ABSTRACT Bulk density (BD) is a soil physical property used as a soil quality indicator and variations in this measurement influence soil water content and carbon stock estimates. This study aims to compile a database of samples of bulk density, textural fractions, and organic carbon values, as well as evaluate the accuracy of published pedotransfer functions (PTF) that predict bulk density, and propose a hierarchical PTF to predict the bulk density of Brazilian Soils. The performance of eleven PTFs and the newly proposed PTFs were evaluated and compared using the root mean square error (RMSE) and coefficient of determination (R2) based on a testing soil database collected from the literature. We noticed a slight improvement in accuracy when organic carbon and coarse and fine sand fractions were included as predictors alongside silt and clay. The best results with existing PTFs were obtained by PTF-A in Tomasella and Hodnett (1998) (RMSE = 0.20 g cm–3) and PTF-F in Benites et al. (2007) (RMSE = 0.17 g cm–3). Our proposed PTFs use textural fractions and organic carbon as predictors in a hierarchical form. The proposed PTF-4, which uses fine sand, coarse sand, clay, and organic carbon, presented the lowest value for RMSE (0.14 g cm–3) for BD prediction.

ABSTRACT Brazilian palm swamps (veredas) are fundamentals in the hydrological balance of watercourses in the Brazilian savanna (Cerrado). The “sponge effect” of their soils is the main factor controlling local hydrology, storing rainwater, and functioning as headwaters. The restricted knowledge of these tropical ecosystems has led to increased losses, poor preservation, and reduction in their ecosystem services. Veredas have become refuges surrounded by croplands, often drained and inappropriately managed. This study shows the impacts of anthropization on soil processes and properties of a vereda in an agricultural area. Two soil profiles were selected and characterized as preserved and anthropized, respectively upstream and downstream of the studied vereda. Morphological, physical, chemical and microbiological properties were analyzed. Principal Components Analysis (PCA) was applied to synthesize the data and provide evidence of the main properties and underlying processes that most responded to the degradation action. The arrangement of this analysis shows three main distinguish drivers: one joining the properties related to the humification of organic matter and relative accumulation of mineral matter versus accumulation of organic matter; the second with properties related to soil chemical reactivity; and the third reflecting the mineralization of organic matter. Our results suggest that the anthropic action has strongly caused the organic carbon reduction (~22 %). After 20 years, the anthropized soil presents not only a great decline in carbon stock (~14 kg m-2), but also strong impacts on several other ecologic functions, such as water holding capacity. Veredas are complex and fragile environments, and they should be fully protected to maintain their ecosystem services.

ABSTRACT Soils in the Brazilian Pantanal classified as Espodossolos and registered in the literature diverge from the central concept of pedogenesis by podzolization, especially due to the high values of pH and basic cations, although the morphology is similar to that of spodic horizons. In this sense, this study hypothesized that the neutral to alkaline conditions along the edges of the saline basins in the Lower Nhecolândia region do not inhibit the podzolization process nor the development of spodic soils. The objectives were to analyze the attributes of soils with spodic features and evaluate whether they correspond to a podzolization process. Four profiles in a transect in Lower Nhecolândia were selected, where the morphology indicated the presence of a spodic horizon. Three profiles (P1, P2 and P4) were located in a bay and one in the transition between a bay and a “cordilheira” (small elevation between lagoons) (P3). The soils were analyzed for particle size, chemistry and X-ray fluorescence (XRF), as well as organic carbon (C org), total carbon (TC) and XRF of the nodules. The complexity of features and characteristics of soil profiles in the Lower Nhecolândia region indicate multiple pedogenetic processes in this environment. The characteristics of all profiles denote a redoximorphic process: poor profile development (except for P3), mottles formed by precipitation of Fe and Mn oxides, as well as presence of Fe and Mn nodules. The organic matter content of these nodules is mostly greater than or equal to that of the surrounding material, and aluminum is relatively not higher than in the fine earth. This reinforces a rexodimorphic process in the horizons with spodic features. The occurrence of sodic and solodic characters, as well as clay accumulation in P3 also characterize the sodification process. The neutral to alkaline pH values in water, high sum of bases and low C org and Al oxide contents of the studied soils all contradict the occurrence of a podzolization process. However, Fe, Al and C org accumulation in the nodules and some B horizons indicate a spodic character. Also, the fact that SiBCS criteria classify the studied soils as Espodossolos indicates the relevance of establishing limits for chemical attributes, which would adjust the taxonomy of soils with a spodic character according to their pedogenesis.

