The role of blue-green algae in a flooded rice ecosystem and the effects on them of some herbicides : a case study in Vietnam | |
| Author | Nguyen Thi Loan |
| Call Number | AIT Diss. no.AS-97-03 |
| Subject(s) | Green algae--Vietnam Herbicides----Environmental aspects--Vietnam Rice Vietnam |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor Technical Science, School of Environment, Resources and Development |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. AS-97-03 |
| Abstract | The economic and environmental costs of the heavy use of chemical nitrogen fertilizer and pesticides in agriculture are a global concern. Sustainability considerations mandate that alternatives to artificial N fertilizers must be found. Biological nitrogen fixation (BNF), a microbiological process which converts atmospheric nitrogen into plant-usable form, offers this alternative. Nitrogen-fixing systems offer an economically attractive and ecologically sound mean of reducing external inputs and improving internal resources. Blue-Green Algae (BGA) well known for N-fixing activities were studied by many researchers. Few studies,however exist on the effect of on BGA density as well as the relationship among BGA and other organisms in rice fields. The main objectives of this study were: To investigate the effects of commonly used herbicides (Butavi, 2,4 D, Ronstar, Sofit, Michelle and Whip'S) on BGA density and their nitrogen fixing activities under condition of lowland rice ecosystem in Vietnam as well as in lab conditions. To study the role of BGA in rice field ecosystems. Studies were done with five consecutive rice mom in several provinces‘in North and Central of Vietnam. Laboratory experiments were also conducted during off rice seasons. The algal diversity in the rice field was studied in Vinh Phu Province. Fifty one species belonged to 4 algal phyla were found. Six species belonged to phylum Chlorophyta make up 12% from total algal composition, Twelve species belonged to Cyanophyta phyla (24%), six species belonged to Euglenophyta phyla (12%) and twenty seven species belonged to Bacillaiiophyta phylum (5 2%). The development of BGA over the whole rice season was investigated during two rice crops (in Duy Xuyen Distfict, Quang Nam— Da Nang Province of Central Vietnam and in Me Tri District-Hanoi city to the North of Vietnam). BGA were more abundant in Duy Xuyen District where weather is' hotter with more sunlight. They were most abundant during the period of 10 DAS to 40 DAS (Days afier Sowing) and again increased after 120 DAS. In Me Tri District, where weather was much cooler at that time, the BGA development was different. The BGA density was highest between the period of 30 DAT to 70 DAT (Days after Transplanting) when weather was getting warmer. Both field and laboratory experiments were done to show the effect of herbicides (Ronstar, Sofit, Butavi, 2,4 D) on BGA density. Butavi has most severe effect follow by Ronstar and Sofit on water and soil BGA, 2,4 D has positive effect on water BGA density and negative effect on soil BGA. A quick method for estimating BGA abundance in the rice field and amount ofN-fixing by BGA was proposed. BGA densities on the soil was estimated by color matching into three levels of densities a, b, c (ranging from slight blue-green to darkest blue-green) and counted by grid iron nets. Number of cells and the amount of N-fixing for each density was counted under microscope and measured by Acetylene Reduction Activity (ARA) method. It was found that the BGA categorised density b has highest N—fixing ability. Density 9 has highest density of BGA but the N-fixing ability was not as high as from density b. The ratio between N-fixing capacity and concentration of cells (million of cell per m2) was almost the same in two places (Duy Xuyen district and Me Tri district). This method has to be studied in more detail before it can be used as a practical method of estimation. The relationship between BGA abundance and other organisms in the rice fields was also investigated. BGA density was in negative proportion to 200plankton. BGA in the rice field serve as food for z00plankton. In the beginning ofthe rice crop, BGA was low and as a consequence zooplankton density was also low. BGA density then increased reaching its highest density creating a favourable condition for zooplankton to grow. When zooplankton have consumed almost all BGA they reach their highest numbers and BGA decrease to lowest density. The influence of difference fertilizers application on BGA densities was also investigated both in field and laboratory condition. It was found that lime and molybdenum have almost an immediate and positive influence on BGA densities. Phosphorus did not show the immediate influence on BGA abundance. Its influence was observed to be very slow and had a real effect probably in the season following the one in which it was applied. The role of BGA in food web was investigated by using labelled 14C which was incorporated into BGA body by growing them in medium containing 14C for three weeks. Two species of these filamentous and monocellular BGA were put into model aquaria and followed through the food chains. The results showed that at first fish consumed BGA followed by both zooplankton and benthos. Benthos consumed more BGA than 200plankton did. Filamentous BGA were consumed much more by three kind of consumers than monocellular ones. The indirect effect of herbicide on fish through herbicide-effected BGA was also studied. BGA were first cultivated in media with different kind of herbicides: Butavi, Ronstar, Sofit, 2;4 D, Michelle and Whip'S. BGA in media containing one kind of herbicide was allowed to grow for three weeks and then served as food for fish. The results showed that fish died over two weeks. Before dying they were sick for 3-4 days. Butavi had the strongest effect followed by 2;4D, Whip'S, Ronstar and Sofit in that order. |
| Year | 1997 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. AS-97-03 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Food, Agriculture and Natural Resources (Former title: Department of Food Agriculture, and BioResources (DFAB)) |
| Academic Program/FoS | Agricultural and Aquatic Systems(AS) |
| Chairperson(s) | Rakshit, K.;Johnsen, S. |
| Examination Committee(s) | Tinsley, R.;Duong Duc Tien; |
| Scholarship Donor(s) | Danish International Development Assistance (DANIDA); |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology |