Estimation of the Actual and Attainable Terrestrial Carbon Budget

Abstract

Organic carbon is a key component of the terrestrial system that affects the physical, chemical, and biological processes. Changes in both the soil and the terrestrial carbon storage occur due to the interactions of natural ecological processes and anthropogenic activities. Research gaps to quantify soil and terrestrial carbon still exist. To discern between the actual and attainable carbon pools is critical to identify suitable adaptation and management alternatives to optimize natural carbon capital in the context of regional imposed changes, such as land use and climate change. Our objectives were to: (i) assess the spatially explicit relationships between observed soil organic carbon (SOC) and environmental factors and (ii) assess actual (TerrCactual) and attainable (TerrCattain) terrestrial carbon capital considering below-ground (soil) and above-ground (biomass) carbon. We collected 234 soil samples in the topsoil (0–20 cm) in 2008 and 2009 across the Suwannee River Basin in Florida, USA, based on a random design stratified by land cover/land use and soil suborders. For above-ground carbon assessment, we derived data from the LANDFIRE project which provided a high-resolution map of year-2000 baseline estimates of biomass carbon. A comprehensive set of 172 soil-environmental and human covariates was assembled from multiple data sources to predict and validate observed SOC stocks and TerrCactual using Random Forest (RF). The STEP-AWBH conceptual model (with S: Soil, T: Topography, E: Ecology, P: Parent material, A: Atmosphere, W: Water, B: Biota, and H: Human factors) provided the conceptual modeling framework to model TerrCattain that was implemented using RF and simulated annealing in combination. In the simulation, the STEP factors were kept constant, but the AWBH factors were varied by ±10, ±20, and ±30 %. The combined factors which amount to the highest modeled terrestrial carbon stocks were postulated to equal the attainable terrestrial carbon stocks. Results suggest that the TerrCattain stocks showed slightly larger amounts than the TerrCactual stocks across the basin. The TerrCactual was 190 Tg C and the maximum TerrCattain was 195 Tg C. Biotic, soil, parent material, topographic, and water-related factors played important roles in determining SOC storage, while human factors were only weak predictors. Although mean annual precipitation and monthly mean temperature in summer months were significant to explain both SOC and terrestrial carbon stocks, they showed moderate/minor influence on carbon storage. The land use/land cover variables were the strongest factors predicting soil and terrestrial carbon stocks. These findings suggest that land use adaptions have much potential to achieve TerrCattain, specifically conversions from cropland to land use systems with larger net primary productivity. Bare soils, which represent marginal soils, also have potential to elevate carbon storage through management adaptions that do not compete with other land uses.