Abstract. Permafrost-affected soils of the Arctic account for 70 %
or 727 Pg of the soil organic carbon (C) stored in the
northern circumpolar permafrost region and therefore play a major role
in the global C cycle. Most studies on the budgeting of C storage and
the quality of soil organic matter (OM; SOM) in the northern circumpolar
region focus on bulk soils. Thus, although there is a plethora of
assumptions regarding differences in terms of C turnover or stability, little knowledge is available on the mechanisms stabilizing
organic C in Arctic soils besides impaired decomposition due to low
temperatures. To gain such knowledge, we investigated soils from
Samoylov Island in the Lena River delta with respect to the
composition and distribution of organic C among differently stabilized
SOM fractions. The soils were fractionated according to density and
particle size to obtain differently stabilized SOM fractions differing
in chemical composition and thus bioavailability. To better understand
the chemical alterations from plant-derived organic particles in these
soils rich in fibrous plant residues to mineral-associated SOM, we
analyzed the elemental, isotopic and chemical composition of
particulate OM (POM) and clay-sized mineral-associated OM (MAOM). We
demonstrate that the SOM fractions that contribute with about
17 kg C m−3 for more than 60 % of the C stock are
highly bioavailable and that most of this labile C can be assumed to
be prone to mineralization under warming conditions. Thus, the amount
of relatively stable, small occluded POM and clay-sized MAOM that
currently accounts with about 10 kg C m−3 for about
40 % of the C stock will most probably be crucial for the quantity
of C protected from mineralization in these Arctic soils in a warmer
future. Using δ15N as a proxy for nitrogen (N) balances
indicated an important role of N inputs by biological N fixation,
while gaseous N losses appeared less important. However, this could
change, as with about 0.4 kg N m−3 one third of the N is
present in bioavailable SOM fractions, which could lead to increases
in mineral N cycling and associated N losses under global warming.
Our results highlight the vulnerability of SOM in Arctic
permafrost-affected soils under rising temperatures, potentially
leading to unparalleled greenhouse gas emissions from these soils.