Whole-watershed nitrogen fertilization effects on tree nitrate reductase activity and stand nitrogen demand

Background/Question/Methods

Small watersheds are powerful tools to study the impacts of N deposition on the N cycle of forests. A small watershed study began in 1989 at the Fernow Experimental Forest in Parsons, WV, to investigate the effects of long-term N additions and soil acidification on whole-watershed ecosystem dynamics. Every year, 35 kg N ha^-1_, as (NH₄)₂SO₄, has been added to one watershed (WS3) while an adjacent unfertilized watershed of similar age (WS7) is used as a reference.  Currently, the stream water NO₃ concentration in the fertilized WS is ~50% higher than the reference WS, consistent with increased rates of NO₃ production. However, there is no detectable difference in measurements of net nitrification when the fertilized WS is compared to the reference. Since the supply of NO_3, via nitrification is not higher in the fertilized forest, then the biotic demand for NO₃ in this WS must be lower. To test this idea, NO₃ reductase activity (NRA) was measured, as an index of NO₃ demand, on roots and leaves of four common overstory tree species growing in these two WS (Acer rubrum, Betula lenta, Liriodendron tulipifera, and Prunus serotina).  As an index of overall N demand, we also used dendrometer bands to measure tree growth of the same trees.

Results/Conclusions

Across species and tissues (leaves & fine roots), there was a trend towards higher NRA (per g tissue) in the fertilized WS compared to the reference (p=0.07), suggesting that trees fertilized with (NH₄)₂SO₄ didn't decrease their reliance on NO₃. Furthermore, when adjusted for the relative importance of the four species, estimates of total NRA (per g tissue) were ~48% higher in the fertilized WS. However, fine root biomass is ~50% lower in the fertilized WS (p=0.03), so root NRA per m² was ~47% lower in the fertilized WS. When pre-fertilization differences in growth between the WS were accounted for, the growth of A. rubrum, L. tulipifera, and P. serotina were 56%, 97%, and 141% lower than expected in the fertilized WS. While NRA data suggest that individual trees do not reduce their reliance on NO₃ when NH₄ availability increases and may even assimilate more NO₃, fertilization and acidification decrease the overall fine root and radial tree growth. Thus, the dominant factor contributing to a ~50% increase in stream water NO₃ in the fertilized WS is the overall effect of fertilization and acidification on tree N demand rather than the form of N used.