the bedrock, or parent material, has a lot of minerals, these minerals can be released with increasing amounts of moisture, temperature, and acids that flow through the soil. Nitric acid is a naturally produced acid from biological nitrogen (N) fixation, which plants like peas and beans with their symbiotic bacteria can do. Across soils that have a lot of N fixing plants, more nitric acid may have helped to weather parent material and release minerals that can help protect SOM, leading to more carbon persistence. This project will investigate the relationship between rock weathering by nitric acid and how it relates to soil C storage from shallow to deep soils, and across ecosystems with low and high weathered parent material. The sites chosen to study the long-term N enrichment gradient in Oregon forests also provide an opportunity to collaborate with ongoing outreach efforts through the Inspiring Girls Expeditions and expand the current trips to include forest ecosystems. This program will be a no-cost educational experience for high school students identifying as women, non-binary, or genderqueer, and selection of students to this program will not be based on traditional academic benchmarks such as grades and test scores. Instead, students will be chosen based on their interest in Critical Zone sciences and the degree to which this program may change their futures. The need to understand the processes that affect mineral-associated organic matter (MAOM) formation and persistence are imperative given the importance of SOM persistence by mineral interactions in C sequestration and storage. Though biological nitrogen (N) fixation and anthropogenic deposition introduce significant quantities of N in ecosystems across the globe, the effects of nitric acid on weathering and subsequent MAOM persistence has not been studied. The long-term goal of this research is to understand how N enrichment and nitric acid weathering affects MAOM abundance and persistence across climates and primary mineral weathering states. This research will quantify both MAOM abundance under long-term soil N-enrichment and MAOM loss after short-term nitrate and nitric acid inputs. These paired experiments will clarify how MAOM formation and destabilization co-occur across increasing N enrichment and between more and less weathered basalt parent material. Because deep soils store a substantial amount of soil C, an additional aim of this proposal is to understand SOM protection by mineral-associations and destabilization with depth. Despite the significant gains in understanding broad patterns of MAOM and mineral associations, key mechanistic questions remain untested. The effects of N enrichment on MAOM constitutes a significant knowledge gap in the understanding of MAOM formation and persistence, and consequently, soil C storage. This proposal advances knowledge in Critical Zone science by linking short and long-term N enrichment with fine-scale geochemical drivers of MAOM. There is an urgent need to understand and predict how much carbon (C) can be stored in soils and the degree to which this carbon persists. This work offers an opportunity to improve the accuracy of MAOM predictions and the accuracy of terrestrial C cycling models in the face of global change. By investigating MAOM abundance in soils across a gradient of soil N, between temperate and tropical climates, and between soils of low and high weathered basalt parent material, these findings can apply to many ecosystems that experience N enrichment. 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.