δ¹³C convergence of glomalin fractions reveals a soil transition from carbon sequestration to functional exhaustion

Abstract

Stable carbon isotope (δ¹³C) fractionation is an integral process in soil organic matter (SOM) dynamics, typically resulting in ¹³C-depleted respired CO₂ and ¹³C-enriched residual pools. However, the long-term isotopic trajectory of specific molecular fractions during ecosystem degradation remains poorly understood. We utilized glomalin-related soil protein (GRSP) as a robust SOM proxy to trace these isotopic shifts across a land-use chronosequence, comprising forest (initial), 5-year fallow (intermediate), and 47-year agriculture (late stage), along with their respective one-year laboratory-incubated counterparts. Our results reveal a progressive, stage-dependent ‘isotopic convergence’ as degradation progresses. In undisturbed forests, the soil micro-architecture maintained high isotopic heterogeneity across sequential GRSP fractions (−27.1‰ to −23.7‰), reflecting the sequestration of distinct carbon inputs. This isotopic diversity narrows moderately in the transitional fallow soils before converging completely toward a uniform, ¹³C-enriched signature (−22.8‰) in the long-term agricultural soils. This isotopic terminality reflects a state of functional exhaustion, where the lack of fresh organic inputs and physical breakdown of the soil matrix is consistent with a potential metabolic shift toward the utilization of historical carbon reserves. Our findings also demonstrate that the “labile-stable” continuum of GRSP is not an intrinsic chemical property, but a state-dependent feature of a healthy soil matrix. Ultimately, this study establishes δ¹³C variance across GRSP fractions as a high-resolution fingerprint for tracking bidirectional state transitions between functional carbon sinks and exhausted soils.

Graphical abstract

Keywords

soil organic carbon / glomalin-related soil protein / stable carbon isotope / δ¹³C fractionation / δ¹³C signature / soil degradation

Highlight

	● Sequential stable carbon isotope signatures act as a soil biogeochemical clock.
	● Undisturbed forest soils maintain high isotopic heterogeneity across glomalin pools.
	● Long-term degradation triggers a progressive isotopic convergence of glomalin.
	● Convergent isotope signatures reveal a metabolic shift toward historical carbon reserves.
	● The labile-stable continuum of glomalin is a state-dependent feature of the soil.

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