Interactive effects of wood biochar application and Thymus species on soil physical quality in irrigated farming

Abstract

Thymus sp. is one of the widely known spicy aromatic and medicinal plants widely used in medicine, cooking and perfumery. Biochar is an amendment to improve soil quality attributes. However, the interactive effect of Thymus cultivation and biochar application rate (BAR) on soil quality indicators in field conditions is not documented yet. This study aimed to investigate the effect of biochar of apple tree branches (BAR of 0, 1 and 2 %), two Thymus species (TS, T. daenensis and T. migricus), and soil sampling zone (SSZ, rhizosphere and bulk soil) on structural stability and water repellency of a silty clay loam soil in the field conditions. The experiment was conducted in a randomized complete block design with a factorial arrangement of the 12 treatments (i.e., 3 levels of BAR × 2 levels of TS × 2 levels of SSZ) with three replicates in Absard plain research site with semiarid climate affiliated with Research Institute of Forests and Rangelands, east of the Tehran. After plant harvest, the soil aggregate stability was characterized by high energy moisture characteristic (HEMC) and percent of water-stable aggregates (WSA), and the soil water repellency was measured by the sorptivity method. Biochar affected soil structural stability indices in both rhizosphere of the two Thymus species and bulk soil. The results showed that an increase in the BAR from 0 to 1 and 2 % enhanced the soil organic carbon (SOC) by 22 and 48 % and consequently increased the HEMC stability ratio from 0.616 to 0.667 and 0.859, respectively. Water holding capacity in the biochar treatments was higher in all matric suctions, therefore, greater HEMC aggregate-structure stability indices were measured in the biochar-amended soils. The rhizosphere soil had significantly greater mean of organic carbon storage (6.74 g kg⁻¹), stability ratio (0.762) and water repellency index (4.08) compared to those in the bulk soil (5.23 g kg⁻¹, 0.666 and 2.17, respectively). The rhizosphere of T. migricus (with greater root development in the upper layers) had significantly greater WSA (59.6 %) compared to that of T. daenensis (51.2 %). However, there was no significant difference between the two plant species for SOC in the rhizosphere. This finding implies that quality (not quantity) and fractions of organic matter might be different in the rhizosphere of the plant species. Water repellency could partly explain the structural stability differences in the rhizosphere of T. daenensis and T. migricus species. Overall, the combination of wood biochar application with a rate of 1 % and T. migricus cultivation is recommended for improving soil physical quality in the region and similar zones.