Synergistic use of biochar and the plant growth-promoting rhizobacteria in mitigating drought stress on oak (Quercus brantii Lindl.) seedlings

The successful early establishment of seedlings is a challenging step for the management of forests in semi-arid regions, such as in the oak (Q. brantii) forests in western Iran. To address this challenge, we tested the separate and combined effects of different biofertilizers: we evaluated the influence of the application of biochar and plant growth-promoting rhizobacteria (PGPR) on the growth of oak seedlings subjected to water stress in controlled conditions. Two acorns were sown in a total of 216 pots distributed according to the following treatments: 3 water treatments (100, 75 and 50 % of field capacity), 4 biochar treatments (0, 1, 2 and 3 % of soil weight), 2 PGPR treatments (absence or presence) in 9 replicates. Biochar was derived from mistletoe (Loranthus europaeus), a widespread hemi-parasite plant in the studied oak forests. After emergence, a total of 20 ecophysiological, morphological and growth traits were measured on oak seedlings. We found that the interaction between irri-gation, biochar and PGPR significantly influenced the ecophysiological attributes of the seedlings. Comparing application to non-application of PGPR, we found that chlorophyll content, photosynthesis rate and biomass of shoots were respectively increased by 21, 50 and 19 % in the absence of water stress and by 54, 14 and 18 % under severe water stress conditions. Moderate biochar concentrations improved seedling growth and ecophysiological responses whereas the highest concentration of biochar had a detrimental effect. A PLS path model revealed the direct and indirect relationships between the treatments, the seedling morpho-physiological traits and the final variable of interest (total dry biomass). The model explained 78 % of the total variance and showed that the root growth indices (path coefficient = 0.665) had a significant direct effect on oak dry matter whereas this effect was reduced with shoot growth indices. We found that the use of moderate levels of biochar alone or in combination with PGPR can benefit oak seedling development in water-limited conditions but further studies are still needed to test the effectiveness and applicability of these treatments in field conditions.