Soil moisture manipulation in a semi-arid pine forest demonstrates large changes in carbon turnover time with no change in soil carbon stock

Soil carbon turnover time (t_SOC), the ratio between soil organic carbon stocks (SOC), and soil heterotrophic respiration (R_h), is a critical factor in determining soil carbon storage and a key parameter in terrestrial carbon models. While t_SOC is generally expected to increase with drying conditions, its interactions with the carbon fluxes and soil moisture are still poorly constrained. Our study centered on a five-year manipulation experiment in the Yatir semi-arid pine forest in Israel, where supplement irrigation eliminated the summer drought. Soil CO₂ fluxes (F_s) and soil organic carbon (SOC) stocks were measured under trees and in open areas in a "control" forest plot (CTRL) and an 0.1 ha “irrigated” plot (IRRI). During the dry period (May-November), daily average F_s in the open areas was near zero in the CTRL but significant in the IRRI plots (0.06 and 2.02 µmol CO₂ m^-2 s^-1 respectively, with a similar trend under the trees). Annual-scale fluxes in the open areas were 82 and 321 g C m^-2 yr^-1 in the CTRL and IRRI plots, respectively (with similar trends under trees). Using published results from the same site enabled us to partition F_s and estimate R_h, which indicated that under the drought conditions (CTRL) t_SOC was x5 longer in the open area (and x2 longer under trees) compared with the non-droughted (IRRI) plot. However, no significant changes in the SOC stock down to 40 cm (the typical soil depth at this site) were observed.  Furthermore, there were no differences between treatments in regard to the ratio between the stable mineral-associated organic carbon fraction and the particulate organic carbon fraction. The stability of SOC stocks, despite the large changes in t_SOC suggests that carbon inputs must have increased proportionally to match the changes in carbon outputs and provided the main source for the increased F_s. The results indicate that changes in the intensity of the seasonal drought can result in large changes in fluxes and t_SOC values with little impact on soil carbon storage and its stability.