Relationship between soil oxidizable carbon and physical, chemical and mineralogical properties of umbric ferralsols

The occurrence of Umbric Ferralsols with thick umbric epipedons (> 100 cm thickness) in humid Tropical and Subtropical areas is a paradox since the processes of organic matter decomposition in these environments are very efficient. Nevertheless, this soil type has been reported in areas in the Southeast and South of Brazil, and at some places in the Northeast. Aspects of the genesis and paleoenvironmental significance of these Ferralsols still need a better understanding. The processes that made the umbric horizons so thick and dark and contributed to the preservation of organic carbon (OC) at considerable depths in these soils are of special interest. In this study, eight Ferralsols with a thick umbric horizon (UF) under different vegetation types were sampled (tropical rain forest, tropical seasonal forest and savanna woodland) and their macromorphological, physical, chemical and mineralogical properties studied to detect soil characteristics that could explain the preservation of high carbon amounts at considerable depths. The studied UF are clayey to very clayey, strongly acidic, dystrophic, and Al-saturated and charcoal fragments are often scattered in the soil matrix. Kaolinites are the main clay minerals in the A and B horizons, followed by abundant gibbsite and hydroxyl-interlayered vermiculite. The latter was only found in UFs derived from basalt rock in the South of the country. Total carbon (TC) ranged from 5 to 101 g kg-1 in the umbric epipedon. Dichromate-oxidizable organic carbon represented nearly 75 % of TC in the thick A horizons, while non-oxidizable C, which includes recalcitrant C (e.g., charcoal), contributed to the remaining 25 % of TC. Carbon contents were not related to most of the inorganic soil variables studied, except for oxalate-extractable Al, which individually explained 69 % (P < 0.001) of the variability of TC in the umbric epipedon. Clay content was not suited as predictor of TC or of the other studied C forms. Bulk density, exchangeable Al3+, Al saturation, ECEC and other parameters obtained by selective extraction were not suitable as predictors of TC and other C forms. Interactions between organic matter and poorly crystalline minerals, as indicated by oxalate-extractable Al, appear to be one of the possible organic matter protection mechanisms of these soils

Enzymatic activity and mineralization of carbon and nitrogen in soil cultivated with coffee and green manures Atividade enzimática e mineralização do carbono e nitrogênio sob solo cultivado com adubos verdes na cultura do cafeeiro

There are great concerns about degradation of agricultural soils. It has been suggested that cultivating different plant species intercropped with coffee plants can increase microbial diversity and enhance soil sustainability. The objective of this study was to evaluate enzyme activity (urease, arylsulfatase and phosphatase) and alterations in C and N mineralization rates as related to different legume cover crops planted between rows of coffee plants. Soil samples were collected in a field experiment conducted for 10 years in a sandy soil in the North of Paraná State, Brazil. Samples were collected from the 0-10 cm layer, both from under the tree canopy and in-between rows in the following treatments: control, Leucaena leucocephala, Crotalaria spectabilis, Crotalaria breviflora, Mucuna pruriens, Mucuna deeringiana, Arachis hypogaea and Vigna unguiculata. The soil was sampled in four stages of legume cover crops: pre-planting (September), after planting (November), flowering stage (February) and after plant residue incorporation (April), from 1997 to 1999. The green manure species influenced soil enzyme activity (urease, arylsulfatase and phosphatase) and C and N mineralization rates, both under the tree canopy and in-between rows. Cultivation of Leucaena leucocephala increased acid phosphatase and arilsulfatase activity and C and N mineralization both under the tree canopy and in-between rows. Intercropped L. leucocephala increased urease activity under the tree canopy while C. breviflora increased urease activity in-between rows.<br>Existe grande preocupação sobre a degradação dos solos agrícolas. Tem sido sugerido que o cultivo de plantas intercalares no cafeeiro aumenta a diversidade microbiana e a sustentabilidade do solo. No presente trabalho foi avaliada a alteração na atividade de enzimas do solo (urease, arilsulfatase e fosfatase) e na mineralização do C e N devido ao cultivo intercalar de diferentes leguminosas de verão na cultura do cafeeiro. Foram feitas amostragens em um experimento de campo de longa duração instalado em Latossolo Vermelho distrófico em Miraselva, PR, na profundidade de 0-10 cm, na projeção da copa e na entrelinha, nos seguintes tratamentos: testemunha, leucena (Leucaena leucocephala), Crotalaria spectabilis, Crotalaria breviflora, amendoim-cavalo (Arachis hypogaea tipo virginia), mucuna-cinza (Mucuna pruriens), mucuna-anã (Mucuna deeringiana) e caupi (Vigna unguiculata). As amostragens de solo foram feitas em quatros estádios de desenvolvimento dos adubos verdes: pré-plantio (setembro), pós-plantio (novembro), florescimento (fevereiro) e pós-incorporação (abril), de 1997 a 1999. O cultivo de adubos verdes influenciou a atividade das enzimas do solo (urease, arilsulfatase e fosfatase) e a mineralização do C e N tanto na projeção da copa como na entrelinha. O cultivo da leucena aumentou a atividade da fosfatase ácida e da arilsulfatase e a mineralização de C e N na projeção da copa e na entrelinha do cafeeiro. O cultivo de leucena aumentou a atividade da urease na projeção da copa, enquanto C. breviflora incrementou a atividade da urease na entrelinha