Planting density regulates root biomass and soil organic carbon stocks in Acacia mearnsii plantations

Root biomass plays a crucial role in soil carbon sequestration in plantation forest ecosystems. While planting density can influence root biomass and soil organic carbon (SOC) accumulation, understanding its effects in short-rotation plantations is important. This study evaluated the effects of planting densities ranging from 0.5 m × 0.5 m to 1.5 m × 1.5 m and soil depth on root biomass, vertical distribution, and SOC stocks in Acacia mearnsii plantations of Ethiopia, using a randomized complete block design with four replications. Fine roots and soil were sampled using soil cores, and coarse roots through pit excavation, within 0–50 cm soil profile. Planting density, soil depth, and their interaction significantly affected root biomass components ( p < 0.05), whereas SOC content and SOC stock were significantly affected by planting density and soil depth only. Root biomass and SOC stocks generally increased with planting density. Compared with the sparsest stands, densest stands increased fine root biomass (FRB), coarse root biomass, and SOC stock by 44%, 85%, and 38%, respectively. Most roots were concentrated in surface soil layers, although denser stands exhibited relatively deeper proportional root distribution, as reflected by higher β values for FRB and SOC stock. Enhanced SOC accumulation under dense stands was associated with greater root biomass and below-ground carbon input. Overall, planting density strongly influenced below-ground carbon dynamics and SOC accumulation, although excessive density may intensify intraspecific competition and requires balanced plantation management strategies that consider broader spatial and temporal scales.