Bidirectional feedbacks between Abies koreana dieback and soil properties slow soil carbon turnover in a subalpine forest

Tree–soil interactions regulate carbon and nutrient cycling in forest ecosystems, but widespread dieback can disturb these processes with biogeochemical consequences. We investigated how Abies koreana dieback both responds to, and feeds back on, soil properties in subalpine forests on Mt. Jiri, South Korea. Soils were sampled at 0–5, 5–10, and 10–15 cm depths across nine stands, categorized into high- (HM) and low-mortality sites (LM). The HM was characterized by steeper southwestern slopes, lower mineralogical diversity, higher sand content, and nutrient-poor soils, with slightly (but not significantly) lower soil pH. These features suggest intrinsically low nutrient availability and water-holding capacity, likely contributing to Abies koreana stress. However, volumetric soil water content ( θ v ) and field capacity did not differ from LM, and trends of relative θ v after rainfall were also insignificant in HM, suggesting that post-dieback organic matter accumulation may have enhanced water retention. Biogeochemical signatures indicate feedbacks of dieback on soil organic matter (SOM) dynamics: the HM exhibited higher soil C:N, more depleted δ 15 N across depths, and older soil organic carbon at 0–5 and 5–10 cm depths, consistent with slowed SOM decomposition. These results suggest reduced rhizosphere activity, suppressed exudation-driven priming, and disturbed litter–soil connectivity. Together, our findings highlight tight, bidirectional linkages between dieback and soil biogeochemistry. Incorporating constraints imposed by both site-specific soil physicochemistry and rhizosphere on SOM turnover will be essential for predicting soil carbon fate under globally increasing subalpine conifer dieback.