Graduate Thesis Or Dissertation

Tracing Sources of Soil Organic Matter Through Time, Across Ecosystems, and Down Soil Profiles

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  • Soils contain the largest pool of carbon that is actively cycling on human timescales, leading many to view soils as a natural climate solution with multiple co-benefits. The field of soil science is rapidly evolving, but without a unified understanding of soil carbon dynamics. This dissertation leverages two distinct long-term monitoring projects that were sampled in their infancy. First, a biomass manipulation experiment in an Oregon Cascade timber farm that contains treatments with a ten-fold difference in residual tree biomass left on site. Standard soil collection methods would otherwise obscure the dynamism occurring in this forest. We observed losses in native soil carbon, buffered by a replacement of newly senesced root-carbon, and the participation of a previously thought ‘stable’ carbon pool. We find that soils are both incredibly resilient, but potentially vulnerable if they are not given enough time to rebuild after perturbations. The second project began during the installation of the National Ecological Observatory Network with 40 sites representing nearly every biome in North America. To the author's knowledge, this is the most broad systematic organic matter inventory and deepest soil horizon assessment that have ever been investigated with the copper oxidation method. Despite the inclusion of desert, grassland, forest and permafrost ecosystems, we find a striking similarity in lignin contributions and soil organic matter composition across sites and down soil profiles. Taken together, this work emphasizes the broad resilience of soils is likely due to a universal ecosystem inertia that transforms diverse plant inputs into homogenized soil organic matter signatures. There were, however, many exceptions which emphasizes that site-specificity will always preclude any potential land management recommendation.
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