Summary Cyanobacteria act as primary producers of carbon and nitrogen in nutrient‐poor ecosystems such as mangroves. This important group of microorganisms plays a critical role in sustaining the productivity of mangrove ecosystems, but the structure and function of cyanobacteria assemblages can be perturbed by anthropogenic influences. The aim of this work was to assess the community structure and ecological drivers that influence the cyanobacterial community harboured in two Brazilian mangrove soils, and examine the long‐term effects of oil contamination on these keystone species. Community fingerprinting results showed that, although cyanobacterial communities are distinct between the two mangroves, the structure and diversity of the assemblages exhibit similar responses to environmental gradients. In each ecosystem, cyanobacteria occupying near‐shore areas were similar in composition, indicating importance of marine influences for structuring the community. Analysis of 16S rRNA sequences revealed the presence of diverse cyanobacterial communities in mangrove sediments, with clear differences among mangrove habitats along a transect from shore to forest. While near‐shore sites in both mangroves were mainly occupied by Prochlorococcus and Synechococcus genera, sequences retrieved from other mangrove niches were mainly affiliated with uncultured cyanobacterial 16S rRNA. The most intriguing finding was the large number of potentially novel cyanobacteria 16S rRNA sequences obtained from a previously oil‐contaminated site. The abundance of cyanobacterial 16S rRNA sequences observed in sites with a history of oil contamination was significantly lower than in the unimpacted areas. This study emphasized the role of environmental drivers in determining the structure of cyanobacterial communities in mangrove soils, and suggests that anthropogenic impacts may also act as ecological filters that select cyanobacterial taxa. These results are an important contribution to our understanding of the composition and relative abundance of previously poorly described cyanobacterial assemblages in mangrove ecosystems.

RESUMONa região de Janaúba, norte de Minas Gerais, não obstante as altas produtividades alcançadas pela bananicultura da região no contexto de uma agricultura altamente tecnificada, com uso intenso de fertilizantes e controle sistemático de doenças e pragas, são comuns os relatos de produtores expressando preocupação com a queda de produtividade, particularmente quando se irriga com água de poços tubulares. Em razão disso, estudou-se o efeito de águas de irrigação provenientes de poços e do rio Gorutuba sobre propriedades químicas de solos sob bananais, onde foram coletadas amostras até 100 cm de profundidade em áreas adjacentes de sequeiro e irrigadas. Observou-se que o uso de águas de poços, de qualidade marginal, causou elevação do pH e dos teores de Ca 2+ e Na + dos solos, elevando as relações Ca/Mg e Ca/K, que se distanciaram da faixa ideal para a cultura da banana. Nos solos irrigados com água de poços houve aumento dos teores de P extraído com H 2 SO 4 , resultante da precipitação do P com o Ca veiculado com a água. Os valores de pH do extrato de saturação do solo foram mais elevados que aqueles registrados na análise de rotina, e a concentração de sais solúveis na solução do solo ocorreu em camadas intermediárias do perfil devido à lixiviação, com destaque para a camada de 30-40 cm dos solos irrigados com água de poços.Termos de indexação: qualidade da água, salinidade, irrigação da bananeira.

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Sedimentology and hydrology seem to play a predominant role in soil formation in the Pantanal, since the fluvial systems of this wetland provided Quaternary sediments and shaped the landscape. However, pedogenic processes are active in this region mainly due to the floods that occur during the summer and fall season. The goal of this work was to study the pedogenesis in a representative transect on the São Lourenço fluvial fan, which has an area of about 16,000 km 2 and is one of the largest sedimentary compartments of the Northern Pantanal, West Central Brazil. A transect of 125 m was studied in the late Pleistocene sedimentary lobe of the fluvial fan, where four landform elements were identified (paleo-channel, paleo-levee, paleo-floodplain and mound) and described to show the soil horizon boundaries along the landform unit. Samples were collected from trenches, auger holes, and boreholes, and were analyzed for soil description and characterization. Soil analyses carried out were: macroand micromorphological, physical, chemical, mineralogical, C isotope fractionation, and OSL dating analyses. The set of evidence shows that the studied transect originated from Pleistocene sediments due to the activity of the São Lourenço fluvial fan. The following soil-landform element relationships developed: Quartzipsamments in the paleo-channel; Plinthaquults in the paleo-levee and paleo-floodplain and; Natrustalfs in the mound. Evidences of pedogenic processes in the transect are: a) clay eluviation and illuviation, resulting in formation of the Bt horizon in the soils at the lower part of the transect and mound; b) gleization and plinthization, mainly at the lower part of the transect, resulting in formation of mottles, reduced soil matrix, and nodules (plinthite and petroplinthite); c) bioturbation, responsible for addition of materials at the soil surface, assisting in the formation/maintenance of the mound; and d) the sodium concentration, observed in soils that have higher clay contents, which are highest in the mound has the highest levels.

