Soil acidity accelerates soil organic matter decomposition in Cryptomeria japonica stands and Chamaecyparis obtusa stands

Background and aims Research has focused on behavior of particulate organic matter (POM) and mineral-associated organic matter (MAOM) in acidic soils, but little attention has been given to the effects of tree species and vertical distribution of these components. With the ultimate aim of preserving soil organic matter, this study clarifies POM and MAOM status throughout the soil profiles of Cryptomeria japonica stands and Chamaecyparis obtusa stands.Methods In 11 C. japonica stands and 7 C. obtusa stands with contrasting soil acidities (i.e., acid buffering capacities, ABC), we collected soil samples from three depths (0-10, 10-20, 20-40 cm) that were then density-fractionated into a light fraction (LF) mainly with POM, middle fraction (MF) mainly with MAOM, and heavy fraction with scarce MAOM. Alkali-extractable compounds within LF and MF were investigated by using fluorescence excitation emission matrices-parallel factor analysis.Results Although POM content was similar between the ABCs for both tree species, MAOM content in the low-ABC soils was higher (C. japonica) or lower (C. obtusa) than in the high-ABC soils. Principal component analysis discriminated fluorescence components in terms of their origin, oxidative degradation, and decomposed structure. Based on these characteristics, POM in the low-ABC soils was more oxidatively degraded than that in the high-ABC soils, whereas MAOM in the low-ABC soils was more plant-derived/highly-decomposed (C. japonica) and more microbially metabolized (C. obtusa) throughout the profiles.Conclusion Our findings revealed that POM and MAOM were more decomposed due to the soil acidity in C. japonica stands and C. obtusa stands, respectively.