Synergistic relationships between the age of soil organic matter, Fe speciation, and aggregate stability

Understanding the formation and stability of soil aggregates is crucial for sustaining soil functions. This study investigates the impact of organic matter (OM), pedogenic Fe (oxyhydr)oxides, and aggregate size on aggregate stability in an arable soil. Samples were collected from the Ap and Bt horizons of a Luvisol after 14 years of bare fallow, and results were compared with a control field under permanent cropping. In the Ap horizon, bare fallow led to a 26% reduction in the median diameter of the 53-250 µm size fraction, indicating decreased stability of larger microaggregates. Simultaneously, the mass of the 20-53 µm size fraction increased by 65%, suggesting reduced stability, particularly of larger soil microaggregates, due to the absence of fresh OM input. The ¹⁴carbon (¹⁴C) fraction of modern C (F14C) under bare fallow ranged from 0.50 to 0.90, lower than the cropped site (F14C between 0.75 and 1.01). This difference was most pronounced in the smallest size fraction, indicating the presence of older C. Higher stability and reduced C turnover in <20 µm aggregates were attributed to their elevated content of poorly crystalline Fe (oxy)hydroxides, acting as cementing agents. Colloid transport from the Ap to the Bt horizon was observed under bare fallow treatment, highlighting the release of mobile colloids. This transport may initiate elemental fluxes with potential unknown environmental consequences. In conclusion, the absence of OM input decreased microaggregate stability, releasing mobile colloids and initiating colloid transport.