Tree species modulate soil carbon recovery in Payments for Ecosystem Services afforestation systems

Costa Rica's Payments for Ecosystem Services (PES) program promotes afforestation to mitigate atmospheric CO2 concentrations through carbon (C) sequestration in aboveground biomass. Although soils store two to three times more C than aboveground biomass, the potential for PES plantations to enhance soil C storage remains poorly quantified. A space-for-time approach was used to evaluate potential soil C storage in tree plantations under PES contracts in the Sarapiquí region of Costa Rica. We measured the quantity, composition, and mineralization of soil organic C (SOC) from PES plantations and adjacent sites representing prior (pastures) and historical (native forests) land uses. PES plantations consisted of one of three native tree species: Hieronyma alchorneoides Allemao (PES-HA), Terminalia amazonia (J.F.Gmel.) Exell (PES-TA) or Vochysia guatemalensis Donn. Sm. (PES-VG). Our results show that SOC recovery was species dependent. Native forests contained higher SOC concentrations than pastures, but SOC levels in PES plantations with Hieronyma and Vochysia were comparable to native forests, whereas Terminalia plantations had lower SOC. Terminalia plantations also had the lowest active C (permanganate oxidizable C, POxC) among species. Pastures had a higher active-to-total C proportion and greater specific respiration (cumulative CO2-C normalized by SOC) than native forests, indicating enhanced microbial turnover. Molecular characterization further showed that SOC in pastures contained compounds with higher nominal oxidation states, suggesting greater thermodynamic favorability for microbial decomposition, particularly in N- and P- containing compounds. In contrast, surface soils in the native forests were enriched in aromatics, consistent with lower specific respiration rates and greater persistence of SOC. SOC metrics in PES plantations were frequently intermediate between pastures and native forests, indicating a gradual transition toward forest-like carbon dynamics. These findings demonstrate that aboveground biomass accumulation alone does not ensure soil carbon restoration. Tree species selection strongly regulates belowground carbon recovery, and long-term climate mitigation benefits of PES programs depend on promoting species that facilitate formation and persistence of soil carbon.