Defining soil carbon benchmark values of agroecosystems across a diversity of tropical soil environments

Abstract

Soil organic carbon (SOC) plays an important role in carbon and nutrient cycling, agricultural productivity, and climate regulation, yet its variability and environmental controls remain poorly constrained in tropical regions characterized by a diversity of soils, climates, and land-use histories. We conducted a regional-scale assessment of SOC across Puerto Rico to evaluate how land use, soil properties, and climate influence SOC variability and to examine the applicability of existing benchmark frameworks in tropical agroecosystems. We compiled SOC data from 586 soil pedons representing nine US Department of Agriculture (USDA) soil orders and multiple land-use categories, with SOC concentrations standardized to the 0-30 cm depth. We evaluated land-use effects and relationships between SOC and environmental variables. SOC concentrations differed by land use, with greater values generally observed in wetlands and forests, intermediate values in pastures or grazing lands, and lower values in agricultural lands. Environmental factors associated with SOC variability differed by land use, with climate and soil suborder 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.

Keywords

Soil organic carbon / land-use change / benchmark modeling / agroecosystems

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