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Diaz-Vallejo et al. Carbon Footprints 2026, 5, 12 Page 15 of 23

Consequently, forest soils today represent a mosaic of successional stages, with many secondary forests still
recovering belowground carbon despite aboveground biomass regrowth. In Puerto Rico, that majority of
forest cover is in different stages of recovery from past agricultural or pasture use , reflecting the island’s
[40]
long history of deforestation and reforestation cycles that continue to shape present-day soil-carbon stocks.
Similar patterns of carbon accumulation in secondary tropical forests have been documented across U.S.
Caribbean forests , where soil organic carbon storage depends strongly on forest age, disturbance history,
[53]
and climatic zone. Climatic variability, particularly in temperature and precipitation, further modulates
decomposition and organic matter inputs , while textural and mineralogical differences influence carbon
[54]
stabilization mechanisms [6,29] . These interacting factors explain the wide range of SOC values observed in our
forest sites and underscore the importance of considering both ecological and historical context when
interpreting land-use effects.

Differences in SOC among forests, pastures, and agricultural systems in Puerto Rico reflect not only
contrasting land-use histories but also fundamentally different pathways of organic matter input and
decomposition environments that regulate carbon stabilization. Forest soils are primarily influenced by
surface-derived inputs from leaf litter and woody debris, which break down within organic horizons and
near-surface mineral layers, producing vertically stratified SOC pools that are sensitive to microclimate, litter
quality, and moisture availability [18,55] . Consistent with this mechanism, forest SOC in our study exhibited
high variability and strong dependence on soil suborder and climate, particularly mean annual temperature
and precipitation, reflecting heterogeneous decomposition environments determined by successional stage
and climatic gradients. In contrast, pasture and agricultural systems are dominated by root-derived carbon
inputs and experience varying degrees of soil disturbance, which alter microbial decomposition efficiency
and promote greater incorporation of organic carbon into mineral-associated organic matter (MAOM)
rather than particulate organic matter (POM) [29,56] . This mechanism is consistent with our results showing
that silt + clay content explained a substantial proportion of SOC variability in pastures and agricultural
lands, while climatic variables were comparatively weak predictors, particularly in pastures. In tropical
systems, the balance between these decomposition and stabilization pathways is further modulated by soil
mineralogy and land-use legacies, leading to divergent SOC trajectories following land-use change .
[19]
Collectively, these mechanistic differences help explain why some pasture soils in our dataset exhibited SOC
concentrations comparable to forests, whereas agricultural soils showed consistently lower SOC and reduced
variability, underscoring the need for benchmark frameworks that explicitly account for land-use - specific
decomposition pathways and edaphic controls when interpreting SOC variability in heterogeneous tropical
landscapes. Although we did not directly quantify SOC fractions or microbial processes in this study, these
established mechanisms provide a useful framework for interpreting the land-use - specific patterns observed
in our dataset .
[57]
Large SOC variance in wetland soils can likewise be attributed to the diverse range of soil orders, hydrologic
regimes, and climatic conditions encompassed in our dataset. Wetland pedons included Entisols, Histosols,
Inceptisols, Mollisols, Ultisols, and Vertisols, spanning mean annual temperatures of 25.4-26.5 °C and
precipitation from 867-1,864 mm. Variations in water saturation frequency and soil characteristics shape the
intensity of redox-driven biogeochemical processes . For example, Vertisols in drier areas contained 1%-7%
[58]
SOC, while Histosols and Entisols in saturated environments such as mangrove swamps, marshes, and
floodplains reached 7%-25% SOC. These contrasts indicate that wetlands act as carbon hotspots, but their
hydrologic uniqueness also means they should be considered a distinct benchmark category rather than
directly compared to well-drained soils. Wetland soils in Puerto Rico also have a long history of cultivation
and other agricultural use, which affects their hydrology and SOC dynamics [59-61] .