Climate warming can increase plant growth in permafrost regions by lengthening the growing season, speeding up plant metabolic processes, and allowing deeper root penetration as permafrost thaws. However, the capacity for additional vegetation to offset the carbon released during permafrost thawing depends on nitrogen supply. Until now, scientists have had a limited understanding of the nitrogen supply-demand balance in permafrost regions under climate warming scenarios.

A study published in PNAS and led by Prof. YANG Yuanhe from the Institute of Botany of the Chinese Academy of Sciences revealed that nitrogen fixation by microbes associated with mosses is a critical factor in sustaining plant growth in permafrost ecosystems under warming conditions, and provided the first direct evidence that the moss-associated biological nitrogen fixation responds much more sensitively to warming than soil nitrogen transformations.

Researchers conducted a whole-ecosystem warming experiment in alpine permafrost regions of the Tibetan Plateau. They quantified 43 indicators related to plant nitrogen demand, nitrogen-use strategies, and soil nitrogen supply, which allowed them to disentangle how nitrogen supply-demand dynamics shift under climate warming scenarios and to identify their key drivers.

Researchers found that warming significantly increased plant nitrogen demand, but did not alter leaf nitrogen resorption efficiency, which indicated that plants primarily relied on soil nitrogen uptake to satisfy their elevated requirements under warming. Using ¹⁵N isotope tracing techniques, they found that soil nitrogen transformations remained largely unchanged, while moss-associated biological nitrogen fixation significantly increased.

Notably, the warming-induced enhancement in moss-associated nitrogen fixation accounted for approximately 48% of the additional nitrogen required by vegetation, underscoring the pivotal role of moss-associated nitrogen fixation in supporting plant growth under warming.

Using quantitative stable isotope probing, researchers uncovered the mechanisms behind the enhanced response of moss-associated nitrogen fixation. Warming significantly increased both the diversity and nitrogen-assimilation capacity of active diazotrophic microorganisms—the microbes that fix nitrogen—living in association with the mosses. These microbial changes were closely linked to warming-induced changes in moss functional traits, suggesting a coordinated plant-microbe response.

These findings reveal how nitrogen supply and demand are restructured in permafrost ecosystems under climate warming, and provide evidence for better understanding carbon-nitrogen interactions in the climate-sensitive regions.

Whole-ecosystem warming experiment shows that moss-associated biological nitrogen fixation is a key pathway supporting vegetation growth in permafrost ecosystems under climate warming. (Image by BAI Yufei)