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.

Canopy soils are unique, diverse, and largely understudied carbon (C) pools whose presence is largely regulated by water and may be impacted by tree-isolation in agricultural pastures. In this study, we investigated how land-use change (LUC) impacts canopy soil C, hydrology, physical properties, and nutrient composition. We used a nested sampling design to compare forest trees to isolated pasture trees from multiple heights within the canopy and distances along branches. LUC did not alter canopy soil hydrology or physical properties but exposed high variation between individual trees and branches within trees. Forest trees contained significantly higher C concentrations (46% ± 2%) compared to pasture trees (42% ± 2%), while pasture trees produced more than twice the cumulative CO2-C (517 ± 75 ug g C-1) compared to forest trees (251 ± 517 ug g C-1) which may be attributed to differences in chemical composition of degradable inputs and/or microbial C use efficiencies between the two land uses. We also quantified the remarkable ability of canopy soil to absorb atmospheric moisture, a characteristic rarely mentioned in current literature. C stocks did not differ between study trees, but on an ecosystem scale, we extrapolated at least a 20 to 22-fold increase in forest tree soil C stocks compared to pasture tree soil C stocks.