Recovery of below-ground associations in restored Brazilian Atlantic Forest

Soil consists of abiotic and biotic components that sustain biodiversity and forest ecosystem functioning. However, soil restoration has been understudied, and restoration efforts often overlook it. Here, we apply an ecological coupling approach using correlation-based network analysis to assess the joint recovery of key abiotic, biotic, and functional soil components during forest restoration in the Brazilian Atlantic Forest. We analyzed soil microbial composition as the biotic component, soil physico-chemical characteristics as the abiotic component, and microbial biodiversity, carbon stocks, and greenhouse gases as functional components. Two restoration methods were studied: (i) active restoration on former sugarcane fields and (ii) assisted restoration on former pasturelands. We examined chronosequences of early-, intermediate-, and late-stage restored forests, comparing them to three reference forests. We show that active forest restoration on former sugarcane fields initially disrupted soil associations, but this disruption was overcome as the forest further developed. Active restoration increased the coupling between soil components and created a co-occurrence network with strong linkages between abiotic and functional soil components. However, the late-stage restored forest remained different from all three reference forests. Assisted forest restoration on pastures already resulted into coupling during the transition from pasture to forest, while coupling and network structure of late-stage restored forest was not different from the native conserved reference forest. The observed coupling was the result of strong links between biotic and functional soil components. Both methods facilitated soil recovery, but coupling in assisted restoration was not different from the native conserved reference forest, while actively restored soils remained distinct from both native conserved and degraded reference forests. We propose that actively restored forests may need more time to converge with reference forests or that active restoration fosters a novel forest soil state.