Soils store substantial amounts of carbon as organic matter. However, the size of this C reservoir may be sensitive to climatic change motivating research on the mechanisms governing soil organic matter stabilization. One existing paradigm for organic matter stabilization suggests that the phyllosilicate “clay” minerals that form in soils protect organic matter from breakdown. However, models are not capable of fully representing this process pointing to knowledge gaps regarding the interactions between organic matter and minerals, their constitutive controls, and their timescales of operation. More broadly, there are persistent challenges in describing the co-evolution of soil mineralogy, structure, and organic matter storage, which limits the treatment of soil dynamics in Earth systems models and their concomitant effects on future climate projections.

Here, we propose to advance our understanding of how clay minerals and organic matter interact at the terrestrial-aquatic interface using the East River watershed as a natural laboratory. In addition to traditional methods, measurements of stable lithium isotopic ratios, which is one of the foremost proxies for quantitatively gauging clay mineral formation, will be central to the proposal. Critically, our approach will enable the detection of “hot spots” and “hot moments” of clay mineral formation by studying different landscape elements (e.g., floodplain vs. hillslope), components of the hydrosphere (e.g., groundwater vs. surface water), and timescales of biogeochemical cycling (e.g., rainfall events, the seasonal cycle, and landscape evolution over millennia). Ultimately, these measurements will be used to improve models that relate field and laboratory measurements with Critical Zone processes.

The East River watershed represents an ideal locality to interrogate these questions because of the extensive study that has already characterized it, a large sample archive, and existing sampling infrastructure that permits geochemical analyses at high spatial and temporal frequencies. Given the outsized role of headwater ecosystems in terrestrial biogeochemical cycles, the proposed work will build a theoretical and methodological framework for soil formation and organic matter stabilization that is applicable to other watersheds and will lead to a more accurate depiction of terrestrial carbon cycling in Earth system models. Lastly and importantly, the project will include the training of 1 graduate student and postdoctoral researcher in laboratory and computational methods.