Improved Modeling of Soil Biogeochemistry in Permafrost

Abstract:

Modeling frozen biogeochemistry in permafrost soils is a major challenge because using observed Q10 values from incubation studies results in unrealistically high carbon emissions from permafrost. Incubation studies of frozen soil show a rapid decline in respiration as temperature decreases below freezing. Permafrost soils contain 1700 Gt of carbon, most of it frozen in permafrost below the active layer. Models with permafrost carbon in the frozen soil layers show unrealistic losses during spinup with observed Q10 values. Greatly increasing the frozen Q10 eliminates the unrealistic emissions, but suppresses winter respiration below observed values. We used a more physical approach in the Simple Biosphere/Carnegie-Ames-Stanford Approach (SiBCASA) model by separating the simulated soil carbon into three pools: thawed, thin film, and bulk frozen. Carbon transfers between thawed, thin film, and frozen pools are controlled by a curve fit of observed liquid water content in frozen soils as a function of temperature, eliminating the frozen Q10 function entirely. This restricts respiration only to the thawed pools while the frozen and thin film pools remain inactive. SiBCASA reproduces observed fluxes from incubation studies and observed winter fluxes. This new parameterization eliminated unrealistic fluxes of permafrost carbon during spinup and resulted in global total amount of frozen carbon much closer to observed values.