An Estimate of Nitrate-N Loading below the Root Zone and Irrigation Efficiency in Onion Fields Under Drip and Furrow Irrigation Systems.

The solubility of nitrates in water causes rapid movement through the soil and is a major source of groundwater pollution in high-value, shallow rooted vegetable crops. The nitrogen fertilizer application is usually high in these crops. Nitrate-nitrogen leaching has been traditionally determined by lysimeters. But as the lysimeter studies are expensive and the method is not practicable for large areas, there is a need for inexpensive method which can be used to determine NO₃ –N leaching over large areas. The objectives of the study were to estimate irrigation efficiency and the deep percolation of NO₃ –N in shallow rooted onion fields under two irrigation systems using the chloride tracer technique. This study was conducted in two onion fields under drip and furrow irrigation systems in Mesilla Valley of Las Cruces, NM. Monthly soil samples were collected at six depths from two fields. A total of sixteen Time domain reflectometry (TDR) sensors (CS 640), eight in each field were installed at four locations at 20- and 40- cm depth. The monthly soil samples collected up to a depth of 110 cm were analyzed for nitrate-nitrogen and chloride. The evapotranspiration was calculated using crop production functions for the onion, measured irrigation and meterological data. Assuming piston flow, Irrigation efficiency, calculated using actual irrigation, precipitation data, and the concentrations of nitrate and chloride, and an empirical factor for root growth, was 87% for onions under furrow irrigation system and 94% under drip irrigated onions. Nitrogen loading below the root zone was 170 kg ha^-1 for furrow irrigated onion field whereas 135 kg ha^-1 for drip irrigated onion field.