Nitrogen isotope discrimination in white spruce fed with low concentrations of ammonium and nitrate

Differences in delta(15)N among ten white spruce [Picea glauca (Moench) Voss] families were examined in hydroponic experiments testing (1) three N sources [100 muM N as (i) NH4+, (ii) NO3- or (iii) NH4NO3] and (2) two supply regimes [200 muM NH4+ (i) maintained steadily or (ii) recurrently drawn-down]. In the N-source experiment, the NH4+ treatment resulted in superior growth and lower C/N ratios. Whole plant delta(15)N was higher on NH4+ and NH4NO3, reflecting higher NH4+ removal rates from the media. Families expressed differences in biomass, C/N, delta(15)N and delta(13)C. Family delta(15)N and delta(13)C were positively correlated in the NH4NO3 treatment and the steady-state regime. Supply regime did not affect total biomass, but higher root/shoot ratios implied N was more limiting under the draw-down regime. Family rank changed with supply regime, but not with N source. Analysis of media isotope enrichment during substrate depletion revealed relationships between net discrimination and external N concentration. Discrimination against (NH4+)-N-15 was about twice that of (NO3-)-N-15 stop. A simple model relating isotope discrimination to relative rates of ion efflux and influx predicted efflux/influx ratios consistent with published values for white spruce. We propose that genetic differences in discrimination are caused by different demands on assimilation and in uptake capacity which interact, influencing the balance between N influx and efflux.