The overlooked role of deep soil in dissolved organic carbon transformation and CO2 emissions: Evidence from incubation experiments and FT-ICR MS characterization

Dissolved organic carbon (DOC) is a major source for CO₂ emission, and strongly involved in the transformation of many pollutants in the environmental medium. Neglecting the transformation of DOC in deep soil (>100 cm) may lead to a high degree of uncertainty in the estimation of the soil C budget, greenhouse gas emission and environmental risk. Using an envelope soil profile in a representative agricultural region of eastern China, this study provides kinetic and molecular evidence for DOC transformation in deep soil. Deep soil remained rich in DOC, with 52.53–65.46% of the DOC sequestered in soil below 100 cm. DOC in deep soil may be derived more from leaching from shallow soil than from the decomposition of in situ SOC. As the incubation process progressed, the DOC changed in three stages: (I) DOC accumulation; (II) DOC decomposition; and (III) slow DOC accumulation, with CO₂ emissions exhibiting corresponding kinetic patterns. Soil CO₂ release from deep soil accounted for a non-negligible portion (12.9–57.4%) of the soil profile. Fourier-transform ion cyclotron resonance mass spectrometry indicated that during the incubation process, less aromatic, and more saturated DOC molecules with lower molecular weights may be preferentially decomposed. During the early stages of incubation, lipids and peptides were preferentially degraded. In the later stages, due to the depletion of active components, lignin began to undergo partial degradation. DOC transformation in deep soil was favored under anaerobic conditions. This study might shed new light on the greenhouse effect and the environmental risk management.