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Abstract:The objective of this work was to evaluate the impact of the application of different levels of sugarcane straw on the aggregation and distribution of organic carbon (C) in aggregate classes of a Red Latosol under mechanical harvest in Paranavaí (PR). For this study, in November 2014, after the mechanical harvesting of cane-plant (variety RB-867515), the following amounts of straw were applied to the soil: 0.00, 3.65, 7.50, and 14.85 Mg ha -1 which corresponded respectively to field maintenance of 0, 25, 50 and 100% of the straw. The experimental design was a randomized block design, with 4 replicates. Approximately one year after application of straw levels on the soil, soil samples were collected in the 0-10 cm layer. In these samples the aggregate mean weight diameters (MWD), the aggregate distribution in the classes of diameters 4-2, 2-1, 1-0.5, 0.5-0.25, and 0.25-0.105 and <0.105 mm by dry route, and the C content in these classes with the exception of class <0.105 mm. In the distribution of the aggregates, there was a predominance of the 0.25-0.105 mm group of aggregates, possibly due to the amount of fine sand (290 g kg ) present in this sieve diameter, although within this class there was no significant effect of the levels of straw. The higher straw deposition on the soil (50 and 100%) increased the percentage of aggregates in the class 4-2 mm (25.7 and 32.0%, respectively) in relation to treatments 0 and 25% (13.5 and 22.7%, respectively), presenting significant quadratic behavior. The same effect was observed for the MWD values, with higher values in the treatments with 50 and 100% (1.65 and 1.60 mm, respectively) in relation to treatments with 25 and 0% straw (1.32 and 1.01 mm, respectively). Within each straw level, the C contents in the aggregate classes presented significant differences among them, except for the treatment with 100% straw, where in general the values were higher in the classes with diameter of 4-2 and 2-1 mm (12.5 and 12.4 g kg -1, respectively) and lower in the class 0.25-0.105 mm (6.6 g kg -1). Thus, the main conclusions were that the application of 50 and 100% (7.55 and 14.85 Mg ha -1 year -1) increases the aggregation of the soil, and that the application of 50% of straw on the soil also (4-2 and 2-1 mm), which are associated to higher soil organic carbon contents (14.4 and 13.2 g Kg -1 , respectively), straw level being more appropriate to keep the surface of the soil to maintain soil structure.Keywords: Management of sugarcane straw; aggregation; classes of aggregates; aggregate organic carbon. ResumoO objetivo desse trabalho foi avaliar o impacto da aplicação de diferentes níveis de palhada da cana-de-açúcar sobre a agregação e distribuição de carbono orgânico (C) em classes de agregados de um Latossolo Vermelho, sob colheita mecânica em Paranavaí (PR). Para esse estudo, em novembro de 2014, após a colheita mecanizada de cana-planta (primeira colheita após o plantio), variedade RB-867515, foram aplicados sobre o solo as seguintes quantidades de palha: 0, 3,65, 7,55...

The aim of this study was to evaluate i) the different cover crops contribution used in no-tillage system (NT) to biogenic aggregation; and ii) the influence of aggregate formation pathways on the compartmentalization and the soil organic carbon origin. Two areas managed under NT with different implementation times (6 and 18 years, NT06 and NT18, respectively) and cover crops were evaluated, totaling six sampling areas: NT06, millet (NT06MI); NT06, brachiaria (NT06BR); NT06, sunn hemp (NT06SH); NT18, millet (NT18MI); NT18, brachiaria (NT18BR); NT18, and sunn hemp (NT18SH). In each sampling area, five pseudo-replicates were collected in the 0.00-0.05 and 0.05-0.10 m layers. The samples were air-dried and sieved using sieves with 9.7 and 8.0 mm mesh, and the aggregates retained within this interval were selected. The percentage of each type of aggregate (physicogenic and biogenic) was quantified. Total organic carbon (TOC) and the natural abundance of δ13C (‰) were analyzed and the physical fractionations of SOM were performed: particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) and density fractionation (free light fraction carbon, FLFC). Physicogenic aggregates were quantified in greater proportion, except for the areas of NT06BR and NT18BR in the 0.00-0.05 m layer. The biogenic aggregates showed the highest contents of TOC, POC, MAOC, FLFC and more negative values of δ13C. The use of grasses, especially Brachiaria spp., as cover plants in NT after 6 and 18 years of adoption favors the formation of aggregates through the biogenic pathway and they influence the compartmentalization and origin of stored organic carbon.

The aim of this study was to evaluate the distribution of chemical and physical fractions of soil organic matter (SOM) in anthropic horizons of soil profiles in the west of Pará. The highest total organic carbon values were observed in the superficial horizons, especially in the antrópico LAd and antrópico CHd (45.0g kg -1 ). Were observed predominance of recalcitrant organic material. It appears that most of the C found in the SOM fractions is associated with C stabilization mechanisms such as the recalcitrance provided by the presence of pyrogenic coal, in addition to its ability to present carboxylic groups that increase its interaction with the mineral fraction of the soil, characterizing the mechanism of chemical protection. It is observed that the SOM fractions can function as indicators that contribute to better understanding of the soil carbon dynamics in soils with antrópico horizons.

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