Aridity drives carbon redistribution among biomass, soil organic and inorganic pools following forestation

Forestation is widely promoted as a nature-based solution for climate change mitigation through carbon sequestration, yet its net carbon outcomes remain uncertain because water limitation may regulate both total carbon storage and carbon allocation among ecosystem pools. Here, we conducted a large-scale paired field investigation across an aridity gradient, using 613 control–forested plot pairs to quantify changes in biomass, soil organic carbon (SOC), soil inorganic carbon (SIC), and soil water content (SWC). We show that aridity reshapes forestation-induced carbon storage by shifting the balance among biomass, SOC, and SIC pools. With increasing aridity, biomass and SOC responses weakened because reduced plant inputs and moisture limitation constrained organic carbon accumulation, whereas SIC increased through enhanced carbonate accumulation driven by reduced leaching and evaporative concentration. These aridity-driven shift from biomass and SOC accumulation toward SIC accumulation explains why forestation did not produce uniform carbon gains, but instead generated a vegetation–soil trade-off and a divergence between SOC and SIC. However, this carbon redistribution was not directly coupled with soil water depletion: although forestation generally reduced SWC, changes in carbon stocks were not significantly related to changes in SWC, indicating a decoupling between carbon and water responses. These findings highlight the importance of incorporating carbon pool interactions, aridity constraints, and carbon–water decoupling into assessments of forestation carbon sequestration potential and support more sustainable forestation strategies under changing environmental conditions.