Studying the effect of vermicompost and biochar on growth, biochemical properties, and leaf element concentrations of juniper seedlings (Juniperus excelsa M. Bieb.) under different irrigation regimes

Introduction and Objective:
Greek juniper (Juniperus excelsa M. Bieb) is a long-lived tree species tolerant to harsh environmental conditions, typically growing at high elevations. Due to its ecological, medicinal, and industrial values, it plays a vital role in mountainous ecosystems. However, decreased rainfall and climate change threaten its survival. Therefore, improving management and nutritional strategies to enhance the drought tolerance of Greek juniper seedlings is essential. The application of soil amendments such as biochar and vermicompost is recognized as an effective approach to mitigate the adverse effects of drought stress in plants. Biochar and vermicompost improve the physical, chemical, and biological properties of the soil, thereby reducing drought stress impacts and promoting plant growth. Although the individual effects of these amendments have been studied in various plants, their combined influence on drought-tolerant species like Greek juniper in arid regions has been less explored. This study aimed to evaluate the separate and combined effects of biochar and vermicompost on growth, biochemical traits, and leaf nutrient concentrations of Greek juniper seedlings under different irrigation levels.
Materials and Methods:
This factorial experiment was conducted on two-year-old J. excelsa seedlings under a completely randomized design in the greenhouse of the Faculty of Natural Resources, University of Tehran (Karaj). Seedlings were subjected to three irrigation levels: 100% field capacity (well-watered), 75% field capacity (mild drought stress), and 50% field capacity (severe drought stress). Additionally, three soil amendment treatments were applied: biochar (5% of pot volume), vermicompost (15% of pot volume), and a combination of biochar (2.5%) with vermicompost (7.5%). Evaluated traits included plant biomass, total chlorophyll content, proline content, antioxidant enzyme activities, and concentrations of macronutrients in leaves.
Results:
The results indicated that in the absence of biochar and vermicompost, drought stress at 50% field capacity significantly reduced growth and nutritional indices of Greek juniper seedlings. Specifically, under this severe stress without amendments, shoot dry weight decreased by 26%, root dry weight by 20%, total chlorophyll by 18%, essential oil yield by 24%, leaf nitrogen by 29%, phosphorus by 25%, and potassium by 28% compared to the 100% field capacity treatment. Conversely, this drought level increased leaf proline content by 24% and antioxidant enzyme activities of catalase and superoxide dismutase by 97% and 105%, respectively. Furthermore, combined application of vermicompost and biochar outperformed individual treatments in improving the measured parameters. Under 50% field capacity stress, this combination increased shoot dry weight by 15%, root dry weight by 14%, total chlorophyll by 13%, proline by 12%, essential oil percentage by 11%, essential oil yield by 25%, and leaf nitrogen, phosphorus, and potassium concentrations by 24%, 21%, and 22%, respectively, compared to untreated controls.
Conclusion:
The simultaneous application of vermicompost and biochar under mild drought stress (75% field capacity) improved growth, nutrient uptake, and essential oil production, thereby enhancing water use efficiency. This combined treatment offers an effective and economical strategy for managing drought stress in Greek juniper seedlings.