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

Abstract

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 ¹³C in controlled and biologically relevant applications. We calibrated CAS system response to increased ¹³CO₂, with an observed ∼4‰ increase in measured Δ¹³C for each 0.03 ppm shift in ¹³CO₂ concentration. We leveraged this calibration to quantify rates of biogeochemical processes using a ¹³C tracer. For example, we monitored microbial respiration of ¹³C-glucose within an agricultural soil at 10 s quantification intervals and results demonstrated 8.6% ± 0.4 of added glucose was converted to ¹³CO₂ within 1.5 h of incubation. We expanded the demonstration by adapting a rhizobox to permit continuous monitoring of ¹³CO₂ 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 a¹³CO₂ tracer and the first signs of respiration by soil biota. These experiments highlight CAS is effective in producing high temporal resolution quantification of ¹³CO₂ and demonstrate potential applications.

Keywords: (13)CO(2); Glucose; Isotopologue; Rhizosphere; Root exudate; Stable isotope analysis; Switchgrass.