Soil carbon stabilization pathways as reflected by the pyrolytic signature of humic acid in agricultural volcanic soils

•Pyrolytic profiles from soil organic matter reveal processes related to C cycle.•New surface density plots from compounds atomic ratios depict humic acid structure.•The proportions of common pyrolytic products are correlated with soils properties.•Up to four different non excluding biogeochemical paths define soil C storage.•An empirical model for soil C storage includes the role of agricultural practices.