Heng Tsai

Pedogenetic features of the soils could be a proxy for revealing the landform surface processes. Our work first analyzed the particle size distributions and lithological discontinuities (LDs) of the soils in the midstream of the Zoushui River, central Taiwan. The results showed that the parental materials of the soils derive from mixed sediments of the Zoushui River and its tributaries, and the LDs of some soils suggested multi-depositional events with homogeneous lithology. Then, we proposed a soil chronosequence of Inceptisols, Ultisols, and Oxisols, over the Pleistocene timescale. There was a very well-defined semilogarithmic relationship between weighted profile development index (WPDI) values and soil age with correlation coefficients (r) greater than 0.9. The age of the soils did not certainly agree with the interglacials of the main marine isotope stage (MIS). However, the soils started to develop only after being aggraded by relatively warm and humid periods or by extreme rain events in cool and dry periods. Irrespective of whether the soils had started to develop, tectonic downcutting, providing clear altitudinal separation of the terrace treads, could inevitably happen later (or almost synchronologically) to ensure stabilities of the tread surfaces.

A well-dated soil chronosequence may allow exploration of the accumulation of soil carbon over time. There are multiple levels of river terraces on the Pakua tableland in Central Taiwan. Unlike many of the reddish or lateritic soils in Taiwan, these soils were recently dated, with absolute ages in the range of 19–400 kyr. This information allowed us to develop an ideal soil chronosequence, with time constraints, through which it is possible to explore soil organic carbon (SOC) storage and its changes over time. In this study, we attempted to establish an SOC time series, and to give an estimate of long-term accumulation of the SOC storage in the red soils of Taiwan. The data on these soils used in this study were taken from the soil profiles presented in our previous studies. Two additional soil profiles were sampled for those soils for which data were not available from the previous studies. The total carbon stock (TCS) for each soil profile was measured and assessed based on the depth categories of 0–30, 30–50, and 50–100 cm. Weighted carbon stock (WCS) measurements were further derived by the total thickness of the soil profile, for better comparison. The overall carbon stocks of the soils in the Pakua tableland were in the range of 2.8–3.2 Tg for TCS and WCS, respectively. In addition, the SOC tended to be highest in the surface soil horizons and decreased with the soil depth. The continuous pattern of the carbon content, in terms of its vertical distribution, was considered in terms of a negative exponential function, which showed that the SOC was highest in the shallowest soil layers and decreased rapidly with the soil depth. This trend was mitigated at a depth of 50–100 cm, which approached a fixed value, denoted as the carbon sequestration value (CSV), below a certain depth. We show here that the values of the CSV, as approximated by exponential fitting, are closely related to soil age. The CSV linearly decreases with age. These findings point to the potential of using carbon storage for chronometric applications.

To elucidate the properties of pedogenic Cr and Ni in serpentine soils in terms of mobilization, three pedons on the shoulder (Entisol), backslope (Inceptisol), and footslope (Alfisol) along a toposequence in eastern Taiwan were examined for metal partitioning and their geochemical origins. The analysis combined bulk soil analysis by selective sequential extraction (SSE) with mineralogical methods, including x‐ray diffraction (XRD) and scanning electron microscopy/energy dispersive x‐ray spectroscopy (SEM/EDX) analyses. Experimental results showed that Cr and Ni were mainly concentrated in chromites and silicates, respectively, and were gradually exposed by weathering of the parent materials. The SEM/EDX analysis indicated that chemical modification of the chromites was more prevalent near the soil surface and that the chemical modification increased as available Cr content increased from the shoulder to the footslope. Landscape position was the most important factor in controlling the trends in Ni and Cr fractions. The footslope accumulated more total Cr and Ni than the shoulder and backslope. Additionally, the soil on the footslope received more effective precipitation as run‐on water from upslope and was potentially more leached than the soils on the other landscape positions. The accumulation of clay and dithionite‐citrate‐bicarbonate (DCB) extractable Fe (Fe _d ) and the increase of exchangeable Ca/Mg ratio correlated with the increased total labile pools of Cr and Ni in the soil from the shoulder and backslope to the footslope. However, the concentrations of acid soluble, reducible, and oxidizable fractions (total labile pool) of Ni were higher than those of Cr, indicating that Ni was more available than Cr in all soils tested by the SSE procedures.

Liuchiu Island is an uplifted coral-reef island located off southwestern Taiwan. A total of four soil pedons, labeled as LC-1 and LC-2 from the Holocene terraces and LC-3 and LC-4 from the Pleistocene terraces, were sampled on the island for this work. These soils were siliceous, and were characterized by enrichment of clay and free iron (Fe_d). According to Soil Taxonomy, pedons LC-3 and LC-4 were classified as Paleudults and pedons LC-1 and LC-2 were Dystrudepts. The soil properties showed progressive changes from pedon LC-1 to pedon LC-4 in morphology, physical and chemical properties, and clay mineralogy. The contents of total Fe and dithionite-citrate-bicarbonate extractable Fe were significantly higher in pedons LC-3 and LC-4 with high weathering degree than in pedons of LC-1 and LC-2 with less weathering degree. Enrichment of kaolinite and gibbsite in pedons LC-3 and LC-4 also suggested high chemical weathering degree of the soils. The estimated soil ages for all studied pedons were consistent with their degrees in pedogenesis, where pedons LC-3 and LC-4 were located at older terraces and pedons LC-1 and LC-2 were located at younger terraces. Namely, it complied with the geologic interpretation of the continuous and simultaneous uplift and tilt of the island over time. Instead of the in situ weathering from the underlying coral reef limestone, all soils developed from siliceous parent materials deposited onto the surfaces. The SiO₂/Al₂O₃ ratios of soils indicated a component of loess may have been incorporated from continental China as part of the parent material, which confirmed a climate change of strong monsoons or severe dust storms occurred before the Holocene. However, soil development increased by the subsequent warm and humid climates of the interglacial stage over time.