Contrasting Energetic Pathways and Successional Trajectories of Soil Food Webs in Natural Succession and Reforestation-Based Restoration.

Forest restoration is increasingly recognised as a crucial strategy for mitigating biodiversity loss and enhancing ecosystem functioning in the face of global change. However, the long-term impacts of different restoration approaches, namely natural succession and reforestation, on the recovery of belowground food web function and energy fluxes remain poorly understood, despite the key role of decomposer food webs in the whole ecosystem. Here, we quantified energy fluxes in soil food webs across microfauna (nematodes) and macrofauna (microarthropods, earthworms) in 30-80-year-old sites under natural succession and reforestation, using a space-for-time substitution approach in southwestern China. We aimed to disentangle how natural succession and reforestation shape belowground energy fluxes and the community maintenance cost associated with sustaining soil animal biomass and biodiversity. We hypothesised that natural succession would be associated with lower maintenance costs than reforestation, while both approaches would follow similar trajectories of energy channelling underpinning belowground ecosystem functioning. Consistent with this hypothesis, natural succession had higher soil animal biodiversity (+50%) and biomass (+112%), while reducing community maintenance costs per unit biomass and biodiversity by 117% and 61%, respectively, compared to reforestation. The two restoration approaches showed divergent energy channelling over time: reforestation increased flux to higher trophic levels (a 5-fold increase in predation-to-basal consumption ratio, approaching natural forest) and shifted from detritivory- (brown) to herbivory-dominated (green) pathways, whereas natural succession enhanced fungal channels and faeces production with little change in predation or green-brown balance. We further found that reforestation increased energy flux primarily through greater litter biomass, whereas in natural succession, the increase was associated with litter quality and abiotic soil conditions. Overall, our findings highlight that natural succession, compared with reforestation, sustains soil communities at lower energetic costs and maintains belowground ecosystem functioning through more balanced and efficient energy fluxes.