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 it
also depends 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 in 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-CO2 production rates of soil samples in two different
types of laboratory incubation experiments. In one experiment, ca. 240 soil
samples from four study areas were incubated using the same protocol (at 5 ∘C, aerobically) over a period of 1 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 ca. 1000 samples
(at 12 ∘C, aerobically) from an additional three study areas. In
these experiments, C-CO2 production rates were measured over the first
4 d of incubation. We have focused our analyses on the relationship
between C-CO2 production per gram dry weight per day (µgC-CO2 gdw−1 d−1) and C content (%C of dry weight) in the
samples, but we show that relationships are consistent when using C ∕ N ratios or
different production units such as µgC per gram soil C per day (µgC-CO2 gC−1 d−1) or per cm3 of soil per day
(µgC-CO2 cm−3 d−1). C content of the samples is
positively correlated to C-CO2 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 of Late Holocene
eolian deposits to alluvial deposits and mineral soils (including
peaty wetlands) to Pleistocene yedoma deposits to C-enriched pockets in cryoturbated soils to 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 low rates of
decomposition in our experiments need urgent attention if we want to better
constrain the magnitude of the thawing permafrost carbon feedback on global
warming.