Chemical characterization of organic matter in soil aggregates under cacao agroforestry systems assessed by solid-state 13 C CPMAS NMR

Cacao-based agroforestry systems can sequester much soil carbon (C), but information on soil organic matter (SOM) chemical composition within aggregates still needs to be available. This study aimed to evaluate (1) the impact of forest or pasture conversion to cacao AFS by assessing the chemistry of the Cm fraction, considering this as the primary C pool in the studied soils, and (2) which C chemical compounds are associated with the aggregate density fractions. Soil chemical characterization was performed using solid-state 13 C CPMAS NMR. The conversion of pasture into cacao AFS did not impact the chemical composition of C. However, converting natural forest into cacao AFS provided differences in the composition of C compounds, mainly in macroaggregates. Notably, the young rubber tree was more prevalent in promoting changes in the chemical composition of C than cacao. The chemical composition of C in Cm density fractions did not vary across the land-use systems, and it was mostly represented by alkyl-C and O/N-alkyl-C forms. The composition of POM fractions was mainly aryl-C. The knowledge of soil organic matter chemical composition proved to be an adequate tool for application in models to establish management strategies and guide policy toward consolidating low-carbon agriculture in Brazil.