9.5 years of whole-soil warming had minor effects on composition of subsoil mineral-associated organic matter

Soil depth and climate shape the amount, sources, and composition of soil organic carbon (SOC), thereby governing its persistence and turnover. However, little is known about how these characteristics respond to changes in climate or plant inputs. SOC is primarily composed of two physicochemical fractions: Free particulate organic matter (fPOM) and mineral-associated organic matter (MAOM). Approximately half of total SOC is stored in subsoils (> 30 cm), where MAOM contributes substantially to total SOC. The relative persistence and predominance of MAOM suggest that this pool may be particularly important in constraining warming-induced SOC loss in subsoils. Nevertheless, empirical evidence for its response to long-term warming remains limited. Leveraging a nearly decade-long whole-soil warming experiment in a temperate forest (Blodgett Forest Research Station, UC Berkeley), we investigated warming-induced shifts in plant sources of SOC and degree of microbial processing in bulk soil, fPOM, and MAOM using fatty acids and n -alkanes. In bulk soil and fPOM, topsoil (< 30 cm) SOC is mainly derived from mixed aboveground and root-derived plant inputs, whereas subsoil SOC was composed predominantly of root-derived material. In contrast, MAOM was dominated by more microbially processed SOC throughout the soil profile. In warmed plots, molecular markers indicated greater root-derived inputs in upper subsoils (30–50 cm), whereas deeper subsoils showed more microbially processed material. At 80–90 cm, MAOM exhibited smaller compositional shifts to warming than fPOM, while fPOM showed greater changes in fatty acid composition and concentration, consistent with stronger microbial transformation. Changes regarding the average chain length of fatty acids further suggested that decomposition products from fPOM may have contributed to shifts in MAOM fatty acid composition. This indicates a possible coupling between decomposition of easily accessible substrates and MAOM formation at depth. Given the substantial fPOM reduction previously observed at 80–90 cm after nearly a decade of warming, it remains unclear whether this pool can continue to mitigate MAOM loss over longer timescales. Overall, our results highlight the importance of including subsoils in studies of soil carbon dynamics under climate change, as these depths may contain SOC pools that are most sensitive to environmental shifts like warming.