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M. S. Mirjat 1 , D.A. Rose 2 , A.S. Chandio 3 1 Visiting Fellow, Department of Agricultural and Environmental Science, University of Newcastle, UK 2 Professor, Department of Agricultural and Environmental Science, University of Newcastle, UK 3 Assistant Professor, Department of Irrigation and Drainage, Sindh Agriculture University Tandojam, Pakistan Abstract The ever-increasing population of the world requires that agricultural production must be increased to meet the food and fibre needs of the population. Fertilizers along with other chemicals are believed to be integral for sustainable agriculture. As a result of their use, both the quality and quantity of crops have improved substantially in the past few decades. But there is a growing awareness about their impact on surface and groundwater quality in developed as well as in developing countries. The public, in these countries, is concerned over the presence of these chemicals in groundwater bodies, which is becoming a serious threat to human health, wild and aquatic life. Particularly, leaching of nitrate into groundwater has become one of the major pollution concerns facing agriculture in the world today. The current practices and excessive rates of fertilizer application are believed to be contributing significantly to the contamination of groundwater in developing countries. In irrigated agriculture, conventional irrigation methods might be the source of nitrate leaching. Although it is practically almost impossible to reverse the environmental pollution problems caused by fertilizers, their leaching potential can be ameliorated through efficient use. In irrigated areas, the irrigation method, amount of water applied and time of application can play a significant role in reducing the downward movement of these chemicals. The present study reports preliminary results on nitrate nitrogen leaching into soil profiles under conventional flood irrigation in Pakistan. The study was conducted at the research station of Sindh Agriculture University Tandojam to determine the movement of nitrate nitrogen under conventional flood irrigation in the presence of a shallow water table. The results reveal that significant amounts of nitrogen are being leached below the root zone that might become a potential threat to shallow ground water. Concentrations of nitrate nitrogen in excess of 10 mg L-1, a threshold limit for health concerns to infants, were observed at 1.2-m soil depths. The research at this stage is still continuing. However, the trends of nitrate concentrations measured below root zone depths reveal that they will continue to leach down towards shallow groundwater tables and consequently contaminate this source. The population living in the rural areas that use these contaminated ground waters for drinking are likely to experience substantial health risks. Introduction Agriculture is a dominant economic activity that contributes over 36% to the gross domestic product (GDP) of Pakistan. About 70 percent of the population living in rural areas is directly or indirectly engaged in agriculture. Agriculture is mainly practised in the Indus Plain formed by the mighty Indus and its tributaries. The increasing population requires that agricultural production must be increased to meet the food and fibre needs of the population. In order to increase agricultural production, the efficient use of fertilizers along with other chemicals has become integral to achieve this goal. As a result of their use, crop quality and quantity have improved substantially in the past few decades. But some questions on the impact on surface and groundwater still remain unanswered. There is growing public awareness over the long-term threat to both surface and ground waters due to leaching of agro-chemicals, which is becoming a serious threat to human health, wild and aquatic life. Particularly, nitrate leaching into ground water has become one of the major pollution concerns facing agriculture in the world today. Active research is being carried out in the United States and in other parts of the world to identify the best possible alternatives, which will not hamper the crop production system either qualitatively or quantitatively, but will reduce their potential threat to environment. Several studies have been conducted to measure the loss of nitrate nitrogen leaching below the root zone down to subsurface drainage (Baker and Johnson, 1981; Kanwar et al., 1988, 1991, 1996; Wright et al., 1992; Mirjat et al., 1997, 1998). Most of these studies indicate that, on average, an equivalent of 20 to 40 percent of the applied nitrogen fertilizers are being lost below the root zone to subsurface waters. Hallberg et al. (1986) even reported the losses of nitrate nitrogen in excess of 50 percent. The current practices of fertilizer application methods and rates are believed to be contributing significantly in the contamination of groundwater. Although it is practically impossible to reverse the environmental pollution caused by fertilizers, the growth of this problem can be mitigated through efficient soil and water management practices. In irrigated areas, the irrigation method, amount of water applied and time of application can play a significant role in reducing the downward movement of these chemicals. They can minimize the leaching of chemicals to groundwater. There is a need to develop alternative strategies, such as improved water, soil, tillage, and nitrogen management practices that will improve the sustainability of agriculture and protect the environment. In Pakistan, the crops are usually irrigated by conventional flood irrigation. This means that the fields are either partly inundated, in the case of furrow irrigation, or entirely inundated, in the case of border or basin irrigation, which require large amounts of water. This increases the opportunity time for infiltration that allows deep percolation of water, but may also cause leaching of nutrients below root zone depths. It is almost impossible to control the volume of percolating water under conventional flood irrigation; however, some