© 2010 "Food Security form Sustainable Agriculture" Proceedings of the 15 th ASA Conference, 15-19 November 2010, Lincoln, New Zealand. Web site www.agronomy.org.au 1 ‘NBudget’ – a nitrogen management tool for cropping systems David Herridge 1 , Stephanie Belfield 2 and Loretta Serafin 3 1 Primary Industries Innovation Centre, University of New England, Armidale, NSW. Email: david.herridge@industry.nsw.gov.au 2 HM Ag, Moree, NSW. Email: steph@hmag.com.au 3 Industry and Investment, NSW, Tamworth Agricultural Institute, Calala, NSW. Email: loretta.serafin@industry.nsw.gov.au Abstract Effective management of plant-available nitrogen (N) by farmers will generally have beneficial productivity, economic and environmental consequences. The reality is that farmers may be unsure as to the nitrate levels in their soils at sowing and often make decisions about what crop to grow and how much fertiliser N to apply using little information relevant to nitrogen supply. Current extension information on N, including legume N 2 fixation, is often confusing, conflicting and inaccurate. With fertiliser N priced at about $1.50/kg, and with increasing pressure to reduce nitrous oxide (N 2 O) emissions in the agricultural sector, farmers need to maximise N inputs from N 2 -fixing legumes and to closely match fertiliser-N inputs to crop requirements.‘NBudget’ is an excel-based decision-support (DS) tool that will help farmers/advisers in Australia’s northern grains region (northern NSW and southern Qld) estimate soil nitrate levels pre-sowing, set target yields and determine fertiliser N requirements for winter cereals and oilseeds. A major difference between ‘NBudget’ and other DS tools for N management is that soil testing for either nitrate, organic carbon or water is not required. Rather, ‘NBudget’ contains rule-of-thumb values and algorithms for estimating the net release or immobilisation of nitrate-N in the soil and for estimating N 2 fixation by a legume crop. Input data to generate the algorithms were derived from published and unpublished experiments conducted in the region during the past 30 years. Input data to run ‘NBudget’ are readily available to farmers. Details of the program are presented in this paper. Key Words Nitrogen budgeting, fertiliser N, legume N 2 fixation, WUE, N mineralisation Introduction Farmers in Australia’s northern grains belt produce 20–25% of the nation’s grain – principally wheat, barley, sorghum, chickpea, fababean and sunflower. All of the crops, except for the N 2 -fixing legumes, chickpea and fababean, require nitrogen (N) to be supplied, either through the in situ mineralisation of soil humus and crop residues or from additional fertiliser N inputs. Matching the supply of N to crop demand remains a challenge for farmers as too little supply reduces crop yields and profits, while too much may result in substantial gaseous and/or leaching losses. Decision support (DS) tools to help farmers and their advisers make decisions about fertiliser N inputs have been developed and promoted during the past 15 years, from the relatively simple paper-based ‘Nitrogen in 95/96’, ‘NITROGEN IN 96’ and ‘Nitrogen budgeting for winter cereals’ to the more complex computer-based APSIM, WhopperCropper and Yield Prophet ® (e.g. Lawrence et al. 2000; Carberry et al. 2009). All rely on a budgeting approach in which the supply of plant-available N for a paddock is determined prior to sowing together with the N demand, i.e. amount of N required to grow the crop (Marcellos and Felton 1994). The difference between N supply and demand is the shortfall that is met by fertiliser N inputs. The amount of plant-available soil N is the sum of N mineralised or immobilised from fresh crop residues and animal manures, N left over (spared) from the previous crop and N mineralized from soil humus and partially decomposed (old) residues. Farmers use various techniques to determine plant-available soil N including deep coring for nitrate, calculating on the basis of soil organic carbon levels and back-calculating on the basis of previous yield x protein outputs, i.e. N replacement. However, farmers don’t always estimate N supply with sufficient accuracy. Surveys of winter cropping paddocks in northern NSW during the past 15 years consistently showed large variations in soil nitrate at the end of the summer fallow and prior to sowing a winter crop. Ranges for two surveys were 17–315 kg nitrate-N/ha (survey in 1996 involving 70 paddocks) and 50–270 kg N/ha (2005–07, 21 paddocks) (GD Schwenke, pers. comm.; Elias 2009). Paddocks in the latter survey were intended to be sown to chickpea, a N 2 -fixing legume that ideally should be grown in low nitrate soils (<50 kg nitrate-N/ha) to ensure nodulation and N 2 fixation are not suppressed. In a benchmarking study of 41 northern NSW durum crops during the 2007–08 winter seasons, N supply (measured soil nitrate plus fertiliser N applied after soil coring) at sowing ranged an astonishing 118–750 kg N/ha (L Serafin, unpublished data). Furthermore, grain proteins varied between 10.8% and 18.0% with the