Mineral neoformation in pore spaces during alteration and weathering of andesitic rocks in humid tropical Indonesia

The morphology, chemistry, and mineralogy of minerals newly formed in voids—products of weathering of andesitic rocks—were studied using optical and submicroscopic techniques on thin sections, combined with scanning electron microscopy (SEM), energy dispersion X-ray analysis (EDXRA), and X-ray diffraction analysis on subsamples.Samples were taken from the core, the outer core, and the soft outer layers of weathered andesitic boulders in the Cigudeg area, West Java, about 75 km south of Jakarta, Indonesia, with udic moisture and isohyperthermic temperature regime.The neoformed minerals observed in planar voids and cavities comprise 1:1 phyllosilicates, goethite, and gibbsite, and juxtaposed coatings of halloysite, mixed layer phyllosilicates, goethite, and gibbsite. The mineral sequences observed confirm former studies on the groundmass of the boulder: in the first weathering layer, an absolute accumulation of gibbsite takes place; further away from the core, the gibbsite is redissolved, and replaced by illuvial phyllosilicate clay. The possibility of inheritance of the 2:1 layer silicates from a hydrothermal alteration is discussed. The sharp boundaries in the juxtaposed coatings point to a stepwise, sequential—rather than a gradual—evolution of the microenvironment.     

Deep weathering and landscape evolution in a tropical plateau

The thick weathering profiles of humid tropical areas are an important, yet generally neglected, source of information on landscape evolution. Six complete profiles of the weathering mantle were sampled by drilling on the three stepped levels of the Campos do Jordão Plateau, on the NW flank of the Continental Rift of Southeastern Brazil. Mineralogical and micromorphological analyses of drill core samples, complemented by geochemical interpretations and by previous data on the upper saprolite, indicate continuity of a general lateritic trend during the entire process of mantle formation. Lateritization phases of different intensity were defined and considered to reflect adjustment to changes in environmental conditions created by the gradual uplift of the plateau to its present position. Older and more superficial materials related to intense lateritic weathering are characterized by allitization with direct formation of gibbsite from silicates, probably related to tropical climates existing immediately before the formation of the continental rift, during the Paleogene, and also before any significant increase in altitude. Monosialitization phase with general kaolinitization and restricted indirect formation of gibbsite from silicates could be associated to less aggressive climates that followed the Neogene (Miocene?) accentuation of uplift rates along the continental rift. The changes produced by uplift in the tropical climate eventually favored the development of a podzolization trend in soils above 1800 m.     

Weathering of chlorite in a soil derived from a chloritoschist under humid tropical conditions

Eight samples from a soil derived from greenschist were chosen to represent the weathering transformation of chlorite and were subjected to chemical and mineralogical analyses by X-ray, D.T.A., selective and acid dissolution techniques. Regularly interstratified chlorite-vermiculite is the major weathering product in the sand and silt fractions, whereas nontronite predominates in the clay fraction. Kaolinite and a “chlorite-like” mineral progressively appear in the clay fraction as the soil surface is approached. The results show that the genetic link suspected between the clay chloritic component and the kaolinite does not withstand a detailed examination. The clay chloritic mineral is a disordered remnant of the regularly interstratified chlorite-vermiculite and it appears in the clay fraction of the solum concommitantly with the argillitization of the silt fraction. Kaolinite occurs in close association with nontronite and would perhaps form with this mineral a true mixed-layer phase. The unequal distribution of aluminium, iron and magnesium amongst the available sites in the parent chlorite appears to be a factor that strengthens the divergent behaviour of these elements during the weathering.     

