NITROGEN AND PHOSPHORUS INTERACTIONS IN AN INTENSIVELY MANAGED NURSERY SOIL-PLANT SYSTEM

Nitrogen in forest tree nurseries is usually applied as topdressing during the growing season, while P is incorporated in the soil before seeding. Combined N and P topdressing as monoammonium phosphate (MAP) may improve nursery stock production. To test this practice, white spruce [Picea glauca (Moench) Voss] seedlings top-dressed with MA.P, or factorial combinations of ammonium nitrate (AN) and phosphoric acid (PA) were grown for two seasons in a loamy sand nursery soil. Seedlings receiving AN alone increased in biomass (34%) but took up less P despite relatively high available P (310 mg kg-1) in the bulk soil. Phosphorus additions stimulated growth (107%) and also N and P uptake. Equivalent N dosages, as MAP or AN + PA, increased dry matter production by 362%, demonstrating a significant positive N x P interaction. Two nutritional responses seem to be involved in this interaction: (i) a countering of N antagonism on P uptake when N only was applied, and (ii) a synergistic effect of N and P uptake when these nutrients were applied together. Lower specific absorption rates of roots and suspected depletion of rhizospheric P probably accounted for the antagonistic effect in N-only treatments, while increased root absorption efficiency and higher P availability in addition to the N supplied in N + P treatments contributed to the synergistic interaction. The N + P treatments resulted in lower seasonal NH4-N and NO3-N levels in subsoil than those from N-only treatments, suggesting less leaching of N from combined N and P topdressings.