Lability classification of soil organic matter in the northern permafrost region

Abstract. The large stocks of soil organic carbon (SOC) in soils and deposits of the northern permafrost region are sensitive to global warming and permafrost thawing. The potential release of this carbon (C) as greenhouse gases to the atmosphere does not only depend on the total quantity of soil organic matter (SOM) affected by warming and thawing, but also on its lability (i.e. the rate at which it will decay). In this study we develop a simple and robust classification scheme of SOM lability for the main types of soils and deposits of the northern permafrost region. The classification is based on widely available soil geochemical parameters and landscape unit classes, which makes it useful for upscaling to the entire northern permafrost region. We have analyzed the relationship between C content and C-CO 2 production rates of soil samples in two different types of laboratory incubation experiment. In one experiment, c. 240 soil samples from four study areas were incubated using the same protocol (at 5 °C, aerobically) over a period of one year. Here we present C release rates measured on day 343 of incubation. These long-term results are compared to those obtained from short-term incubations of c. 1000 samples (at 12 °C, aerobically) from an additional three study areas. In these experiments, C-CO 2 production rates were measured over the first four days of incubation. We have focused our analyses on the relationship between C-CO 2 production per gram dry weight per day (µgC-CO 2  gdw −1  d −1 ) and C content (%C of dry weight) in the samples, but show that relationships are consistent when using C / N ratios or different production units such as µgC per gram soil C per day (µgC-CO 2  gC −1  d −1 ) or per cm of soil per day (µgC-CO 2  cm 3−1  d −1 ). C content of the samples is positively correlated to C-CO 2 production rates but explains less than 50 % of the observed variability when the full datasets are considered. A partitioning of the data into landscape units greatly reduces variance and provides consistent results between incubation experiments. These results indicate that relative SOM lability decreases in the order: Late Holocene eolian deposits > alluvial deposits and mineral upland soils (including peaty wetlands) > Pleistocene Yedoma deposits > C-enriched pockets in cryoturbated soils > peat deposits. Thus, three of the most important SOC storage classes in the northern permafrost region (Yedoma, cryoturbated soils and peatlands) show low relative SOM lability. Previous research has suggested that SOM in these pools is relatively undecomposed and the reasons for the observed resistance to decomposition in our experiments needs urgent attention if we want to better constrain the magnitude of the thawing permafrost carbon feedback on global warming.