Accurate evaluation of the Birch effect requires continuous CO2 measurements and relevant controls

The influence of dry-wet cycles (DWC) on soil organic carbon (SOC) decomposition is still debated given the somehow controversial results observed in the literature. The objective of this study was to evaluate the effects of DWC on SOC mineralization relative to various moisture controls in 7 treatments from two long-term French field experiments presenting contrasted SOC concentrations. We conducted a laboratory incubation to quantify CO₂ emissions upon four soil moisture scenarios: continuously wet (WET), continuously moderately wet (MWET), continuously dry (DRY) and dry-wet cycles (DWC). We also calculated the SOC mineralization that would correspond to the average water content in DWC (mean_DWC). Our results showed that across all treatments the daily carbon mineralization rate increased with soil moisture (WET>MWET>DRY). In DWC scenario, mineralization rates fluctuated with the changes in soil moisture. As soils dried, daily mineralization rates decreased and the subsequent soil rewetting, to pF 1.5, caused a rapid mineralization flush or "Birch effect". However, these flushes did not compensate for the low mineralization rates in the drying phase as the cumulative mineralization was not higher in the DWC scenario compared to the mean_DWC which was the scenario with equivalent water content as the DWC. We also observed that not accounting the CO₂ emissions in the drying phase could lead to an overestimation of the effect of DWC. We recommend to measure continuously the soil respiration during dry-wet experiments and to compare the CO₂ emitted in DWC with a control that has a water content equivalent to the average water content in DWC. In addition, we questioned the importance of the effect of dry-wet cycles on overall soil respiration.