Divergent carbon sequestration pathways: biochar and maize stover mediate DOC properties and soil carbon accumulation

Abstract

Biochar and stover incorporation are common ways to achieve long-term increases in soil organic carbon (SOC) storage. However, the effects of biochar and stover application on the dissolved organic carbon (DOC) molecular chemodiversity and origin of SOC still remain unclear. We explored the DOC molecular chemodiversity, plant derived carbon and microbial necromass carbon by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), lignin phenols and amino sugars by using a 10-year maize field experiment receiving three treatments (control, biochar application at a rate of 2.63 t ha−1 annually, hereafter termed BC, stover application at a rate of 7.5 t ha−1 annually, hereafter termed SR). Both biochar and stover incorporation increased SOC contents significantly (P < 0.05), with no significant difference in the topsoil between BC and SR treatments (0–20 cm, BC: + 49.70%, SR: + 48.87%, P > 0.05), while stover showed a greater SOC increase in the subsurface soil (20–40 cm, SR: + 105.90%, BC: + 72.81%, P < 0.05) and deep soil layer (40–60 cm, BC: + 4.74%, SR: + 32.35%). Biochar and crop stover application not only elevated DOC contents but also altered the molecular composition. Biochar increased the H/C ratio and decreased the nominal oxidation state of all C atoms (NOSC), which means biochar increased the aromaticity and decreased the bioactivity of DOC. Stover incorporation improved the bioavailability of DOC, as reflected by NOSC. Biochar and stover both increased the microbial necromass carbon contents, but the potential mechanisms involve different regulatory pathways. Biochar application decreased microbial carbon pump efficiency, whereas stover increased it. SR enriched plant derived carbon contents, while BC reduced plant derived carbon contents by promoting more plant carbon decomposition. Large amounts of stable carbon were introduced into soil by biochar application and manifested as other C, the native carbon and plant debris were decomposed by microorganisms stimulated by biochar incorporation. So the plant derived carbon in biochar treatments was lower than control in the 0-20 cm and 20–40 cm soil layers. Partial least-squares path model (PLS-PM) revealed SR enhanced SOC via aggregate, plant derived carbon and microbial necromass carbon (MNC) synergy driven by active microbial cycling, while BC sequestered SOC through direct stable carbon input and indirect MNC accumulation, with lower MCP efficiency. This study demonstrates that BC outperforms SR for long-term carbon sequestration by stabilizing carbon pools, while SR fosters active soil carbon cycling, providing insights for tailored carbon management in agricultural ecosystems. Biochar and stover increased SOC via divergent pathways over a decade. Stover incorporation mainly relies on accumulation of microbial necromass carbon, while biochar mainly relies on its own stable carbon input and microbial necromass carbon accumulation. Biochar lowered DOC bioactivity through reduced NOSC, whereas stover raised DOC bioactivity. Biochar is superior for long-term carbon sequestration by elevating stable carbon pools, while stover fosters active carbon cycling through microbial processing. Biochar and stover increased SOC via divergent pathways over a decade. Stover incorporation mainly relies on accumulation of microbial necromass carbon, while biochar mainly relies on its own stable carbon input and microbial necromass carbon accumulation. Biochar lowered DOC bioactivity through reduced NOSC, whereas stover raised DOC bioactivity. Biochar is superior for long-term carbon sequestration by elevating stable carbon pools, while stover fosters active carbon cycling through microbial processing.