Estimates of the apparent net mineralisation of legume N and comparisons of the subsequent recovery of legume or fertiliser nitrogen by wheat

Mark Peoples1, Tony Swan1, Laura Goward1, James Hunt1,2

1 CSIRO Agriculture & Food, Black Mountain Laboratories, GPO Box 1600 Canberra, ACT 2601, Australia; [email protected]

2 Current address: Department of Animal, Plant and Soil Sciences, La Trobe University, Bundoora, VIC 3086, Australia

Abstract

Results from experimentation undertaken near Junee in southern New South Wales, Australia indicated that concentrations of soil mineral (inorganic) nitrogen (N) measured just prior to sowing wheat in 2012 (0-1.6m) were 42 or 92 kg N/ha greater following lupin grown for either grain or brown manure (BM) than where the preceding crop in 2011 had been wheat or canola. The apparent net mineralisation of lupin organic N over the 2011/12 summer fallow was calculated to be equivalent to 0.11-0.18 kg N/ha per mm rainfall and 7-11 kg mineral N per tonne lupin shoot residue dry matter (DM), representing 22-32% of the total residue N estimated to be remaining at the end of the 2011 growing season. The higher concentrations of soil mineral N after the 2011 lupin treatments resulted in 55-80 kg N/ha more N being accumulated by the 2012 wheat crop (50-74% increase) compared to wheat following wheat and improved grain protein contents from ~9.8% to 12.4-13.6%. The additional N uptake was equivalent to 28% of the lupin residue N from 2011. The uptake of N by wheat grown after either the 2011 wheat or canola treatments was 25-30 kg N/ha higher (21-28% increase) when top-dressed with an additional 51 kg fertiliser-N/ha prior to stem elongation. This represented an apparent recovery of 47-59% of the fertiliser N.