Effects of ecological succession on surface mineral horizons in Japanese volcanic ash soil

Abstract

Japanese volcanic ash soils have very thick and dark-colored A horizons with large amounts of black humic acids which are characterized by their extremely high aromatic structure and stabilities such as black carbon. Nevertheless, the disappearance of the melanic epipedon with a decreasing aromatic C and increasing alkyl C proportion of humic acids was observed in ecological succession for only over 20–30 years. To obtain further details regarding the chemical characteristics of this extraordinary changing process quantitatively, humus quantitative analysis, high-performance size exclusion chromatography (HPSEC), and liquid-state 13C nuclear magnetic resonance ( 13C NMR) analysis were performed, including those for fulvic acids at the surface of mineral soils (0–20 cm), in the grassland/forest ecotone of volcanic ash soil in Japan. In addition, to obtain information regarding the stability and origin of C which gives the key of changing mechanism of humus along with ecological succession, resistance to hydrogen peroxide oxidation (percent decolorization) and the stable isotope ratio of carbon (δ 13C) were measured, respectively. The study sites were managed as grassland for several hundred years. Grassland at site 1 was still being maintained by mowing, while the maintenance of sites 2 and 3 was discontinued approximately 30 and more than 100 years ago, respectively. The dominant vegetative cover at site 1 was Miscanthus sinensis, that at site 2 was Pinus densiflora with an understory of Sasa spp., and the vegetation comprised mainly Quercus crispula with an understory of Sasa spp. at site 3.The concentration of humic acids decreased significantly from 60.4 to 47.4 C g kg − 1 in site 2, and then decreased continuously from 47.4 to 30.2 C g kg − 1 in site 3. These trends paralleled those of the total C and N content in soils, and C:N ratio increased along with succession from 11.1 to 17.0 to 19.2, respectively, supporting the hypothesis that changing vegetation from grassland to forest may promote N losses by plant uptake. Simultaneously, the concentration of fulvic acids and humin decreased only slightly throughout the series relative to humic acids. Physicochemical properties appeared to be affected by the succession especially in humic acids. In particular, the apparent content of aryl C moieties in the humic acids of site 2 decreased significantly from 30.0 to 14.9 C g kg − 1 , a level similar to site 3. Percent decolorization of humic acids corresponded to their aromaticity; site 1 had a lower value (55.3%) than those of sites 2 and 3 (65.8% and 66.4%, respectively). The δ 13C values of humic acids ranged from − 19.6 to − 22.6‰; however, the values for sites 1 and 2 were exactly the same. The findings of this study point to the fact that humic acids, but not other humus fractions, significantly changed with decreasing aryl C moieties and stability over the first 30 years of conversion from managed grassland to forest. Furthermore, the disappearance of the melanic epipedon in the surface volcanic ash soil evidently contributed to the aryl C, but not alkyl C, content of humic acids, and there might be a difference in the major mechanism of chemical change of humic acids before 30 years and more than 100 years. At least for the first 30 years, it is strongly suggested that C losses and changes in chemical properties of humic acids could be due to extraordinary microbial degradation processes through accelerated plant N uptake along with ecological succession from grassland to forest in earlier stages.