Nitrogen fertilizer and urine patch interaction-use of APSIM to aid experimental design

Abstract

Previous studies have shown that leaching losses from pasture-based systems occurs predominantly from urine deposited by grazing animals, but also following fertiliser application. However, there is limited understanding of the interaction and fate of urinary N and fertiliser N. The objective of this study was to use APSIM modelling software to generate pre-experimental data to aid with the design of a lysimeter experiment that will endeavour to study the interactions of fertiliser urinary N on N leaching and N₂O emissions. APSIM simulations were run to explore the likely fate of several combinations of urine and fertiliser N under experimental conditions normally imposed on lysimeters, using the weather records for Hamilton from 1972 to 2009. The soil type used was a Horotiu silt loam with a urine-N rate of 800 kg/ha and fertiliser rates of 0 to 500 kg N/ha per year. The modelled data suggested two hypotheses, including that:  NO₃-N leaching from autumn-deposited urine will be greater than from spring-deposited urine at the same urinary fertiliser N application rates.  There will be a greater N fertiliser effect on NO₃-N leaching from under spring-deposited urine than under autumn-deposited urine. Based upon prior knowledge and examination of data from the pre-experimental modelling, we were able to determine that the most appropriate fertiliser rates for the lysimeter experiment should include a control rate of 0 kg N/ha per year; a middle rate of 200 kg N/ha per year and an upper rate of 400 kg N/ha per year. Due to the cost and time limitations of a lysimeter experiment, a one year investigative period has been proposed. Analysis of the modelled data using climate inputs for each year from 1972 to 2009, concluded that the likelihood of this single year design producing anomalous results is low. The modelled data also indicated that leaching from the lysimeters should continue to be monitored for 18 to 20 months in order to capture leachate associated with both the autumn and spring-applied urine. The information from the modelled data has been used in the final design of the experiment. The lysimeter experiment was commenced in February 2011, and will test the above-mentioned hypotheses.