efficient and improved irrigation methods, such as trickle and sprinkler, have the potential to reduce leaching of chemicals. Minimizing leaching losses out of the root zone could decrease the pollution potential of nitrogen fertilizers without decreasing the profitability of crop production. Therefore, it is necessary to study the better management practices that would reduce these losses into shallow ground water by minimizing their movement below root zone depths. The introduction of modern irrigation methods and development of better management practices necessary to prevent contamination of surface and groundwater by nitrates will require knowledge of soil nitrogen dynamics and the development of models that accurately predict nitrate movement in soils on a site-specific basis. Movement of field-applied nitrate into surface and ground water is a complex process and is dependant upon the weather, nitrogen application method, farm management practices, soil properties, and hydrologic factors. A large proportion of irrigated land in Pakistan is affected by water logging due to continuous irrigation in the absence of appropriate drainage. Water tables have risen from 8 m depth to about 2 m in many areas (Tarar, 1995). According to recent reports quoted by Mirjat et al. (1999) nearly 2.4 million hectares have water tables located within 2 m or less of the soil surface during summer season. In such areas, irrigation by conventional flooding is not only decreasing water table depth but may be causing fast leaching of nitrate to shallow ground tables and contaminating them. It is believed that over 70 percent of the population living in such areas use this source for drinking purpose without knowing the hazards of such contaminated water. To the best of our knowledge, very little work has been done to determine the potential for nitrate leaching under conventional flood irrigation. Therefore, this study was aimed to determine the movement of nitrate into the soil profile and towards shallow ground waters under conventional flood irrigation. Materials and Methods The experiments were conducted at the research station of Sindh Agriculture University Tandojam to determine the movement of nitrate nitrogen under conventional flood irrigation. Nine field lysimeters were used to control ground water levels at 0.6, 0.9, and 1.2-m depths. The square-shaped field lysimeters, as shown in photograph, with each side measuring 1.83 m, were constructed during 1999 at the research site. A rectangular pit measuring 9.5 x 6.5 m was excavated to 2-m depth in layers of 30 cm each. Each layer was separated placing a plastic sheet between them so that it could be used to refill the lysimeter after construction with the same soil. A double layered plastic sheet was laid at the bottom of pit and was filled with 20-cm thick mortar of cement and concrete. The pit was divided into three sections and the lysimeters were constructed in the outer sections of the pit while the middle section was used to install water-controlling mechanism, to collect water samples, and to fix measurement equipment. Each lysimeter has been bounded by 15-cm thick masonry wall. To minimize the seepage from the bottom and leakage from the walls, caustic soda was added with the mortar. In addition, the walls were also coated with bitumen. Four pieces of Galvanised Iron (GI) pipe with 1.27-cm diameter were fitted at 60, 90, 120, and 150-cm depths in the inner wall of each lysimeter. The pipes installed at 60, 90 and 120-cm depths were used to control water levels as well as to collect water samples. These pipes were connected with perforated poly vinyl chloride (PVC) pipes extending inside the lysimeters The perforated PVC pipes were wrapped by nylon cloth to restrict silt entry.  The pipes located at 150-cm depth (at the bottom of lysimeter) were connected to perforated PVC pipes at the inner sides and connected to water supply tanks at the outer sides. The tanks were equipped with a float mechanism that controls water level in the lysimeter and regulates the water flow in the tank. A water meter has been fitted in the pipe supplying water to the tank. This supply pipe also extends to the surface of each lysimeter to provide water for surface irrigation. The lysimeters were checked for seepage or leakage before soil filling. They were completely filled with water and left for 72 hours. During this period they were covered with a plastic sheet to minimize the evaporation losses. The lysimeters were filled with soil excavated earlier. At the bottom of each lysimeter, a 5-cm thick layer of well-graded boulders with 10-25 mm diameter was placed around the perforated drainage pipes. Another layer of 5-cm thick gravel with 1-10 mm diameter was placed over the boulders. A 5-cm thick third layer of river sand was placed over the second layer. This combination provided a 15-cm thick filter at the bottom of each lysimeter. Water was infiltrated from the bottom to provide proper settling of the filter material. Once the filter material was placed, the soil filling process was initiated. Each lysimeter was filled with a representative soil layer. After the placement of each layer, water was allowed from the bottom to move upward until it appeared on the top of the soil. Soil was allowed to dry up for one week or when it reached to the plastic limit. Samples were taken for bulk density determination. If the bulk density differed from the original one, the soil was gently compacted until the desired bulk density of the layer was achieved. A 2-cm thick layer of fine graded hill sand was also placed around the network of perforated PVC pipes installed at 60, 90, and 120 cm. All the lysimeters were equipped with piezometers, made up of 25-mm diameter PVC pipes, to collect water samples. The piezometers were installed at 30, 60, 90, and 120-cm depths. Water samples were collected before fertilizer application and continued until harvest. The piezometers were pumped out one day before sampling, the water samples were collected on the following day and stored in a cold chamber at 4o C for later analysis. Soil samples were also collected from the 0-30, 