Decomposition kinetics and organic geochemistry of woody debris in a ferralsol in a humid tropical climate

Large inputs of woody debris to soil can improve the soil. We examined the fate of woody debris buried in soil after fire‐free forest conversion to cropland in French Guiana. We measured the mass loss of woody debris > 4 mm on five sampling dates for 4 years after deforestation. Composition of the organic matter of woody debris was analysed with Rock‐Eval pyrolysis, which enabled us to distinguish a labile carbon pool (C_lab) and a resistant carbon pool (C_res). Decomposition of woody debris followed a first‐order function with a half‐life of 17.6 months. During the decomposition of woody debris > 4 mm, the C:N ratio, hydrogen index (HI) and pyrolysed carbon below 400°C (R400) decreased, suggesting that decarboxylation and dehydrogenation of woody debris occurred. Both C_lab and C_res stocks decreased with time, but the decrease in C_lab was faster. There was little humification of the debris and no long‐term biogeochemical preservation of a woody debris fraction, which accords with the first‐order decay observed. We conclude that the benefits of ligno‐cellulosic inputs for soil organic carbon contents in a tropical humid climate occur during the first year following deforestation. Résumé Les apports massifs de débris de bois au sol peuvent améliorer le sol. Nous avons examiné sur un site en Guyane le devenir de débris de bois enfouis dans le sol, suite à la conversion sans brûlis de forêt en cultures. Nous avons mesuré le stock des débris de bois > 4 mm jusqu'à 4 années après déforestation. Les transformations de la matière organique des débris ont bois ont été analysées par la pyrolyse Rock‐Eval, nous permettant de distinguer un compartiment de carbone labile (C_lab) et un compartiment de carbone résistant (C_res) dans ces débris. La décomposition des débris de bois suit une décroissance exponentielle de premier ordre, avec une demi‐vie de 17,6 mois. Durant la décomposition des débris, le rapport C/N, l'indice d'hydrogène (HI) et le carbone pyrolysé avant 400°C (R400) diminuent, suggérant des processus de décarboxylation et déshydrogénation qui se mettent en place rapidement après l'enfouissement des débris. Les stocks de C_lab et C_res diminuent tous deux au cours du temps, avec une cinétique plus rapide pour C_lab. Ces résultats indiquent une faible polymérisation de la matière organique des débris, et une absence de préservation biogéochimique d'une partie des débris, en accord avec la décroissance exponentielle observée. Cette étude suggère que les bénéfices des amendements ligno‐cellulosiques pour la matière organique des sols sont de courte durée en milieu tropical humide.     

Modification of the environment by animal houses in a hot humid climate

A survey was conducted in West Java to study the modification of the environment by traditional sheep and goat houses. The houses were all small and had a thick sloping roof with large overhangs, slatted walls and a slatted floor raised an average of 0.6 m above the ground. The mean wind speed outside the houses was low (0.3 m s⁻¹) and on average the wind speed inside the houses was 37% of that outside. The mean temperature outside the houses was 21°C in the morning and 27°C in the afternoon. The corresponding relative humidities were 93 and 70%. The internal temperature was only 0.3°C higher than the external temperature on average and the internal humidity only 0.3 g m⁻³ higher than the external humidity, indicating that in general the houses were well designed to maintain the internal environment as cool and dry as possible. Comparisons between houses showed that a high internal wind speed, a low number of animals per unit area and a long roof overhang favoured a low internal temperature. Houses with higher platforms and more open walls had higher internal wind speeds than lower, more enclosed houses. Heat losses from the animals in the houses by long-wave radiation and evaporation were larger than the heat losses by convection.     

Nutrient demand modeling of a soilless cucumber crop under greenhouses conditions in a humid tropical climate

ABSTRACT

This experiment was conducted in a greenhouse in Vinces Los Rios (Ecuador). The aim of this work is to evaluate the relationship between different parameters and the nutrient demand of a greenhouse soilless cucumber production system in a tropical climate as well as to propose an empirical model for the design of the nutrient supply to the system. Considering the results obtained in this experiment, there was a high correlation between nutrient demands of the system with nutrient supplies and water uptake but not with leaf area index. With respect to the models, nutrient supplies and leaf area index were the most significant variables for nitrate (NO₃^?), chloride (Cl^?) and manganese (Mn). For calcium (Ca²⁺), magnesium (Mg²⁺) and zinc (Zn), nutrients supply was the most significant variable and for the rest of nutrients [phosphates (H₂PO₄^?), sulfates (SO₄²)^?, potassium (K⁺), sodium (Na⁺) and iron (Fe)], the three variables assessed were significant.     

