Tracing carbon stabilization pathways in soil incubation using isotopes, REOs, and phase-space modelling

Mineral-associated organic carbon (MAOC) constitutes the largest and most persistent pool of soil organic carbon, yet the pathways through which new plant inputs are stabilised in MAOC remain actively debated. In particular, the relative contributions of microbial processing versus direct physical transfer from particulate organic carbon (POC) from the plant are still poorly constrained. Here, we combined rare earth oxide (REO) labelling, stable carbon isotopes (δ¹³C), and controlled soil incubations to trace carbon redistribution between POC and MAOC in two soils with contrasting carbon status. Reconstituted soils containing independently REO-labelled POC and MAOC were amended with sugarcane mulch that had undergone different water-processing treatments to systematically manipulate carbon solubility and microbial accessibility. Carbon dynamics were monitored over a 180-day incubation using physical fractionation, respiration measurements, isotopic mass balance, and REO recovery, and interpreted within a POC–MAOC phase-space framework to track the different pathways of carbon stabilization (Manzoni and Cotrufo, 2024). To better align this framework with the objectives of the present study, we adapted its application to explicitly resolve the fate of newly added plant-derived carbon. Rather than treating POC as a single composite pool, we quantified the incorporation of mulch-derived carbon separately within POC and MAOC using isotope-based partitioning. This integrated approach enables direct comparison of physical redistribution and microbial transformation pathways, providing new mechanistic constraints on MAOC formation under contrasting input bioavailability and soil conditions.