Toward Spatially Resolved Mapping of Subsurface Biological Processes Using Isotopic Signatures.

Microbial processes are responsible for converting micro- and macronutrients into forms thatcan be utilized by plants, as well as nutrient loss channels, for example through denitrification(which releases nitrous oxide) and methanogenesis (which releases methane). Thebiological activity in the soil is therefore a direct window into the health of the soil. Theisotopic composition of soil trace gases reflects the microbial pathways that produce them,making these gases effective messengers of bioactivity. Here we present results fromlaboratory testing of new diffusive soil probes coupled to a nitrous oxide/methane isotopeTILDAS (Tunable Infrared Laser Direct Absorption Spectrometer). The TILDAS issensitive to the common N2O, 15N14N16O, 14N15N16O, 14N14N18O, common CH4, and13CH4 species. The number of probes in the system is scalable, opening up thepossibility of spatially and temporally-resolved mapping of subsurface biologicalactivity. Tests using abiotic silica sand demonstrates the efficacy of these probes. We will also present results from sampling gases in tropical rainforest soil fromBiosphere 2. Drying the soil, irrigating it, and exposing it to different oxygen levelsreveals variations in N2O and CH4 concentrations, as well as isotopic signatures. [ABSTRACT FROM AUTHOR]