Coupling tree-ring δ13C and δ15N to test the effect of fertilization on mature Douglas-fir (Pseudotsuga menziesii var. glauca) stands across the Interior northwest, USA

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 C-13. We used stable isotopes of carbon (C-13) 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 (N-15) in tree rings were also used to assess the presence and activity of fertilizer N. We hypothesized that N fertilization would (i) increase N-15 and N concentration of stemwood relative to non-fertilized stands and (ii) increase stemwood C-13 as photosynthetic gas exchange responded to the additional N. This experiment included two rates of urea addition, 178kgha(1) (low) and 357kgha(1) (high), which were applied twice over a 6-year interval bracketed by the 18years 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 N-15 by 1.3 parts per thousand at the low fertilization rate and by 2.4 parts per thousand 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 C-13 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.