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Impact of irrigation and fertilization on NO 3 leaching and soil-ground water contamination in Lebanon

The farmer’s practice in two different pedoclimatic conditions of Lebanon was evaluated in relation to nitrate contamination. Peasants apply in the soil up to 600-kg N ha using mainly macrosprinklers in Central Bekaa plain while they use mostly drip system in the northern parts of the plain. Increasing nitrate concentration in groundwater was observed in the first while secondary soil salinization was detected in the second area. The objective of the work was to fallow the impact of the current agricultural activities on the soil and ground water contamination. In Central Bekaa, nitrogen and water use efficiency in conventional production (Tc) were compared to fertigation (Tf) in potato field on clay soil using N-tracer technique. Tensiometers inserted at 30 and 60 cm depth were used for irrigation scheduling. Water consumption was assessed using neutron probe. Nitrates movement was monitored using suction ceramic cups inserted at different depths: 30-60-80 and 100 cm. Results for 1997, 1998 and 1999 showed no significant difference in the yield of fresh potato (Spunta), which fluctuated around 30 t ha. For total applied water of 859 mm by sprinklers and 496 mm by drip, consumed water ranged between 343 and 351 mm without significant differences. A comparable N use efficiency (41.2%) was noticed in both treatments. The removed soil solution showed significant nitrate leaching beyond the root zone in Tc which amounted to 54.3 kg N ha. Consequently, the current farmer practice could be an early indicator of soil and groundwater vulnerability to degradation. Similar farmer fertilization practice was extrapolated to northern Bekaa area. But, in the above-mentioned two climatic zones, final results are different concerning the fate of NO3 and its pathway towards the groundwater. While in central Bekaa a rainfall of 600 mm is enough for NO3 leaching towards a relatively high water table, nutrients accumulate mostly on the soil surface in the drier Northern Bekaa (less than 250 mm. of rainfall). This has resulted in the appearance of Solontchaks in the area of Petric Calcisols of Northern Bekaa. Shifting towards drip irrigation and the soil-plant-fertilizer-integrated management resulted in 55% N recovery in the Central part of the plain. However, the introduction of modern irrigation techniques in its Northern part did not show positive steps in combating soil salinization. This is due mainly to the mismanagement of the drip irrigation and poor fertilization policy.