Cultivation reduces soil carbon stocks but enhances thermal stability in Mollisols, Northeast China

Land-use change (LUC) can reduce soil organic carbon (SOC) storage, but its effects on SOC thermal stability remain unclear. We collected paired non-cultivated and cultivated Mollisols in Heilongjiang Province, China, and determined SOC, total nitrogen (TN), particulate organic carbon (POC), and mineral-associated organic carbon (MAOC). Molecular characteristics of SOC were assessed by analyzing humic acid (HA) using FTIR-spectroscopy and Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS). Fe and Mn concentrations were determined in POC and MAOC. Thermal stability was evaluated using TG-T50 and DSC-T50 derived from TG–DSC analysis. Structural equation modeling (SEM) was used to examine relationships among molecular composition, TN, MAOC thermal stability, and carbon pools. Cultivation reduced SOC and TN by 41% and 34%, respectively. POC and MAOC declined by 74% and 22%, respectively. LUC shifted SOC molecular composition toward more aliphatic moieties, characterized by increased alkane abundance and decreased aromatic and nitrogen-containing compounds in arable soils. MAOC thermal stability increased after cultivation, with TG-T50 increasing from 368.67 to 375.92 °C and DSC-T50 from 392.58 to 399.91 °C, whereas POC thermal stability changed little. The SEM explained 34% of the variation in MAOC thermal stability and 25% of the variation in the carbon pool. Molecular composition (β = −0.47) and TN (β = −0.26) were negatively associated with MAOC thermal stability, while MAOC thermal stability was negatively associated with the carbon pool (β = −0.38). Molecular composition and TN had no significant direct effects on the carbon pool. These results show that increased MAOC thermal stability occurred alongside declining carbon content, indicating a partial decoupling between MAOC thermal stability and carbon storage under cultivation.