Epsilon Archive for Student Projects

Abstract

The Mekong Delta is one of two rice baskets in Vietnam. Due to the convenient
environmental conditions for rice cropping, people in this region have cultivated two or three rice crops a year. The intensive rice cultivation may have a negative impact on the soil environment and soil microbial diversity. To start investigations on how to reduce negative impacts on the soil environment and maintain the soil fertility in a long term perspective, this study was made. Rice stubble was collected from two different rice fields, Hoa An (Acid sulphate soil) and Vinh Nguon in the Mekong Delta. The functional traits and molecular diversity of cultivable bacteria and fungi as well as their effects on the next rice crop in terms of germination and radicle length were investigated. The effect of increasing number of species and functional groups was also studied to understand the relationship between microbial diversity and function. Microcosms containing sterile rice straw inoculated with taxonomically and functionally different microorganisms were used for this purpose.

A total of 259 bacterial strains and 45 fungal isolates were identified, with various functional characteristics. Cellulolytic and chitinolytic activity was abundant among bacteria but fluorescent isolates were absent. The rice stubble from the Hoa An site exhibited a higher functional diversity of bacteria compared to that at the Vinh Nguon site mainly because of high abundance of chitinolytic and cellulolytic bacteria while the fungal isolates in stubble from the Vinh Nguon site showed a higher functional diversity than that of the Hoa An site. Bacteria that were either deleterious, neutral or beneficial to rice seed emergence and radicle growth were found. The proportion of deleterious bacteria was almost 3 times higher than that of beneficial ones. In contrast, the majority of the isolated fungi seemed to be beneficial at the inoculum concentration tested. Molecular diversity of 259 bacterial and 45 fungal isolates was assessed by sequencing the 16s rDNA and the ITS rDNA respectively and showed that bacterial strains could be sorted into 17 families and the fungal isolates belonged to 9 families. Taxonomic groups on the family level generally had a high bootstrap support in a
Neighbour-joining analysis. The bacterial communities differed significantly among the two
sites with the highest molecular diversity in the acid soil. Bacillaceace, Burkholderiaceae, Enterobacteraceae and Pseudomonadaceae were the most common families. Functional traits were found in phylogenetically diverse families. Cellulolytic activity was found in the Bacilliaceae, Enterobacteriaceae, Flexibacteraceae, Microbacteriaceae and the Paenibacillaceae. The chitionolytic activity was most prevalent within the members of Flexibacteraceae but present in Burkholderiaceae, Enterobacteriaceae, Oxalobacteriaceae, Staphylococcaeae and Xanthomonadaceae. Fungal diversity did not differ among the two sites but species composition was different. Potential rice pathogens within the Nectriaceae and Trichocomaceae families were also isolated from the straw.

The results from the microcosm study revealed that the straw inoculated with either single
cultures or various mixtures exhibited a dry weight loss up to 32% depending on the
treatment. There was a positive relationship between the number of species inoculated and the weight loss. There was also a positive correlation between the number of functional groups and weight loss as well as between functional dissimilarity and weight loss. Multiple regression analysis showed that in addition to the number of species and functional groups, 5 fungal species and one bacterial species contributed significantly to the decomposition of rice straw. Highest weight loss was achieved in microcosms inoculated with fungi. The decomposition by bacterial inocula was lower than for fungi indicating that the presence of bacteria had an inhibiting effect on the total decomposition during the experimental period. My results using mixtures of fungi and bacteria agree well with previous observations reporting a positive response of the number of species on degradation rates for either fungal or bacterial biodiversity. Functional redundancy was not a general phenomenon in our experiment since many single cultures and species mixtures showed low levels of degradation. The results are consistent with the well accepted view that fungi are the most important contributors to the degradation of recalcitrant plant material in terrestrial environments. In this experiment, interactions between fungi and bacteria seemed to be mostly neutral but with examples of both positive and negative interactions. It is likely that both facilitative interaction and species effects contribute to the positive relationship between
species/ functional biodiversity and rice straw decomposition.