Disentangling carbon stabilization in a Calcisol subsoil amended with iron oxyhydroxides: A dual-13C isotope approach

Calcisols pose some unique challenges, particularly relating to their low organic carbon (C) content and low C storage ceiling. To address this, we investigated the role of iron (Fe) oxyhydroxides – goethite and ferrihydrite (0.36, 0.72, 3.6, and 7.2 g kg−1 soil) in the presence of a labile C substrate (glucose) to simulate rhizodeposition, on C-cycling. As there were three potential C sources: (i) glucose-C, (ii) native SOC, and (iii) soil inorganic C (SIC), a novel dual-13C isotope approach (δ13C-enriched glucose of 29 and 81‰) was implemented to accurately differentiate these three C sources from a Calcisol subsoil (δ13SOC, −23‰; δ13SIC, −3.6‰). Over 28 days, across the glucose and Fe oxyhydroxide treatments, 34.8–41.7% of the supplied glucose-C (1.0 g C kg−1 soil), 7.5–9.6% of the native SOC (3.7 g kg−1 soil), and 0.11–0.19% of the SIC (48 g kg−1 soil) were lost as CO2. Goethite and ferrihydrite generally stabilized organic C (including glucose-C and native SOC) which occurred primarily within the first 10 days following amendment with Fe oxyhydroxide, and the stabilization effect generally increased with increasing Fe oxyhydroxide dose. This is likely due to rapid Fe-OC adsorption that protected the OC from microbial decomposition. Ferrihydrite (cf. goethite) had a smaller effect on suppressing positive priming of SOC mineralization induced by glucose, possibly resulting from the lower C use efficiency and less stable Fe-OC associations due to the higher dissolution rate of ferrihydrite. The SIC loss increased after glucose addition, which was further enhanced by Fe oxyhydroxides. We conclude that Fe oxyhydroxides may be useful amendments for increasing SOC in highly alkaline Calcisols.