UC Davis

Climate and parent material are considered the primary factors determining the distributions of soil clay (secondary) minerals, but their influence has not been rigorously elucidated for tropical volcanic soils. Herein, we investigated soil secondary mineral distributions in volcanic regions of Java and Sumatra islands representing large variations in climatic (mean annual temperature (MAT): 13 to 27°C; precipitation: 1910 to 3950 mm) and parent material conditions (rhyolitic-to-basaltic tephra). Soil secondary minerals were assessed by selective extractions, X-ray diffraction analysis, and differential thermal analysis. Moreover, the thermodynamic stabilities of minerals were evaluated based on the ion activities of equilibrated soil–water suspensions. Factor analysis of climate and soil geochemical (e.g., total Si, Fe and K) properties identified temperature, dry season intensity, and parent material as the primary factors regulating secondary mineral distributions. A negative correlation between oxalate extractable Al and Fe (Alo and Feo) and the temperature factor indicates low temperature promoted the formation and preservation of short-range-order (SRO) minerals and organo-Al/Fe complexes, which resulted in Alo + 1/2Feo ≥ 20 g kg−1 (andic property criterion) at MAT <21°C. Desiccation in the dry season, represented by excess precipitation for the driest quarter of the year, was related to soil H4SiO40 activity of soil–water suspensions. High H4SiO40 activity resulting from intense seasonal desiccation coincided with a higher Si/Al ratio of SRO aluminosilicates (Si-rich allophane) and smectite. In contrast, low H4SiO40 activity enhanced the formation of SRO aluminosilicates with a low Si/Al ratio (Al-rich allophane) and gibbsite. The influence of parent materials was evident in high free Fe(hydr) oxide content in soils derived from mafic materials and the occurrence of mica, which altered to hydroxy-Al interlayered vermiculite under continuous leaching conditions, in soils from felsic materials. Overall, we demonstrated strong temperature and seasonal desiccation controls on secondary mineral distributions in the humid, tropical volcanic soils of Java and Sumatra islands.