Linking below- and aboveground indicators under different forest restoration interventions in a mosaic landscape with different land-use legacies

Deforestation and forest degradation continue at alarming rates. Landowners often clear forest patches for alternative use, creating landscape mosaics of different land-use legacies. Ecological restoration is usually monitored by aboveground processes, while belowground processes are far less studied. We addressed how the abundance and activity of nutrient improvement bacteria contribute to restoration success, considering different restoration interventions and land-use legacies. We assessed the abundance and potential activity of N-fixing and P-mineralizing bacteria in the rhizosphere of 279 two-year-old Quercus mexicana plantlets in a disturbed oak forest in central Mexico. We analyzed the differences across landscape patches (eroded landslide, pastureland, and forest) of four common restoration interventions: passive restoration, artificial shading, topsoil translocation, and combining both treatments. Restoration interventions modified soil functioning differently across landscape patches with a significant increase of P-mineralizing bacteria abundance and potential activity in the landslide and pastureland. In contrast, N-fixing bacteria increased only in the landslide site where we registered the lowest organic matter content. Surprisingly, interventions that enhance bacterial activity and abundance do not necessarily improve short-term seedling performance. We recommend considering the landscape heterogeneity to better match the restoration interventions with the prevailing degradation factor in each landscape patch. Also, combining restoration strategies does not necessarily result in synergy and may imply a useless expense of resources. Finally, monitoring soil health provides novel insights to understand restoration trajectories that remain undetected when focusing only on seedling performance.