Forest harvesting alters the biogeochemical character and protection of mineral associated carbon in a Podzolic soil.

Abstract

Forest harvesting can alter the biogeochemical processes that determine the quantity and chemical nature of stored soil organic carbon (SOC). This SOC is distributed amongst organic compounds in soils of varying biological and physical complexity, with a large fraction of this SOC associated with metals and minerals. The objective of this study was to investigate changes in fractions of SOC associated with metals and minerals and their chemical character through depth and time for Podzolic soils of a harvested temperate red spruce (Picea rubens Sarg.) forest chronosequence to understand biogeochemical changes in SOC and its sensitivity to forest harvesting disturbance. A standard analytical approach was used to sequentially extract C from mineral pools of increasing SOC retention strength through depth (0-5, 5-10, 10-15, 15-20, 20-35, 35-50 cm) and time (1, 15, 45, 80, and 125⁺-yr) following forest harvesting. The biogeochemical character of the largest pool was assessed using aromaticity (specific UV absorbance: SUVA₂₅₄) and stable C-isotopes (δ¹³C). Results show that the organo-metal complexed fraction accounted for the largest pool examined (~ 59-85% of mineral associated SOC) and was most susceptible to harvesting losses, particularly in the deeper mineral soil. Biogeochemical patterns of SOC within this pool through depth showed distinct changes including: i) a lower aromaticity with lower C content across all sites within the deep mineral soil (20-50 cm), and ii), a correlation between decreases in aromaticity and increases in δ¹³C across all sites. This pattern of lower aromaticity and enriched δ¹³C can be interpreted as a shift towards more microbially processed and/or simpler compounds. Overall, the observed patterns support the conceptual hypothesis that forest harvesting alters the biogeochemical character and protection of the mineral associated fraction of SOC in soils of this region.