[H-CG26] Adaptation to climate change and its social implementation

[HCG26-P01] Metal supply control on formation of organo-mineral assemblage from crushed rocks and leaf compost via wet and dry cycles

*Kaori Matsuoka¹, Jo Jinno¹, Hiroaki Shimada ¹, Rota Wagai ¹ (1.Institute for Agro-Environmental Sciences, National Agriculture and Food Research Organization)

Keywords:Basalt, Density fractionation, Granite, Organic carbon, STXM-NEXAFS, Weathering rate

Soil represents a main terrestrial carbon pool where organic, mineral/metal and biological components interact together to form ‘organo-mineral assemblage’. Organic carbon (OC) in organo-mineral assemblage could be protected via assemblage with aluminum (Al) and iron (Fe). These minerals are ultimately supplied by weathering of parent materials such as rocks. From the view of pedogenesis, we hypothesized that supply of Al and Fe from crushed rocks promotes the formation of organo-mineral assemblage. We selected two end-member igneous rocks (granite and basalt) that were crushed to have 38–75 µm size for both rocks and 20–38 µm size for basalt. We also used sand (100–300 µm) as the control. As an organic matter (OM) source, we used tree leaf compost (100–250 µm). The mixture of rock and leaf compost (rock-OM mixture) incubated with natural soil microbial community and was subjected to eight wet-and-dry cycles with artificial rainwater over 55-day experiment. At the end of experiment, the mixtures were fractionated by density to separate particulate organic matter (< 1.8 g cm^–3: low density), organo-mineral assemblage (1.8–2.4 g cm^–3: meso density) and mineral fraction (> 2.4 g cm^–3: high density). The OC and extractable Al and Fe in the bulk and isolated fractions were quantified by the combustion method and the sequential extraction with sodium pyrophosphate, acid oxalate, and dithionite-citrate. The result showed that OC in meso-density fraction significantly increased from 0 day (0.00014–0.023 mg C g^–1) to 55 days (0.11–1.3 mg C g^–1) with the highest concentration in basalt (20–38 µm), followed by basalt (38–75 µm), granite (38–75 µm), and sand (100–300 µm). The C:N ratios showed density-dependent decline for all rock-OM mixtures were lower as increase in density (low-density fraction 39.7–40.3, meso-density fraction 15.2–19.6, and high-density fraction 3.6–7.1), indicating the dominance of microbially-derived OM in higher density fractions. For the basalt samples, we further showed that the smaller particle size of basalt (20–38 µm) led to greater enrichment of extractable Al and Fe in meso-density fraction as well as higher concentration of sum of weathered ions (Na, Ca, K, plus Mg) in the leachate, suggesting the faster supply of Al and Fe from basalt (20–38 µm) sample via the chemical weathering. We also confirmed the presence of micron-scale co-localization of OC, Al, and Fe in meso-density fraction of basalt (38–75 µm) by using scanning transmission X-ray microscopy and near-edge X-ray absorption fine structure. Overall, our findings showed a rapid formation of organo-mineral assemblage promoted by the microbial OM transformation and minerals released from rock weathering under the experimental condition, which supports organo-metallic glue hypothesis (Wagai et al., 2020, SOIL) proposed based on the analyses of natural soils.