ABSTRACT Chemical soil analysis data can be expressed by weight (i.e., gravimetric basis) or volume (i.e., volumetric basis) of the fine earth (sieved ≥2 mm), resulting in different units, cmolc kg-1 and cmolc dm-3, respectively. The research problem is that the difference between methods to express the same soil properties hinders the comparison of results and database or dataset standardization. This paper aims to develop pedotransfer functions (PTF) to obtain the density of fine earth, which will then be used for conversion data expressed in volumetric to gravimetric basis, or vice versa, that will be applied to compare results and to standardize databases with different units. Soils samples, including profiles of the main soil orders in Brazil such as Latossolos (Ferralsols or Oxisols) and Argissolos (Acrisols or Ultisols), from the states of Rondônia, Roraima, and Mato Grosso do Sul (132 horizons) were selected and weighed (in triplicate) to obtain the fine earth mass contained in a volume of 10 cm3. The mass values were used to calculate the fine earth density. Spearman’s correlation analysis was used between the density and nine soil properties (coarse sand, fine sand, total sand, silt, clay, clay dispersed in water, clay dispersion, particle density, and organic carbon). The total sand, clay, and organic carbon showed the best correlations, therefore they were selected to construct the pedotransfer functions. Nonlinear regression techniques were used to obtain the models (PTFs) to predict density, which was used for unit conversion. As a result, the residual standard error (RSE) statistics of the models were: 0.0920, 0.1231, and 0.1633 g cm-3, respectively for PTF1 (using total sand as a predictor), PTF2 (using clay), and PTF3 (using organic carbon). Independent data was used to evaluate the accuracy of the models by residue analysis and the RSE. For the validation, the lowest RSE obtained was from the PTF1, so the best performance. Thus, to convert values of the chemical properties from a volumetric to gravimetric basis, the value must be divided by the predicted density. While, the conversion from gravimetric to volumetric basis requires that the value be multiplied by the predicted density. The PTFs using the properties total sand, clay, and organic carbon as predictor variables, allowed conversion of analytical data of soil samples expressed in the volumetric basis to gravimetric and vice versa, which can be used for dataset or database standardization.

ABSTRACT Soils with high organic carbon content, such as those with “humic A horizon”, occur in different regions of Brazil. This study aimed to determine the quantitative and qualitative characteristics of organic matter regarding the humic A horizon under different land uses in Bom Jardim, in Rio de Janeiro State, Brazil. Samples from forest, pasture, Eucalyptus plantation, coffee cultivation, olericulture, and also passion fruit soils were sampled and analyzed. In October 2011, undisturbed samples from humic A horizons, were collected for soil density analysis. In addition, disturbed samples were collected for the determination of organic carbon (Corg) content, total carbon, humic substance fractions contents and natural abundance of 13Carbon (δ13C). Low Corg contents were observed in areas under olericulture, pasture and passion fruit cultivation. The fulvic acid fraction was higher than other fractions, regardless of land use. The small variability in δ13C indicated maintenance of original Corg and suggest high resistance of Soil Organic Matter in humic A horizons.

RESUMO O Planalto da Borborema é caracterizado pelos diferentes estágios de evolução do relevo, modificando o clima e a vegetação, sendo observadas florestas tropicais, (brejos de altitude), circundado por caatinga. O objetivo deste trabalho foi caracterizar os solos de uma topoclimossequência no agreste pernambucano, avaliandoa influência do relevo e do clima na pedogênese. Foi selecionada uma topoclimossequência e abertas trincheiras nas feições geomorfológicas de brejo de altitude (P1), entre brejo de altitude e pediplano (P2) e em superfície de pediplanação (P3 e P4). Foi realizada a descrição morfológica e a caracterização física, química e micromorfológica. De maneira geral, os solos são arenosos, com predomínio da estrutura grão simples ou fraca agregação. Maiores valores de pH, S, V% e P assimilável foram quantificados nas partes mais baixas da paisagem. Na análise micromorfológica detectou-se o desenvolvimento pedogenético incipiente de estrutura nos horizontes C de P1 e P3; translocação de argila em P2; e degradação de micronódulos de ferro em P4. Os perfis P1 e P3 foram classificados como Neossolos Regolíticos Distróficos espessarênicos (Regosols); P2 como Argissolo Amarelo Distrófico típico (Alisol); P4 como Planossolo Háplico Eutrófico arênico (Planosol). A variação dos solos na paisagem foi determinada pelo clima, relevo e material de origem. A micromorfológica foi eficiente na detecção de atributos não observados em campo, como agregação incipiente nos Neossolos Regolíticos e degradação de micronódulos de ferro no Planossolo Háplico.

