Beer, and 6 co-authors, reported in September, 2022 that Permafrost thaw will release additional carbon dioxide into the atmosphere resulting in a positive feedback to climate change. However, the mineralization dynamics of organic matter (OM) stored in permafrost-affected soils remain unclear. The authors used physical soil fractionation, radiocarbon measurements, incubation experiments, and a dynamic decomposition model to identify distinct vertical pattern in OM decomposability. The observed differences reflect the type of OM input to the subsoil, either by cryoturbation or otherwise, e.g. by advective water borne transport of dissolved OM.
In non-cryoturbated subsoil horizons, most OM is stabilized at mineral surfaces or by occlusion in aggregates. In contrast, pockets of OM-rich cryoturbated soil contain sufficient free particulate OM for microbial decomposition. After thaw, OM turnover is as fast as in the upper active layer. Since cryoturbated soils store ca. 450 Pg carbon, identifying differences in decomposability according to such translocation processes has large implications for the future
global carbon cycle and climate, and directs further process model development.
The authors concluded that Our findings demonstrate a specific vertical pattern of decomposability of OM in permafrost-affected soils (figure 3). The majority of OM translocated to the lower active layer by an advective transport of DOM with the water phase has long mean residence times,
hence low decomposability due to stabilization by organo-mineral associations. In contrast, the fPOM mixed down in cryoturbated layers will decompose
fast (short mean residence times) after thawing, similar to fPOM of the upper active layer. An explanation might be that cryoturbation transports all physical soil fractions of a topsoil horizon fast enough to the subsoil, such that we can still find a high amount of particulate OM there.
Since ca. 70% of the 1000 Pg carbon stored in permafrost affected soils is located in the upper organic horizons and in cryoturbated layers (Harden et al 2012, Hugelius et al 2014) these links among carbon transport, stabilization and decomposition mechanisms need to be understood for estimating the future carbon cycle and climate. For example, cryoturbation and carbon reservoirs like fPOM need to be considered in land surface models. Furthermore, considering the interaction between these processes enable a reliable initialization of soil carbon stocks in land surface models, which is a pre-condition for projecting the future greenhouse gas release from permafrost-affected soils and related biogeochemical feedback mechanisms. More topsoil and subsoil accounting of fPOM and DOM fractions of soil OM is required as these fractions are most vulnerable to fast mineralization on warming.
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