Mineral-Organic Interactions and Soil Gradients Drive Plant Community Assembly in Brazilian Semiarid Inselbergs

Abstract

This study investigated a soil catena along a colluvial slope of a southeast-facing granitic inselberg to evaluate the influence of soil property variations on vegetation composition across altitudinal and distance gradients from the inselberg. Four soil profiles were described and sampled along a toposequence. Soil analyses included texture, acidity, exchangeable bases, phosphorus, nitrogen, and organic carbon contents. Total porosity, macroporosity, and microporosity were determined using undisturbed soil samples. Tree and shrub individuals were identified within vegetation plots around the soil profiles, and plant diversity and evenness were subsequently assessed. The toposequence comprised an Umbrisol (SP1), a Phaeozem (SP2), and two Regosols (SP3 e SP4), extending across different geomorphic positions (summit, shoulder, backslope and toeslope). Principal Component Analysis (PCA) and Spearman correlation were applied to assess the bivariate and multivariate relationships between soil, geomorphological, and vegetation variables. Results indicate that soil weathering intensity, leaching processes, and organic carbon contents increase with elevation and proximity to the inselberg. The higher organic carbon contents observed at the summit (C = 9.93% in SP1) and on the shoulder (C = 10.89% in SP2) of the colluvial slope are attributed to reduced accessibility of organic substrates to decomposers, coupled with intensified mineral-organic interactions, including sorption and chelation processes. Species from the Myrtaceae, Rubiaceae, Fabaceae, and Sapindaceae families were predominantly associated with soils rich in organic carbon, whereas species belonging to the Cactaceae and Euphorbiaceae families were more abundant in soils with lower organic carbon contents in the toeslope (SP4). Peripheral areas in inselbergs host more developed soils that support denser and structurally complex vegetation, functioning as ecological microrefugia relative to the surrounding Caatinga landscape. Overall, the findings of this study demonstrate that inselbergs can be considered essential environments for the conservation and maintenance of Caatinga biodiversity.