Niloufar Nasrollahi, James R Hunt, Caixian Tang, David J Cann
Department of Animal, Plant and Soil Sciences, AgriBio Centre for AgriBiosciences, 5 Ring Rd, La Trobe University, Bundoora 3086, Australia N.Nasrollahi@latrobe.edu.au
Most dryland growers in Australia retain all or most of their crop residues to protect the soil from erosion and improve soil water balance but retaining stubbles with high C:N ratio can decrease nitrogen (N) availability to crops and nitrogen-use efficiency (NUE). A simulation experiment was conducted using different N application rates (0, 25, 50, 75, 100, 150 and 200 kg N/ha) over 27 years at four locations (Mildura, Birchip, Longerenong and Lake Bolac) in Victoria, Australia to capture interactions between sites and seasons, with and without retention of crop residues to investigate how residue retention influences NUE. Immobilization of nitrogen was the biggest source of inefficiency at all simulated sites at rates of N fertilizer application likely to be used by growers. Leaching became a bigger source of inefficiency at Mildura due to low soil water holding capacity, but only at rates of fertilizer application much higher than would be commercially applied. At all simulation sites, immobilization started to decline above annual additions of 50 kg N/ha at Mildura, 75 kg N/ha at Birchip and 200 kg N/ha at Lake Bolac and Longerenong. At all sites, C:N ratio of stubbles decreased with increasing N rates which reduces the immobilization of N. This research shows that NUE could be improved by reducing immobilization, and further research is necessary to evaluate strategies to minimise immobilization of N.