ABSTRACT The Borborema Plateau is characterized by different stages of relief evolution, which modify the climate and vegetation, and where high-altitude tropical forests can be seen surrounded by caatinga. The aim of this study was to characterize the soils of a topo-climosequence in the Agreste region of the State of Pernambuco, and evaluate the influence of the relief and climate on the pedogenesis. A topo-climosequence was selected, and trenches were opened in the geomorphological features of high-altitude forest (P1), between forest and pediplane (P2) and on a pediplanation surface (P3 and P4). A morphological description and a physical, chemical and micromorphological characterization were carried out. In general, the soils are sandy, with the predominance of a single-grain structure or weak aggregation. Higher values for pH, S, V% and assimilable P were found on the lower parts of the landscape. From the micromorphological analysis, the incipient development of pedogenic structures was detected in the C horizon in P1 and P3, clay translocation in P2 and the degradation of iron micronodules in P4. The P1 and P3 profiles were classified as Neossolos Regolíticos Distróficos espessarênicos (Regolsols), the P2 profile as a Argissolo Amarelo Distrófico típico (Alisol), and P4 as an Planossolo Háplico Eutrófico arênico (Planosol). Soil variation in the landscape was determined by climate, relief and parent material. Micromorphology was efficient in detecting attributes not seen in the field, such as incipient aggregation in the Neossolos Regolíticos (Regosols) and the degradation of iron micronodules in the Planossolo Háplico (Planosol).

ABSTRACT: The evaluation of soils in representative landscapes constitutes an opportunity to evaluate spatial distribution, discuss formation processes, and apply this knowledge to land use and management. In this sense, from the perspective of an environmentally diversified region, the aim of the present study is to evaluate the occurrence and understand the formation of soils in different geomorphic surfaces of a landscape from a mountain region in the state of Rio de Janeiro. The study was developed in the Pito Aceso microbasin in the municipality of Bom Jardim, composed of narrow valleys and a rugged mountain domain, with elevation between 640 and 1,270 m. In a representative landscape, the geomorphic surfaces were obtained from the slope segments and flow lines. On the geomorphic surfaces, soil profiles were described by their morphological properties, collected, and analyzed to describe the chemical and physical properties of each horizon. Geomorphological aspects and possible variations of the parent material directly affected pedogenesis and led to distinct soil classes in the landscape. Variation in the geomorphic surfaces directs the processes for soil formation under current conditions, as well as the preservation of polygenetic soils. Soils of lower development and with greater participation of the exchangeable cations were identified at the summit (talus deposit) (Neossolo Litólico and Cambissolo Húmico) and toeslope (colluvial-alluvial) (Neossolo Flúvico), whereas more developed soils with lower nutrient content occur in the concave (Argissolos Vermelho and Amarelo) and convex (Latossolo Amarelo) backslope, except for the Argissolo Vermelho-Amarelo in the shoulder, which had high exchangeable cations contents.

RESUMO O objetivo deste trabalho foi quantificar os diferentes compartimentos da matéria orgânica de solos sob floresta no estado do Acre. Perfis de solo foram descritos e coletados nos terços superior, terço médio e terço inferior da paisagem localizados nos municípios de Sena Madureira, Manoel Urbano e Feijó, no estado do Acre. Os solos foram caracterizados quanto à composição química, física, frações húmicas e frações oxidáveis. Os teores de carbono orgânico estão concentrados nos horizontes superficiais dos solos, destacados pelo decréscimo abrupto em relação aos demais horizontes. Os teores da argila e de cátions atuam de forma mais efetiva na dinâmica da matéria orgânica em detrimento à posição na paisagem e classe de solo. As análises multivariadas indicam maior similaridade entre os pontos de Sena Madureira (T1) e de Feijó (T3), exceto o ponto do terço superior de Feijó, que é semelhante ao Manoel Urbano (T2).

