Elucidating the Fate, Transport, and Processes Controlling Belowground Carbon on the Landscape: Biogeochemistry Tools for the 21st Century.

Globally, more carbon is stored belowground as soil organic matter (SOM) than in terrestrial vegetation and the atmosphere combined. A critical scientific question is how soils serve as sources and sinks for atmospheric CO₂ and how these sinks will evolve with expected changes in atmospheric CO₂ concentrations, climate, and land-use. The process behind SOM stabilization and loss, which span broad temporal and spatial scales, are poorly constrained in many dynamic land surface models. At LLNL, we have developed a suite of analytical tools that allow us to follow the movement of carbon at the cell to landscape scale, including: ‘Chip-SIP’, ‘STXM-SIMS’, and new sample interfaces for accelerator mass spectrometry (AMS).  Field-based and in vivo experiments allow us to further the mechanistic understanding of factors that control the fate, transport, and sequestration potential of belowground carbon. Chip-SIP allows us to interrogate which microbial species in a complex community incorporate specific substrates in order to better elucidate energy and carbon transfers in wetlands and soils. A combination of high-resolution microspectroscopy (STXM-NEXAFS), electron microscopy (SEM), and nano-scale imaging mass spectrometry (nanoSIMS) collectively known as STXM-SIMS, allows us to disentangle the complex interactions at soil-microbial-mineral interfaces with minimal disruption. This approach allows us to track labeled litter, exudates and microbial necromass onto soil surfaces and elucidate how SOM source and environmental conditions influence the physical and molecular fate of SOM.   Isotopic characterization (¹⁴C, ¹³C, ²H) of CH₄, CO₂, dissolved organic carbon (DOC) and physical sources of carbon provide mechanistic fingerprints of the biogeochemical pathways that cycle carbon through the landscape. Building on our expertise in accelerator mass spectrometry (AMS), we are developing methods that will enable more widespread and routine use of AMS in biological and environmental applications. Applied examples of these novel techniques, addressing critical questions in soil biogeochemistry, will be presented.