Soils and slopes in a humid temperate environment: A simulation study

A three dimensional simulation of the landsurface system is presented, which includes the processes of weathering, soil creep and fluvial erosion. Soils represent a deposit in equilibrium between the processes of weathering and erosion. The simulated drainage basin with a fixed exit point reproduces the “closed” system dynamics of the Davisian model, in which however slopes still attain a dynamic equilibrium form. Within the basin the overall mean soil depth very rapidly reaches a constant value even though a considerable redistribution of material is still to occur. This implies a rapid establishment of a balance between the process of weathering and erosion. By contrast the detailed pattern of soil depths on individual slopes is established much more slowly, only after the slopes themselves come to equilibrium. The equilibrium configuration of soils under soil creep shows a slight downslope increase in depths, and major increases in soil depth are restricted to locations where external constraints force an accumulation of material.     

Carbon limitations to nitrous oxide emissions in a humid tropical forest of the Brazilian Amazon

The availability of labile organic C for microbial metabolic processes could be an important factor regulating N₂O emissions from tropical soils. We explored the effects of labile C on the emissions of N₂O from a forest soil in the State of Rondônia in the southwestern quadrant of the Brazilian Amazon. We measured emissions of N₂O from a forest soil after amendments with solutions containing glucose, water only or NO₃^–. Addition of glucose to the forest soil resulted in very large increases in N₂O emissions whereas the water only and NO₃^– additions did not. These results suggest a strong C limitation on N₂O production in this forest soil in the southwestern Amazon.     

Trace metals and lanthanides in a tropical river environment

The river Periyar in the State of Kerala, India is subjected to pollution from a number of industrial units such as zinc smelter, rare earth plant, phosphatic fertilizer unit etc. The levels of Zn and Cd in water and sediments in the river have in recent years increased ten fold as compared to a study conducted earlier during 1970. The levels of these trace metals in water increase at the backwater zone under high salinity in summer due to solubilisation from the sediments. Among the lanthanides Ce is the most predominant in water. Elements such as Ce, La, Sm, Gd and Y show a concentration factor of 10³ in sediments. The trace metals show maximum concentration at the top layer of sediment column extending up to 20 cm from the surface. Particle size analysis of river sediments in the industrial outfall area shows a mass median diameter of 11 μm with a geometric standard deviation of 2.4 for Zn and Cd.     

Interactive influence of livestock grazing and manipulated rainfall on soil properties in a humid tropical savanna

Purpose

The effect of uncontrolled grazing and unpredictable rainfall pattern on future changes in soil properties and processes of savanna ecosystems is poorly understood. This study investigated how rainfall amount at a gradient of 50%, 100%, and 150% would influence soil bulk density (ρ), volumetric water content (θ_v), carbon (C), and nitrogen (N) contents in grazed (G) and ungrazed (U) areas.

Materials and methods

Rainfall was manipulated by 50% reduction (simulating drought—50%) and 50% increase (simulating abundance—150%) from the ambient (100%) in both G and U areas. Plots were named by combining the first letter of the area followed by rainfall amount, i.e., G150%. Samples for soil ρ, C, and N analysis were extracted using soil corer (8 cm diameter and 10 cm height). Real-time θ_v was measured using 5TE soil probes (20 cm depth). The EA2400CHNS/O and EA2410 analyzers were used to estimate soil C and N contents respectively.

Results and discussion

The interaction between grazing and rainfall manipulation increased θ_v and C but decreased N with no effect on ρ and C:N ratio. Rainfall reduction (50%) strongly affected most soil properties compared to an increase (150%). The highest (1.241?±?0.10 g cm^?3) and lowest (1.099?±?0.05 g cm^?3) ρ were in the G50% and U150% plots respectively. Soil θ_v decreased by 34.0% (grazed) and 25.8% (ungrazed) due to drought after rainfall cessation. Soil ρ increased with grazing due to trampling effect, therefore reducing infiltration of rainwater and soil moisture availability. Consequently, soil C content (11.45%) and C:N ratio (24.68%) decreased, whereas N increased (7.8%) in the grazed plots due to reduced C input and decomposition rate.

