The rhizosphere of tropical grasses as driver of soil weathering in embryonic Technosols (SE-Brazil)

Abstract

The rhizosphere is a highly biogeochemically active site influencing several soil processes that are crucial for soil formation (e.g., mineral weathering). We studied weathering processes in embryonic Technosols constructed with limestone mine spoils, cultivated with sugarcane for two and six years (illitic siltstone areas: SC2 and SC6) and forage grasses (mainly Urochloa spp.) for 20 years (smectitic black shale area: P20). Rhizosphere and bulk soils were characterized through biological, chemical, and mineralogical analyses. We found contrasting biogeochemical environments when comparing rhizosphere and bulk soils. The pH values were about 1 unit lower, dissolved organic carbon concentrations were up to 3 times higher, and the average enzymatic activity was about 3 times higher in the rhizosphere soils compared to the bulk soils of the studied Technosols. A higher biological activity in the rhizosphere triggered the weathering of primary minerals (dolomite, feldspar, muscovite, quartz), causing the release of K, Ca, Mg, and Si from the mineral structure into the soil solution. The enrichment of the rhizospheric soil solution with Si and bases (mainly Ca and Mg) triggered the transformation of clay minerals and led to the formation of amorphous Si phases. Under sugarcane, smectite content increased from 18% in the siltstone to 44% in the SC6 rhizosphere. Under forage grasses, there was an increase in smectite contents from 58% in the black shale to 75% in both the rhizosphere and bulk soils. The main clay transformation process was a topotactic transformation from illite to smectite through progressive interstratification, supplied by the high availability of Si, Ca, and Mg in the rhizosphere. Our results highlight the pivotal role of rhizospheric processes in mineral weathering of Technosols developed from limestone mine waste. These processes may accelerate pedogenesis, and, consequently, the recovering of the soil functions essential for mine reclamation processes.