Changes in aggregate-associated carbon and microbial respiration affected by aggregate size, soil depth, and altitude in a forest soil

This study investigates the variations of aggregate-associated organic carbon (OC) and microbial respiration (MR) with respect to aggregate size, soil depth, and land altitude in a forest soil. The research was conducted in Arasbaran forest, East Azerbaijan province, Iran. Four sites were selected at different altitudes: 0–600 m, 600–1200 m, 1200–1800 m, and 1800–2400 m above sea level (a.s.l.), along the north-facing slope of the forest region. Soil samples were collected at five depths: 0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm, and 80–100 cm, with three replications for each depth. The soil samples were then fractioned into four aggregate size classes: <0.053 mm, 0.053–0.25 mm, 0.25–2 mm, and >2 mm. Both soil samples and aggregate fractions were analyzed for OC content and MR. The results revealed that OC content and MR in both soil samples and aggregate fractions increased with altitude and decreased with soil depth. Moreover, aggregate-associated OC increased with aggregate size. Specifically, as soil aggregate sizes increased from <0.053 mm to >2 mm at different altitudes, the mean occluded OC content of aggregates increased by approximately four times. Similarly, the mean aggregate-associated MR intensified eight times with the increase in aggregate size. A significant positive correlation was also observed between OC and MR rates of all aggregates. Furthermore, aggregates larger than 2 mm exhibited the highest contribution to the total soil OC storage (47.47–57.22%) and CO2 production rate (20–21%). These findings underscore the critical role of large macro-aggregates (>2 mm) in soil carbon storage and microbial activity, particularly in surface soils at higher altitudes.