Developing a Soil Health Index for Mangrove Soils under Contrasting Anthropogenic Influence

Mangroves provide ecosystem services such as carbon sequestration and contaminant immobilization, both linked to soil biogeochemical processes. Anthropogenic pressures associated with urbanization can disrupt these dynamics and compromise soil health. This study aimed to develop a soil health index (SHI) for mangroves subjected to different degrees of anthropogenic influence, assessing differences in soil geochemical functioning and their implications for soil health. Physical, chemical, and biological soil attributes were evaluated in two mangrove forests along São Paulo State coast (Cananéia, a preserved forest, and São Vicente, an urbanized forest. The SHI was constructed using indicators relevant to local geochemical functioning, integrating soil texture, redox potential (Eh), pH, iron (Fe) mineral fractions, soil organic carbon (SOC), and total nitrogen (TN). Significant differences were observed between the forests. Cananéia exhibited more reducing conditions (Eh = 15 ± 14 mV), higher proportions of fine particles, and higher Fe contents (8.130 ± 1.478 mg kg⁻¹), with dominance of Fe oxides and pyritic minerals. Comparatively, São Vicente showed more oxidizing conditions (Eh = 27 ± 26 mV), predominantly sandy textures, lower Fe contents (5381 ± 1061 mg kg⁻¹), higher SOC (10.5 ± 5.8%) and TN (0.4 ± 0.2%) contents, and higher C:N ratios (26.3 ± 6.7). These contrasting biogeochemical functioning, shaped by local conditions, were reflected in SHI values, which were significantly higher in Cananéia (1.00 ± 0.03) than in São Vicente (0.58 ± 0.02). Additionally, the SHI is highlighted as a useful integrative tool for assessing soil health and supporting environmental monitoring in mangrove ecosystems.