Biotic soil-plant interaction processes explain most of hysteric soil CO₂ efflux response to temperature in cross-factorial mesocosm experiment

Ecosystem carbon flux partitioning is strongly influenced by poorly constrained soil CO₂ efflux (F_soil). Simple model applications (Arrhenius and Q₁₀) do not account for observed diel hysteresis between F_soil and soil temperature. How this hysteresis emerges and how it will respond to variation in vegetation or soil moisture remains unknown. We used an ecosystem-level experimental system to independently control potential abiotic and biotic drivers of the F_soil-T hysteresis. We hypothesized a principally biological cause for the hysteresis. Alternatively, F_soil hysteresis is primarily driven by thermal convection through the soil profile. We conducted experiments under normal, fluctuating diurnal soil temperatures and under conditions where we held soil temperature near constant. We found (i) significant and nearly equal amplitudes of hysteresis regardless of soil temperature regime, and (ii) the amplitude of hysteresis was most closely tied to baseline rates of F_soil, which were mostly driven by photosynthetic rates. Together, these findings suggest a more biologically-driven mechanism associated with photosynthate transport in yielding the observed patterns of soil CO₂ efflux being out of sync with soil temperature. These findings should be considered on future partitioning models of ecosystem respiration.