Elsevier

Bioresource Technology

Volume 132, March 2013, Pages 320-326
Bioresource Technology

Chemical and spectroscopic characterization of water extractable organic matter during vermicomposting of cattle dung

https://doi.org/10.1016/j.biortech.2013.01.006Get rights and content

Abstract

This work illustrated the characteristics and transformation of water extractable organic matter (WEOM) during vermistabilization (Eisenia fetida) of cattle dung by means of chemical and spectroscopic methods. The independent experiment conducted in triplicate was sampled at the 0, 7, 14, 21, 35, 60 and 90 days. Results showed that the DOC kept steady around 2.7 g kg−1 after day 60 and the DOC/DON ratio decreased from 19.77 to 5.26 till the end of vermicomposting. On the other hand, vermicomposting decreased the aliphatic, proteinaceous, carbohydrates components and increased the aromaticity and oxygen-containing functional groups in the WEOM. Moreover, fluorescence spectra and fluorescence regional integration (FRI) results indicated that protein-like groups were degraded and fulvic and humic acid-like compounds were evolved during the vermicomposting process. In all, this study suggested the suitability of WEOM for monitoring the organics transformation and assessing the maturity in the vermicomposting.

Highlights

► Vermicomposting degraded the protein-like groups while increased the humic acid-like compounds in the WEOM. ► Fluorescence regional integration (FRI) can reveal the transformation and humification process during the vermicomposting. ► WEOM is very useful to monitor the organics transformation and assess the maturity in the vermicomposting.

Introduction

In China, the intensification of cattle breeding has resulted in an increase in the production of biodegradable organic wastes, which need to be efficiently recycled due to the environmental problems associated with their indiscriminate application to agricultural fields (Bernal et al., 2009). Vermicomposting is a popular technique characterized by the combined action of earthworms and microorganisms, thereby accelerating the stabilization of organic matter and greatly modifying its physical and biochemical properties (Aira et al., 2002, Lazcano et al., 2008). This method is easy to operate, odorless, cost effective, pathogen free and environmental friendly (Khwairakpam and Bhargava, 2009, Lazcano et al., 2008, Li et al., 2011). Additionally, the significant higher nitrogen, phosphorus and humic contents in the end products of vermicomposting can help to improve soil fertility and stimulate plant growth (Arancon et al., 2005).

Although earthworms accelerate the vermicomposting process by modifying the substrate and stimulating the microbial metabolic activity (Lazcano et al., 2008), the biochemical degradation of the organic matter is carried out by the microorganisms, either living free and/or dwell in the earthworm gut (Aira et al., 2002, Benitez et al., 1999). Since most of the organic matter is transformed by microorganisms in the water-soluble phase (Caricasole et al., 2010), WEOM stands for the most active fraction of the organic waste. Moreover, WEOM was most subject to change and it could directly reflect the organic matter transformation processes (Said-Pullicino et al., 2007, Caricasole et al., 2010). Consequently, the composition of WEOM has been suggested as a better indicator of stability for the organic matter than that of the solid phase (Caricasole et al., 2010). For example, the evolution of WEOM extracted from aerobic composting has been regarded as a good indicator of the overall transformation and maturity of organic matter (Said-Pullicino et al., 2007). Therefore, studying the changes of WEOM is very helpful to understand the bio-stabilization process.

Previous studies on vermicomposting focused on the stabilization of various solid wastes, the application of vermicompost as a soil organic amendment or fertilizer and the earthworm population dynamics (Garg et al., 2006, Aira et al., 2007, Renuka Gupta, 2008). In addition, the characteristics of WEOM in the initial and final vermicomposts of sewage sludge and cow dung were also reported (Xing et al., 2012). However, the chemical and spectroscopic behaviors of WEOM during vermicomposting process have not been fully understood.

WEOM consists of a heterogeneous mixture of compounds with different molecular sizes and complexities, ranging from simple sugars and organic acids to complex proteins and humic colloids (Traversa et al., 2010). An integration of various techniques is a prevalent way to characterize the features of WEOM. Many indices, including dissolved organic carbon (DOC), specific ultra-violet absorbance (SUVA), and fluorescence excitation–emission matrix (EEM) spectroscopy etc., were used to investigate the properties of WEOM (Shao et al., 2009, He et al., 2011a). DOC can act as a general indicator of WEOM, while SUVA254, E4/E6 and Fourier transform infrared spectra (FT-IR) can provide more detailed information on WEOM, such as the aromaticity and humification degree of derived compounds (Weishaar et al., 2003, Saadi et al., 2006, He et al., 2011a). Furthermore, fluorescence excitation–emission matrix (EEM) spectroscopy combining with fluorescence regional integration (FRI) technique can provide an overall view of fluorescent properties of WEOM in a selected spectral range, which has been employed in structural identification and stability assessment of organic wastes (Zhu et al., 2011).

Keeping in light of the above facts, we hypothesized that the WEOM could show obvious changes during the vermicomposting process and be also very helpful to understand the stabilization process in the vermicomposting. Therefore, the aims of this study were to examine the chemical and spectroscopic characteristics of WEOM at different stages during vermicomposting of cattle dung by using various analytical approaches, as well as to investigate the transformation of WEOM and the vermicompost stability based on the WEOM information obtained.

Section snippets

Vermicomposting process and sampling

The fresh cattle dung (FCD) was obtained from a cattle farm in Pudong district, Shanghai, China. In order to avoid the damage of the high moisture content and anaerobic fermentation to earthworms, the cattle dung was naturally dried under sunlight for 1 week with periodic turning over before used. Eisenia fetida maintained in the laboratory with cattle dung as culturing substrate were randomly picked from several stock cultures. E. fetida was chosen because it had wide tolerance of environmental

DOC

DOC is the most biological and chemical active fraction in organic waste (Caricasole et al., 2010). As the DOC content of the WEOM decreases, the substrates become more stable (He et al., 2011b). The concentrations of DOC extracted from different stages are shown in Table 1. In the initial 14 days, the DOC concentration showed no significant change, after then the DOC decreased rapidly from 4.30 to 2.70 g kg−1 until the day 60, which was possibly attributed to the fast degradation of easily

Conclusions

WEOM extracted from different vermicomposting stages were characterized by chemical and spectroscopic techniques. The DOC concentration kept steady around 2.7 g kg−1 after day 60. Moreover, the aromaticity of WEOM significantly increased and the fraction with molecular weight between 103 and 106 Da became the main part of WEOM. Vermicomposting decreased the aliphatics, alcohols, protein-like materials and polysaccharides, whereas increased the humic-like and fulvic-like materials in the WEOM.

Acknowledgements

The research was funded by the National Natural Science Foundation of China (NSFC, No:51109161), the PhD Programs Foundation of Ministry of Education of China (20110072120029), the Fundamental Research Funds for The Central Universities (0400219187), the Open Analysis Fund for Large Apparatus and Equipments of Tongji University (No. 2012055), the National Spark Program of China (2010GA680004). The author would like to thank Shaobo Liang (University of Idaho) for reading through the manuscript

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