Assessment of postfire soils degradation dynamics: Stability and molecular composition of humic acids with use of spectroscopy methods

Abstract

The effect of wildfires on the soils of the south taiga and forest‐steppe environments of Central Russia (Histic Spodosols and Eutric Fluvic Arenosols) was investigated in terms of the content and quality of humic acids (HAs) using instrumental spectroscopic methods (solid‐state carbon‐13 nuclear magnetic resonance and electron spin resonance). The bulk elemental composition of HAs was not essentially altered in postfire soils; however, the organic matter of fire‐affected superficial soil layers was characterized by changes in the structural composition and biochemical activity levels. Solid‐state carbon‐13 nuclear magnetic resonance spectroscopy showed that there is an intensive increase in aromatic compounds in HA molecules in soil from both the south taiga and forest‐steppe environments. There is a pronounced and statistically significant decline of aliphatic chain content in response to exposure to fire. The free radicals content and the degree of molecular stabilization assessed with electron spin resonance showed an essential alteration of the HAs, expressed in the increase in the radical's portion, in postfire soils compared with that found in soils not exposed to fire. It was also shown that the accumulation of aromatic compounds indicates only apparent stabilization of HAs due to the loss of periphery alkylic carbon species, which was confirmed by destabilization of the molecules as illustrated by the increase of free radicals.

Stability of soil organic matter in Cryosols of the maritime Antarctic: insights from <sup>13</sup>C NMR and electron spin resonance spectroscopy

“…While both were elevated, the TOC was higher in the superficial levels relative to the lower ones. Previous studies describe high variability in the TOC content from the soils of King George and Galindez islands, mainly depending on the diversity of the ecotopes and the sources of organic matter (Abakumov, 2010;González-Guzmán et al, 2017;Moura et al, 2012;Parnikoza et al, 2016). TOC was previously found higher in ornithogenic soils of rocky platforms compared to non-ornithogenic soils (Moura et al, 2012).…”

“…Also, analyses of cryptogam extracts were conducted towards identification of individual organic precursors (Chapman et al, 1994). This feature was then shown to be typical for soils from different regions of the Antarctic (Abakumov, 2010), including soil formed on the penguin rockeries (Abakumov and Fattakhova, 2014). The northernmost soil of the Arctic polar biome shows the same trend in organic molecule organization: higher prevalence of aliphatic structures over aromatic ones.…”

“…The stocks of SOM in the Antarctic are underestimated compared to the Arctic because of the lack of data for many parts of this continent, due to the high content of stones in the soils and the high variability in the carbon content of the fine earth. Stocks of organic carbon in the Antarctic soil have been reported as 0.5 kg m −2 in its polar deserts, about 1.0 kg m −2 in its barrens, up to 3-5 kg m −2 in the sub-Antarctic tundra, and up to 30 kg m −2 in the penguin rockeries of the maritime islands (Abakumov, 2010;Abakumov and Mukhametova, 2014;Abakumov et al, 2016). To date, investigation on structural composition of the SOM from both superficial and partially isolated areas has only been performed on Cryosols of the Kolyma Lowland (Lupachev et al, 2017), where the organic matter of modern and buried soils vary greatly in terms of their molecular composition and quality.…”

“…The northernmost soil of the Arctic polar biome shows the same trend in organic molecule organization: higher prevalence of aliphatic structures over aromatic ones. The diversity of the individual components in aromatic and aliphatic areas is usually higher in Arctic soil because of the increased diversity of humification precursors (Ejarque and Abakumov, 2016;Abakumov, 2010). A selective preservation of the alkyl moieties in the deeper soil layers has been suggested, and little transformation processes of the SOM are detectable because soil temperatures are not high enough to stimulate further microbial breakdown, even in the summer (Beyer et al, 1997).…”

Ash and fire, char, and biochar in the environment

“…The free radicals content and the degree of molecular stabilization assessed with electron spin resonance showed an increase in the radical's portion in postfire soils. The accumulation of aromatic compounds indicates only apparent stabilization of humic acids due to the loss of peripheral alkylic groups, which was confirmed by destabilization of the molecules as illustrated by the increase of free radicals (Abakumov et al, ).…”