Invasion of Hawaiian rainforests by an introduced amphibian predator and N₂‐fixing tree increases soil N₂O emissions

Abstract

Invasions of introduced species have homogenized ecological communities worldwide, leading to losses of native species and the services they provide. Some of these invaders substantially alter nutrient cycling, which changes conditions for all other organisms, but less is known about the potential influence of these species on nitrogen (N) trace gas emissions that affect atmospheric processes. We used a natural experiment to explore whether the establishment of an introduced nitrogen (N) fixing tree (Falcataria moluccana) and recent invasion of an amphibian predator, the Caribbean tree frog (Eleutherodactylus coqui), into native Hawaiian rainforests have affected soil emissions of nitrous oxide (N2O) and nitric oxide (NO), two atmospherically important trace gases produced by soil microorganisms. Soil N2O and NO emissions and rates of soil N cycling were significantly higher in F. moluccana‐dominated stands compared to native Metrosideros polymorpha (Ohi'a) stands. Additionally, invasion of E. coqui frogs moderately increased soil N2O emissions, primarily in non‐native F. moluccana forests where soil N availability was already elevated. N2O emissions were positively and significantly related to net potential N mineralization, and total N2O+NO fluxes increased with soil nitrate (NO3−) concentration and rates of nitrification. Previous work in these Hawaiian rainforest sites has shown that F. moluccana substantially increases N availability by increasing ecosystem N supply compared to uninvaded stands, and E. coqui accelerates N availability and litter decomposition, although moderately, due to enhanced fluxes of nutrient‐rich waste products. Here, we show that acceleration of nutrient cycling by introduced species can also alter biosphere–atmosphere exchange of N‐oxides.

Nitrogen‐fixing trees increase soil nitrous oxide emissions: a meta‐analysis

“…Finally, N-fixing trees also influence the magnitude of the soil carbon pool (N-fixing trees enrich the soil carbon pool relative to non-fixing trees; Binkley 2006) as well as soil moisture and temperature (N-fixing trees have higher water use efficiency than non-fixing trees (Adams et al 2016)), all of which control soil N 2 O emissions. Empirically, there are examples for both increased (Hall et al 2018) and decreased (Van Haren et al 2013) soil N 2 O emissions under N-fixing trees relative to both non-fixing trees and soils.…”