Conclusions

The combined effect of grazing and rainfall variability will likely increase soil θ_v, thereby enhancing C and N input. Grazing during drought will induce water stress that will destabilize soil C and N contents therefore affecting other soil properties. Such changes are important in predicting the response of soil properties to extreme rainfall pattern and uncontrolled livestock grazing that currently characterize most savanna ecosystems.

     

Effects of carbon additions on iron reduction and phosphorus availability in a humid tropical forest soil

In the highly weathered soils of humid tropical forests, iron (Fe) plays a key role in ecosystem biogeochemical cycling through its interactions with carbon (C) and phosphorus (P). We used a laboratory study to explore the role of C quantity and quality in Fe reduction and associated P mobilization in tropical forest soils. Soils were incubated under an ambient atmosphere headspace (room air) with multiple levels of leaf litter leachate or acetate additions. Net Fe reduction occurred in all the treatments and at every time point. The more complex mixture of organic compounds in leaf litter leachate stimulated Fe reduction as much acetate, an easily fermentable C source. At the end of the experiment, Fe reduction was generally greater with higher C additions than in the low C additions and controls. The microbial biomass P had increased significantly suggesting rapid microbial uptake of P liberated from Fe. This occurred without increases in the available (NaHCO₃) P pool. The immobilization of P by microbes during the incubation provides a P conservation mechanism in these soils with fluctuating redox potential, and may ultimately stimulate more C cycling in these highly productive ecosystems. Iron cycling appears to be an important source of P for the biota and can contribute significantly to C oxidation in upland tropical forest soils.     

Oxalate-soluble compounds of Al, Fe, and Si in soils of cold humid regions as a function of weathering

Original and published data on the contents of X-ray amorphous oxalate-soluble compounds of Al, Fe, and Si in mesomorphic eluvial soils of cold, moderately cold, and moderately warm continental humid and semihumid regions are generalized. The groups of soils developed from mafic igneous, metamorphic, and pyroclastic rocks are considered. It is shown that the content of oxalate-soluble oxides (OSOx) in the horizons of their maximal accumulation varies from less than 1% to 20–30%; the Al_ox/Fe_ox ratio varies from 1 to 6.5. The leading factor dictating the amount and quality of the OSOx in the soils is the presence or absence of volcanic glass in the parent materials. The boundary between the soils with and without volcanic glass corresponds to the OSOx content of 5% and the Al₂O_3ox/Fe₂O_3ox ratio equal to 2. These criteria are more reliable than the Al_ox/Fe_ox ratio used by foreign soil scientists to specify Andosols (Al_ox/Fe_ox > 2). The contents of oxalate-soluble oxides of Al and Fe do not depend on the total contents of these oxides in the parent material and seem to be related to the presence of these elements in minerals with different resistance to weathering. Under the natural conditions described in this paper, the content of OSOx shows a very weak response to zonal (temperature-controlled) climatic changes and/or to changes in the degree of humidity and the continentality of the climate.     

Some aspects of frequency and stability wind roses in a tropical region

The frequency wind roses and stability wind roses based on meteorological data at Nairobi International Airport are calculated. The stability is estimated using a Pasquill-Gifford Scheme. The results show that in the tropics the winds have generally high speeds and are highly constant in direction throughout the year. The high wind speed in these regions has a great influence on the stability estimation scheme that is used. The high wind speeds and persistence in the tropics are viewed as favorable factors in the air pollution problem since the dispersive ability of the atmosphere is enhanced and, in terms of long term planning, the wind variability is much less than is the case with middle latitudes.     

