| 04 Jan 2026
Thawing Siberian permafrost stabilizes organic carbon from recent plant litter inputs
Abstract. Greenhouse gas release due to microbial decomposition of thawing permafrost organic matter receives ample attention but the other side of the permafrost soil carbon budget, the stabilization of organic matter due to rising plant litter input in a greening Arctic has hardly been addressed. Here we explore whether thawing permafrost may act as a long-term sink of fresh plant litter carbon. To identify the magnitude and drivers of litter carbon stabilization in thawing permafrost, we incubated permafrost samples under oxic and anoxic conditions with 13C-labelled plant litter for nine years, used the microbial CO2 and CH4 production to calibrate a carbon decomposition model, and finally fractionated the remaining organic matter into a dissolved, a mineral-associated and a particulate fraction. At the end of the experiment, on average 38.9 ± 21.1 (mean ± SD, oxic) and 59.1 ± 12.2 % (anoxic) of the added litter-C was still present in the thawed permafrost. The mean residence times of the stable litter carbon pool of 17.6 (22.3) yr (median (IQR), oxic) and 52.4 (54.2) yr (anoxic) indicate a substantial stabilization of fresh litter carbon in thawing permafrost. Most of the remaining litter carbon (82.5 (35.3) % oxic and 83.8 (21.4) % anoxic) was part of the mineral-associated fraction, but in contrast to current understanding, litter decomposability is positively correlated with the size of the mineral bound litter pool. Although the fraction of mineral-bound permafrost carbon (64.4 (20.0) % oxic and 68.0 (17.0) % anoxic) was significantly smaller than of litter carbon, the mean residence times of the stable permafrost carbon pools were more than 10-fold higher. We identified interactions between new litter carbon and pre-existing mineral-bound permafrost carbon as an important driver of litter carbon stabilization. Such interactions may reduce net carbon emissions from thawing permafrost and add complexity to the permafrost carbon climate feedback.
The manuscript fits the scope of the journal and represents clear added value to biogeochemical community. Decade-long incubation experiments with permafrost soils are indeed extremely rare and this study contributes to better understanding of main governing factors and magnitude of occurring processes during plant litter interaction with permafrost soils.
My main criticism is lack of information on soil minerals, the key components controlling C storage. The identity of these minerals should e established via combination of XRD and total chemical analysis, and their nature should be mentioned already in the Abstract (which clays, which oxides – Fe, Al, Mn?, amorphous, allophanes?)….
Furthermore, SEM observations of post-reacted minerals can be useful to identify possible changes on the surfaces of these minerals after incubation
Another major comment is experimental setup (section 2.2): For the incubation conditions, 4 °C over 9 years is not what one expects in soils of the Samoylov Island or other places in Yakutia. Discuss the role of annual freezing cycles on laboratory modeling
L375-380 Again, the identity of clay (or oxide?) minerals, their specific surface area and their capacity to adsorb soil DOM become the crucial issues for understanding related mechanism. See for instance some published experiments in this direction (https://doi.org/10.1016/j.geoderma.2021.115601)
L 409-410 What are the possible mechanisms of such binding mode?
The role of Fe oxides, especially at the variable redox conditions, on OM binding to soil minerals, is not discussed