Understanding of the organomineral interactions in soil can elucidate the humic substances (HS) formation and the humification process. This study aimed to structurally characterize the humic acids (HA) formed in tropical soils with different pedogenesis degrees. The HA fraction was extracted from soil samples in the superficial horizons of an Inceptisol and a Mollisol in Rio de Janeiro state. The HA was characterized using spectroscopic techniques following by chemometric analyses: principal component analysis, descriptive analysis, and multivariate curve resolution. The results show that HA extracted from superficial horizon of Inceptisol and Mollisol are structurally different. The Inceptisol HA has a lower E4/E6 ratio than the Mollisol HA. The chemical groups show that most -CH groups belong to -CH2 and -CH3 and hydroxyls (-OH) for the Inceptisol HA, whereas the Mollisol HA predominantly contains aromatic C=C groups. The Inceptisol HA is mainly composed of CAlkyl-H,R and -C=O structures, whereas the Mollisol HA contains a higher quantity of CAlkyl-O,N, CAlkyl-O and CAromatic-H,R groups. The Inceptisol HA has a higher aliphatic content and hydrophobicity than the Mollisol HA. In Inceptisol, organic carbon stabilization can be facilitated by hydrophobic interactions, whereas in Mollisol, different interaction sites can stabilize proteinaceous compounds.

Resumen La determinación del carbono (C) potencialmente mineralizable es realizada mediante la evolución del C-CO2, la cual refleja la actividad de la biota del suelo en la descomposición de la materia orgánica (MOS) y puede ser utilizada como un parámetro para encontrar diferencias en sistemas de producción agroecológicos. El presente trabajo tuvo como objetivo cuantificar el C mineralizable en agregados con diferentes vías de formación (biogénica y fisiogénica) en sistemas de manejo agroecológicos del municipio de Seropédica, estado de Rio de Janeiro, Brasil. Cinco áreas fueron seleccionadas para la colecta de las muestras: (1) sistema agroforestal con 10 años (AFS), (2) café en pleno sol con 15 años (C-SOL), (3) café sombreado con 15 años (C-SOM), (4) calles con Flemingia macrophylla, con 10 años (AL-FLE) e (5) sistema de plantación directa con maíz e berenjena con 6 años (PD), en un Argisol Rojo-Amarillo (Ultisol) en Seropédica-RJ. Los agregados biogénicos y fisiogénicos fueron identificados de acuerdo con su vía de formación y atendiendo a sus patrones morfológicos. En ellos fue determinada la fertilidad, el carbono orgánico total (COT) y el nitrógeno total (NT). La evolución y acumulación de C-CO2 fue determinada en laboratorio después de pasados 35 días de incubación. Los agregados biogénicos y fisiogénicos obtenidos en los diferentes sistemas de manejos agroecológicos mostraron picos de evolución de C-CO2 hasta el vigésimo día de incubación y una estabilización a los 35 días de evaluación. El área de manejo agroecológico PD presentó mayor acumulación de C-CO2 entre todos los sistemas evaluados en los agregados biogénicos y menor acumulación en los agregados fisiogénicos.

Abstract Determination of mineralizable carbon (C) is based on the evolution of C-CO2, reflecting the activity of soil biota in soil organic matter (SOM) decomposition, and can be used as a testing parameter in agroecological production systems. The objective of the present work was to quantify mineralizable C in aggregates of different biogenic and physiogenic formation pathways in agroecological management systems in the city of Seropédica, state of Rio de Janeiro. Five sampling areas were selected: (1) 10-year agroforestry system (AFS), (2) 15-year full sun coffee (C-SUN), (3) 15-year shaded coffee (C-SHA), (4) 10-year Flemingia macrophylla (FLE), and (5) 6-year no-till planting system with corn and eggplant (NT) in Red-Yellow Argisol in Seropédica-RJ. Morphological patterns were used to identify the formation pathway (biogenic and physiogenic) of aggregates. In these, fertility, total organic carbon (TOC), and total nitrogen (TN) were evaluated. The evolution and accumulation of C-CO2 were determined in the laboratory after 35 days of incubation. The biological and physiogenic aggregates of the different agricultural management systems resulted in peaks of C-CO2 evolution up to the 20th day of incubation, with stabilization at 35 days of evaluation. The NT area under agroecological management presented higher C-CO2 accumulation among the systems with biogenic aggregates and less accumulation in the physiogenic aggregates.

Resumo O objetivo deste trabalho foi identificar as vias de formação de agregados biogênicos, intermediários e fisiogênicos, bem como sua estabilidade e seus atributos químicos em diferentes sistemas de manejo agroecológico. Foram escolhidas cinco áreas para coleta de amostras, em Argissolo Vermelho-Amarelo, no Município de Seropédica, RJ: SAF, sistema agroflorestal, com 10 anos de implantação; C-SOL, café a pleno sol, com 15 anos; C-SOM, café sombreado, com 15 anos; AL-FLE, aleias com flemíngia (Flemingia macrophylla), com 10 anos; e PD, sistema plantio direto, com milho e berinjela, com 6 anos. As amostras foram coletadas às profundidades de 0,00-0,05 e 0,05-0,10 m, e a fração dessas amostras retida em peneiras de 9,7 e 8,0 mm foi utilizada para identificar os diferentes tipos de agregado. Após a separação, foram avaliados a estabilidade e os atributos químicos de cada tipo. Os sistemas de manejo não produziram efeito sobre a proporção média de cada tipo de agregado, e a contribuição relativa dos agregados intermediários para a massa total de agregados foi superior à dos demais. Os sistemas C-SOL e C-SOM apresentam os maiores teores de carbono orgânico total, carbono orgânico associado aos minerais, carbono orgânico particulado, independentemente dos sistemas e das profundidades avaliadas.

Abstract The objective of this work was to identify the formation pathways of biogenic, intermediate, and physiogenic aggregates, as well as their stability and chemical characteristics in different agroecological management systems. Five areas were chosen for sampling in Argissolo Vermelho-Amarelo (Ultisol), in the municipality of Seropédica, in the state of Rio de Janeiro, Brazil: SAF, 10-year agroforestry system; C-SOL, 15-year full-sun coffee; C-SOM, 15-year shaded coffee; AL-FLE, 10-year alleys with flemingia (Flemingia macrophylla); and PD, 6-year no-tillage system with corn and eggplant. Soil samples were collected at the 0.00-0.05 and 0.05-0.10-m soil depths, and the fraction retained in sieves of 9.7 and 8.0 mm were used to identify the different types of aggregates. After separation, the stability and chemical characteristics of each aggregate type were evaluated. The proportion between the aggregate types was not affected by the managements systems, and the relative contribution of intermediate aggregates to the total aggregate mass was higher than that of the other ones. The C-SOL and C-SOM systems show the highest total organic carbon, organic carbon associated with minerals, and particulate organic carbon, irrespectively of the evaluated systems and depths.