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The importance of the composition of soil organic matter (SOM) for carbon (C) cycling is still under debate. Here a single soil source was used to examine the specific influence of its composition on stability ( 14 C activity) of SOM fractions while constraining other influential C turnover factors such as mineral, climate and plant input. The following SOM fractions were isolated from two soil samples: four humic acids, two humins, non-hydrolyzable carbon, and the demineralized fraction. We examined the isotope ratios of SOM fractions in relation to composition (such as aliphatic and aromatic C content) using solid state 13 C nuclear magnetic resonance (NMR) and thermal analysis. The Δ 14 C values of the fractions isolated from both an albic soil (SOMs-A) and a black soil (SOMs-B) correlated negatively with their peak temperature of decomposition and the temperature where half of the total heat of reaction was evolved, implying a potential link between thermal and biogeochemical stability of SOM fractions. Aryl C contents of SOMs-A determined using 13 C NMR varied inversely with δ 15 N values and directly with δ 13 C values, suggesting that part of aryl C of SOMs-A might be fire-derived. The Δ 14 C values of SOMs-A correlated positively with aliphatic C content and negatively with aromatic C content. We therefore concluded that fire-derived aromatic C in SOMs-A appeared to be more stable than microbially-derived aliphatic C. The greater decomposition of SOMs-B fractions weakened the relationship of their Δ 14 C values with alkyl and aryl C contents. Hence, the role of the composition of SOM fractions in regulating stability might be dependent on the source of specific C forms and their stage of decomposition. [ABSTRACT FROM AUTHOR]

The function of biochar (BC) as an eco-friendly adsorbent for environmental remediation is gaining much attention. However, the pristine BC had limited abilities for the removal of As (III, V). Towards this issue, this study synthesized biochar/micro-nanosized α-MnO 2 (BM) composites with different mass ratios of biochar to MnO 2 . Comprehensive characterizations confirmed the successful loading of micro-nanosized α-MnO 2 onto the BC surface and the obvious specific surface area enhancement (7.5-13.5 times) of BM relative to BC. BM composites exhibited 5.0-13.0 folds higher removal capacity for As (III, V) than pristine BC since the composites gave full play to the oxidation contributed by micro-nanosized α-MnO 2 substrate and adsorption functions provided by the Mn-OH, BC-COOH, and BC-OH functional groups. Moreover, BM was well reused maintaining a relatively high removal efficiency for As (III, V). Regardless of reaction time and initial As (III) concentration (C 0 ), the removal of As (III) by pristine BC was negligibly contributed by the oxidized As (V) remaining in solutions, with the relative contribution <15.0%. For the BM composites, relative contribution of adsorbed As (III, V) dominated over that of oxidation to mobile As (V) remaining in solution, and exhibited the decreasing trend with increasing C 0 . These findings demonstrated BM as a promising candidate in remediating As (III, V)-polluted water, and provide mechanistic insights into the role of oxidation and adsorption in As (III, V) removal., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)