Characterizing the impact of diffusive and advective soil gas transport on the measurement and interpretation of the isotopic signal of soil respiration

By measuring the isotopic signature of soil respiration, we seek to learn the isotopic composition of the carbon respired in the soil (δ¹³C_R-s) so that we may draw inferences about ecosystem processes. Requisite to this goal is the need to understand how δ¹³C_R-s is affected by both contributions of multiple carbon sources to respiration and fractionation due to soil gas transport. In this study, we measured potential isotopic sources to determine their contributions to δ¹³C_R-s and we performed a series of experiments to investigate the impact of soil gas transport on δ¹³C_R-s estimates. The objectives of these experiments were to: i) compare estimates of δ¹³C_R-s derived from aboveground and belowground techniques, ii) evaluate the roles of diffusion and advection in a forest soil on the estimates of δ¹³C_R-s, and iii) determine the contribution of new and old carbon sources to δ¹³C_R-s for a Douglas-fir stand in the Pacific Northwest during our measurement period. We found a maximum difference of −2.36‰ between estimates of δ¹³C_R-s based on aboveground vs. belowground measurements; the aboveground estimate was enriched relative to the belowground estimate. Soil gas transport during the experiment was primarily by diffusion and the average belowground estimate of δ¹³C_R-s was enriched by 3.8-4.0‰ with respect to the source estimates from steady-state transport models. The affect of natural fluctuations in advective soil gas transport was little to non-existent; however, an advection-diffusion model was more accurate than a model based solely on diffusion in predicting the isotopic samples near the soil surface. Thus, estimates made from belowground gas samples will improve with an increase in samples near the soil surface. We measured a −1‰ difference in δ¹³C_R-s as a result of an experiment where advection was induced, a value which may represent an upper limit in fractionation due to advective gas transport in forest ecosystems. We found that aboveground measurements of δ¹³C_R-s may be particularly susceptible to atmospheric incursion, which may produce estimates that are enriched in ¹³C. The partitioning results attributed 69-98% of soil respiration to a source with a highly depleted isotopic signature similar to that of water-soluble carbon from foliage measured at our site.