Increasing use of automated soil respiration chambers in recent years has demonstrated complex diel relationships between soil respiration and temperature that are not apparent from less frequent measurements. Soil surface flux is often lagged from soil temperature by several hours, which results in semielliptical hysteresis loops when surface flux is plotted as a function of soil temperature. Both biological and physical explanations have been suggested for hysteresis patterns, and there is currently no consensus on their causes or how such data should be analyzed to interpret the sensitivity of respiration to temperature. We used a one-dimensional soil CO 2 and heat transport model based on physical first principles to demonstrate a theoretical basis for lags between surface flux and soil temperatures. Using numerical simulations, we demonstrated that diel phase lags between surface flux and soil temperature can result from heat and CO 2 transport processes alone. While factors other than temperature that vary on a diel basis, such as carbon substrate supply and atmospheric CO 2 concentration, can additionally alter lag times and hysteresis patterns to varying degrees, physical transport processes alone are sufficient to create hysteresis. Therefore, the existence of hysteresis does not necessarily indicate soil respiration is influenced by photosynthetic carbon supply. We also demonstrated how lags can cause errors in Q 10 values calculated from regressions of surface flux and soil temperature measured at a single depth. Furthermore, synchronizing surface flux and soil temperature to account for transport-related lags generally does not improve Q 10 estimation. In order to calculate the sensitivity of soil respiration to temperature, we suggest using approaches that account for the gradients in temperature and production existing within the soil. We conclude that consideration of heat and CO 2 transport processes is a requirement to correctly interpret diel soil respiration patterns.

Permafrost soils currently store approximately 1672 Pg of carbon (C), but as high latitudes warm, this temperature‐protected C reservoir will become vulnerable to higher rates of decomposition. In recent decades, air temperatures in the high latitudes have warmed more than any other region globally, particularly during the winter. Over the coming century, the arctic winter is also expected to experience the most warming of any region or season, yet it is notably understudied. Here we present nonsummer season (NSS) CO2 flux data from the Carbon in Permafrost Experimental Heating Research project, an ecosystem warming experiment of moist acidic tussock tundra in interior Alaska. Our goals were to quantify the relationship between environmental variables and winter CO2 production, account for subnivean photosynthesis and late fall plant C uptake in our estimate of NSS CO2 exchange, constrain NSS CO2 loss estimates using multiple methods of measuring winter CO2 flux, and quantify the effect of winter soil warming on total NSS CO2 balance. We measured CO2 flux using four methods: two chamber techniques (the snow pit method and one where a chamber is left under the snow for the entire season), eddy covariance, and soda lime adsorption, and found that NSS CO2 loss varied up to fourfold, depending on the method used. CO2 production was dependent on soil temperature and day of season but atmospheric pressure and air temperature were also important in explaining CO2 diffusion out of the soil. Warming stimulated both ecosystem respiration and productivity during the NSS and increased overall CO2 loss during this period by 14% (this effect varied by year, ranging from 7 to 24%). When combined with the summertime CO2 fluxes from the same site, our results suggest that this subarctic tundra ecosystem is shifting away from its historical function as a C sink to a C source.

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[1] Measurement of the isotopic composition of soil and soil-respired CO 2 (d 13 CO 2 ) has become an invaluable tool in understanding ecosystem carbon-cycling processes. While steady state work has been indispensable in understanding the effects of diffusive transport on soil CO 2 isotopic composition, it is crucial that researchers studying temporally dependent processes, such as soil CO 2 efflux, realize that these systems are rarely at steady state. Non-steady-state effects could result in misinterpretation of isotopic data, but have not been addressed in the literature, despite their fundamental importance to researchers who use isotopes in diffusive, non-steady-state environments. Here, we use an isotopologue-based model to study dynamic fractionation, which we propose is a byproduct of transient changes in environmental variables. Time varying soil characteristics and processes such as biological production rate, soil pore space, diffusivity and atmospheric concentration were all found to induce non-steady-state gas transport conditions in the soil leading to transient changes in the isotopic composition of soil CO 2 flux. The main driving force behind this transport related fractionation of CO 2 is the rate of the change in 12 CO 2 gradient compared to that of 13 CO 2 . These numerical simulations show that dynamic fractionation exists under non-steady-state diffusive conditions and suggest that isotopic data collected in non-steady-state, natural environments, cannot be properly interpreted without considering dynamic fractionation effects.