Functionality of the porous network of Bt horizons of soils with and without cohesive character

Abstract

Knowledge on pore functionality is important for the management of soils in the environment. The present study considered the hypothesis that Bt horizons with cohesive character have a different porous network regarding its functionality because, among other aspects, their pores are preferentially oriented in the horizontal direction. This study aimed to evaluate pore functionality in cohesive and non-cohesive Bt horizons, including the functional evaluation of pores arranged vertically and horizontally in soils of the Coastal Tablelands of the Ceará state, Brazil. In an Argissolo Vermelho-Amarelo Distrófico típico (PVAd) and in an Argissolo Amarelo Eutrocoeso abrúptico (PAex), soil samples were collected in cohesive and non-cohesive Bt horizons (two soil collection arrangements: horizontal and vertical), in seven replicates, considering undisturbed soil structure (28 samples in each soil, a total of 56 samples in both soils). A disturbed sample was collected from each soil horizon for grain size analysis. The following variables were analyzed: granulometry, soil air intrinsic permeability (Kair), pore continuity, soil-water characteristic curve and average pore length. The data of each soil profile were analyzed considering the completely randomized design, with seven replicates. For soil air intrinsic permeability, a 2×2×5 factorial scheme was used (two horizons: cohesive Bt and non-cohesive Bt; two pore arrangements: horizontal and vertical; five matric potentials: −2, −6, −10, −33 and −100kPa); for average pore length, a 2×2 factorial scheme was used (two horizons: cohesive Bt and non-cohesive Bt; two pore arrangements: horizontal and vertical). The data were subjected to the Shapiro-Wilk test to verify normality, to F test for the analysis of variance, and to the Tukey test for the comparison of means, all at 0.05 probability level. The sand fraction prevailed in both soils. Horizons with cohesive character have lower Kair in comparison to the non-cohesive ones. For pore orientation, in general, Kair was superior for those arranged horizontally. Pores in cohesive horizons are less continuous compared with horizons without cohesive character. As to the arrangement, in general, pores are more continuous horizontally. Regarding soil water characteristic curve, the expressive difference between cohesive and non-cohesive horizons is within the matric potential range from saturation up to approximately −8kPa to −10kPa. In the rest of the curve, in cohesive horizons, especially in the situation where pores are vertically arranged, there is more water available to plants at the same matric potential. It was concluded that pores in Bt horizons with cohesive character are less functional than those in Bt horizons without cohesive character, with lower permeability to air, length and connection between them. Regardless of how well the pores are oriented, the highest percentage of blocked pores, i.e., those that do not contribute to gas and heat fluxes, occurs in the horizons with cohesive character. In horizons with cohesive character, horizontal porosity proved to be functionally better than vertical porosity. The fact that the pores in cohesive horizons show lower permeability to the air, shorter length and less connectivity evidences the limitation of these soils for the exploration by plant roots, since they characterize an environment with restriction to gas and heat exchanges - although they can function as a barrier to water drainage, supplying water in greater amount in comparison to a non-cohesive soil.