What Do Observational Data Products Tell Us about Near-future Changes in Soil Organic Carbon?

Abstract:

Soil is the largest terrestrial carbon (C) reservoir, which can potentially sequester or release large amounts of CO₂ from or to the atmosphere. Evaluating the trajectories in soil organic carbon (SOC) storage is typically carried out using process-based simulation models, which often do not perform well when confronted with observations due to process and parametric uncertainty. In recent years, many global observational data products have been released, and these products can be used to (1) create new benchmarks for typical C cycle model components, such as SOC allocation among pools and their turnover rates, as well as (2) evaluate near-future SOC trajectories. We demonstrate that soil C ages, stocks, and heterotrophic respiration impose constraints on the fractions of fresh C allocated to faster-decomposing SOC pool, revealing that these fractions exceed 96% for most areas around the globe. Combined with the reasonable range of labile SOC proportions (1-20%), these observational products also impose constraints on decay rates of labile and recalcitrant SOC. Lastly, we combined the derived model parameter distributions with soil C input inferred from satellite data products to evaluate the magnitudes of near-future SOC changes, assuming that no substantial change in C input and climatic conditions would occur within a decade. Maximum likelihood changes in SOC over a 10-year period amounted to a 27.2 Pg C loss, however they were highly uncertain, ranging from a 95.6 Pg C loss to a 32.3 Pg C gain. The biggest decadal SOC loss is projected to occur in croplands (4.94 Pg C), and the smallest – in temperate forests (0.34 Pg C). Reducing the uncertainty in near-future SOC changes requires further constraining of the magnitudes and spatial variability of labile SOC fractions and soil C input.