SUMMARYThe Pantanal region can be characterized as a quaternary floodplain with predominant sedimentation in the form of alluvial fans. In the geomorphologic and sedimentary evolution, the avulsion process is inherent to this depositional system and its dynamics, together with surface water floods, influence soil sedimentation on this plain. The knowledge and differentiation of these two events can contribute to a better understanding of the variability of soil properties and distribution under the influence of these sedimentation processes. Therefore, this study investigated the genesis of soils in the Northern Pantanal with textural contrasts in deeper horizons and their relationship with the depositional system dynamics. We analyzed four soil profiles in the region of Barão de Melgaço, Mato Grosso State, Brazil (RPPN SESC Pantanal). Two profiles were sampled near the Rio Cuiabá (AP1 and AP4) and two near the Rio São Lourenço (AP10 and AP11). In AP11, the horizons contrast in particle size between the profile basis and the surface. In AP1, AP4 and AP10, the horizons overlaying the sand layer have similar particle size properties, mainly in terms of sand distribution. In the first case, floods (surface water) seem to have originated the horizons and layers with contrasting texture. In the second case, avulsion is the most pronounced process. Therefore, the two modes can form soils with contrasting texture that are discriminable by soil morphology, based on the distinct features associated to the specific sedimentation processes.Index terms: alluvial soils, hydromorphic soils, fluvial deposition system, alluvial fans, avulsion.(

Salt-affected soils play an important role in the Pantanal wetlands ecosystem, in Brazil, but their occurrence on higher landforms is not well understood. In order to investigate the major processes involved in salt-affected soil formation, a trench was opened in a representative transition between the floodplain and a paleochannel-paleolevee complex not currently reached by floodwater. The trench was 6 meters long and 2 meters deep, and 121 soil samples were collected in a 0.25 x 0.25 m regular grid and submitted for soil (particle-size, pH, exchangeable cations) and solution extract (major cation and anions) analyses. Analytical results were subjected to geostatistical and principal components analyses (PCA). For selected horizons, an undisturbed sample was collected to perform thin sections and micromorphology description. The presence of degraded carbonates in the soils indicated an ancient formation. The regional environmental settings of the alluvial fan and particle size variation along the soil profiles suggested that the sediments of the study site were deposited during the Late Pleistocene and Holocene, in a river channel-levee complex and an abandoned channel by avulsion processes. The depressed topography of the abandoned channel, accompanied by moderately drier climates during early Holocene, likely favored low leaching of salts and higher evapotranspiration, enabling the development of salt-affected soils. Later erosion of the region, which is expected in older lobes of alluvial fans, triggered a relief inversion in the area, preserving the more clayey areas as a slightly higher landform with remaining salt-affected soils. Currently, the seasonal flooding mainly reaches the footslope of this landform, triggering redoximorphic processes, expressed as redox concentrations and depletions, and solodization processes, expressed by lower values of pH, cation exchange capacity (CEC), base saturation and exchangeable sodium percentage (ESP) than at the summit. Thus, the occurrence of salt-affected soils on higher landforms is the reflection of a complex sedimentological history. In this way, the genesis of salt-affected soils on higher landforms, which seems to be poorly understood in many situations worldwide, should be carefully investigated in order to address formation mechanisms, especially in extremely dynamic environments of tropical wetlands.

ABSTRACT:The simultaneous occurrence of high levels of exchangeable sodium percentage (ESP) and alkalinity in soils imposes restrictions on plant development and affects physical properties such as porosity, bulk density, permeability, and hydraulic conductivity. Although sodic soils are frequent in the flood plain of the São Lourenço River, northern Pantanal, Brazil, few studies focus on their formation and classification, especially with regard to specific processes and detailed classification into lower categorical levels by the different systems available. The aim of this study was to identify the predominant pedogenetic processes occurring in sodic soils of the flood plain of the São Lourenço River to understand their genesis and assess how taxonomic classification systems contemplate the variations in soil properties. Five profiles were selected in sites with different progressive stages of dissection from erosion (P1, P2, P3, P4, and P5). At each site, a pit was dug for morphological description of the profiles and for collecting samples for chemical, particle size, mineralogical, micromorphological, and chronological analyses. Each profile was classified according to the Soil Taxonomy, World Reference Base (WRB), and Brazilian Soil Classification System (SiBCS/ Sistema Brasileiro de Classificação de Solos). Argilluviation is the predominant process, with a localized and intense ferrolysis action in the E/Bt transition zones in profile P5. Soils showed signs of lithologic discontinuity. This makes it difficult to distinguish how much of the textural gradient is inherited from fluvial sedimentation processes and how much is the result of pedogenetic processes. In the most advanced stage of alteration, P5 had a paler color, thickening of the E horizon, and an abrupt and irregular transition entering the Bt horizon in the form of a "tongue". When passing from the most preserved to the most eroded area, ferrolysis becomes more intense in the E/Bt transition, the electrical conductivity values decrease, and the ESP values increase, suggesting the sodification process. Under Soil Taxonomy criteria, P5 was classified as Natraqualf and the other profiles were classified as Natrudalf; under the WRB, however, all profiles were classified as Solonetz. The SiBCS exhibited variation at the Order level, with P5 classified as Planossolo and the others as Luvissolo. Despite an indication of different processes at the Order level, the SiBCS does not yet contemplate the sodic character in the studied Luvissolos. We propose inclusion of the sodic character at the Great Group level, as has already occurred with other SiBCS classes.