Black soils in the Araripe basin, Northeast Brazil: Organic and inorganic carbon accumulation in a Chernozem-Kastanozem-Phaeozem sequence

Abstract

The Global Soil Partnership with the Food and Agriculture Organization of the United Nations (FAO) defines Chernozem, Kastanozem, and Phaeozems as black soils characterized by high natural fertility and dark surface horizons enriched in organic matter. Chernozems and Kastanozems are also characterized by carbonate accumulation. In the Araripe basin located in northeastern Brazil, black soils formed from limestone are present in the scarps of the Araripe Plateau. Despite their importance, particularly in dry climatic regions, there are no studies characterizing the soils of this region. Thus, our study aimed characterizing the macro and micromorphological, physical, and chemical attributes of three soil profiles (Chernozem, Kastanozem, and Phaeozem, respectively in the upper, middle, and lower third) in a toposequence of the Araripe region. In addition, we identified the main mechanisms of organic and inorganic C accumulation in these profiles. Our results showed that the combination of high mineral-associated organic matter (MAOM) content, high MAOM/soil organic C ratio, and low C/N ratio indicates a high degree of transformation and stabilization the organic matter in these soils. The high 2:1 clays and exchangeable Ca2+ benefits the stabilization of organic C by means of organic–mineral interactions. Additionally, the dominance of carbonates of geogenic origin (limestone particles in the sand fraction) is indicative of the low dissolution of these carbonates. Through micromorphological evaluation, pedogenic carbonates were observed at the top of the B horizon of these soils, where the carbon dioxide (CO2) pressure is lower. The variation in relief demonstrated an ostensible influence on inorganic C constrained to the Phaeozem (lower third). This favored high humidity and root activity at depth, increasing the CO2 pressure and hindering the re-precipitation of carbonates despite the high pH and abundance of exchangeable Ca2+.