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 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. </jats:p>

Lignin Preservation and Microbial Carbohydrate Metabolism in Permafrost Soils

“…Prater et al. (2020) underlined that with deepening of the active layer in permafrost soils the large and rather undecomposed OM pool becomes accessible to microorganisms, while OM occluded in aggregates or associated with clay‐sized minerals may be less mobilized. However, thawing along with formation of water‐logging and reducing conditions could also unlock carbon associated with metastable Fe phases due to the activity of Fe(III)‐reducing bacteria (Patzner et al., 2020).…”

Soil Particulate and Mineral-Associated Organic Matter Increases in Organic Farming under Cover Cropping and Manure Addition

“…Cover cropping also promotes increased microbial activity [75]. Microbes with higher activity in organic systems secrete extracellular polymetric substances that promote the formation of stable aggregates, resulting in slower cycling of organic matter [14,76]. During 2008-2018, an average of 0.50 Mg C ha −1 y −1 was applied to the soil with composted manure, which is less than the amount applied to the soil by cover crops (0.74 Mg C ha −1 y −1 ).…”

“…The narrower C/N ratios in MAOM indicate an increasing degree of humification of organic [49]. The proportion of microbially derived organic matter with its characteristically low C/N ratio increases after the depolymerization of plant-derived organic macromolecules [76]. At the same time, enrichment with N-rich metabolites occurs in MAOM [49,76], and microbial residues bind to the surface of soil minerals, making them less available [96,97].…”

“…The proportion of microbially derived organic matter with its characteristically low C/N ratio increases after the depolymerization of plant-derived organic macromolecules [76]. At the same time, enrichment with N-rich metabolites occurs in MAOM [49,76], and microbial residues bind to the surface of soil minerals, making them less available [96,97]. As MAOM is mostly of microbial origin [20], the MAOM-C/N variability is lower than that of POM (Table 5).…”

“…after the depolymerization of plant-derived organic macromolecules [76]. At the same time, enrichment with N-rich metabolites occurs in MAOM [49,76], and microbial residues bind to the surface of soil minerals, making them less available [96,97].…”