ABSTRACT This study aimed to quantify the different compartments of organic matter from under forest soils in Acre state. Soil profiles were described and sampled on shoulder, backslope and footslope landscape positions located in the counties of Sena Madureira, Manoel Urbano and Feijó, Acre state. Soils were characterized by chemically and physically attribute, humic and oxidizable fractions. The organic carbon is concentrated in the superficial soil horizons, highlighted by the sharp decrease in relation to other horizons. The clay and cations act more effectively on organic matter dynamics than the landscape position and soil class. Multivariate analyzes indicate high similarity between the sites of Sena Madureira (T1) and Feijó (T3), except the shoulder site from Feijó, which is similar to Manoel Urbano (T2).

Resumo O objetivo deste trabalho foi avaliar os efeitos de diferentes usos da terra nas características e nos atributos de Latossolos da região Oeste do Estado da Bahia. Os Latossolos apresentavam texturas com fração areia amplamente predominante, e foram avaliados em áreas de cerrado, algodão, soja e feijão, no Município de Luís Eduardo Magalhães. A caracterização morfológica e a coleta de amostras de solos para avaliação dos atributos físicos e químicos por horizontes foram feitas em minitrincheiras, enquanto a avaliação da condutividade hidráulica saturada foi feita com permeâmetro de Guelph em duas profundidades: 0,0-0,20 e 0,20-0,40 m. Os Latossolos avaliados, além de uma estrutura maciça, apresentaram horizonte genético adensado, que, sob uso agrícola, torna-se mais compactado, espesso e superficial, e forma torrões quando revolvido. O uso agrícola reduz a condutividade hidráulica saturada dos Latossolos avaliados nas duas camadas, com exceção do cultivo de feijão.

Abstract The objective of this work was to evaluate the effects of different land uses on the characteristics and attributes of Oxisols in the western region of the state of Bahia, Brazil. The Oxisols had textures with largely predominant sand fraction, and they were evaluated in areas of cerrado, cotton, soybean, and common bean in the municipality of Luís Eduardo Magalhães. The morphological characterization and the collection of samples for evaluating soil physical and chemical attributes of horizons were carried out in small trenches, while the evaluation of field saturated hydraulic conductivity was done using the Guelph permeameter at two depths: 0.0 to 0.20 and 0.20 to 0.40 m. The evaluated Oxisols, besides their massive structure, showed a dense genetic horizon, which, under agricultural use, becomes more compacted, thick, and shallow, forming clods when plowed. Agricultural use reduces the saturated hydraulic conductivity of the evaluated Oxisols in both layers, except for common bean cultivation.

Resumo O objetivo deste trabalho foi avaliar o efeito da eutrofização do solo em fragmentos remanescentes de vegetação nativa sobre a comunidade florística de arbóreas em paisagens agrícolas. Foram amostrados oito sítios de vegetação arbórea na Mata Atlântica, no Estado do Rio de Janeiro, em pares formados de acordo com o uso do solo em seu entorno - intensivo (UI) ou extensivo (UE) - e com diferentes tamanhos: dois fragmentos pequenos, com cerca de 10 ha; dois médios, com 30 ha; dois grandes, com 100 ha; e outros dois sítios localizados dentro de uma área contínua de floresta. Um conjunto de parâmetros bióticos e de solos, além de métricas da paisagem, foi obtido e analisado segundo duas hipóteses: H01, a fertilidade do solo dos remanescentes naturais adjacentes a áreas agrícolas de uso intensivo é alterada; e H02, a eutrofização do solo dos remanescentes naturais adjacentes a áreas agrícolas de uso intensivo altera a composição da sua comunidade de arbóreas. Os teores de cálcio foram cerca de sete vezes maiores nos sítios adjacentes a UI, quando comparados aos fragmentos adjacentes a UE. Os teores de fósforo foram de 1,5 a 2,5 vezes maiores nos fragmentos pequenos e médios adjacentes a áreas de UI. A análise de ordenamento evidenciou que P, K, Ca, luz difusa, abertura de dossel, relação área/perímetro e isolamento dos fragmentos explicam 59% da composição das comunidades arbóreas.

