Moisture effect on soil humus characteristics in a laboratory incubation experiment

Cuilan Li, Shuqing Gao, Jinjing Zhang, Lanpo Zhao and Lichun Wang
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Cuilan Li: National Engineering Laboratory for Improving Quality of Arable Land, Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing, P.R. China;
Shuqing Gao: College of Resource and Environmental Science, Jilin Agricultural University, Changchun, P.R. China
Jinjing Zhang: College of Resource and Environmental Science, Jilin Agricultural University, Changchun, P.R. China
Lanpo Zhao: College of Resource and Environmental Science, Jilin Agricultural University, Changchun, P.R. China
Lichun Wang: Institute of Agricultural Resources and Environments, Jilin Academy of Agricultural Sciences, Changchun, P.R. China

Soil and Water Research, 2016, vol. 11, issue 1, 37-43

Abstract: A 180-day laboratory incubation experiment (30°C) was conducted to investigate the quantitative and qualitative characteristics of humic fractions in a Mollisol at different moisture conditions. The soil moisture contents were 30, 60, and 250% field water-holding capacity (WHC), which represented the low, middle, and high moisture levels, respectively. The results showed that the carbon contents of the total soil and corresponding humic fractions generally decreased with increasing soil moisture. A significant difference was observed between the 250% WHC and the two other moisture levels. By contrast, the carbon content of the water soluble fraction significantly increased with increasing soil moisture levels. The solid-state 13C nuclear magnetic resonance (NMR) spectra showed that the alkyl C/O-alkyl C, aliphatic C/aromatic C, and hydrophobic C/hydrophilic C ratios were in the order of 250% WHC > 30% WHC ≍ 60% WHC, 30% WHC ≍ 60% WHC > 250% WHC and 250% WHC > 60% WHC ≍ 30% WHC for humic acid, and 250% WHC > 30% WHC ≍ 60% WHC, 60% WHC ≍ 250 % WHC > 30% WHC and 30% WHC ≍ 250% WHC > 60% WHC for humin, respectively. These results indicated that a high moisture level was unfavourable for the carbon accumulation of the total soil and humic fractions, whereas it was favourable for the accumulation of water soluble carbon. Although soil moisture levels had a distinct effect on the chemical composition of humic acid and humin, the decomposition degree of the two humic substances components, as indicated by the alkyl C/O-alkyl C ratio, were both higher at a high moisture level than at a low moisture level. Therefore, the lower soil organic carbon content at a high moisture level than at a low moisture level can be ascribed to the higher water soluble carbon content and larger decomposition degree of humic acid and humin in the former. Our results are important for understanding the behaviour and mechanisms of humic substances at specific soil moisture conditions.

Keywords: humic acid; humin; humus composition; soil moisture; solid-state 13C NMR (search for similar items in EconPapers)
Date: 2016
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DOI: 10.17221/21/2015-SWR

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