COS 207-2 - Are nitrogen-fixing trees different than non-fixing trees in how they allocate nitrogen to photosynthesis and water use efficiency? Results from a field fertilization experiment

Background/Question/Methods

Nitrogen (N)-fixing trees, which can access atmospheric N₂ through symbiotic bacteria, frequently have higher leaf N content than non-fixers. Higher leaf N is thought to benefit growth and survival by leading to higher photosynthesis and/or water use efficiency (WUE). However, recent work across species suggests that N fixers are different from non-fixers because N fixers use leaf N to increase WUE, but not photosynthesis. It is unclear if this pattern is consistent at the within-species scale where physiological plasticity operates and if it is mediated by N limitation status. We asked: (1) how do leaf N per area (N_area), photosynthesis per area (A_sat), WUE (intrinsic, WUE_i, and δ¹³C), and leaf biomass respond to N inputs (fertilization and N fixation) within N-fixing and non-fixing species? And (2) how do A_sat and WUE respond to N_area in N-fixing and non-fixing species?

We grew six N-fixing and four non-fixing tree species in field experiments in Hawaii, Oregon, and New York at three levels of N addition (0, 100, and 150 kg N ha^-1 yr^-1). We measured A_sat and WUE_i with a LI-COR photosynthesis system, N fixation by ¹⁵N isotope dilution, and leaf biomass by harvesting trees and developing allometric equations.

Results/Conclusions

Both N_area and leaf biomass increased with fertilization in non-fixers but not in N fixers. N_area (and leaf N per mass) for a given leaf mass per area was greater in N fixers than in non-fixers. A_sat and WUE (both WUE_i and δ¹³C) did not respond directly to fertilization but significantly increased in N fixers with both total tree N fixation and with %N from fixation (%N_dfa). Leaf biomass also increased with both total tree N fixation and %N_dfa. This suggests that N fixation may be more important than soil N supply in determining tree allocation to carbon capture and water use.

A_sat, WUE_i, and δ¹³C increased significantly with N_area. There was not a significant difference between fixer and non-fixer log-log slopes, which were < 1 (0.38 for A_sat, 0.30 for WUE), indicating concave-down relationships. δ¹³C also responded to N_area in a saturating manner. Thus, N fixers are not different from non-fixers in how they use leaf N for A_sat and WUE. Instead, N fixers simply have higher leaf N, and A_sat and WUE respond to leaf N in a saturating manner.