Abstract This objective of this work was to evaluate the effect of soil eutrophication in remaining patches of native vegetation on the floristic community of trees in agricultural landscapes. Eight sites of arboreal vegetation were sampled in the Atlantic Forest, in state of Rio de Janeiro, Brazil, in pairs formed according to land use in their boundary - intensive (UI) or extensive (UE) -, and with different sizes: two small sites, with about 10 ha; two medium-sized ones, with 30 ha; two large ones, with 100 ha; and two sites located within a continuous forest area. A set of biotic and soil parameters, besides landscape metrics, was surveyed and analyzed considering two hypotheses: H01, soil fertility is changed in forest remnants next to areas of intensive agricultural use; and H02, soil eutrophication in forest remnants next to areas of intensive agricultural use changes the composition of its tree community. Calcium contents were about seven times higher in sites next to UI areas, in comparison with those next to UE areas. Phosphorous contents were 1.5 to 2.5 times higher in the small and medium-sized patches next to UI areas. The ordering analysis showed that P, K, Ca, diffuse light, canopy opening, area/perimeter ratio, and patch isolation account for 59% of the tree community composition.

Resumo O objetivo deste trabalho foi selecionar um método para estimar o N-NH3 perdido por volatilização e determinar essas perdas em fertilizantes mistos contendo ureia. Um experimento foi realizado em casa de vegetação, em bandejas preenchidas com solo arenoso, com as doses de ureia de 0, 50, 100 e 200 kg ha-1 de N. A amônia volatilizada foi quantificada por três métodos: CSA, coletor semiaberto; Sale, coletor semiaberto livre estático; e CA, coletor aberto. Em um segundo experimento, avaliou-se a volatilização de amônia proveniente de 18 fertilizantes mistos com ureia, ácidos húmicos e zeólita, em solos com textura arenosa e argilosa, na dose de 200 kg ha-1 de N. O CSA foi o método que recuperou mais N, seguido do Sale e do CA. O Sale é mais adequado para experimentos com elevado número de unidades amostrais, por ser mais prático. Os fertilizantes minerais mistos com ureia e ácido húmico (5 e 10%) e com ureia e zeólita (10%) apresentaram os melhores resultados na mitigação da volatilização da amônia, com redução nas perdas de N-NH3 de até 38%. Os tratamentos são mais efetivos no combate à volatilização no solo de textura arenosa.

Abstract The objective of this work was to select a method for estimating N-NH3 lost by volatilization and to determine these losses in mixed fertilizers containing urea. An experiment was conducted in a greenhouse in trays filled with sandy soil, with urea rates of 0, 50, 100, and 200 kg ha-1 N. Volatilized ammonia was quantified by three methods: CSA, semi-open collector; Sale, free and static semi-open collector; and CA, open collector. In a second experiment, volatilization of ammonia from 18 fertilizers mixed with urea, humic acids, and zeolite was evaluated in sandy and clayey soils, at 200 kg ha-1 N. CSA was the method with the highest N recovery, followed by Sale and CA. Sale is more suited for experiments with a high number of experimental units, since it is more practical. The mixed fertilizers with urea and humic acids (5 and 10%) and with urea and zeolite (10%) had the best results in the mitigation of ammonia volatilization, reducing N-NH3 losses in up to 38%. The treatments are more effective in mitigating volatilization losses in the sandy soil.

Resumo Os solos leves ocupam cerca de 8% do território brasileiro e são epecialmente expressivos na nova e na última fronteira agrícola do país: a região de Matopiba, nos estados do Maranhão, do Tocantins, do Piauí e da Bahia, onde representam 20% da área. Esses solos enquadram-se nas classes texturais areia e areia franca ou francoarenosa, até a profundidade de 0,75 m ou mais, e são representados principalmente pelos Neossolos Quartzarênicos e, em parte, por Latossolos e Argissolos. O entendimento do funcionamento desses solos depende do estabelecimento de critérios distintivos sobre: dinâmica da matéria orgânica; teor e mineralogia da fração argila; teores de areia grossa e de areia total, em relação aos de areia fina; diâmetro médio da fração areia; e capacidade de retenção de água. Esses critérios podem contribuir para o zoneamento e para o manejo conservacionista e da fertilidade dos solos leves, bem como para estimação de seu potencial agrícola. Sistemas integrados de produção, como os de integração lavoura-pecuária e lavoura-pecuária-floresta, além do plantio direto com rotação de culturas, dos plantios florestais mistos com espécies leguminosas, e do uso de adubos verdes e cultivos de cobertura, são relevantes para o manejo adequado desses solos. O objetivo deste artigo de revisão foi caracterizar os solos leves e apontar os principais desafios em relação a seu potencial agrícola, a seu manejo e conservação e sua fertilidade, frente à expansão e à consolidação da nova fronteira agrícola.

