Soil organic matter is an important part of the surface organic carbon reservoir in terrestrial ecosystem with dissolved organic matter (DOM) as the most active fraction. Dynamic changes of DOM have a great impact on the global carbon cycle. In addition,DOM has redox, photochemical reactivity and biological activity, which influence the transport, transformation, bioavailability and toxicity of pollutants in the environment.
Due to the complexity of the DOM composition, understanding of its chemical composition and structure is still very limited. In recent years, with the development of high-resolution mass spectrometry, especially the Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), it is possible to characterize the DOM composition at molecule level. Three parts of studies were carried out in the present study, including extraction method of soil DOM, molecular diversity of soil DOM and molecular fractionation behaviors of DOM at soil mineral surfaces.
Different reagents have been used to extract DOM from soils in previous studies,and ultrapure water and NaOH are the most widely used reagents. Besides, chemical reagents such as dithionite, hydroxylamine and pyrophosphate have also been used to selectively extract mineral-bound soil DOM. However, whether the extractions affect the molecular composition of DOM is still unclear. It was found in this study that dithionite, hydroxylamine and pyrophosphate did increase the extraction efficiency of soil DOM based on the total organic carbon. But we also observed that chemical reagents could react with DOM and then induced the changes in the molecular composition of DOM. The reaction of dithionite and DOM led to the increase of sulfur-containing molecules in DOM, while hydroxylamine reacted with the aldehydes in DOM to form oxime compounds, which greatly increased the nitrogen-containing molecules in DOM. Compared with strong reducing reagents, pyrophosphate had less effect on the composition of DOM molecules. Therefore, when selecting chemical reagents to extract soil DOM, particular attention should be paid to the reactions between the reagents and DOM molecules to avoid the generation of artificial "false DOM" during extraction. To avoid such influence, water-extractable organic matter (WEOM) was used in the following researches.
To further reveal the molecular diversity of soil DOM, the molecular composition of 42 soil DOMs were analyzed by FT-ICR MS. A total of 10,039 unique molecules were detected in all the samples, much more than what has been reported for the aquatic DOM, of which CHO molecules accounted for 36.5%,nitrogen-containing molecules accounted for 34.9%, and sulfur-containing molecules accounted for 18.7%. Cluster analysis showed that there were obvious geographical differences in the composition of soil DOMs. The molecular composition of DOM in the red soils was characterized as low in aromaticity, saturation and oxidation, which was significantly different from other soil DOMs. Multivariate statistical analysis showed that soil pH, iron carbon ratio (Fe/C) and aluminum carbon ratio (Al/C) were the key factors affecting the molecular diversity of DOM. With increase of soil pH, condensed polycyclic aromatics, polyphenols, highly unsaturated and phenolic compounds (O/C＞0.5) increased, while highly unsaturated and phenolic compounds (O/C＜0.5) and aliphatic compounds decreased. With increase of Fe/C and Al/C ratios,the highly unsaturated and phenolic compounds (O/C<0.5) and aliphatic compounds increased, while condensed polycyclic aromatics, polyphenols, highly unsaturated and phenolic compounds (O/C>0.5) decreased. These results suggested that selective retention of polycyclic aromatics and polyphenols compounds by iron and aluminum oxides highly determined the organic carbon storage in soils.
The selective adsorption of DOM by soil minerals causes molecular fractionation of DOM at the water-soil interfaces, which further affects molecular diversity of soilDOM; however, the main factors controlling the molecular fractionation behavior of DOM are still unclear. The adsorption and molecular fractionation behavior of peat leachate (PL) onto different soils and soil minerals were then studied. The results showed that there were obvious differences in adsorption kinetics among DOM components. At first, the red soil rapidly adsorbed all the DOM components with the highest proportion of polycyclic aromatics. Then, the adsorption of polycyclic aromatics and polyphenols increased while aliphatic compounds decreased with the increasing time, indicating that aliphatic compounds were gradually replaced by polycyclic aromatics and polyphenols. When reaching adsorption equilibrium, PL showed the pronounced adsorption fractionation onto the red soil. High molecular weight compounds, and compounds high in unsaturation and polarity had higher affinity to the red soil. Low molecular weight compounds, compounds low in unsaturation and polarity were preferentially maintained in solution. Further comparison of the molecular fractionation of DOM derived from adsorption on different soils and minerals indicated that molecular fractionation of DOM was mainly attributable to the high content of iron oxide in the soils and minerals, and the adsorption of DOM onto quartz and clay minerals was not selective. Furthermore, the composition of DOM molecules in supernatants after adsorption onto the red soil was similar to WEOM in red soils, which suggested that molecular fractionation of DOM at water-soil interfaces was probably one of the reasons why the molecular diversity of the red soil DOMs was significantly different from that of other soil DOMs.