Symbiosis Type in Nitrogen Fixing Trees Determines Soil Greenhouse Gas Emissions: A Global Meta‐Analysis

Abstract Nitrogen‐fixing (N‐fixing) trees are widely planted in forests and agroforestry ecosystems due to their benefits in soil fertility and carbon sequestration. However, their effects on soil fluxes of nitrous oxide (N 2 O), methane (CH 4 ), and carbon dioxide (CO 2 ) compared to non‐fixing trees remain uncertain, potentially challenging their assumed role in greenhouse gas (GHG) mitigation. Through a meta‐analysis of 276 observations from 55 publications, we found that N‐fixing trees increased soil N 2 O emissions (Hedge's d = 0.42) and enhanced CH 4 uptake (Hedge's d = −0.59) without significantly affecting CO 2 emissions and non‐CO 2 global warming potential. The type of symbiotic bacteria was critical: actinorhizal N‐fixing trees increased N 2 O emissions (Hedge's d = 0.70) but had no effect on CH 4 or CO 2 fluxes, whereas rhizobial N‐fixing trees increased N 2 O emissions (Hedge's d = 0.37), CH 4 uptake (Hedge's d = −0.66) and CO 2 emissions (+13%). The effects of N‐fixing trees on N 2 O are mainly influenced by elevation and clay content, on CH 4 by clay content and bulk density, and on soil CO 2 fluxes by clay content, mean annual temperature, and soil organic carbon. This study highlights the importance of symbiotic relationships in N‐fixing trees when designing climate change mitigation strategies, as different types have distinct effects on ecosystem GHG balances.