Buried organic horizons represent amino acid reservoirs in boreal forest soils

► Composition of amino acid pool is similar in horizons and two successional stages. ► Glu, Gln, Ala, Asn, Asp, and His accounted for 80% of total free amino acid pool. ► Glutamine proportions increased with depth, suggesting roots are the source. ► Concentration of amino acids followed horizon series Oe/Oa > BOHs > mineral. ► BOHs are potential biological hot spots and nutrient reserves to 60 cm depth.