Nitrogen isotope discrimination in white spruce fed with low concentrations of ammonium and nitrate
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Differences in δ15N among ten white spruce [Picea glauca (Moench) Voss] families were examined in hydroponic experiments testing (1) three N sources [100 μM N as (i) NH4+, (ii) NO3− or (iii) NH4NO3] and (2) two supply regimes [200 μM 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 δ15N was higher on NH4+ and NH4NO3, reflecting higher NH4+ removal rates from the media. Families expressed differences in biomass, C/N, δ15N and δ13C. Family δ15N and δ13C 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 15NH4+ was about twice that of 15NO3−. 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.