The conversion of natural grassland to cropland drives thermodynamic destabilization of subsoil DOM over decades

The conversion of natural grassland to cropland is a widespread practice that threatens soil carbon stocks. However, its impact on the dynamics of dissolved organic matter (DOM), particularly the thermodynamic stability of subsoil DOM, remains poorly quantified. This study investigated the distribution and biochemical characterization of DOM and its transformation processes following 11 and 40 years of conversion from natural grassland to agricultural land on a Haplic Chernozems soil in northern China. The post-conversion agricultural management was no-tillage, focused on rain-fed cultivation of wheat and oats. The chemical characterization of DOM was conducted using UV–visible and fluorescence spectroscopy and Fourier-transform ion cyclotron resonance mass spectrometry combined with PARAFAC analysis and substrate-explicit modeling. The results showed a critical depth-dependent shift of DOM content and stability: while topsoil (0–20 cm) dissolved organic carbon (DOC) decreased by 88 %, its thermodynamic stability increased. Conversely, subsoil (80–100 cm) DOC increased by 1.8-fold, yet this accumulated pool was characterized by lower molecular weight, enrichment of a blue-shifted humic-like component (C2), and significantly higher thermodynamic degradability. Molecular-level evidence revealed that this destabilization was primarily driven by the transformation of complex subsoil organic matter into labile DOM, a process strongly linked to nutrient enrichment. Although derived from a single representative soil type, our findings provide a mechanistic framework for assessing carbon vulnerability in agroecosystems following land-use change. • Depth-resolved (0–100 cm) study after 11- and 40-year grassland-to-cropland conversion. • Topsoil DOC decreased while stability increased; subsoil (80–100 cm) showed the opposite. • Subsoil DOM destabilization following conversion is driven by in-situ transformation. • Blue-shifted humic-like C2 enrichment linked to nutrient status and in-situ transformation.