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

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.