Abiotic CO2 exchange between soil and atmosphere and its response to temperature
Soil CO2 flux, commonly referred to as soil respiration, is mainly derived from microbial oxidation of soil organic matter and litter fall and root. However, abiotic soil CO2 flux can also be observed on desert soil, though neither of them is generated from conventional biological processes. The mechanism of soil respiration is mainly based on the tricarboxylic acid cycle in mitochondria metabolism, its response to soil temperature has been studied extensively. The influence of soil temperature on abiotic soil CO2 flux is not explained as soil respiration, because the biotic processes in desert soil are extremely faint. Using an LI-8100 automated soil CO2 flux measurement system with a LI-8150 multiplexer and four 8100-104 long-term chambers, we measured abiotic soil carbon flux and soil temperature on sterilized soil at 2-day intervals in the Mu Us Desert, northwestern China from May to October in 2012. The questions asked were (1) whether the abiotic CO2 exchange process between soil and atmosphere could induce carbon sequestration, (2) how explore the response of this exchange process to soil temperature.
The results showed that negative abiotic soil CO2 flux (CO2 absorption) occurred at night while positive abiotic soil CO2 flux (CO2 emission) during the day. Net CO2 sequestration during the six months totaled 64.37 g CO2 m-2 (0.0920 μmol m-2 s-1). On the seasonal scale, abiotic soil CO2 flux was weakly correlated with soil temperature (γ = 0.15, p< 0.01), but showed a strong positive correlation with the rate of change in soil temperature (ΔT/Δt (°C h-1)) (γ = 0.82, p< 0.01). On the diurnal scale, abiotic soil CO2 flux showed clear hysteresis loops with respect to soil temperature, but ΔT/Δt explained more than 85% of the diurnal variations in abiotic soil CO2 flux for different months. The results indicate that a non-trivial CO2 exchange process between soil and atmosphere can induce carbon sequestration and this exchange process is modified by ΔT/Δt. CO2 absorption and emission is driven by the rate of falling and rising of soil temperature, respectively. The findings may more accurately explain the role of soil temperature played in the abiotic CO2 exchange between soil and atmosphere and may facilitate to interpret the formation of this CO2 exchange process.