Synergistic relationships between the age of soil organic matter, Fe speciation, and aggregate stability in an arable Luvisol

Background: Knowledge of soil aggregate formation and stability is essential, as this is important for maintaining soil functions. Aims: This study aimed to investigate the influence of organic matter (OM), the content of pedogenic iron (Fe) (oxyhydr)oxides, and aggregate size on the stability of aggregates in arable soil. Methods: To this end, the Ap and Bt horizons of a Luvisol were sampled after 14 years of bare fallow, and the results were compared with a control field that had been permanently cropped. Results: In the Ap horizon, bare fallow decreased the median diameter of the 53-250 mu m size fraction by 26%. Simultaneously, the mass of the 20-53 mu m size fraction increased by 65%, indicating reduced stability-particularly of larger soil microaggregates-due to the lack of input of fresh OM. The range of (14)carbon (C-14) fraction of modern C ((FC)-C-14) under bare fallow was between 0.50 and 0.90, and thus lower than the cropped site ((FC)-C-14 between 0.75 and 1.01), which is particularly pronounced in the smallest size fraction, indicating the presence of older C. This higher stability and the reduced C turnover in <20 mu m aggregates is probably also due to having the highest content of poorly crystalline Fe (oxy)hydroxides, compared to the other size fractions, which act as a cementing agent. Colloid transport from the Ap to the Bt horizon was observed under bare fallow treatment. Conclusions: The lack of input of OM decreased the stability of microaggregates and led to a release of mobile colloids, the transport of which can initiate elemental fluxes with as-yet unknown environmental consequences.