Consideration and Prelimilary Test of Using Vetiver For Water Ultrophication Control in Taihu Lake of China

 

Enviromental Group (Institute of Soil Science, Academia Sinica, Nanjing 210008)

 

1. Background of Taihu Lake

Taihu Lake, one of the largest lakes in China is located at around N31.20' and E120.16' with a total area of 2,420 sq. km, total content for 4,870 million cubic meters, and inputted flow of 195.0 cubic meters/sec., involving Jiangsu Province, Zhejiang Province, and Shanghai City. The Taihu Lake includes numerous smaller lakes, rivers, and channels. The Taihu region is a relatively developed area in the country with high population (986 persons/sq.km). Under subtropical climate, it has a mean temperature of 10 C and mean rainfall of 1,100 - 1,400 mm. With intensive land cultivation farmers harvest their crops 2 to 3 times a year under high chemical fertilizer input (N 345 kg/ha and P 18 kg/ha in average annually).

2. Water Ultrophication in Taihu Lake

Based on the research in 1982 - 1990, the agricultural non-point nitrogen pollution in Taihu Lake was as high as 35,000 t N/yr, accounting for 25.20% of annual total N application, resulting in water ultrophication, while phosphorous content and chemical oxygen consumption (COD) was relatively low. The water ultrophication is usually concentrated in the closed or semi-closed lakes and slow-flowing rivers (flow speed less than 1 m/min.), which account for about one third of the whole Taihu Lake. The ultrophication led to high content of N, P, and C elements in the water and nearby soils and resulted in rapid growth of blue algae, green algae, etc. Consequently, the algae blooms occurred while the water transparency and soluble oxygen content was declined, leading to the death of many water organisms and aquatic life. In many places, a terrible smell was released from the water, influencing people's life.Studies showed that in the period of 1981-1988, the content of NO3-N in the water of Taihu Lake was doubled. The content of NO3-N in the water of wells and rivers during dry seasons was increased by 5 - 10 times. In the main lakes of Taihu Lake, such as Tai lake, Yiangcheng lake, Dshu lake, Jinji lake, Ge lake and Jiu lake, the mean annual inorganic nitrogen content in the water was more than 0.25 mg/L, which was 2.5 times of national parameter of surface water. As a result, the control of water ultrophication of Taihu Lake become a critical environmental issue in the region.

3. Possibility of using Vetiver Grass For Water Ultrophication

ControlResearch indicated that vetiver can survive and grow well in wet land. It can grow much faster in the soil rich in nitrogen. Besides, vetiver has dramatic biomass up to 176,798 kg/ha to 353,596 kg/ha in 6 months (P.E. Lgbokwe, 1991) which can consume nitrogen considerably in the soil along the lake and in the area where water surface fluctuate seasonally. Because the high nitrogen content is concentrated in the water near the banks of rivers and lakes, planting vetiver grass in wet land along rivers and around lakes can relieve ultrophication problem as vetiver contains 0.44-0.68% crude protein and 0.068-0.076% P (R.D.Hill, 1992). The vetiver grass planted on banks and slopes along rivers and around lakes can reduce soil erosion caused by backwash generated by boat traffic. Besides, the grass will be harvested for pulp at a price of 80 Yuan RMB /ton dry matter (about USD10), which can increase farmers' income.

4. Preliminary test

The preliminary test was conducted in Nanjing. Pots were used for vetiver culture from March 1997 up to now (July 1997). Vetiver grass were floated on the water surface using floating plastic plates. The test showed that the grass grew very well on water surface, even better than that growing on earth. The further treatments included: (1) ultrophicated water with grass on it, (2) ultrophicated water without grass, (3) natural water (non-ultrophicated). Two days later after testing, ultrophicated water with grass of treatment (1) became quite clear with a layer of green matter consisting of algae deposited at the bottom of the pot. It seemed that the algae died which needs further identification. The treatment (2) was unchanged. The water was still unclear with dark-green color mixed with algae. The treat (3) remained clear (without algae). The further test will include field trial and laboratory analysis.Vetiver nursery, demonstration, training, and extension are expected to be conducted. Also, the test will include to culture vetiver on the water of fish ponds with ultrophicated water to clear the polluted water and to use vetiver to feed fish. The test will be cooperated by Aquatic Products Institute of Anhui Agriculture Academy.