From fibrous plant residues to mineral-associated organic carbon – the fate of organic matter in Arctic permafrost soils

Abstract

<jats:p>Abstract. Permafrost-affected soils of the Arctic account for 70 % or 727 Pg of the soil organic carbon (C) stored in the 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 (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, only 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 analysed 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 bioaccessible 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 account currently 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 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 bioaccessible 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. </jats:p>

Particulate organic matter as a functional soil component for persistent soil organic carbon

“…The regions were integrated and an alkyl C:O alkyl C ratio (−10-45/45-110 ppm) was computed to describe the degree of aliphaticity of the different fractions 43 . Lastly, the obtained spectra were transformed into OM compound classes via the molecular mixing model 18,44 where δ 13 C resp emission gives the δ 13 C for the current CO 2 emission between the two samplings (‰ V-PDB), δ 13 C control is the average δ 13 C of the control soils at the time of measurement, and δ 13 C litter is the δ 13 C signature of the labeled litter. Finally, the respired C originating from the soil was computed (Eq.…”

Global patterns of geo-ecological controls on the response of soil respiration to warming

“…However, based on our results, we would predict that in colder environments, warming will create over time a more reactive soil matrix, similar to those found in temperate climates. Examples for the expected changes in arctic soils are for example, higher rock-derived nutrient release due to (bio-) chemical weathering, higher potential to stabilize carbon with minerals, thicker soils for higher water retention capacity and larger rooting zones [50][51][52] . It is thus likely that in many of these changed future soils of arctic, antarctic or alpine environments, plant productivity will increase, C stabilization through various mineral related physico-chemical mechanisms 53 will improve and microbial communities will respond to the changed climatic conditions with, for example, higher carbon use efficiency 43 .…”

Soil organic carbon under lockdown: Fresh plant litter as the nucleus for persistent carbon

“…The regions were integrated and an alkyl C/O alkyl C ratio (−10-45/45-110 ppm) was computed to describe the degree of aliphaticity of the different fractions 34 . Lastly, the obtained spectra were transformed into OM compound classes via the molecular mixing model 15,35 with the following chemical shift regions: 0-45, 45-60, 60-95, 95-110, 100-145, 145-165 and 165-215 ppm.…”