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The existing models for estimating cation exchange capacity (CEC) from easy‐to‐measure hygroscopic water content (θh) were based on a single water activity (aw) value rather than on the processes that govern soil water vapor adsorption for a distinct aw range. Here, we present a new CEC estimation model based on θh data of 119 soils with different clay mineralogy (i.e., illitic [IL], montmorillonitic [ML], and kaolinitic [KA] samples) and organic carbon (OC) contents for the aw range from 0.23 to 0.57 (Δθ0.23–0.57) and validate its performance. Based on the hypothesis that multilayer adsorption exhibits a higher correlation with CEC than monolayer adsorption and capillary condensation, the aw range from 0.23 to 0.57 was chosen with CEC calculated as CEC = k × Δθ0.23–0.57. The performance of the new model is compared to the Arthur (2017) model and the Torrent (2015) model, which considers a single θh value. The average proportionality coefficient (k) varied with the dominant clay mineralogy of the investigated soils. For soils dominated by 2:1 clay minerals (i.e., IL and ML), the new model showed a good estimation accuracy (Nash‐Sutcliffe model efficiency [E] ≥ 0.85; root mean squared error [RMSE] ≤ 4.18 cmol(+) kg−1). The new model performed better for IL and ML samples than for KA samples, and yielded more accurate CEC estimations than the Arthur model and Torrent model for soils with 2:1 clay minerals. For soil with high OC content (>23.2 g kg−1), the new model slightly underestimated CEC (E = 0.66; RMSE = 5.87). [ABSTRACT FROM AUTHOR]

The soil mantle of the tropical karstic landscapes of Southern Mexico was shaped by specific processes of pedogenesis and long-term human impacts of ancient Maya agriculture. To understand the interaction between natural and human-induced soilforming processes in the calcareous mountains of Chiapas state, we studied soil toposequences around the Classic Maya site of Budsilhá and related them to the archaeological evidence of settlement and land-use distribution. Soil chemical analysis, micromorphological observations, and clay mineral identification were carried out in key soil profiles at the main geoforms. Limestone hills are occupied by shallow Rendolls which are usually perceived as incipient soils. However, high content of silicate clay composed of kaolinite and vermiculite and ferruginous clayey soil material observed at macro- and microscale backed the hypothesis that these soils were formed from the residues of thick Terra Rossa after their erosion. Swampy lowlands are occupied by thick clayey gleyic soils with clay mineral assemblages similar to those in the upland Rendolls. We suppose that the mineral matrix of the lowland soils is largely derived from the pedosediments of eroded upland Terra Rossa, which lost original ferruginous pigmentation and aggregation due to redoximorphic processes. Some wetland soils contain neoformed gypsum that is atypical for humid tropics; sulfidesulfate transformation under fluctuating redox conditions could promote gypsum synthesis. Ancient Maya land use was closely related to soil-geomorphic conditions: settlements with homegardens occupied calcareous hills, whereas the primary agricultural domain was developed on lowland soils after their drainage by artificial canals. [ABSTRACT FROM AUTHOR]

Comparisons of soil aggregation and aggregate-protected organic matter among soil textures in the same climatic zone and of the same mineralogical composition are somewhat limited. Two undisturbed, semiarid sites in northeastern Wyoming were selected to determine the differences in aggregation and organic matter between loam and sandy clay loans soils. Additionally, data from these two sites were used to illustrate differences between two statistical approaches to data analysis: Bayesian modeling in the context of a two-sample comparison of means and conventional t-tests. Examination of posterior distributions (Bayesian modeling) and confidence intervals (conventional t-tests) resulted in similar conclusions about the influence of soil texture on aggregation and C and N concentrations. Loam soils generally had more structure (as indicated by higher macroaggregation) and aggregate-protected C and N than sandy clay loam soils, which had greater amounts of available C and N not protected by aggregates. For soils in undisturbed semiarid grasslands with similar clay contents, in the range of 11 to 24%, we suspect that aggregate stability was primarily affected by sand content, while aggregate C and N accumulations were influenced by silt. A higher percentage of silt in the loam soils appeared to have provided enough of a difference in small pore space between the two soil types to result in higher C and N storage in loam soils relative to sandy clay loam soils. Soil texture is a major driving force behind aggregation as well as C and N storage in semiarid soils of the same mineralogy. [ABSTRACT FROM AUTHOR]

The minerals of the clay fraction in estuarine plains are mainly detrital, being a mixture of marine and continental sediments, but can also be authigenic. Because of the importance of mangrove ecosystems in tropical estuarine areas and the relatively few existing studies of the mineralogical composition of soils in these environments, the aim of this study was to determine the mineralogical assemblage and identify potential contrasts along the coast of the State of São Paulo. Soils from 11 mangroves distributed along the coastal plain of the State of São Paulo were sampled at depths of 0 to 20 and 60 to 80 cm, and samples of suspended sediments from the Ribeira do Iguape River were collected for analysis. Mineralogical analyses were performed on the clay and silt fractions by x-ray diffraction (XRD) and transmission electron microscopy, and fresh soil samples were analyzed by scanning electron microscopy-energy dispersive spectrometry and suspended sediments by XRD. The silt fraction contained quartz, feldspars, gibbsite, kaolinite, illite, and vermiculite, and the clay fraction contained smectite, kaolinite, illite, gibbsite, quartz, and feldspars. Locally, vermiculite, biotite, anatase, halloysite, and goethite may occur because of recent transport of sediments to the system. Pyrite was identified in fresh samples. The allochthonous minerals found either were terrestrial and transported by rivers or had originated from the continental platform by past transgressive events. We suggest that the neoformation of smectite and kaolinite occurs in mangrove soils. Different geomorphological settings along the São Paulo coast appear to regulate mineral distribution in mangrove soils. [ABSTRACT FROM AUTHOR]

Serpentinite-derived soils give rise to botanically distinct systems primarily because of inadequate parent material Ca content. We hypothesized that Ca content varies widely in what have been mapped as soils derived from serpentinite. An exchangeable Ca/Mg ratio <0.7 is often used to relate the imbalance of these nutrient elements in serpentinite-derived soils. We sampled six parent materials and soils from the Coast Ranges of California in Henneke soil series (clayey-skeletal, magnesic, thermic Lithic Argixerolls) modal location map unit polygons. Parent material total CaO content varied from 1.0 to 230 mg kg-1, and CaO/MgO varied from <0.1 to 4. A combination of x-ray diffraction (XRD), polarized light microscopy (PLM), and electron microscopy was used to identify the Ca-bearing accessory minerals diopside, grossularite, andradite, and tremolite. Accessory mineral content was often too low to be detected by XRD or minerals were too finely disseminated and difficult to detect in thin section by PLM. Electron microscopy, in concert with XRD and PLM, was needed to fully characterize the mineral assemblage. Two sites, Napa and Tehama, contained no serpentine minerals, were not serpentinites, and were tectonic inclusions in the serpentinite landscape. Napa rocks contained almost no Ca-bearing minerals and would be identified as a serpentinite if relying on elemental analysis CaO/MgO ratio alone. Tectonic inclusions and Ca-bearing accessory minerals affect Ca distribution and presumably its availability for plants. Careful mineralogical analysis may be required to identify Ca-bearing accessory minerals. [ABSTRACT FROM AUTHOR]

Examines the effects of litter quality and soil clay content on carbon (C) and net nitrogen (N) mineralization rates in mineral soils. Comparison of aspen litter quality and pine litter quality; Release of soil C in pine and aspen soils; Microbial biomass under pine soil; Relationship between net N mineralization rates and species or soil mineralization rates.