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more strongly associated with SOC in forests, soil texture and suborder in pastures, and a combination of soil order,
texture, temperature, and pH in agricultural systems. We also compared the effectiveness of two SOC benchmarking
approaches: the Soil Health Gap model and a Scores Benchmark based on empirical cumulative distributions applied
to soils of the tropics in our database. The limited availability of primary forests poses a challenge for applying
benchmarks that require undisturbed reference conditions, while the island’s broad climatic range allows for testing
the robustness of SOC benchmarks across a diversity of soil environmental conditions. The Soil Health Gap showed
differences among soil orders and climate, whereas the Scores Benchmark provided a more flexible framework for
contextualizing SOC across heterogeneous tropical landscapes. Our findings support the use of context-specific
SOC benchmarks to evaluate land-use change and soil assessment efforts in tropical regions without assuming
undisturbed reference conditions.

INTRODUCTION
Soil organic carbon (SOC) plays a fundamental role in ecosystem functioning, influencing soil structure,
water retention, nutrient availability, and microbial processes . As one of the largest terrestrial carbon
[1,2]
reservoirs , changes in SOC directly affect atmospheric concentrations of carbon dioxide and methane.
[3]
Understanding how SOC responds to environmental and land-use changes is therefore essential for both
climate change mitigation and agricultural sustainability. Soil organic carbon is widely used as a core soil
health indicator because it influences multiple biological, chemical, and physical soil processes, including
nutrient cycling, microbial activity, and structure, and because it responds predictably to management and
environmental gradients . However, predicting SOC dynamics at regional scales remains challenging due
[4,5]
to the complex interactions among climate, soil properties, and biotic factors [2,6,7] . Ignoring these interactions
can result in large uncertainties in model outcomes . Developing predictive soil health tools, such as
[8]
benchmarks, provides a means to establish reference conditions for SOC across land uses, climates, and soil
types, helping to reduce these uncertainties.

Globally, the conversion of forests to agricultural lands typically reduces SOC, with a few exceptions in
well-managed pastures with large belowground inputs [9-12] . Such land-use changes contribute substantially to
greenhouse gas emissions and smaller SOC stocks through reduced organic inputs and increased soil
disturbance, which accelerates decomposition [13,14] . These effects are particularly pronounced in tropical
regions, where deforestation is estimated to cause the loss of more than half of the originally stored soil
organic carbon in the upper soil layer . Yet, SOC responses to land-use change in the tropics remain highly
[12]
variable , partly because of uneven research coverage across the region’s diverse climates, soils, and
[9]
management systems . To address these biases, large-scale, integrative assessments of SOC across tropical
[11]
environmental gradients are urgently needed.

Recent efforts to define soil health indicators and benchmarks have advanced our ability to assess how land
use affects soil properties and functions [2,15] . Recognizing SOC as a core indicator of soil health underscores
the importance of understanding its variability across tropical climates and soil environments . Such
[4,5]
understanding can guide sustainable land management and policy design aimed at reducing soil carbon
losses and enhancing soil resilience.

Several benchmark frameworks have been developed to contextualize SOC values, including
reference-condition approaches such as the Soil Health Gap model and score-based benchmarks derived
from empirical SOC distributions. In this study, benchmarks are defined as soils used to establish expected
SOC ranges under comparable environmental and pedogenic conditions, derived from empirical regional
data. The Soil Health Gap is defined as the difference between soil health in undisturbed systems and that in
managed croplands within the same agroecosystem . However, the concept of “undisturbed” reference soils
[16]