Nitrogen fixation responds to soil nitrogen at low but not high light in two invasive understory species

Abstract

Light and soil nitrogen availability can be strong controls of plant nitrogen (N) fixation, but data on how understory N‐fixing plants respond to these drivers are limited despite their important role in ecosystem N cycling. Furthermore, ecosystem N cycling can be altered by the introduction of species with nutrient use patterns that differ from natives. We assessed how N fixation of two exotic, understory species responded to varying light and soil N environments. We sampled leaf tissue from Mimosa pudica L., Desmodium triflorum (L.) DC., and a nonfixing reference plant (Axonopus) growing in control and two N fertilization treatments under either N‐fixing or non‐N‐fixing trees, which may alter local soil nutrient cycling, across a range of light conditions. We measured N fixation with 15N isotope dilution, and ensured that N‐fixing neighbour trees were in fact fixing N. All understory plants were wild‐growing species not native to the study location. Desmodium and Mimosa acquired 82.6% and 71.6% of their nitrogen from fixation (%Ndfa) in the control, compared to 66.8% and 58.1% in the +10 g N m−2 year−1 treatment and 73.1% and 64.7% in the +15 g N m−2 year−1 treatment. These subtle %Ndfa differences across fertilization treatments were more apparent at low light availability and disappeared at high light availability. The amount of N fixed by neighbouring trees did not influence %Ndfa in the understory species. Synthesis. Our study shows some differences in N fixation across different nutrient environments at low light for two N‐fixing species, though the changes were small, and both species derived most of their N from fixation. These findings imply that introduced N‐fixing species could exacerbate ecosystem N enrichment, particularly under high soil N conditions.

Tree symbioses sustain nitrogen fixation despite excess nitrogen supply

“…The light interpretation is also consistent with another study in our experimental plots in Waiakea. Mimosa pudica and Desmodium triflorum, two ground-layer legume species that recruited naturally under and near our Gliricidia, Casuarina, and Psidium trees, had %N dfa values similar to those of our trees (averages of 58%-82%), but their %N dfa in the fertilized plots decreased with shade (Schmidt et al, 2023).…”

Leaf nitrogen affects photosynthesis and water use efficiency similarly in nitrogen‐fixing and non‐fixing trees

“…However, Funk et al (2013) also observed that N fixers tended to have a greater proportion of leaf N associated with photosynthesis than non‐fixers. This may be due to N fixers specializing in high‐light environments (McCulloch & Porder, 2021; Schmidt et al, 2023; Taylor & Menge, 2018; and therefore having lower shade tolerance than non‐fixers; Menge et al, 2010), where N associated with photosynthesis is likely to be more beneficial.…”

Allometric relationships for eight species of 4–5 year old nitrogen-fixing and non-fixing trees

“…The control treatment received 0.1 g N m −2 y −1 as ammonium nitrate in years 2 and 3 and none in all other years. This small addition of ammonium nitrate was isotopically labeled in order to facilitate the measurement of symbiotic N fixation, which was a major goal of the overall experiment [ 63 ], though not our focus here. As is standard in the enriched isotopic pool dilution method of measuring symbiotic N fixation [ 64 ], we needed to add a small amount (0.1 g N m −2 y −1 ) of labeled N to measure N fixation.…”