Century of forest conservation sequesters wood-derived particulate organic matter

Forest soils are substantial sinks for carbon, yet the effectiveness of management practices such as strict conservation to sequester more soil organic carbon is unknown. A complication is the increase in forest disturbance due to climate change, key to which is the loss of canopy cover. We sampled paired control-canopy gap plots in drought-sensitive Norway spruce stands on Stangosols developed on Buntsandstein in the Black Forest National Park (2000 mm MAP, 5.5°C MAT). Three pairs are located within the management zone with sanitation logging of beetle infestations (gaps < 5 years old with little deadwood), and three more are in the core zone without logging since 1911 (gaps < 15 years old with moderate deadwood).

In the management zone, canopy gaps had warmer and slightly wetter forest floors (+0.45 °C and +1.7 % Vol), but thickness, carbon stocks and C/N ratios in the forest floor did not differ between canopy treatments. In the core zone by comparison, circa 50 Mg ha^-1 more carbon was found under closed canopies than canopy gaps as well as both canopy types in the management zone. Density fraction revealed most changes occurred in the free and occluded light fractions, which constituted circa 50 and 40% of SOC, respectively, in the core zone.

Lignin-derived phenols were extracted with cupric oxide oxidation to trace the source of soil organic matter (SOM). Lignin markers in the forest floor came mostly from coniferous wood and at times was less oxidized in canopy gaps. Additionally, subsoil horizons exhibited surprisingly little lignin oxidation regardless of canopy treatment, resulting in lignin-derived phenols constituting up to circa 20% of SOM. This applied to other CuO-extractable phenols, which in subsoil accounted for a further 10% of SOM.

The increase in core zone SOC stock is of a similar magnitude to accumulated deadwood if averages of 10 m³ ha^-1 year^-1 annual growth increment and 33% mortality are assumed. Wood-derived lignin absent in canopy gaps likely underwent less-oxidative photodegradation and leaching both deeper into mineral soil and laterally into catchments. Sequestration of wood-derived particulate SOM is thus possible in moist, acidic sandy soils, but such SOM is sensitive to disturbance-driven microclimatic changes.