A new study has pinpointed tree symbiotic relationships with different types of mycorrhizal fungi—arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM)—as a critical predictor of woody biomass carbon (C) accumulation when nitrogen (N) is added to ecosystems. The research was recently published in Soil Biology and Biochemistry.

Led by researchers from the South China Botanical Garden of the Chinese Academy of Sciences, the team synthesized data from 125 independent global nitrogen-addition experiments. These experiments covered 71 tree species and included 189 paired observations, forming a comprehensive dataset for analysis.

A key breakthrough of the study was its first-ever quantification of "C_perN"—the absolute amount of woody biomass carbon gained per unit of nitrogen added—for trees associated with AM and ECM fungi. The researchers revealed that C_perN increased with latitude across global forest systems, and on average, AM-associated trees exhibited a C_perN six times higher than ECM-associated trees (17.2 kg C per kg N vs. 2.9 kg C per kg N).

This divergence, the researchers explained, stems from the distinct nitrogen-acquisition strategies of the two fungal types. ECM fungi can directly access organic nitrogen in soil, meaning their host trees are less dependent on external nitrogen inputs and thus show weaker responses to nitrogen addition. In contrast, AM fungi rely primarily on inorganic nitrogen, making their host trees more responsive to increased nitrogen availability.

Furthermore, the study highlighted the practical implications of these findings for global carbon cycle modeling. Using a global map of tree-mycorrhizal associations, the team calculated that ignoring the role of mycorrhizal type leads to significant overestimates of nitrogen-induced tree carbon sequestration: a 12% overestimation globally, equivalent to 9.8 teragrams (Tg) of carbon per year, and a 17% overestimation in temperate forests, accounting for 9.4 Tg of carbon per year.

This overestimation is driven largely by the dominance of ECM-associated trees in many temperate forest ecosystems, particularly in Europe.

This study carry implications for understanding future forest carbon sinks under ongoing nitrogen deposition. The researchers emphasized that shifts in the relative abundance of AM and ECM trees could strongly alter forest carbon sequestration capacity. Trees with "acquisitive" nutrient strategies (e.g., AM-associated species) are likely to sequester more carbon under increased nitrogen availability compared to those with "conservative" strategies (e.g., ECM-associated species).