Differential responses of soil organic carbon fractions to warming: Results from an analysis with data assimilation

This study was aimed to assess the decomposition temperature sensitivity (Q₁₀) of C fractions cycling from yearly through decades' and up to centennial timescales using a data assimilation approach. A three-pool C-cycling model was optimally fitted with previously-published data from a 588-day long soil incubation experiment conducted at two temperatures (25 and 35°C) for 12 soils collected from six sites arrayed across a mean annual temperature gradient from 2.0 to 25.6°C. Three sets of key parameters of the model, which are initial C pool fractions, decomposition rates and Q₁₀ of individual pools, were estimated with a Markov chain, Monte Carlo technique. Initial C pool fractions were well constrained with pool 1 (the most labile pool), pool 2 (more recalcitrant pool) and pool 3 (the most recalcitrant pool) accounting for 4.7%±2.6% (mean±SD), 22.4%±16.1% and 72.9%±17.6%, respectively, of the total initial C pools. Mean residence time (MRT) was 0.19±0.17, 2.71±2.34 and 80.15±61.14 years for pool 1, pool 2 and pool 3, respectively. Q₁₀ values increased from pool 1 to pool 3 for individual soils or across all the soils. When Q₁₀ values were plotted against MRT after the data were log-transformed, Q₁₀ for the three pools formed three clusters and increased with MRT. Higher Q₁₀ for decades-old C fractions implies that a major portion of soil C may become a source of atmospheric CO₂ under global warming in the 21st century.