Landscape-scale patterns of soil organic carbon mineralization: Spatial, temporal, and vertical variability

Abstract. The mineralization of soil organic carbon (SOC) through microbial respiration is controlled by a complex interplay of environmental conditions, stabilization mechanisms, and substrate availability. SOC mineralization varies with land use, soil depth, and environmental conditions, but how these factors interact across entire soil profiles and throughout the year remains insufficiently understood. We investigated spatial, seasonal, and vertical patterns of SOC mineralization in coniferous forest, meadow, and cropland soils across a 20 × 20 km landscape in the Fichtelgebirge Mountains, Bavaria, Germany. Soil samples were collected 120 times within one year from depths down to 1 m and sliced into 10 cm increments. On the day of sampling, samples were incubated for multiple hours at field temperature and moisture to estimate mineralization under in situ conditions. Additional standardized laboratory incubations (10 days, 22°C and 50 % WHC) were used to determine potential SOC-specific mineralization as an indicator of SOC stability.

Potential SOC-specific mineralization did not consistently decrease with depth. Instead, it was higher in the deepest subsoil (70–100 cm) than in the topsoil (0–30 cm) and upper subsoil (30–70 cm) in coniferous forest and cropland, indicating that subsoil SOC is not necessarily more stable than topsoil SOC. Under in situ field conditions, mineralization rates were significantly higher in meadow soils than in cropland and coniferous forest soils across all depth intervals, demonstrating that land use influences SOC mineralization throughout the upper meter of the soil profile. Despite pronounced seasonal variation in soil temperature and moisture, no clear seasonal pattern in SOC mineralization emerged, potentially reflecting contrasting constraints imposed by temperature and water availability. Mineralization rates under field conditions of temperature and moisture declined with depth in cropland but remained comparatively stable with depth in meadow and coniferous forest soils, despite decreasing SOC stocks.

Our findings highlight that SOC stocks alone do not adequately explain patterns of mineralization and the resulting CO₂ release across space, time, and depth and that the relationships between SOC stability, land use, soil depth, and environmental conditions are complex. Accounting for these interactions is important for improving assessments of soil carbon dynamics across landscapes and for understanding the potential vulnerability of subsoil carbon to mineralization.