Parâmetros físicos de solos coesos sob cultivo de eucalipto em topossequência na zona da mata nordestina

Abstract

The soils of Coastal Tablelands may present a cohesive character, which limits the effective depth for water dynamics, soil aeration, and root growth. Changes in topography and the shape of the relief can promote variations in the morphological, physical, chemical, mineralogical, and biological attributes of the soil, affecting the development, growth, and productivity of crops, which justifies studies along toposequence. The planting of eucalyptus makes it possible to use slope areas that are not feasible to plant other crops, due to its greater slope; allied to that, there is a great diversity in the market for its exploration. However, in these areas, eucalyptus cultivation has shown productivity problems, as a result of changes in soil properties due to its position in the landscape. In this context, the objective of the research was to evaluate if there are limitations in the production of eucalyptus due to the physical attributes of the soil in a toposequence, characterizing its variability according to the position in the landscape and identifying the one with the worst use conditions. The research was carried out in an experimental area located in the Coastal Tablelands region of Alagoas, with clonal stands of Eucalyptus sp. The plots used in the experiment were: 0 year (T0), when sugar cane is removed and eucalyptus planted; 6 years (T6), which is the harvest age; in addition to an area of native forest of conserved Atlantic Forest, considered as reference toposequence (TR). Thus, in the three selected toposequences (T0, T6, and TR), the landscape positions were defined: top or upper third of the slope (TSE); the middle third of the slope (TME); and lower third of the slope (TIE). In these, two horizons were studied: one with a cohesive character (HC); and one without, located below that, defined as the selected horizon (HS). For each toposequences, three soil profiles were opened in each position in the landscape, totaling nine profiles per toposequence (3 positions  3 trenches), where the samples were collected: structured (soil cores and aggregated) and unstructured. The total of samples collected in all study areas was 432 samples: 16 per profile, 8 per horizon, with HC and HS, resulting in 48 samples per landscape position and, totaling 144 samples per toposequence (48 samples  3 positions). In the respective samples, the following tests were carried out: distribution of particle size fractions; total porosity and its pore size distribution (macro, meso, structural and biological micropores, textural micropores); bulk density; aggregate stability indexes; tensile strength of aggregates; simulation of soil resistance to root penetration; saturated hydraulic conductivity. Soil quality indexes (IQS) were elaborated, defining the role that attributes have in the soil, from an environmental or agricultural point of view. The statistical analysis of the data along the toposequences revealed that the TSE presented the best results, as observed in the area under cultivation with eucalyptus, six years ago (T6) in the HS, and with less than one year (T0) in the HC, being the worst conditions found in the TME of the T6 area of HC and the TIE of the TR area of HS. These results are according to those obtained by the analysis of the attributes by IQS, demonstrating the effectiveness of this method for assessing soil quality under different managements.