Rock weathering is a critical control over Earth’s climate system and biogeochemical cycling. Chemical weathering reactions at Earth’s surface occur at the molecular level, but the effects of weathering ripple across spatial and temporal scales. In this dissertation I explored weathering across scales, including investigations of (1) field-scale impacts of enhanced silicate weathering on carbon dioxide (CO2) removal over one winter rainy season and then over two years; (2) mesocosm-scale plant-soil-water interactions in a 3-month trial with olivine, red clover, and an irrigation gradient; and (3) long-term rock nitrogen inputs to soils at five sites in mesic, mixed conifer forests.
Silicate weathering via reaction with dissolved CO2 has traditionally been inferred as a key driver of climate regulation over the long-term carbon cycle. Proponents of enhanced silicate weathering aim to accelerate this process by applying crushed silicate minerals to agricultural soils, leading to CO2 removal over shorter time scales. However, significant questions remain concerning weathering rates, environmental controls, plant and soil impacts, and suitable methodologies for carbon removal quantification. In Chapters One and Two, I used lysimeters to sample soil pore water at a five-acre agricultural field site in Northern California amended with crushed olivine and metabasalt. During an extreme drought in the first winter season following lysimeter installation, I observed a 2.6 to 2.9-fold increase in in-situ bicarbonate alkalinity in response to additions of metabasalt and olivine fines. Over the entire two-year study, I estimated net carbon removal of 0.14 to 0.48 t CO2 ha-1 yr-1 with crushed rock additions to the soil; however, these estimates were limited by field-level spatial variability, complexities associated with irregular precipitation, unknown plant interactions, alkalinity inputs from irrigation water, and the uncertain fates of weathering products.
I further explored mechanisms and controls on enhanced weathering in Chapter Three, where, in a mesocosm study in a controlled environment, I assessed weathering across an irrigation gradient with and without red clover in pots amended with 10 wt. % finely crushed olivine. During the 85-day experiment, silicate weathering resulted in removal of -0.212 to 0.177 t CO2 ha-1 yr-1. I found interacting effects of clover and water on weathering fluxes and CO2 removal. There was no detectable influence of olivine on plant growth and microbial biomass, but water-stressed clover accumulated more Fe, Ni, and Cr. Results from this study illustrated complex enhanced weathering interactions with soil, biological processes, and hydrologic controls, as well as differences among quantification approaches used to estimate CO2 removal via enhanced weathering.
In Chapter Four, I investigated natural weathering at a larger spatial scale, exploring long-term release of rock-derived nitrogen in the Northern Coast Ranges of California. Rock-derived nitrogen is a critical nutrient pool to the terrestrial biosphere but is not well understood. By quantifying rock N release across five sites with mean annual precipitation ranging from 950 to 3107 mm and outcrop rock N from 77 to 1007 ppm, I constructed geochemical mass balances for rock N release in these systems. I separated contributions of fixed NH4+ from total soil N and assessed loss of N from parent material. At wetter sites underlain by schist, bedrock N measurements were lower at the bedrock-soil interface than in outcrop rock samples, suggesting deep chemical weathering and mobilization of rock N from silicates. Overall, annual fluxes of rock-derived N at the four schist sites ranged from 1.55 to 5.30 kg ha-1 yr-1, with negligible fluxes at the lowest rock N site underlain by diorite. Increased rock N fluxes at these sites were associated primarily with higher denudation rates. Continued field assessments of rock nitrogen are critical to constrain this nutrient pool that is globally widespread but still poorly understood.
