In a groundbreaking study from the Institute of Botany at the Chinese Academy of Sciences (CAS), researchers have uncovered how forest canopy leaves absorb atmospheric nitrogen in ways that depend on leaf age. This discovery sheds new light on how trees in China's vast tropical and subtropical forests manage nutrient intake amid rising atmospheric pollution. Led by Dr. Wang Xin, the team used innovative field techniques to demonstrate that while young and old leaves show similar overall uptake rates, the underlying processes differ significantly, with implications for ecosystem health and carbon storage.

The research, published in Plant, Cell & Environment, highlights the role of foliar nitrogen uptake as a supplementary nutrient pathway, especially relevant in regions with high nitrogen deposition like southern China.

Understanding the Nitrogen Cycle in Forests

The nitrogen cycle is fundamental to forest ecosystems, where nitrogen (N)—a key nutrient for growth—is often limited. Atmospheric nitrogen deposition, primarily from industrial emissions, agriculture, and vehicle exhaust, has surged in China, reaching 20-40 kg N per hectare per year in the south. Traditionally, scientists focused on root uptake from soil, but recent evidence shows canopies intercept up to 80% of this deposition, processing it through leaves.

In China, where forests cover 23% of land and sequester vast carbon, understanding canopy contributions is crucial. Elevated N can boost growth but risks soil acidification, biodiversity loss, and altered carbon dynamics. This study addresses a gap: how leaf age influences uptake efficiency and mechanisms.

The Study's Innovative Approach

Conducted at Dinghushan Biosphere Reserve (subtropical) and Jianfengling National Forest Park (tropical) in southern China, the experiment leveraged canopy cranes for access to treetops. Researchers selected 13 dominant species, including Castanopsis chinensis and Schima superba, spraying leaves with 15N-labeled ammonium nitrate—a stable isotope tracer mimicking deposition.

After 48 hours, leaves were washed, dried, and analyzed for 15N enrichment via mass spectrometry. Mature leaves (fully expanded, green) and old leaves (yellowing, pre-senescence) were compared. Leaf traits like stomatal density, nitrogen content per area (Narea), and vein density were measured, alongside soil properties (texture, fertility) and meteorology (PAR, temperature, humidity).

Key Findings: Variation Across Species and Leaf Ages

Foliar uptake ranged from 0.01 to 0.3 mg N m⁻² leaf day⁻¹, varying widely among species. Species with high N demand and dense stomata showed superior uptake, especially in mature leaves. Average rates were comparable between ages, but drivers diverged.

• Mature leaves: Driven by intrinsic traits—stomatal density (r=0.62, p<0.01), Narea, and vein density facilitated entry and transport.
• Old leaves: Environment dominated—PAR positively correlated (higher light boosts), soil clay content enhanced (better retention), while fertility suppressed uptake less.

PAR emerged as the top meteorological driver for both, underscoring light's role in stomatal opening and transport.

Photo by HAN CHUANG on Unsplash

Mechanisms Unraveled Step-by-Step

1. Deposition Interception: Canopies capture gaseous (NOx, NH3) and particulate N, with wet deposition dissolving into leaf surfaces.

2. Stomatal Entry: In mature leaves, dense stomata allow diffusion; cuticles may also absorb dissolved forms.

3. Xylem Transport: Nitrogen moves to mesophyll for assimilation into amino acids.

4. Age Shift: As leaves senesce, reduced transpiration and metabolism make traits less relevant; external factors like light and soil moisture dictate.

This step-by-step insight refines models of forest N budgets.

Ecological Implications for China's Forests

China's forests, critical for carbon sequestration (sequestering ~1 Pg C annually), face N saturation. Canopy uptake supplies 10-50% of foliar N needs, alleviating soil limitation but potentially accelerating saturation. In N-limited old-growth stands, this boosts productivity; in polluted south, it may exacerbate leaching.

Linked to carbon: Enhanced N raises photosynthesis, but excess risks fine root decline, reducing sequestration. Models ignoring canopy uptake underestimate impacts by 20-30%.

Real-world case: Dinghushan, with ~30 kg N ha⁻¹ yr⁻¹ deposition, shows canopy retention aiding resilience amid climate stress.

CAS Institute of Botany's Pivotal Role

The Institute of Botany, CAS (IBCAS), founded 1928, leads plant science in China. With 500+ researchers, it hosts Key Labs in Vegetation Change and Plant Resources, training 1000+ PhD students yearly via UCAS—the graduate university of CAS.

Wang Xin's team exemplifies this: combining field ecology with isotope tech. UCAS students co-authored, gaining hands-on training in canopy access via cranes—a rarity globally. This research underscores IBCAS's contribution to China's 'ecological civilization', informing restoration like Grain-for-Green.

In higher ed, IBCAS-UCAS model integrates research-training, producing leaders in forest ecology. Programs emphasize N cycling for sustainable forestry.Read the full study here.

Stakeholder Perspectives and Challenges

Foresters view it positively: canopy uptake extends N benefits beyond roots, aiding plantations. Ecologists caution saturation—China's N dep peaked ~2010s, now declining in north but high south.

Challenges: Climate change alters PAR/soil; aging forests (mean age 40+ years) may shift dynamics. Policymakers use for N emission controls, targeting 10% cut by 2030.

Photo by Travis Chen on Unsplash

Future Outlook and Actionable Insights

Future research: Scale to whole-tree/canopy models, test under dep scenarios. For Chinese unis: Integrate into botany curricula, field stations like Dinghushan for student training.

Actionable: Monitor leaf age in N models; prioritize light management in agroforestry; UCAS-like programs expand canopy ecology expertise.

This advances China's forest science, positioning CAS/UCAS as global leaders. Explore botany research roles at AcademicJobs.com/research-jobs.