Pathways for losses of fertilizer nitrogen from a Rhodes grass pasture in south-eastern Queensland

Abstract

The importance of surface run-off water, leaching and evolution of gases on losses of nitrogen fertilizer from a Rhodes grass pasture in south-eastern Queensland were assessed.

Field microplots encased in steel tubes 21 cm in diameter and 60 cm deep were equipped to collect surface run-off, fertilized with ¹⁵NH₄¹⁵NO₃ prills at the rate of 150 kg nitrogen ha^-1 and destructively sampled at 4, 8, 12, 16 and 40 weeks after fertilizing. The recovery of ¹⁵N in the soil-plant system, losses of ¹⁵N in surface run-off and movements of ¹⁵N down the soil profile were measured. Open pasture plots were fertilized with NH₄NO₃ at rates of 0 and 150 kg nitrogen ha^-1 and harvested at the same times as the microplots. The results were used to calculate the apparent recovery of fertilizer nitrogen by the plant tops. Pasture cores of 11.5 cm diam. and 12.0 cm deep were given the same fertilizer treatment as the microplots, placed in gas-tight growth chambers for periods of 4 weeks starting at 0, 4, 8, 12 and 16 weeks after fertilizing, and used to measure gaseous losses of ¹⁵N. The effects of soil water content ranging from field capacity to waterlogged on these losses were studied on a second series of cores. The apparent recovery of fertilizer nitrogen and the recovery of ¹⁵N in plant tops were usually well below 20%, and the recovery of ¹⁵N in the soil-plant system of the microplots was always below 50% of the amount applied. Most of the loss of ¹⁵N occurred during the first 4 weeks. A large part of the ¹⁵N lost from the field microplots was not traced, but the results demonstrated that surface run-off and leachate should not be ignored during nitrogen balance studies on pastures in south-eastern Queensland. Surface run-off generally removed less than 5% of the ¹⁵N, but the loss was 40% from one microplot. Losses due to leaching were not quantified, but a small significant excess of ¹⁵N in soil layers below 60 cm suggested that they did occur. Gaseous losses of ¹⁵N from waterlogged pasture cores reached 27%, but they were small or absent from cores with a soil water content at or below field capacity. Detailed work in the gas-tight growth chambers to define the soil conditions associated with gaseous losses of nitrogen are needed to relate laboratory findings to field conditions.