Maki Asano

Distinctive organic matter pools among particle-size fractions detected by solid-state 13C-NMR, δ13C and δ15N analyses only after strong dispersion in an allophanic Andisol

Abstract We previously showed the first clear evidence of aggregate hierarchy in an Andisol by comparing the particle-size fractions released upon different levels of dispersion energy up to the maximum dispersion – sonication at 5 kJ mL−1 following sodium saturation. While smaller particles (< 2 μm) appeared to act as major binding agents, the variation in organic matter (OM) chemistry among the size fractions remains unstudied. Here, we focused on comparing the carbon structure and carbon and nitrogen stable isotope ratio (δ13C and δ15N) among the particle-size fractions isolated by limited dispersion (mechanical shaking) and by the maximum dispersion treatments for the allophanic Andisol previously examined. Both solid-state carbon-13 nuclear magnetic resonance (13C NMR)and stable isotope ratios showed clear differences among the size fractions after the maximum dispersion but not after the limited dispersion. From 2–53 μm toward the < 0.2 μm fraction, we observed a progressive decline in the proportion of aromatic-C and an increase in that of O-alkyl-C. Similarly, the enrichment of 13C and 15N toward the smaller particle size fractions was observed after the maximum dispersion. While δ15 N had progressive enrichment from 3.6‰ (53–4000 μm) to 6.4‰ (< 0.2 μm fraction), δ13C showed a 2.5–3.0‰ enrichment from the 53–4000 μm fraction (−24.0‰) to 2–53 μm fraction and remained largely constant between 0.2–2 μm and < 0.2 μm fractions. The sonication-induced redistribution and chemical alternation of OM appeared to be minor due to the small pool sizes of low-density materials (e.g., plant litter) and microbial biomass. The emergence of the size-dependent changes in C chemistry after the maximum dispersion was consistent with the Andisol aggregate hierarchy model we previously proposed. The observed difference in C/N ratio and isotopic ratios as well as C composition implies that the OM present in the sonication-resistant particles of < `2 μm sizes (that account for roughly 70% of total C and N) are highly recycled by and/or largely originated from soil microbes. The applicability of current findings to other samples (e.g., non-allophanic Andisols) should be examined to establish the unique role of OM-enriched, micron to submicron particles in the aggregate hierarchy of Andisols.

Soil Diversity in Mongolia

Mongolia is located in a vegetation transition zone between the Siberian taiga, forest steppes, steppes, desert steppes, and the Central Asian desert region where the vegetation changes from the northern forest to the central steppe and southern desert. There is also a unique combination of geological conditions and topography. These variations in environmental factors in different parts of the country specify Mongolia’s soil distribution, which changes from north to south following a longitudinal zonal schema. The zones include (1) a mountain taiga zone with cryomorphic taiga soils, (2) a mountain forest-steppe zone with Chernozems, dark Kastanozems, dark-colored forest soils, and derno taiga soils, (3) a dry-steppe zone with Kastanozems, (4) a semidesert zone with brown semidesert soils, and (5) a desert zone with gray-brown desert soils and extremely arid desert “borzon” soils. Typical forest-steppe and steppe soil is a chestnut soil, Kastanozem. In the Gobi-Steppe, calcisols are distributed. In this chapter, Mongolian soils are characterized and the pedogenesis is summarized.

Chronology of anthropedogenesis in the Omiya tableland, Japan, based on a 14C age profile of humic acid

Volcanic ash soils along the western edge of the Omiya tableland, Japan, are covered with thick anthropogenic soil horizons. The formation of anthropogenic soil horizons occurs because of the soil dressing practice known as “Dorotsuke,” where alluvial soil materials are deposited on fields and mixed with volcanic ash topsoil by tillage over the years. To clarify the chronology of this anthropedogenesis, carbon-14 (14C) age profiles were estimated using humic acid fractions from three pedons: an anthropogenic soil, an undressed Andosol, and a Fluvisol. Soil charcoal fragments were also dated to estimate maximum burial age. Charcoal fragments displayed vertically random age distributions, indicating that the fragments may have had multiple origins. However, the age of charcoal in the lower part of the anthropogenic soil horizons indicated that the initiation of anthropedogenesis occurred later than the late 13th century. The 14C age profile of humic acid in the Andosol exhibited little variation in age with depth in the subsoil. The 14C age profile of humic acid in the Fluvisol suggested that the humic acid fraction included allochthonous old carbon (C), although the soil itself had been formed from recent sediments. The 14C age profile of humic acid in the anthropogenic soil showed features of its two component soils. The 14C ages in the volcanic ash subsoil matched with those in the Andosol, whereas the ages increased in the anthropogenic soil horizons because of supplementation with old C from alluvial soil materials. However, the peak 14C ages occurred in the lower part of the anthropogenic horizons, whereas the middle part on the peak position displayed a gradual age-depth gradient. This feature was interpreted as a sign of 14C activity equilibrium throughout anthropedogenesis. On the basis of this postulated 14C activity equilibrium, the linear age-depth gradient at the peak position was derived from differences in burial time, and burial ages were calculated by estimating steady-state 14C. The calculated ages were lower than the charcoal ages. These age estimates suggest that anthropedogenesis was initiated in the Middle Ages and reached an intermediate stage before or during the first half of the Edo period.