[M-IS09] Biogeochemistry

[MIS09-09] Enhanced rock weathering in the buckwheat field after amending basaltic powders observed by quantitative X-ray diffraction analysis

Kazuki Azuma¹, *Atsushi Nakao¹, Kohei Kurokawa¹, Katashi Kubo², Junta Yanai¹ (1.Graduate School of Life and Environmental Sciences, Kyoto Prefectural University, 2.National Agriculture and Food Research Organization)

Keywords:Enhanced rock weathering, X-ray powder diffraction analysis, Plagioclase, Field experiment, Rhizospheric effect

Enhanced rock weathering (ERW) by amending rock powders to agricultural field is paid attention as a measure to convert atmospheric CO₂ into inorganic carbon species, such as pedogenic carbonates and aqueous bicarbonate ions. Basaltic powder application is of special interest as it contains many weatherable minerals and few toxic elements. Direct measurement of minerals using X-ray powder diffraction (XRPD) is expected to be effective for identifying the types of minerals present in soil, but few studies have tested the accuracy of XRPD-based quantitative values. We first determined the primary mineral content in agricultural soils all over Japan using two XRPD-based quantitative methods, i.e., the mineral intensity factor (MIF) method and full-pattern summation (FPS) method performed with powdR package for R. These data were compared with content data obtained using a conventional wet chemical method to test the accuracy of the determination. According to the tests, the FPS method was found to be more accurate especially for micas, plagioclase and K-feldspars. With this method, we determined changes in mineralogical composition of soils before and after 2.5-month cultivation in a buckwheat field with or without applying basaltic powder. The soil type in the field is a Brown Lowland soil. The basalt powder contained 26.6 wt.% Ca-rich feldspar, 6.6 wt.% Na-rich feldspar, 3.3 wt.% K-feldspar, 3.8 wt.% serpentine, 5.1 wt.% pyroxene, 5.6 wt.% carbonates and the other minor minerals. The field experiment was carried out from July to October, 2022 in a decontaminated field in Fukushima. Buckwheat yield was not significantly different between with or without basaltic powder application. The content of Ca-rich plagioclase, although it was increased by the basalt application, turned to decrease after cultivation. The decreasing amount in root zone was about six times higher than that in non-root zone soils. We concluded that XRPD-method is useful to detect the in situ weathering of basaltic powders amended and that the result of weathering should be separately investigated between root zone and non-root zone for accurate carbon accounting in the ERW system.