Abiotic and biotic controls on soil organo-mineral stability

Reversing the trend of decreasing soil carbon stocks is important to help mitigate current environmental challenges. Improving knowledge on the mechanisms that control the stabilisation and persistence of soil organic carbon will provide a foundation to tackle the issue. This includes the mechanisms controlling the stability of organo-mineral associations, considered to be the most persistent pool of soil carbon. Uncertainties remain in how the composition of carbon involved in mineral associations can control the persistence of this soil organic carbon pool. Carboxylic acid richness of organic carbon (OC) are important indicators for OC stability in ocean sediments, however, it remains to be seen whether this is replicated for organo-minerals in terrestrial soil settings. Experiments were designed to investigate if OC carboxyl richness controls organo-mineral stability by using synthesised model organo-minerals. These model organo-minerals consist of simple homogenous carboxylic acids of varying carboxyl richness to more complex heterogeneous microbial necromass OC associated to ferrihydrite. Organo-mineral OC stability was tested by measuring the change in solid organo-mineral OC from before to after destabilisation treatments. These treatments were NaOH and NaOCl chemical washes that induce desorption and oxidation of OC, and redox fluctuations that enable the reductive dissolution of ferrihydrite. This accompanied a soil mesocosm incubation experiment containing 13C labelled necromass organo-minerals from different microbial types. Results indicate that carboxyl rich OC has greater stability in organo-mineral associations, compared to carboxyl poor OC. Findings also indicate that heterogeneous necromass OC follows this trend, where fungal necromass organo-minerals has greater organo-mineral stability compared to other microbial necromass organo-minerals. These results imply that carboxyl rich OC can enhance the persistence of the mineral associated pool of soil organic carbon.