Condutividade hidráulica saturada de solos: limites metodológicos em função da geometria porosa no diagnóstico de resiliência de áreas degradadas

Abstract

Saturated hydraulic conductivity (Ksat) expresses the movement of water in the soil pores, constituting an important tool for the evaluation of water dynamics, nutrients and pollutants in the soil. It is an attribute that varies according to the characteristics of the percolating fluid and its values may be influenced by the porous matrix (texture, bulk density, pore size distribution and aggregate stability). The porous soil geometry, in turn, is sensitive to changes promoted by the use and management of cultivated areas. Such modifications lead to difficulties for the development of fauna and flora, for agricultural production and natural environmental recovery, or for anthropic intervention. As a result, this research aimed to define the methodological limits for the determination of the Ksat of soils, in function of the alterations associated with its porous matrix, in the diagnosis of the resilience of degraded areas. The experimental area consisted of three systems under different managements: cactus forage; resting area (herbaceous cultivation); and under preserved Caatinga. The samples were collected at a depth of 0-20 cm, with: deformed samples, via the dutch type auger; and structured, using a volumetric cores, inserted into the soil with the aid of a Uhland sampler. In depth, samples were also collected in the form of clods. Ksat was determined through the application of different hydraulic loads, applied via constant-head permeameter (PCC) and falling head permeameter (PCD). The tests to obtain the other physical attributes of the soils were carried out by using the following methods: distribution of the particle size fractions of the soil, via modified desimeter; aggregate stability, assessed by wet; soil density, by the volumetric core; soil resistance to root penetration, bench penetrometer; total porosity, via saturation; and pore size distribution, using the tension table. Data processing was performed using statistical analysis software, obtaining Pearson's correlation (p <0.01; p <0.05; p <0.1), descriptive statistics, means tests (p < 0.05) for the investigated properties and, finally, the soil quality index (SQI), defined for the management systems through the analysis of all evaluated attributes. The Ksat results are shown to be significant by the Tukey test (p<0.05) in both methods used, and have positive and negative relationships with the studied attributes, referring to the porous soil matrix. In general, observing each management in isolation, it is observed that, with the application of the different hydraulic loads, it was possible to identify similar values for both methodologies, i.e., the increase in the hydraulic load did not influence the Ksat results, which did not present differences significant to each other, at 5% by the Tukey test. This result suggests that the resilience assessment of the studied areas can be performed by Ksat, and that its results are not influenced by the choice of hydraulic load during the respective tests. On the other hand, the physical attributes that best met the SQI classification were: the aggregate stability index; micro-aggregates; Ksat, when obtained by PCD, in loads of 10 and 40 cca. Thus, the values of the SQI for the Caatinga were lower when compared to the evaluated management systems. Therefore, it is noticeable that the actions of deforestation of the Caatinga in the study area, and its subsequent use for cultivation, promoted changes in the original condition of its soil structure, where a decade of conservation has not yet been sufficient to detect the resilience of the area.