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Purpose Returning of rice straw into paddy field, which not only can increase the organic carbon content of soil but also can introduce dissolved organic matter (DOM) into soil, is a popular farm management strategy. However, chemical and structural heterogeneities of DOM derived from rice straw are not well characterized. The aim of this paper was to study the chemical and structural characteristics of DOM derived from rice straw at the different stages of decay. Materials and methods A total of ten different DOM samples were obtained from aerobic biodegrading rice straw for ten different incubation times. Glass bottles contained a certain amount of rice straw, quartz sand, inoculum, and distilled water were placed on an incubator. At each time of interval, five replicate reactors were taken from the incubator and each was filled with 60 ml distilled water and shaken. After centrifugation, the supernatant was withdrawn from each replicate reactor and filtered through a 0.45-μm membrane filter. The filtrates were analyzed for chemical and structural characteristics of the DOMs using elemental analyzer, Fourier transform infrared (FTIR) spectroscopy, and 1 H, 13 C-nuclear magnetic resonance spectroscopy. Results and discussionThe results showed that the contents of dissolved organic carbon decreased dramatically in the early stage (from day 0 to day 3), then increased nearly linearly till day 63, and finally leveled off at about 22-24 mg g −1 straw (from day 63 to day 180). The FTIR spectroscopy data indicated that the functional groups of the DOM samples were characterized by carboxyl, aliphatic, and aromatic groups. The elemental composition showed that the H/C atomic ratios decreased as a function of decay time, indicating aromaticity of DOM increased with decay time. The 1 H, 13 C-NMR spectra showed that the contents of carbohydrates decreased while the contents of aromatic groups increased as straw decay proceeded, suggesting carbohydrates were consumed faster than aromatic groups by microorganisms. Conclusions Our results indicate that the DOM derived from rice straw are highly heterogeneous in terms of their structural and functional groups, and aromatic groups remain longer than other groups do, suggesting that the aromatic groups represent the stable portion of DOM.

Leather tanneries around the world, including China, introduce chromium (Cr) and other metals into the environment. In China, the population pressure compels the utilization of every piece of available land for food production. In this study, we investigated the content, leachability and possible storage sinks for Cr and other metals in soils around facilities of leather industry in southern China. It was found that Cr in soils impacted by tannery can be as high as 2484 mg Cr kg⁻¹ soil, and the mean contents of other metals such as Zn (214 mg Zn kg⁻¹ soil), Cd (5.4 mg Cd kg⁻¹ soil), As (17 mg As kg⁻¹ soil) exceeded the soil quality standards and guidelines in China and Canada. Simulated leaching studies (i.e., Synthetic Precipitation Leaching Procedure) indicated that these soils could release Cr and other metals in concentrations above the environmental quality guidelines and standards for water in China and Canada. As a result, the mobility of metals from these soils can potentially contaminate both groundwater and surface water. We also found differential leachability of metals with soil properties such as total metal and total carbon contents. Principal component analysis of the total contents of 32 elements showed that the possible major sinks for Cr are organic matter and oxides of Fe/Mn/Al, while sulfates and phosphates are potential storage of Cd, Zn, Cu and Pb. The information obtained from this study can be valuable for the restoration of ecosystem functions (i.e., food production) in the study area.

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