Mungbeans: response to applied nitrogen

Abstract

Mungbeans are becoming an increasingly important component of northern grains region farming systems. Currently, most growers inoculate crops with commercial rhizobia to promote nodulation and nitrogen (N) fixation, however, poor nodulation commonly results in N deficiency and significant yield reductions. To counteract these issues a proportion of the industry apply N to maximise yield. In response, a series of eight trials were implemented to quantify the response of mungbeans under different soil N concentrations and the impact this had on N fixation, from Emerald in Queensland to Narrabri in New South Wales. There was a strong indication that mungbeans will not respond to applied nitrogen fertiliser under a wide range of environmental conditions except for low starting mineral nitrogen and high yielding conditions under irrigation. This lack of response was due to rapid mineralisation and the plants ability to fix atmospheric N if well-nodulated. All treatments utilised mineral N (starting soil N + applied N), including the 0 kg N/ha treatments which utilised from ~50 to 75 kg N/ha except for one site. The percentage of N fixation decreased as the N rate increased at both the Emerald and Hopeland summer sites with the higher N rates not fixing any N at all, supporting the concept that mungbeans will not fix N if planted in a soil with high levels of available N.