Nitrate effects on N₂ fixation, growth and feed quality of lucerne and perennial lupin

Abstract

The effects of NO₃¯ supply (0–500 kg N/ha) on total plant dry weight (DW), shoot N content and nutritional quality, and the proportion of plant N derived from the atmosphere (%Ndfa) were determined for lucerne and perennial lupin using ¹⁵NO₃¯ under glasshouse conditions. Fodder beet was used as a non-legume reference plant. In both the initial and repeat experiments, total plant DW, shoot N% and shoot nutritional quality for lucerne and perennial lupin were not affected by NO₃¯ supply. Total plant DW increased 10-fold and shoot N% tripled for fodder beet with increased N supply. In the initial experiment, the %Ndfa for lucerne decreased from 89 to 37% with increased N supply from 0 to 500 kg N/ha, where comparable values for perennial lupin were 96 to 64%. In the repeat experiment, %Ndfa decreased from 90 to 49% and 93 to 65% for lucerne and perennial lupin, respectively, with increasing NO₃¯ supply from 0 to 500 kg N/ha. Both legumes showed an increased reliance on NO₃¯ with increased soil NO₃¯ level, but even at 500 kg N/ha (similar to amount of N in sheep urine patch), perennial lupin obtained much of its N from N₂ fixation.