Tree Species Effects on SOC and Soil Microbial Properties: Case Study From Beech and Spruce Stands in Bohinj Valley, Slovenia

Abstract

Climate change and forest management strategies in Central Europe are driving the decline of spruce in forests, while beech is expected to expand its range. Beech is seen as a key species for converting spruce-dominated forests to mixed forests, aiming to improve forest resilience. The objective of our study was to examine the long-term effects of a spruce stand and a beech stand that transitioned from a conifer-dominated stand on soil organic carbon (SOC), microbial biomass and the abundance of total bacteria, archaea and fungi. In contrast to most other studies, we used a horizon-based soil sampling approach, which provides better insights into how changes in soil chemical properties influence microbial community composition, and consequently, microbial-based processes like C-sequestration. Composite soil samples from two depths, corresponding to the A horizon (approx. 0–10 cm) and the B horizon (approx. 10–20 cm), representing the entire shallow soil profile, were collected from a European beech (Fagus sylvatica L.) stand and a Norway spruce (Picea abies [L.] Karst.) stand sharing the same soil group on limestone and dolomite. In the top A horizon, the spruce stand exhibited significantly higher levels of total organic carbon (C), total nitrogen (N), dissolved organic C and dissolved N compared to the beech stand (11.5% vs. 9.0%; 0.63% vs. 0.52%; 15.3% vs. 9.5 mg C kg⁻¹ dry soil; 2.9 vs. 1.6 mg N kg⁻¹ dry soil; respectively). The beech stand had significantly higher base saturation (84.6%) in the A horizon compared to the spruce stand (43.6%), primarily due to increased levels of exchangeable Ca²⁺. The soil pH did not show statistically significant differences between the stands, indicating a strong buffering capacity of the soil and its slow response to changes in the composition of tree species in the stand. Microbial biomass C (MBC) in the A horizon was significantly higher in the spruce than in the beech stand (585 vs. 492 mg C kg⁻¹ dry soil, respectively). While the abundance of bacteria and fungi did not differ significantly between the stands, a higher abundance of archaea was observed in the spruce compared to the beech stand. Total SOC stock in the entire soil profile (A and B horizons) was significantly lower in the beech than in the spruce stand (71.20 ± 3.08 t ha⁻¹ and 85.35 ± 2.84 t ha⁻¹, respectively), similar to the total MBC stock (0.42 ± 0.01 t ha⁻¹ and 0.48 ± 0.02 t ha⁻¹, respectively), with no significant differences observed in the B horizon. In conclusion, 20 years after the transition to a beech stand, significant differences in soil properties compared to spruce stand remain limited and confined to the A horizon. This reflects the gradual nature of changes driven by the litter input. The transition from a conifer-dominated to a beech-dominated stand leads to a reduction in SOC stocks. In comparison to beech-dominated stands, mixed forests-including both broadleaf and conifer species-may offer a promising strategy to mitigate SOC loss while enhancing forest resilience to climate change and natural disturbances.