Effect of irrigation management on pasture yield and nitrogen losses

While the benefits of irrigation for agricultural production are well established, the effects of irrigation management on production and environmental consequences are not well quantified. We used the APSIM (Agricultural Production Systems sIMulator) model to simulate the effect of six different irrigation management scenarios on pasture production and nitrogen (N) losses from both urine patches and non-urine affected areas using New Zealand farming systems as a test case. Nitrogen losses included nitrate (NO3) leaching, and gaseous emissions via ammonia (NH3) volatilisation and denitrification (nitrous oxide (N2O) and dinitrogen (N-2) emissions). The effects of different climate and rainfall regimes were simulated for three different soil types - deep, poorly drained (Otokia), deep, well drained (Templeton) and shallow, well drained (Eyre) soils, using 20 years of data from two climate stations with mean annual rainfall amounts of 600 and 800 mm. In the shallow soil with the least plant available water, more frequent irrigation with higher total annual amounts resulted in a significant increase in predicted pasture production compared with less frequent irrigation due to less plant water stress. However, predicted N losses through leaching, denitrification and N2O emissions also increased. In the deep soils pasture growth was not affected by the frequency of irrigation, whereas denitrification increased with higher frequency irrigation, especially in the poorly drained soil, leading to increased N2O emissions. Based on these modelling results, N losses can be reduced with little effect on production by irrigating less frequently and maintaining soil water deficits.