Physical soil degradation induced by deforestation and slope modification in a temperate‐humid environment

Between February 1998 and March 1999 physical soil degradation was studied in temperate‐humid NW Spain after deforestation and slope modification. Changes in bulk density, total porosity, pore size distribution, aggregate stability and soil mechanical strength indicated the degradation process. Slope levelling produced the greatest impact. A rapid recovery of physical properties was observed, with the exception of those related to the soil mechanical strength. The increased soil erosion needs agronomic studies to understand the long‐term benefits and adverse aspects of the land‐use changes. Copyright © 2001 John Wiley & Sons, Ltd.     

Trace metals in a tropical river environment —speciation and biological transfer

Speciation of trace metals in the river water indicates possible organic bonding. Copper is highly complexed with the humic matter in the river sediments. Trace metals show higher levels in the milk of the cows fed on a contaminated pasture. Analysis of fish from the river shows highest enhancement in concentration for Cd. Tissuewise, the concentrations are highest in bone and liver. Calculation of the projected daily intake of trace metals, from consumer items produced locally, shows the intake of Zn and Cd to be high.     

Rock weathering rates at the soil surface in an alpine environment

Mass loss from 60 g samples of crushed rhyodacite exposed on 71 plots at two localities above treeline in the San Juan Mountains, Colorado are used to estimate weathering rates at the soil surface. The results define an average rate of loss of 0.079 ± 0.012 %/yr (mean of 42 plots over 5 years, ± 2 standard errors), with an apparent slowing of the rate as the period of observation increased. Two features of the soil surface environment influence the observed weathering rates: the soil pH (which is dependent on the vegetation cover); and the volume and residence time of water moving through and over the soil. Both of these are controlled basically by the distribution of the winter snow cover.     

Weathering of primary silicate minerals in the natural soil environment in relation to a chemical weathering model

A theoretical model for the weathering rate of silicate minerals in the laboratory and the natural soil environment has been developed where the rate is assumed to be proportional to the exposed surface area of the mineral, the soil moisture saturation and the chemical driving force, given as a kinetic expression. The kinetic expression describes the dependency of the rate on the solution concentration of reactants and has the general form of a sum forward rates minus a sum backward rates. Data from the Gårdsjön watershed in Southern Sweden were used to estimate weathering rate of several silicate minerals, and the result is in good agreement with laboratory values. The weathering model developed by the authors was applied to the watershed to predict the present day weathering in the area, and the predicted values coincide with the observed weathering rates.     

Improvement of the physical fertility of a degraded Alfisol with planted and natural fallows under humid tropical conditions

Abstract. Topsoil (0–15 cm) bulk density, aggregate stability, soil dispersibility, water retention and infiltration were measured between 1989 and 1996 on an Alfisol under rehabilitation in southwestern Nigeria. The planted leguminous species were Pueraria phaseoloides, Senna siamea, Leucaena leucocephala, Acacia leptocarpa and A. auriculiformis. Also, plots with natural fallow and maize/cassava intercropping were included. Level (minimum) and mound tillage with hoes was adopted for the cultivated areas under study after 4 and 6 year fallow periods. Under fallow, the soil bulk density decreased from1.56 to 1.11 t m73.The continuously cropped treatment (level tillage) had significantly higher bulk density than the fallowed subplots after 6 years. Mean soil penetrometer resistance ranged from 75 to 157 kPa for fallowed plots and from 192 to 295 kPa for the continuously cropped (level tillage) subplot. Surface soilwater contentswere similar for all the treatments during the soil strength measurements. Although soil aggregates were generally of low stability and not well formed, they were improved by fallowing.
Soil structural improvement by planted fallows was similar to that by natural fallow, but the trees were more promising for long-term fallow (>6 years) than the herbaceous P. phaseoloides. However, the improvement in soil structure after 4 or 6 year fallow could not be maintained in subsequent cropping. Furthermore, the significant improvement in soil bulk density caused by A. auriculiformis and natural fallow was more rapidly lost on the cultivated subplots compared with other fallow treatments. Thus, soil structure recovery under a fallow does not imply a sustained improvement when stress is applied to this soil. Post-fallow soil management options such as residue incorporation and tillage to ameliorate compaction or soil strength will be necessary to enhance the improvements by fallow species.