Search

Soil carbon turnover time (tSOC), the ratio between soil organic carbon stocks (SOC), and soil heterotrophic respiration (Rh), is a critical factor in determining soil carbon storage and a key parameter in terrestrial carbon models. While tSOC 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 (Fs) 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 Fs 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 Fs and estimate Rh, which indicated that under the drought conditions (CTRL) tSOC 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 tSOC suggests that carbon inputs must have increased proportionally to match the changes in carbon outputs and provided the main source for the increased Fs. The results indicate that changes in the intensity of the seasonal drought can result in large changes in fluxes and tSOC values with little impact on soil carbon storage and its stability.

The modulation of the leaf energy budget components to maintain optimal leaf temperature are fundamental aspects of plant functioning and survival. Better understanding these aspects becomes increasingly important under a drying and warming climate when cooling through evapotranspiration (ET) is suppressed. Combining novel measurements and theoretical estimates, we obtained unusually comprehensive twig-scale leaf energy budgets under extreme field conditions in droughted (suppressed ET) and non-droughted (enhanced ET) plots of a semi-arid pine forest. Under the same high mid-summer radiative load, leaf cooling shifted from relying on nearly equal contributions of sensible ( H) and latent ( LE) energy fluxes in non-droughted trees to relying almost exclusively on H in droughted ones, with no change in leaf temperature. Relying on our detailed leaf energy budget, we could demonstrate that this is due to a 2× reduction in leaf aerodynamic resistance. This capability for LE-to-H shift in leaves of mature Aleppo pine trees under droughted field conditions without increasing leaf temperature is likely a critical factor in the resilience and relatively high productivity of this important Mediterranean tree species under drying conditions.