30-60, 60-90, and 90-120-cm depths. They were collected before fertilizer application, after 15 days of fertilizer application and at the time of harvest. Urea nitrogen fertilizer was surface applied in two equal splits at the rate of 200 kg-N ha-1 during every year. The first dose was applied at the time of first irrigation while the second dose applied at the time of fourth irrigation. A single dose of Di-Ammonium Phosphate (125-kg ha-1) was applied during seedbed preparation. The seeds of cotton variety Nayyab-78 were manually sown on 17 May 2000 and on 10 May 2001. The seeds were soaked overnight and were sown in three rows spaced at a distance of 60 cm with a 30-cm plant to plant spacing. This arrangement resulted in a total of 18 plants in each lysimeter. Pesticides were sprayed whenever an attack of pests was witnessed during both study years. Results and Discussion Table 1 gives the average nitrate nitrogen concentration at various depths for three water table depths for both years. The nitrate nitrogen concentrations were different at different water table depths during growing season. The overall data shown in this table indicate that the concentrations at shallower water table depths of 0.6 m were lower than those at 0.9 m and 1.2 m depths for both years. A possible reason for decreased nitrate concentration at shallow water table depth is increased rate of denitrification because of anaerobic conditions that help the denitrification of nitrate nitrogen in the soil profile. Also the average nitrate nitrogen concentrations decreased with increased soil depth for all water table depths. The data show that nitrate concentrations were higher at the time of fertilizer application and generally decreased with increased depth and time during the growing season. It was also observed that flood irrigation expedited movement of nitrates into the soil profile, which resulted in higher concentrations at deeper soil depths at the time of harvest.  Since heavy doses under flood irrigation cause deep percolation that enhances the leaching potential of fertilizers from surface layers to deeper depths, therefore higher concentrations at these depths could be anticipated. The presence of nitrate nitrogen in excess of 10 mg L-1 at deeper depths, in particular, is dangerous because nitrate will keep moving towards shallow ground water tables and will consequently contaminate them. The population living in the rural areas that use these contaminated ground waters for drinking are likely to experience substantial health risks. Data on cotton yield (not shown here) were collected during both study years. The yields were higher at deeper water table depths. The highest yields of 2820 kg ha-1 were observed at 1.2-m water table depth as compared to 2552 kg ha-1 and 1946 kg ha-1 at 0.9 and 0.6 water table depths, respectively during 2000. Almost similar trends were observed during 2001 and the highest yields were obtained at 1.2-m water table depths. The yields obtained during this year were 2678, 2423, and 1898 kg ha-1 at 1.2, 0.9, and 0.6-m depths respectively. Conclusions Results from two years data show that nitrate nitrogen concentrations were lower at shallow water table depths during the growing season. The average nitrate nitrogen concentrations generally decreased with increasing depth and time during the growing season. Results reveal that significant amounts of nitrogen are being leached below the root zone that might become a potential threat to shallow ground water. The average nitrate concentrations were higher than the EPA's water quality standards of 10 mg L-1 under all water table depths. The presence of concentrations in excess of safe drinking limit at deeper soil depths warns that it will continue to leach-down blow root zone into shallow ground water tables and contaminate them, which is hazardous for human health. The population living in the rural areas using these contaminated ground waters for drinking is likely to experience substantial health risk. Acknowledgement This research is a part of the project funded by University Grants Commission Islamabad, Pakistan. References . Baker, J.L., & Johnson, H.P., 1981. Nitrate nitrogen in tile drainage as affected by fertilization. Journal of Environmental Quality, 10:519-522. . Hallberg, G.R., 1986. Overview of agricultural chemicals in groundwater. In: Agricultural Impacts on groundwater, 1-67. National Water Well Association, Warthington, OH. . Kanwar, R.S., Baker, J.L., & Baker, D.G., 1988. Tillage and split N-fertilization effect on subsurface drainage water quality and corn yield. Transactions of the ASAE, 31(5):1423-1429. . Kanwar, R.S., & Baker, J.L., 1991. Long term effects of tillage and reduced chemical application on the quality of subsurface drainage and shallow groundwater. In. Proceedings of Environmental Sound Agriculture, Eds. A.B. Bottcher. . Kanwar, R.S., Melvin, S.W., Kalita, P.K., & Mirjat, M.S., 1996. Water table management effects on NO3-N and Atrazine leaching to ground water. Proceedings of 6th Drainage Workshop on Drainage and the Environment, Ljubljana Slovenia, pp.23-30. . Mirjat, M.S., Kanwar, R.S., & Brohi, A.D., 1997. Plant growth as affected by temporary water logging. II. A Lysimeter Study. Pakistan Journal of Agriculture Agricultural Engineering and Veterinary Sciences, 13(1):6-10. . Mirjat, M. S., Kanwar, R.S., & Melvin, S.W., 1998. Transport of NO3-N into groundwater as affected by water table management. Proceedings of Seventh International Drainage Symposium held at Orlando, Florida, USA, March 8-11. . Mirjat, M.S., Khaskheli, M.U., Chandio, A.S. & Junejo, B., 1999. Design and construction of automatic water level indicator.Pakistan Journal of Agriculture Agricultural Engineering and Veterinary Sciences, 15(1):37-40. . Tarar, 1995. Drainage system in Indus Plain - An over view. Proceedings of national workshop on drainage system performance in Indus Plain and future strategies, 39(II)1-45. . Wright, J.A., Shirmohammadi, A., Magette, W.L., Fouss, J.L., Bengtson, R.L., & Parsons, J.E., 1992. Water table management practice effects on water quality. Transactions of the ASAE, 35(3):823-831. |