Influence of Solenopsis invicta Buren on soil chemical properties, silicon pools, and phytolith assemblages in organic agricultural soils

Ants mediate ecosystem services that can modulate crop performance and overall agroecosystem functioning. Our study investigated how the activity of Solenopsis invicta Buren (Hymenoptera: Formicidae) influences soil chemical properties, soil silicon pools, and soil phytoliths. We hypothesized Solenopsis invicta Buren activity would increase the macronutrient content, available silicon for plants (PASi), and amorphous silicon (ASi) in nest soils, which is related to changes in the phytolith assemblage. This study was conducted on agricultural soil under organic management, covering an area of approximately 70 ha in Rio de Janeiro State, Brazil. Thirty paired soil and nest samples were collected at a depth of 0.20 m and analyzed to obtain values for soil macronutrients and silicon pools (PASi and ASi) and to perform phytolithic analyses. Phytolith extraction from the samples was undertaken using a protocol that involved removed coatings, clay fractions, and iron oxides through density separation, followed by the determination of the phytolith concentration and identification of a minimum of 200 phytoliths per sample using a ZeissAxioskop 40 optical microscope. Results indicated an increase in total organic carbon (TOC), phosphorus (P), and potassium (K) levels and in all forms of Si studied. Ants also affected phytolith assemblages, and there was an increase in morphotypes associated with dicotyledonous plants in the nest soil. We assumed these results were derived from bioturbation promoted by ants, predatory activity, and the interaction of some prey and phytoliths, leading to biologically enhanced weathering. By uncovering and describing this novel role for a widely distributed and highly abundant organism in Neotropical soils, we propose that these alterations, particularly in the biogeochemical cycle of silicon, should be added to the list of ecosystem services provided by ants. This study presents the first evidence of alterations in silicon pools and phytolith assemblages caused by the action of ants.