Capillary absorption spectroscopy for high temporal resolution measurements of stable carbon isotopes in soil and plant-based systems.

Capillary Absorption Spectroscopy (CAS) is a relatively new analytical technique for performing stable isotope analysis. Here, we demonstrate the utility of CAS by recording and quantifying variation in C in controlled and biologically relevant applications. We calibrated CAS system response to increased CO, with an observed ∼4‰ increase in measured ΔC for each 0.03 ppm shift in CO concentration. We leveraged this calibration to quantify rates of biogeochemical processes using a C tracer. For example, we monitored microbial respiration of C-glucose within an agricultural soil at 10 s quantification intervals and results demonstrated 8.6% ± 0.4 of added glucose was converted to CO within 1.5 h of incubation. We expanded the demonstration by adapting a rhizobox to permit continuous monitoring of CO in a soil (as distinct from plant) headspace to track the timing and quantify respiration rates of fresh plant photosynthate and observed a 3.5 h delay between plant exposure to aCO tracer and the first signs of respiration by soil biota. These experiments highlight CAS is effective in producing high temporal resolution quantification of CO and demonstrate potential applications.