Nitrogen (N) fertilization causes long-term increases in biomass production in many N-limited forests around the world, but the mechanistic basis underlying the increase is often unclear. One possibility, especially in summer-dry climates, is that N fertilization increases the efficiency with which a finite water supply is consumed to support photosynthesis. This increase is achieved by a reduction in the canopy-integrated concentration of internal CO2 and thus discrimination against 13C. We used stable isotopes of carbon (δ13C) in tree rings to experimentally test the physiological impact of N fertilization on mature Douglas-fir (Pseudotsuga menziesii Franco var. glauca) stands across the geographic extent of the Intermountain West, USA. The concentration and the stable isotopes of N (δ15N) in tree rings were also used to assess the presence and activity of fertilizer N. We hypothesized that N fertilization would (i) increase δ15N and N concentration of stemwood relative to non-fertilized stands and (ii) increase stemwood δ13C as photosynthetic gas exchange responded to the additional N. This experiment included two rates of urea addition, 178 kg ha−1 (low) and 357 kg ha−1 (high), which were applied twice over a 6-year interval bracketed by the 18 years of wood production measured in this study. Foliar N concentrations measured the year after each fertilization treatment suggest that the fertilizer N had been assimilated by the trees (P < 0.001). The N fertilization significantly enriched stemwood δ15N by 1.3‰ at the low fertilization rate and by 2.4‰ at the high rate (P < 0.001) despite variation in soil N between sites. However, we found no significant effect of the N fertilizer on δ13C of the annual rings (P = 0.76). These data lead us to suggest that alternative mechanisms underlie the growth response to fertilizer, i.e., increase in canopy area and shifts in biomass allocation.
Keywords: carbon isotopes; dual-isotope; nitrogen; nitrogen fertilization; nitrogen isotopes; stable isotopes; tree rings
Journal Article. 7778 words. Illustrated.
Subjects: Plant Sciences and Forestry
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