Subsurface approaches for measuring soil CO₂isotopologue flux: Theory and application

Abstract

Measurements of the stable isotope composition of soil flux have many uses, from separating autotrophic and heterotrophic components of respiration to teasing apart information about gas transport physics. While soil flux chambers are typically used for these measurements, subsurface approaches are becoming more accessible with the introduction of field-deployable isotope analyzers. These subsurface measurements have the unique benefit of offering depth-resolved isotopologue flux data, which can help to disentangle the many soil respiration processes that occur throughout the soil profile. These methods are likely to grow in popularity in the coming years and a solid methodological basis needs to be formed in order for data collected in these subsurface studies to be interpreted properly. Here we explore the range of possible techniques that could be used for subsurface isotopologue gas interpretation and rigorously test the assumptions and application of each approach using a combination of numerical modeling, laboratory experiments, and field studies. Our results suggest that methodological uncertainties arise due to poor assumptions and mathematical instabilities but certain methods, particularly those based on diffusion physics, are able to cope with these uncertainties well and produce excellent depth-resolved isotopologue flux data.

Observations of 14CO2 in ecosystem respiration from a temperate deciduous forest in Northern Wisconsin

“…Soil surface isotopic flux was computed from subsurface Δ 14 CO 2 and δ 13 CO 2 profiles measured at interfaces of genetic horizons at depths of approximately 8, 15, 22, 30, 50, and 70 cm, using the soil respiration gradient method adapted for isotopologues, as derived by Nickerson et al . [].…”

Environmental forcing does not induce diel or synoptic variation in the carbon isotope content of forest soil respiration

“…To test the hypothesis that barometric pressure change leads to ventilation of the soil pore space, we compared soil pore space observations (C S , δ S ) to model simulations produced using a steady-state soil gas transport model (Cerling, 1984;Nickerson et al, 2014). The model was based on Fick's second law of diffusion:…”

Biological and physical influences on soil <sup>14</sup>CO<sub>2</sub> seasonal dynamics in a temperate hardwood forest

“…The effects of soil moisture and gas diffusion on respired 14 CO 2 are also largely unexplored. Although soil moisture and gas diffusion can play roles in regulating deep versus shallow CO 2 production (Davidson et al, 2006;Phillips et al, 2012), gas diffusion is often neglected in favor of biological explanations for why sources of soil respiration vary through time. A simultaneous assessment of the relative influences on 14 CO 2 by soil physical factors in addition to plant and microbial activity provides a check on existing assumptions and tendencies.…”