Soil organic matter persistence in hyperhumic colluvial soils caused by palaeofires, root inputs and mineral binding

Understanding the formation of long-term persistent soil organic matter (SOM) is key to optimizing soil management and predicting the response of the terrestrial organic carbon (OC) pool to climate change, yet our knowledge of the soil-type dependent weight of different stabilization pathways (e.g., recalcitrance and mineral binding) is fragmentary. Owing to their stratigraphy, the exceptionally SOM-rich (up to 2 m of mineral soil with >5% OC) colluvial slope deposits of Atlantic Europe (Haplic Umbrisol [colluvic/hyperhumic]) are archives of palaeo-environmental conditions including SOM formation pathways. The objective of this study was to determine how the different drivers of persistent SOM formation influenced the formation of these organic-rich soils. For this purpose, we use Holocene (∼9000 yrs) molecular composition records obtained from pyrolysis-GC–MS (Py-GC–MS) and thermally assisted hydrolysis and methylation (THM-GC–MS). The results emphasize three pathways to stability (i.e., persistence on millennial timescales): 1) palaeofires that generated recalcitrant pyrogenic SOM, 2) release of root-derived aliphatic macromolecules (suberin-like SOM), and 3) formation of microbial necromass. Pathways 1 and 2 are controlled by land use: Pathway 1 was relatively important under intense anthropogenic fire regimes and pyrophytic shrubland expansion; Pathway 2 was stimulated during early forest phases and under pasture conditions, when past societies focused vegetation management on grazing instead of fire; Pathway 3 was controlled by binding with aluminium-dominated mineral phases. However, we found indications that Pathway 2 (suberin input and preservation) relied partially on sorptive preservation as well. Aided by structured equation modeling (SEM), we show that the formation of persistent SOM pools was driven by balanced weights of i) microbial vs. plant-derived SOM and ii) intrinsic chemical properties of SOM (recalcitrance continuum) vs. mineral binding/occlusion, which varied in keeping with interactions between past land use, topography and vegetation. These findings are inconsistent with the prevalent paradigm of persistent SOM formation by sorptive/occlusive preservation of microbial necromass alone.