Losses of carbon from mineral-associated soil organic matter pools in podzolic horizons following soil climatic changes associated with forest clear-cut harvesting

Abstract:

Losses of soil organic matter (SOM) from mineral soils through depth have been documented following shifts in soil climate associated with intensive forest harvesting practices such as clear-cut harvesting, with serious implications for this important global carbon (C) pool. SOM was previously thought to be primarily stabilized in mineral soils by binding with a suite of metal hydroxide mineral pools (ranging from organo-metal complexes (OMC) to crystalline hydroxide minerals. Recent evidence has determined that SOM associated with particular mineral pools is dynamic and may be vulnerable to destabilization via decomposition to carbon dioxide (CO₂ respiration) and mobilization (as dissolved organic carbon (DOC)). In order to improve the mechanistic understanding of SOM losses from mineral soils, the distribution and character of mineral-associated SOM pools through depth from adjacent Mature (110 yr since clear-cut harvest) and Young (35 yr) forests were measured, and short-term respiration responses and shifts in DOC chemistry were analysed for podzolic horizons through depth (A_e, B_f and BC) across a range of soil climate conditions. Organo-metal complex pools dominated the distribution of mineral SOM C storage and were highest in illuvial B_f horizons of the Mature compared to the Young site. Short-term incubations revealed that temperature sensitivity (Q₁₀) and bioavailability were greater in the surface mineral horizons compared to subsoil (B_f>A_e>>BC) at both sites. Extreme climatic disturbances (dry-rewetting and freeze-thaw) were shown to enhance mobility of soil C from surface horizons and increase decomposition losses of soil C at depth. DOC was the likely source of respiratory substrate compounds as indicated by congruent shifts in the aromatic character of DOC and δ¹³C of respired CO₂. Together, these results suggest that mineral-associated SOM in horizons dominated by OMC, such as illuvial B_f horizons, can be destabilized via changes in environmental conditions. These findings challenge our assumptions about the stability of SOM associated with minerals.