Ecosystem carbon flux partitioning is strongly influenced by poorly constrained soil CO 2 efflux (F soil). Simple model applications (Arrhenius and Q 10) 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 CO 2 efflux being out of sync with soil temperature. These findings should be considered on future partitioning models of ecosystem respiration. A major challenge in terrestrial carbon science is identifying atmospheric CO 2 source and sink dynamics across numerous timescales 1-3. Because CO 2 efflux (F soil) can be the largest and most variable component flux in many ecosystems 4 , F soil drives regional carbon dynamics 5,6. Accurate measurements of F soil are critical for partitioning net ecosystem CO 2 flux (NEE) and modeling local-to-global carbon dynamics 4,7,8. Nighttime ecosystem respiration (R eco) can be un-measurable using the eddy covariance (EC) technique because of a lack of turbulence and atmospheric mixing, requiring gap-filling procedures to produce credible sums 7. These missing nighttime