Movement of nitrate-N and transformations of urea-N under field conditions

Abstract

Summer and winter sodium nitrate treatments at 400 kg N/ha were given to ryegrass pasture on Horotiu sandy loam. The movement and adsorption of nitrate were studied under natural precipitation and ground water flow. Movement of nitrate to depths of 400 mm could be explained by miscible displacement when the soil was near field capacity, and movementwas faster under unsaturated flow conditions. Adsorption measurements showed that the soil had an anion exhange capacityof 0.85 me./100 g in 0.025m CaCl2. These positively charged sites were able to retard nitrate movement below 400 mm. Simulated urine treatments were also made at 2-monthly intervals at rates of 200–400 kg N/ha as urea. Thetransformations in exchangeable nitrogen were studied. Hydrolysis of urea was complete in 2 days in summer and 3 days inwinter. Nitrification was not appreciable until after 7 days in all seasons. At soil temperatures of 7.5–10°c nitrification continued to be slow and was not complete until 60 days, but at temperatures above 15°c it was rapid, once initiated, and was complete in 30 days. Apparent volatilisation losses of ammonia of 20–37% were recorded, rate of application having the main effect. Leaching of nitrate from under sodium nitrate or simulated urine treatments was accompanied by substantial Mg movement. © 1977 Taylor & Francis Group, LLC.