The role of the molecular composition of organic matter in its (co-)precipitation by aluminium, podzolization, and carbon sequestration

This thesis describes the role of the molecular composition of organic matter (OM) in Al-OM (co-) precipitation, podzolization, and carbon sequestration. First, the molecular OM composition appeared to be less important for Al-OM precipitation than previously assumed and the large differences in OM composition needed for large effects (e.g. large amount of Al-OM precipitates) were not found for leaf litter derived DOM. Second, podzolization can be seen as ecosystem property, with environmental factors such as pH and M/C ratio determining which Al-OM complexation mechanism dominates (charge neutralization vs. adsorption to Al hydroxides). The role of the molecular OM composition depends on the dominating Al-OM complexation mechanism. Lastly, the molecular OM composition was also found to be important for carbon sequestration, as differences in the SOM composition of forest soils were found upon forest conversion due to a change in litter input chemistry (OM composition). Concluding, although the role of molecular composition is controversial as explanation for OM stability, the molecular composition of OM does play an important role in: (1) Al-OM (co-) precipitation, (2) podzolization, and (3) carbon sequestration. The molecular composition of OM can affect (co-) precipitation and adsorption, e.g. through the number of carboxyl groups or the amount of aromatic-C. This then indirectly influences stabilization by spatial inaccessibility and/or mineral/metal interactions. The findings of this thesis fit within the current paradigm of OM persistence as ecosystem property, but stresses that it remains important to include the molecular composition of OM in future studies.