Thu, Aug 18, 2022: 8:15 AM-8:30 AM
515B
Background/Question/MethodsNitrogen (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 (Narea), photosynthesis per area (Asat), WUE (intrinsic, WUEi, and δ13C), and leaf biomass respond to N inputs (fertilization and N fixation) within N-fixing and non-fixing species? And (2) how do Asat and WUE respond to Narea 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 Asat and WUEi with a LI-COR photosynthesis system, N fixation by 15N isotope dilution, and leaf biomass by harvesting trees and developing allometric equations.
Results/ConclusionsBoth Narea and leaf biomass increased with fertilization in non-fixers but not in N fixers. Narea (and leaf N per mass) for a given leaf mass per area was greater in N fixers than in non-fixers. Asat and WUE (both WUEi 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 (%Ndfa). Leaf biomass also increased with both total tree N fixation and %Ndfa. This suggests that N fixation may be more important than soil N supply in determining tree allocation to carbon capture and water use.Asat, WUEi, and δ13C increased significantly with Narea. There was not a significant difference between fixer and non-fixer log-log slopes, which were < 1 (0.38 for Asat, 0.30 for WUE), indicating concave-down relationships. δ13C also responded to Narea in a saturating manner. Thus, N fixers are not different from non-fixers in how they use leaf N for Asat and WUE. Instead, N fixers simply have higher leaf N, and Asat and WUE respond to leaf N in a saturating manner.
Results/ConclusionsBoth Narea and leaf biomass increased with fertilization in non-fixers but not in N fixers. Narea (and leaf N per mass) for a given leaf mass per area was greater in N fixers than in non-fixers. Asat and WUE (both WUEi 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 (%Ndfa). Leaf biomass also increased with both total tree N fixation and %Ndfa. This suggests that N fixation may be more important than soil N supply in determining tree allocation to carbon capture and water use.Asat, WUEi, and δ13C increased significantly with Narea. There was not a significant difference between fixer and non-fixer log-log slopes, which were < 1 (0.38 for Asat, 0.30 for WUE), indicating concave-down relationships. δ13C also responded to Narea in a saturating manner. Thus, N fixers are not different from non-fixers in how they use leaf N for Asat and WUE. Instead, N fixers simply have higher leaf N, and Asat and WUE respond to leaf N in a saturating manner.