Soil sampling during early forest growth on coastal sand showed little indication of nitrogen accumulation in the mineral soil. In spite of 7 years’ intermittent lupin (Lupinus arboreus
Sims) growth and regular 6-monthly applications of fertiliser (a total of 900 kg N/ha, plus other nutrient elements) over a 10-year period, total-nitrogen levels remained below 0.03%.
A bioassay of soil samples using Pinus radiata D. Don seedlings showed that potentially-available-nitrogen was influenced by experimental treatment (fertiliser addition, lupin exclusion) to a greater extent than total-nitrogen. Potentially-available-nitrogen responses to the presence of lupins and to fertiliser were observed in the second year of the trial and from the fifth year onwards. The lupin effect was very similar to that of fertiliser treatment. Highest values resulted from the combined influence of lupins and fertiliser, but the effect was additive rather than interactive.
Foliar-nitrogen levels declined during the first 5 years of tree growth and showed no effect of lupins or fertiliser treatment. In the sixth year the onset of nitrogen stress in trees growing without fertiliser or lupins (foliar-nitrogen range 0.9-1.2%) was associated with significant and recurring treatment differences. The effect of fertiliser (foliar-nitrogen range 1.1-1.5%) was sometimes, but not always greater than that of lupins (foliar-nitrogen range 1.0-1.4%) and highest values (1.2-1.6%) were recorded where trees were influenced by both lupins and fertiliser. The lupin effect lasted for at least 3 years after lupins had died out of the understorey at tree age 4 years and the fertiliser effect for at least 4 years after the final application.
Evidence from foliar analysis indicated that tree nitrogen demand was not met by fertiliser treatment alone, and the efficiency of fertiliser-nitrogen utilisation appeared to be low. All improvements in tree nitrogen status attributable to lupins were achieved without additional management costs.