Abstract Light soils occupy 8% of the Brazilian territory and are especially expressive in the new and last agricultural frontier in Brazil: the Matopiba region - in the states of Maranhão, Tocantins, Piauí, and Bahia -, where they represent 20% of the area. These soils fit into the textural classes of sand and loamy sand or sandy loam, down to 0.75-m soil depth or deeper, and they are mainly represented by Neossolos Quartzarênicos (Quartzipsamments) and, partly, by Latossolos (Oxisols) and Argissolos (Ultisols). The understanding of soil functioning depends on the establishment of distinguishing criteria for: organic matter dynamics; content and mineralogy of the clay fraction; coarse sand and total sand contents, in relation to those of fine sand; mean diameter of the sand fraction; and water retention capacity. These criteria can contribute for the zoning and for the conservation and fertility management of light soils, as well as for the estimation of their agricultural potential. Integrated production systems, such as crop-livestock and crop-livestock-forestry integration, besides no-tillage with crop rotation, mixed forestry planting with legumes, and the use of green manure and cover crops are relevant for the proper management of these soils. The objective of this review was to characterize light soils and to highlight the main challenges regarding their agricultural potential and their conservation and fertility managements, in face of the expansion and consolidation of the new Brazilian agricultural frontier.

ABSTRACT Soils formed in high mountainous regions in southern Brazil are characterized by great accumulation of organic matter (OM) in the surface horizons and variation in the degree of development. We hypothesized that soil properties and genesis are influenced by the interaction of parent materials and climate factors, which differ depending on the location along the altitudinal gradient. The goal of this study was to characterize and classify the soil, evaluate soil distribution, and determine the interactive effects of soil-forming factors in the subtropical mountain regions in Santa Catarina state. Soil samples were collected in areas known for wine production, for a total of 38 modal profiles. Based on morphological, physical, and chemical properties, soils were evaluated for pedogenesis and classified according to the Brazilian System of Soil Classification, with equivalent classes in the World Reference Basis (WRB). The results indicated that pedogenesis was strongly influenced by the parent material, weather, and relief. In the areas where basic effusive rocks (basalt) were observed, there was formation of extensive areas of clayey soils with reddish color and higher iron oxide contents. There was a predominance of Nitossolos Vermelhos and Háplicos (Nitisols), Latossolos Vermelhos (Ferralsols), and Cambissolos Háplicos (Cambisols), highlighting the pedogenetic processes of eluviation, illuviation of clay, and latosolization in conditions of year-long, large-volume, well-distributed rainfall and stability of land forms. In areas with acid effusive rocks (rhyodacites), medial or clayey soils were observed with lower iron oxide content, invariably acidic, and with low base content. For these soils, relief promoted substantial removal of material, resulting in intense rejuvenation, with a predominance of Cambissolos Háplicos (Cambisols) and lesser occurrence of Nitossolos Brunos (Nitisols) and Neossolos Litólicos (Leptosols). Soils formed from sedimentary rocks also tended to be more acidic, but with higher sand content, and the soils identified were Cambissolos Háplicos and Húmicos (Cambisols). Cluster analysis separated the soil profiles into three groups: the first and largest was formed by profiles originating from sedimentary rocks and rhyodacites; the second, smaller group was formed by four profiles in the Água Doce region (acidic rocks); and the third was formed by profiles derived from basalt. Discriminant analysis was effective in grouping soil classes. Thus, the study highlighted the importance of geology in the formation of soils in this landscape associated with climate and relief.

Em solos tropicais, a disponibilidade de fósforo (P) pode ser regulada pela decomposição e mineralização da fração lábil de P orgânico (Po), tornando necessários estudos para saber a sua real contribuição no solo. O objetivo deste trabalho foi avaliar o teor de Po total e lábil em horizontes superficiais de diferentes classes de solo e quais as propriedades do solo exercem controle na acumulação dessas frações de P. Foram estudadas amostras de diferentes horizontes diagnósticos superficiais, a saber: H e O hístico; A chernozêmico; e A húmicos, coletados em vários estados do Brasil. Para a determinação das frações totais de P inorgânico (Pi) e orgânico (Po), foi usado o método da extração-sequencial ácido-alcalina, e a fração lábil de P foi determinada pela extração com bicarbonato de sódio. A taxa de recuperação de Pi + Po em relação à extração nítrico-perclórica do P total (PT) variou de 46 a 99 %. O Po total variou entre 35 e 1077 mg kg-1, com uma média de 298 mg kg-1. Para a fração lábil do Po, os teores variaram entre 7,2 e 99,5 mg kg-1, com uma média de 27,1 mg kg-1. O Po variou, em média, de 36 a 46 % do P total extraído. Em relação ao P lábil, o Po representou mais de 70 % para todos os horizontes diagnósticos. O PT foi o principal atributo controlador da acumulação de Po nos solos pelas análises de correlação e regressão múltipla. Verificou-se que horizontes com baixa capacidade de adsorção de P proporcionaram alta labilidade de Po.

