RESUMO

The capacity of soils to sequestrate carbon (C) is mainly related to the formation of organo-mineral complexes. In this study, we investigated the influence of soil management systems on the C retention capacity of soil with an emphasis on the silt and clay fractions of two subtropical soils with different mineralogy and climate. Samples from a Humic Hapludox and a Rhodic Hapludox, clayey soils cultivated for approximately 30 years under no-tillage (NT) and conventional tillage (CT) were collected from six layers distributed within 100-cm soil depth from each site and from an adjacent native forest. After the removal of particulate organic matter (POM), the suspension ( 53 µm) was sonicated, the silt and clay fractions were separated in accordance with Stokes' law and the carbon content of whole soil and physical fractions was determined. In the Humic Hapludox, the clay and silt fractions under NT showed a higher maximum C retention (72 and 52 g kg-1, respectively) in comparison to those under CT (54 and 38 g kg-1, respectively). Moreover, the C concentration increase in both fractions under NT occurred mainly in the topsoil (up to 5 cm). The C retention in physical fractions of Rhodic Hapludox varied from 25 to 32 g kg-1, and no difference was observed whether under an NT or a CT management system. The predominance of goethite and gibbsite in the Humic Hapludox, as well as its exposure to a colder climate, may have contributed to its greater C retention capacity. In addition to the organo-mineral interaction, a mechanism of organic matter self-assemblage, enhanced by longer periods of soil non-disturbance, seems to have contributed to the carbon stabilization in both soils.

Assuntos

RESUMO

The capacity of soils to sequestrate carbon (C) is mainly related to the formation of organo-mineral complexes. In this study, we investigated the influence of soil management systems on the C retention capacity of soil with an emphasis on the silt and clay fractions of two subtropical soils with different mineralogy and climate. Samples from a Humic Hapludox and a Rhodic Hapludox, clayey soils cultivated for approximately 30 years under no-tillage (NT) and conventional tillage (CT) were collected from six layers distributed within 100-cm soil depth from each site and from an adjacent native forest. After the removal of particulate organic matter (POM), the suspension ( 53 µm) was sonicated, the silt and clay fractions were separated in accordance with Stokes' law and the carbon content of whole soil and physical fractions was determined. In the Humic Hapludox, the clay and silt fractions under NT showed a higher maximum C retention (72 and 52 g kg-1, respectively) in comparison to those under CT (54 and 38 g kg-1, respectively). Moreover, the C concentration increase in both fractions under NT occurred mainly in the topsoil (up to 5 cm). The C retention in physical fractions of Rhodic Hapludox varied from 25 to 32 g kg-1, and no difference was observed whether under an NT or a CT management system. The predominance of goethite and gibbsite in the Humic Hapludox, as well as its exposure to a colder climate, may have contributed to its greater C retention capacity. In addition to the organo-mineral interaction, a mechanism of organic matter self-assemblage, enhanced by longer periods of soil non-disturbance, seems to have contributed to the carbon stabilization in both soils.(AU)

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RESUMO

A toxicidade do alumínio é um dos principais fatores limitantes do desenvolvimento das plantas emsolos ácidos. Pelo fato da utilização de corretivos da acidez do solo não ser a estratégia mais viável emmuitas situações com solos ácidos (por razões técnicas e econômicas), o desenvolvimento de genótipostolerantes ao Al tem sido o caminho mais focado, assim a investigação dos mecanismos de tolerânciabem como as bases genéticas da tolerância ao Al têm merecido atenção especial pela pesquisa científica.Nos últimos anos, foi gerado um significativo progresso no entendimento das bases dos mecanismos detolerância ao Al, assim como no desenvolvimento de cultivares mais adaptados as condições de solosácidos. Os mecanismos de tolerância ao Al conhecidos se resumem basicamente em duas classes: os queagem no sentido de expulsar o Al depois de absorvido ou de impedir sua entrada pela raiz e os mecanismosde desintoxicação, complexando o Al em organelas específicas da planta, principalmente nos vacúolos.Em inúmeras espécies, mecanismos fisiológicos tem sido reportados como responsáveis pela ativaçãode ácidos orgânicos (principalmente citrato e malato) que atuam como agentes quelantes do Al, porémmuitos processos ainda não são bem entendidos e esclarecidos. Atualmente, se começa a entendermelhor um segundo mecanismo de tolerância ao Al que envolve a desintoxicação interna do Al atravésda complexação por ácidos orgânicos e o seqüestro destes complexos pelos vacúolos. Outros mecanismospotenciais são alvo de especulações e discussões.

Aluminum toxicity is one of the major limiting factor regarding plant development in acid soils. The use of liming for correcting soil pH is not viable for some of acid soil areas (technique or economic reasons),making the development of Al tolerant genotypes the best alternative. Thus, the tolerance mechanisms as well as the genetic basis of Al tolerance has deserved special attention in the scientific community. In the last years, a significant progress has been achieved towards these goals, as well as in developing cultivars adapted to acid soils. The Al tolerance mechanisms are divided basically in two classes: the exclusion mechanisms that act after absorption or blocking its entry in the root system and those involved in detoxification, complexing the Al in specific organelles, mainly in the vacuoles. In man yspecies, physiological mechanisms have been reported as responsible for the activation of organic acids(mainly citrate and malate) that act as Al quelating agents, however many process are not yet understood and cleared. Currently, the basis for the internal detoxification is becoming clear through organic acid complexes and there sequestering by the vacuoles. Other potential mechanisms are the target for discussions

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