Chemical,physical and micromorphological properties of termite mounds and adjacent soils along a toposequence in Zona da Mata,Minas Gerais State,Brazil

Little is known about the effects of neotropical mound-building termites in soil chemical and physical properties. The influence of soil termite activity on soil characteristics was studied by assessing chemical, physical and micromorphological properties of a toposequence of Latosols (Oxisols). Soil samples were collected from the walls and inner parts of termite mounds and also from adjacent soil. A high diversity of termite genera was found in the mounds along the toposequence, together with the inquiline termites and other soil-dwelling arthropods. Chemical analyses showed that pH and the contents of organic C and N, P, Ca and Mg were significantly higher in termite mounds compared with adjacent areas, with an inverse trend for Al content. Significant differences in pH and exchangeable Al were observed between soil and mound across the slopes. The mound density across the landscape was higher at the upper slope segment, followed by the hill top, middle slope and lower slope segments. Considering a lifespan of 30 years and dimensions of termite mounds found in the toposequence we conclude that the textural and chemical uniformity of Latosols may be increased, following the pedobiological turnover during mound building, with local rates varying from 2.1 to 7.5 m³ ha^− 1.     

Adsorption and Desorption of Arsenate in Different Soils and Gold Mining Substrates of Minas Gerais State,Brazil

Background  Arsenic (As) availability in natural environment is related to the element’s adsorption and desorption processes in soils. Total As is better related to available As in temperate soils than in tropical soils. In tropical soils, total As is not very significant in terms of availability, therefore justifying the necessity for studies into As dynamics. Knowledge of As dynamics in soil as well as development of new analytical methodologies involving tropical soils are insufficient and necessary for future mitigation projects. Objective  The objectives of this study were: (1) To adjust methodologies which may assist in understanding arsenate dynamics in tropical soils and substrates; (2) To evaluate the adsorption and desorption of arsenate in soils and substrate samples, and to find a minimum value of arsenate available in soil which is lethal to sorghum plants. Material and Methods  Samples of three soils from Minas Gerais State (YL, RYL, and CS) and two sulfide substrates of gold mining (B1 and B2) were used in the assays. All the material was physically and chemically characterized. Remaining As (As-rem) and remaining P (P-rem) of each material, along with MAC_P and MAC_As (using the Langmuir isotherms), were obtained. After agitation to obtain MAC_P and MAC_As, arsenate was extracted by anionic resin and Mehlich-III to evaluate arsenate desorption of the material retained on the filter paper. Subsequently, arsenate desorption curves for the different materials were obtained, and arsenate availability was determined through a bioassay with sorghum plants. Samples of soils and substrate B1 were incubated with six levels of As doses. Plants were grown under greenhouse conditions for 30 days. The plants were then harvested, dried and weighed. Available As in the soils and substrate was determined by Mehlich-III. Results and Discussions  As-rem level decreased from YL (sandy) to RYL (clayey) soil samples, which always showed lower values than P-rem. Among the soils and substrates evaluated, RYL showed the highest MAC_As and MAC_P, followed by CS, YL and Bl. The results were in accordance with the values observed for As-rem and P-rem and confirm the idea that the ability of the assayed materials to remove As from the soil/substrate solution is higher than the ability to remove P. On the other hand, the binding energy (a) between soil/substrate and As is weaker than the binding energy of P. Given the fact that the studied soils present a real ability to remove As from the solution, only a small part of As would be unavailable considering MAC_As as a reference. As-Mehlich-III values were higher than As-resin for substrate Bl. Mehlich-III seemed to be more appropriate to extract labile forms of arsenate in substrate B1 as well as in the soils. Available As by Mehlich-III (26.9 mg/dm³) was considered a reference of As LCL to sorghum plants. CC50 was sensitive to the buffering capacity of each soil, showing values varying from 1.34 mg/dm³ As (clay soil with lower As-rem) to 12.31 mg/dm³ As (sandy soil with higher As-rem). Conclusions  The adaptation of the As-rem and MAC_As methodologies was satisfactory and of great value in the study of adsorption, desorption and As availability for soils and mining substrate. Mehlich-III was also satisfactory to estimate available As and was sensitive to soil buffering capacity. Nevertheless, resin can also be used as an alternative. MAC_As varied among soils and was higher than MAC_p. However, As showed higher lability than P. Using Mehlich-III, we determined the value corresponding to CC50 that showed a good reference of toxicity to available As. Outlook  The environmental implications of the As behavior are quite serious. Beside the fact that arsenate is removed very fast from the soil solution, an anthropogenic input of the element, being part of the soil quantity factor, may remain in a reversible form for a long time. As may therefore return to the soil solution and becomes available to plants, animals and the entire environment. Considering that CC50 is the maximum contents of available As the environment can tolerate to allow some vegetal biomass production, the maximum capacity of As immobilization in each soil is reduced when compared to the soils’ MAC_As values. Therefore, the maximum and safe values of reference to be used in the evaluation of incidental discharge of the element in soils must be reduced.     

Human modification of stream valleys in the western plateau of São Paulo,Brazil: implications for environmental narratives and management

Deforestation, agricultural land uses and urbanization have altered stream channels and floodplains in many regions of the world, yet occurrences of post‐settlement alluvium (PSA) and benchmark pre‐settlement horizons (Palaeosols) are unreported in many tropical and subtropical developing countries. Twentieth century land uses, motivated strongly by state subsidies in the cases of coffee and cotton cultivation and soy – wheat double cropping, caused soil erosion and the corresponding formation of PSA and stream‐channel changes in small catchments of the Western Plateau of São Paulo State, Brazil. Analysis of land‐use changes suggests necessary alteration of regional environmental management for restoration planning, water quality monitoring and control of gully erosion. Narratives of regional environmental change should address the impacts of agropastoral land uses on soil and water resources, rather than concentrating on deforestation and forest land cover. Copyright © 2000 John Wiley & Sons, Ltd.