Date of Award
Doctor of Philosophy (PhD)
Diane M McKnight
Suzanne P Anderson
Michael D SanClements
The result of plant and microbial decomposition, organic matter (OM) is a complex, heterogeneous mixture of organic compounds found in all soil and aquatic systems. In soils, OM plays an important role in nutrient cycles. In aquatic systems, dissolved organic matter (DOM) is also integral to nutrient cycling, and also complexes with metals. While OM is important for ecosystem functioning, DOM can cause toxic byproducts at water treatment plants. Concentrations of DOM have been increasing in many northern latitudes and an understanding of this phenomenon can help us better predict future water quality. Many hypotheses for the increase in DOM suggest that changes in the terrestrial landscape are affecting DOM concentration and chemistry in aquatic systems.
As part of the Boulder Creek Critical Zone Observatory, weekly samples of DOM in analyzed with fluorescence spectroscopy. In addition, water-soluble soil organic matter from the soil of Gordon Gulch, a subcatchment, was also analyzed. This multi-year dataset indicated that while Boulder Creek and Gordon Gulch both exhibit DOM dynamics commonly found in alpine ecosystems, the first-order stream responds more quickly to inputs from the catchment. Soil samples displayed a change in WSOM chemistry with depth that suggest a correlation between microbial input to the OM pool and oxidation state of that pool, indicating OM could be involved in microbial weathering of bedrock. Additionally, there was a noticeable difference in the chemistry of soil and stream OM, most pronounced in samples from the riparian zone.
Two tracer studies were performed to investigate how in-stream processing alters the chemical character of soil organic matter. Chloride was used as a conservative tracer and freshly leached soil OM from the riparian zone as a reactive tracer. The DOM chemical signature from the reactive tracer, as measured with fluorescence spectroscopy, attenuated quickly, and in-stream processing of the WSOM appeared to almost completely remove the tracer signal within 35 m. Modeling with OTIS showed considerable differences between reaches which where clear and reaches which were clogged with plant debris. These results indicate that while catchment character is a significant driver of DOM chemistry, in-stream processing has a substantial impact.
Gabor, Rachel Sarah, "A Spectroscopic Investigation of the Chemistry of Organic Matter During Biogeochemical Cycling Through the Soil, Groundwater, and Surface Water of the Boulder Creek Watershed" (2013). Environmental Studies Graduate Theses & Dissertations. 13.