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• Marsh degradation reduced large-aggregate ratio and SOC content in aggregates. • Dominant aggregate size of SOC preservation decreased under marsh degradation. • SOC active functional groups in aggregates increased along degraded marsh gradient. • Marsh degradation weaken organic C stability and sequestration in soil aggregates. Alpine marsh wetlands in the Qinghai-Tibetan Plateau are ecological hotspots for global carbon (C) cycling in Central Asia; however, the soil C sinks are increasingly limited by marsh degradation. To assess the physical mechanisms by which changes in the soil C sequestration occur, the distribution and functional groups of soil organic C (SOC) in aggregates along a marsh degradation gradient on the Zoige Plateau in China were investigated using soil samples collected from differently degraded marshes. Our results showed that, beyond the state of light degradation, the large aggregate content decreased significantly by 31.1–55.2%, while the micro-aggregate content increased significantly by 23.1–93.1%. The SOC content in the aggregates decreased significantly by 39.1–84.8% and the dominant aggregate size for SOC preservation decreased to small or micro-aggregate. Marsh degradation also decreased the relative ratio of aliphatic-C while increasing that of alcohol and phenolic-C, and polysaccharide-C. Index I (relative decomposition of SOC in aggregates) was 73.5–429.0% higher for degraded marshes than for relatively pristine marsh (RPM), while Index Ⅱ (relative recalcitrance of SOC in aggregates) was 13.8–79.5% lower in comparison. Therefore, we concluded that marsh degradation probably reduces the organic C sequestration capacity of soil by increasing the decomposition of SOC in aggregates after both the dominant aggregate size for SOC preservation and aggregate stability decreased. This result represents the physical mechanism by which the decrease in SOC content occurs in response to marsh degradation in alpine regions. [ABSTRACT FROM AUTHOR]

• Marsh degradation reduced large-aggregate ratio and SOC content in aggregates. • Dominant aggregate size of SOC preservation decreased under marsh degradation. • SOC active functional groups in aggregates increased along degraded marsh gradient. • Marsh degradation weaken organic C stability and sequestration in soil aggregates. Alpine marsh wetlands in the Qinghai-Tibetan Plateau are ecological hotspots for global carbon (C) cycling in Central Asia; however, the soil C sinks are increasingly limited by marsh degradation. To assess the physical mechanisms by which changes in the soil C sequestration occur, the distribution and functional groups of soil organic C (SOC) in aggregates along a marsh degradation gradient on the Zoige Plateau in China were investigated using soil samples collected from differently degraded marshes. Our results showed that, beyond the state of light degradation, the large aggregate content decreased significantly by 31.1–55.2%, while the micro-aggregate content increased significantly by 23.1–93.1%. The SOC content in the aggregates decreased significantly by 39.1–84.8% and the dominant aggregate size for SOC preservation decreased to small or micro-aggregate. Marsh degradation also decreased the relative ratio of aliphatic-C while increasing that of alcohol and phenolic-C, and polysaccharide-C. Index I (relative decomposition of SOC in aggregates) was 73.5–429.0% higher for degraded marshes than for relatively pristine marsh (RPM), while Index Ⅱ (relative recalcitrance of SOC in aggregates) was 13.8–79.5% lower in comparison. Therefore, we concluded that marsh degradation probably reduces the organic C sequestration capacity of soil by increasing the decomposition of SOC in aggregates after both the dominant aggregate size for SOC preservation and aggregate stability decreased. This result represents the physical mechanism by which the decrease in SOC content occurs in response to marsh degradation in alpine regions. [ABSTRACT FROM AUTHOR]