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Page 16 of 23 Diaz-Vallejo et al. Carbon Footprints 2026, 5, 12

Agricultural soils exhibited the lowest SOC concentration and variance as expected. This limited variability
likely results from management homogenization, where residue removal and high biomass export constrain
organic matter accumulation [40,62] . Although the return of crop residues can replenish SOC , our dataset
[62]
lacked detailed information on crop type or management intensity, preventing finer-scale interpretation.
These findings suggest SOC in croplands is primarily constrained by climate and soil, but management
homogenization reduces within-class variability.

In contrast, pasture lands showed substantially greater SOC variability reflecting the interactive influence of
management and environment. The literature documents divergent SOC responses to forest-to-pasture
conversion, with increases [63,64] and decreases [64,65] both reported. In our study, some pastures and agricultural
sites, particularly those on Alfisols, Mollisols, and Aridisols, exhibited SOC levels comparable to or even
exceeding nearby forests. These patterns suggest that long-term pasture stabilization and continuous
vegetation cover can promote SOC retention under favorable soil conditions [54,66,67] . Differences in the
magnitude and direction of SOC between pastures and successional forests have been attributed to soil
properties (including soil mineralogy), pasture age, grazing intensity and historical legacies of cultivation
pre-reforestation . Overall, grasslands and pastures contribute substantial organic inputs through root
[68]
turnover and permanent cover , reinforcing that SOC trajectories following land conversion are not
[69]
unidirectional but depend on soil type, climate, and management history.

Soil carbon environmental controls vary within land use types
The environmental controls on SOC varied among land use types, revealing distinct drivers of SOC
dynamics in soils of the tropics. In forest systems, variability was mainly associated with soil suborder,
texture, and temperature, while pH exerted little influence. Many forest pedons in this study are derived
from karst, a common substrate in the northern part of Puerto Rico. These soils contain free calcium
carbonates, as indicated by strong effervescence reactions both in the field and laboratory [7,70] . These
calcareous soils, characterized by high pH and calcium content, can protect organic matter from
decomposition through several mechanisms, including cation bridging and mineral-organic
complexation [6,32,70,71] . The resulting aggregates enhance physical protection of organic matter . However, in
[72]
our dataset, acidic soils such as Oxisols also contained high SOC, suggesting that carbon stabilization in
forest soils operates through multiple, overlapping pathways rather than a single linear relationship with pH.

Despite representation of a large rainfall gradient in the sites sampled (750-3,200 mm MAP), precipitation
had a significant but modest effect on SOC in forests, explaining about 11% of the variance. SOC accretion
rates during tropical forest succession are typically greater in moist and wet life zones than in dry forests ,
[69]
and temperature can also influence carbon storage during succession . Thus, climatic factors remain
[21]
important regulators of organic matter turnover and vegetation productivity, even when their statistical
influence appears weaker relative to edaphic variables. The relatively low explanatory power of climate
variables compared to soil properties emphasizes that SOC dynamics in forest soils depend on complex
interactions between climate, mineralogy, and past land use, especially in a geologically diverse region .
[7]

In agricultural systems, climate and soil properties jointly influenced SOC. Mean annual precipitation
explained nearly 20% of the variability, and MAT, soil order, pH, and the proportion of silt + clay together
accounted for roughly 60% of SOC variance. These results align with global patterns indicating that SOC in
croplands is primarily governed by environmental constraints and secondarily by management practices .
[9]
Similar relationships have been observed historically in Puerto Rico, where early soil surveys documented
strong associations between organic matter and fine-textured fractions under agricultural use , and
[73]
subsequent studies linked climate and land-use legacies to SOC distribution across the island’s reforested and
cultivated landscapes . The unexplained variability likely reflects differences in cropping systems,
[74]