Phosphorus availability in tropical soils may be regulated by the decomposition and mineralization of the labile organic phosphorus fraction (Po); therefore, studies assessing the actual contribution of Po to soil composition are necessary. The objective of this study was to evaluate the total and labile Po content in diagnostic surface horizons of different soil types and to assess which soil properties control the accumulation of these P fractions. Samples of different diagnostic surface horizons were studied, namely histic H and O, chernozemic A, and humic A, collected from several Brazilian states. The total fractions of inorganic P (Pi) and organic P (Po) were determined using the sequential acid-base extraction method, and the labile P fraction was determined by extraction with sodium bicarbonate. The recovery rate of Pi + Po relative to nitric-perchloric extraction of total P (TP) ranged from 46 to 99 %. Total Po ranged from 35 to 1077 mg kg-1 (mean of 298 mg kg-1). The labile Po fraction values ranged from 7.2 to 99.5 mg kg-1 (mean of 27.1 mg kg-1). Po represented, on average, 36 to 46 % of the total P extracted and over 70 % of the labile P in all diagnostic horizons. TP was the main property driving Po accumulation in soils according to correlation and multiple regression analyses. Horizons with low P adsorption capacity were observed as leading to high Po lability.

O objetivo deste trabalho foi quantificar o carbono orgânico e as frações húmicas de horizontes superficiais de perfis de solo em três topolitossequências no ambiente de Mar de Morros, no município de Pinheiral, RJ. Foram selecionadas as topolitossequências: T1 (basalto), T2 (muscovita-biotita-gnaisse) e T3 (gabro). Os horizontes superficiais de 14 perfis de solo, sob cobertura de pastagem (T1 e T3) e pastagem mista não manejada e leguminosa (pasto sujo/capoeira) (T2), foram descritos e coletados para análises químicas, físicas e do teor de carbono nas frações húmicas. Os teores de carbono orgânico variaram de acordo com o material de origem e decrescem na seguinte ordem: T3 > T1 > T2. Os teores de carbono orgânico aumentam do topo para a várzea na T1, ocorrendo o inverso na T3 e sem um padrão definido na T2. A fração humina predominou em todos os perfis de solo das topolitossequências e foi seguida pela fração ácidos fúlvicos na T1 e de forma equitativa entre as frações ácidos fúlvicos e ácidos húmicos na T2 e T3. Considerando o ambiente similar em termos da vegetação original e o uso agrícola, as mudanças no carbono do solo e nas frações húmicas podem refletir as interações com os componentes minerais e processos pedogenéticos, influenciados pelo material de origem e o relevo.

The aim of this study was to quantify the organic carbon and humic fractions of the surface horizons of soil profiles in three topolito-sequences from the Mar de Morros environment in the town of Pinheiral, in the state of Rio de Janeiro, Brazil. The topolito-sequences selected were: T1 (basalt), T2 (muscovite-biotite-gneiss), and T3 (gabbro). The surface horizons of 14 soil profiles under pasture (T1 and T3), and unmanaged mixed pasture and leguminous plants (shrub/vegetation) (T2), were marked out and collected for chemical and physical analysis, and evaluation of the carbon content of the humic fractions. The organic-carbon content varied according to the source material, decreasing in the order: T3 > T1 > T2. The organic-carbon content increased from the highest point to the lowlands in T1, with the opposite occurring in T3, and no definite pattern being seen in T2. The humin fraction predominated in all the topolito-sequence soil profiles, being followed by the fulvic acid fraction in T1, and by the fulvic and humic acid fractions equally in T2 and T3. Considering the environments as being similar in terms of original vegetation and agricultural use, changes in the carbon and humic fractions of the soil may reflect interactions with mineral components and pedogenic processes, influenced by the original material and the landscape.