A research team from the Institute of Applied Ecology (IAE) of the Chinese Academy of Sciences has clarified how deposited nitrogen is retained in forest ecosystems and how this process contributes to carbon sequestration across China.
The findings were published in Global Change Biology on January 30.
China experiences some of the highest rates of atmospheric nitrogen deposition worldwide. Yet, until now, the capacity of its forests to retain this deposited nitrogen-and the magnitude of the associated carbon gains-has remained poorly constrained, largely due to the absence of a comprehensive, ecosystem-scale national assessment. This uncertainity has limited accurate evaluation of carbon-nitrogen coupling in Chinese forests under ongoing global change.
To address this knowledge gap, the researchers led by Prof. FANG Yunting and Dr. Geshere Abdisa Gurmesa at IAE synthesized decades of ecosystem-scale N15 tracer experiments to provide the first national assessment of nitrogen retention and its carbon consequences in Chinese forests.
By integrating datasets from 18 forest sites spanning four biomes (boreal, temperate, subtropical, and tropical) and three successional stages (primary forests, secondary forests, and plantations), the researchers revealed that Chinese forests retain ~65% of deposited nitrogen, with nearly two-thirds stored in soils. Retention shows strong spatial variation, reaching up to ~90% in boreal forests but declining to 50-70% in subtropical and tropical forests due to long-term nitrogen saturation.
The study also identified forest successional stage and nitrogen form as key regulators of nitrogen partitioning within ecosystems. Primary forests preferentially retain deposited nitrogen in mineral soils, whereas plantations retain more in soil organic layers. In addition, nitrate is more efficiently taken up by plants, while ammonium is mainly retained in organic soil pools, reflecting differences in nitrogen mobility and biological assimilation pathways.
Using a stoichiometric upscaling approach, the researchers further quantified how nitrogen deposition contributes to China's forest carbon sink. The carbon-nitrogen response ranges from seven to 40 kg C per kg deposited nitrogen, with the strongest responses observed in boreal forests and young plantations. At the national scale, nitrogen deposition enhances forest carbon sequestration by approximately 0.11 Pg C yr⁻¹, accounting for 20-30% of China's total forest carbon sink.
As the first tracer-based national synthesis of deposited nitrogen retention in Chinese forests, the study advances understanding of carbon-nitrogen coupling and providing a scientific basis for forest management and climate change mitigation strategies.
Nitrogen-induced forest carbon sequestration (C-N response) (a) and its decline with increasing nitrogen deposition (b) across Chinese forest ecosystems (Image by Geshere Abdisa Gurmesa)
