Spatial and seasonal variabilities of the stable carbon isotope composition of soil CO₂ concentration and flux in complex terrain

Abstract

Biogeochemical processes driving the spatial variability of soil CO2 production and flux are well studied, but little is known about the variability in the spatial distribution of the stable carbon isotopes that make up soil CO2, particularly in complex terrain. Spatial differences in stable isotopes of soil CO2 could indicate fundamental differences in isotopic fractionation at the landscape level and may be useful to inform modeling of carbon cycling over large areas. We measured the spatial and seasonal variabilities of the δ13C of soil CO2 (δS) and the δ13C of soil CO2 flux (δP) in a subalpine forest ecosystem located in the Rocky Mountains of Montana. We found consistently more isotopically depleted values of δS and δP in low and wet areas of the landscape relative to steep and dry areas. Our results suggest that the spatial patterns of δS and δP are strongly mediated by soil water and soil respiration rate. More interestingly, our analysis revealed different temporal trends in δP across the landscape; in high landscape positions δP became more positive, whereas in low landscape positions δP became more negative with time. These trends might be the result of differential dynamics in the seasonality of soil moisture and its effects on soil CO2 production and flux. Our results suggest concomitant yet independent effects of water on physical (soil gas diffusivity) and biological (photosynthetic discrimination) processes that mediate δS and δP and are important when evaluating the δ13C of CO2 exchanged between soils and the atmosphere in complex terrain.

Estimating Soil Respiration in a Subalpine Landscape Using Point, Terrain, Climate, and Greenness Data

“…In the Rocky Mountains, subalpine forests are a regionally important ecosystem for C sequestration and sensitive to future changes in snowpack amount and duration (Schimel et al, ; Sexstone et al, ; Winchell et al, ). Previous efforts within subalpine forests have focused on characterizing C pools and fluxes across individual sites (Bradford et al, ; Liang et al, ; Scott‐Denton et al, ). Larger landscape‐scale assessments are lacking, but they would improve our understanding of how C flux‐environmental relationships gleaned at individual sites drive variability in regional C cycling.…”

Multi‐century stasis in C3 and C4 grass distributions across the contiguous United States since the industrial revolution

“…Similarly, applications that depend on δ 13 C data often fail to consider the spatial grain at which different δ 13 C proxies integrate C (Auerswald et al., ). For example, the δ 13 C composition of soil surface layers is related to soil texture and organic matter over relatively small areas (~m 2 ; Bai et al., ; Liang, Riveros‐Iregui, & Risk, ; Wynn et al., ), while herbivore tissues correspond to vegetation composition over larger spatial extents (~10 s of km 2 ; Auerswald et al., ; Kohn & Fremd, ; Meagher, ; Widga, Walker, & Stockli, ). As a result, the spatial scale of C integration may impact how well δ 13 C proxies represent vegetation at the spatial extents and spatial grains that they are often used.…”

Technical Note: Isotopic corrections for the radiocarbon composition of CO<sub>2</sub> in the soil gas environment must account for diffusion and diffusive mixing

“…In the model, isotopologues of CO 2 are treated as independent gases, with their own specific concentration gradients and diffusion rates (Cerling et al, 1991;Risk and Kellman, 2008;Nickerson and Risk, 2009). We assume total CO 2 to be 12 CO 2 because of its high abundance.…”