Molecular characteristics of vermicompost and their relationship to preservation of inoculated nitrogen-fixing bacteria

https://doi.org/10.1016/j.jaap.2013.05.015Get rights and content

Highlights

  • We monitor changes on molecular composition over the vermicompost process.

  • The increment of hydrophobic compounds occurs with maturation.

  • Microorganisms’ persistence was related with vermicompost chemical composition.

  • Understanding chemical nature makes possible increase vermicompost value and activity.

Abstract

The chemical nature of organic matter during the process of vermicomposting of cattle manure and filter cake from a sugar factory was characterized by thermochemolysis. The pyrolysates were mainly constituted of lignin moieties from propanoic acid units and short-chain (<C20) fatty acids (as methyl esters); alkanes, alkenes, terpenes and steroids were minor compounds. Nitrogen containing compounds were noticeable in filter cake vermicompost (VC) as were carbohydrate moieties. Despite thermochemolysis had shown low sensitivity for carbohydrates, the presence of a number of carbohydrate derivatives was indicative of extensive biological transformation of organic matter during vermicomposting. A high content of long-chain alcohols was found only in filter cake VC. At the end of VC maturation, the content of hydrophobic compounds (lignins plus fatty acids) in filter cake residues was higher than in cattle manure. This mature VC exhibited the highest natural density of culturable diazotrophic bacteria compared to cattle manure VC (approximately 104 times cells g−1 VC), and when the diazotrophic bacterium Herbaspirillum seropedicae was introduced to both types of VC, the population numbers were higher in filter cake VC at 10 months of storage after inoculation. We found an apparent relationship between molecular characteristics of organic matter and the harboring or preservation of diazotrophic bacteria introduced to VC, which is a step toward understanding the relationship between the molecular characteristics of organic matter and the microbial activities.

Introduction

A large number of organic wastes can be ingested by earthworms and excreted as vermicompost (VC), recycling nutrients and reducing environmental constraints [1]. This process accelerates organic residue stabilization, producing a greater proportion of hydrophobic organic matter, i.e., final stabilized organic products enriched with humic acids [2] having high biological activities [3], [4]. VC is the final product of organic wastes processed by earthworms and can be used to alleviate soil contamination by heavy metals, PAHs and herbicides [5], [6], [7] due to its high chemical and biological activities. In another hand, there are a number of studies demonstrating the possibility of VC use as a vehicle for bioinoculants [8], [9]. Microbiological processes such as biological nitrogen fixation, phosphate solubilization, and biostimulation can be improved with progressive knowledge about the relationship between the chemical nature of organic matter and the viability and activity of populations of natural or introduced beneficial microorganisms.

Herbaspirillum seropedicae is an endophytic diazotrophic bacterium that colonizes mainly graminaceous plants such as sugarcane, rice, wheat, sorghum, and maize [10]. Canellas et al. [11] showed that inoculation of maize with H. seropedicae in the presence of humic acids isolated from VC increased the bacterial population associated with the plant root as well maize grain yield. In another study under a technological perspective stressing the importance of organic matter in driving a microbial process, Busato et al. [12] inoculated VC from cattle manure with a mixture bacteria able to solubilize phosphate and fix atmospheric nitrogen, resulting in increased phosphorus content for the agricultural substrate produced. However, in this study, the population of the introduced microorganisms decreased during VC maturation. A more effective plant growth medium using VC enriched with selected microorganisms depends on the maintenance of high levels of the selected population, since VC is a medium rich in microorganisms and microbial diversity.

Detailed knowledge of the changes occurring in organic matter during vermicomposting can be useful for increasing the efficiency of VC use in many different processes, including VC enrichment with selected microorganisms or bioactive substances. Molecular characterization of organic matter can be improved using pyrolysis combined with methylation in the presence of tetramethylammonium hydroxide (TMAH) [13]. This method is efficient for transesterification of esters and methylation of fatty acids and lignin derivatives [14]. In fact, information obtained by preparative off-line pyrolysis (TMAH thermochemolysis) allows the use of large quantities of samples, allowing structural investigation of complex forms of natural organic compounds. Despite the importance of organic matter characterization to VC quality and the recognized efficiency of thermochemolysis in compost characterization [15], [16], such approach using preparative off-line pyrolysis on VCs is still rare.

The aim of this study was to monitor and compare the dynamics of organic matter transformation during vermicomposting of two raw materials (cattle manure and sugarcane filter cake) using thermochemolysis analysis. At the end of VC maturation, we assessed the size of the natural culturable diazotrophic population based on counts in semi-solid malate medium as well as by monitoring the survival of an introduced diazotrophic strain of H. seropedicae during storage using both VCs as inoculant carriers. A putative relationship between the molecular composition of a VC and its ability to preserve a microbial population was observed, opening a new trend for design of inoculant formulations based on the chemical composition of different VCs. This design should allow more effective application of microorganisms promoting plant growth for sustainable agriculture.

Section snippets

VC preparation, sampling, and analysis of total C and N

Two different VCs were prepared using cattle manure and filter cake from a sugarcane factory. Filter cake is the pulp resulting from the grinding of sugarcane. The sugarcane juice is treated with sulfur to clean it and with calcium to promote colloid flocculation. The resulting colorless cleaned juice is evaporated and the broth goes to vacuum filtration. The remaining solid stacked in the filter is called filter cake. Each cattle manure and filter cake were placed on a concrete cylinder (100 cm

Results

During the vermicomposting process, the total organic carbon (TOC) content decreased (Fig. 1A) whereas the total nitrogen increased in the first 30 days and afterward remained stable in cattle manure and increases in filter cake VC. At the end of the incubation time, both VCs showed a higher N content than at the initial time, and this increase was higher in filter cake VC (+128%, Fig. 1B). As a consequence, the C/N ratio decreased, and at the end of the maturation period, the VC could be

Discussion

Piccolo [30] postulated that recalcitrant hydrophobic components derived from plant degradation and microbial activities are able to randomly incorporate more polar molecules and hence protect them against degradation. This postulation is based on the concept that heterogeneous organic matter, such as humic substances, are present as supramolecular self-assembly with relatively loose binding of oligomers via hydrophobic interactions, π–π stacking, and hydrogen bonding [31], [32], [33]. It was

Conclusion

The molecular changes during vermicomposting of cattle manure and sugarcane filter cake include intense transformation of lignin compounds. Alkyl C compounds were relatively well preserved, enhancing the hydrophobic nature of VC, especially from filter cake, which showed an increase in nitrogenated compounds and carbohydrate preservation, probably due to hydrophobic protection. Comparison of the natural diazotrophic bacterial population and monitoring the survival of an introduced strain of H.

Acknowledgements

This work was supported by the following institutions: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ), Coordenadoria de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Instituto Nacional de Ciência e Tecnologia (INCT) para a Fixação Biológica de Nitrogênio, Internacional Foundation of Science (IFS) and OCWP.

References (34)

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