Sources and drivers of particulate organic matter and mineral-associated organic matter along a calcareous soil chronosequence

Abstract

Understanding how plant-derived carbon (PDC) and microbial necromass carbon (MNC) are partitioned and stabilized within particulate organic matter (POM) and mineral-associated organic matter (MAOM) is essential for predicting soil organic carbon (SOC) persistence. However, the sources and controlling factors of POM and MAOM remain poorly understood along a calcareous soil chronosequence. Here, biomarker analysis, solid-state ¹³C NMR spectroscopy, qPCR, and enzyme activity assays were combined to elucidate the dynamics and drivers of POM and MAOM along the chronosequence. Results showed that SOC and exchangeable cations (Ca²⁺, Mg²⁺) decreased significantly in both fractions over succession. PDC dominated POM at the early stage (38.52%) but declined sharply in the middle stage (6.40%−7.33%), whereas MNC consistently exceeded PDC in MAOM, with fungal necromass contributing most in the early and middle stages and bacterial necromass increasing in the later stages. Structural equation modeling (SEM) revealed divergent pathways: POM formation was mainly driven by microbial activity and enzymatic processing, while MAOM accumulation depended on Ca²⁺/Mg²⁺-mediated mineral protection. The negative coupling between PDC loss in POM and MNC enrichment in MAOM indicates that plant residues are progressively transformed into mineral-associated microbial necromass. Overall, despite a decline in SOC, calcareous soils exhibited a compositional and stabilization shift from plant-residue-dominated POM toward microbially processed and mineral-protected MAOM, highlighting the synergistic roles of microbial transformation and mineral stabilization in SOC persistence.

Graphical abstract

Keywords

mineral-associated organic matter / particulate organic matter / plant-derived carbon / microbial necromass carbon / calcareous soil chronosequence / microbial functional genes

Highlight

	● SOC decreased significantly along the calcareous soil chronosequence.
	● FNC dominates the early stages, whereas BNC increases in the later stages.
	● During succession, calcareous soils shift from plant-residue accumulation in POM to microbial-necromass sequestration in MAOM.
	● Mineral cations (Ca2+, Mg2+) synergistically stabilize plant-derived and microbial necromass carbon in MAOM.

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