Estimates of lupin below-ground biomass nitrogen, dry matter, and nitrogen turnover to wheat

The amount of lupin below-ground biomass (BGB), BGB nitrogen (N) content, and utilisation of BGB-N by subsequent wheat was estimated from lupins grown in soil columns. Lupin plants were enriched in situ with N-15-labelled urea through a cotton wick inserted through the stem. Of the applied N-15, 92% was recovered in the lupin plant-soil system at maturity; 87% of this N-15 was in lupin aboveground biomass and 13% in the soil columns. Total mature lupin dry matter (DM) approximated 11 t/ha, with 3.0 t/ha (27%) of this DM below ground. Total mature lupin N approximated 321 kg/ha, of which 91 kg/ha (28%) resided below ground. In terms of N and DM, BGB was the largest lupin residue component even though only 35% of this was recoverable as root material. About 13% of the BGB-N was in inorganic form at maturity. The net mineralisation of lupin BGB-N after 2 consecutive years of wheat growth was 27%, and wheat assimilated about 74% of this N (i.e. 20% of BGB-N), with equal quantities assimilated in each year. The contribution of lupin BGB-N to the N in wheat tops ranged from 40% for soil columns receiving no fertiliser N to 15-20% for soil columns fertilised with 30 kg N/ha. The net mineralisation of BGB-N and the assimilation of BGB-N by wheat were unaffected by the application of fertiliser N.