A 15-year experiment in northern China suggests trees use a hidden nitrogen-saving pathway that becomes increasingly important as forests mature.

For decades, scientists have largely understood tree nitrogen use as a two-part economy: roots take nitrogen from the soil, and leaves recycle nitrogen before they fall.

A new study suggests that view may be missing a crucial player: roots themselves.

Now, a team led by Professor Tao Yan at the Institute of Applied Ecology, Chinese Academy of Sciences, has found that aging roots can also recycle nitrogen before they die. The study, published in New Phytologist, proposes a new triadic nitrogen economy for trees — one that brings together nitrogen uptake by roots, nitrogen resorption from leaves, and nitrogen resorption from roots. This overlooked process may reshape how scientists understand forest nutrient cycling, especially in plantation forests exposed to long-term nitrogen deposition.

The research was conducted in Saihanba, a vast plantation forest region in northern China. There, the team used a 15-year nitrogen-addition experiment in Larix principis-rupprechtii plantations across young, intermediate and mature stands. This long-term design allowed the researchers to ask a simple but important question: do trees of different ages manage nitrogen in the same way? The answer was no.

In young stands, trees appeared to follow a more selective strategy. Root nitrogen uptake traded off with nitrogen recycling from both leaves and roots, suggesting that young trees leaned more strongly on external nitrogen acquisition when nitrogen limitation was relatively weak. At the same time, leaf and root nitrogen resorption worked together, showing that internal recycling was still coordinated.

As forests matured, the system changed. Mature trees showed stronger nitrogen limitation and faced higher carbon costs for obtaining nitrogen through roots. Under these conditions, trees appeared to rely less on purely self-directed uptake and more on a combination of internal nitrogen recycling and cooperation with ectomycorrhizal fungi. The relationship between nitrogen uptake and nitrogen recycling shifted from trade-off to synergy.

That shift matters because plantation forests are expected to do many things at once: produce timber, store carbon and support ecological restoration. Yet many plantations face nutrient imbalance caused by simplified species composition, repeated harvest and rising atmospheric nitrogen deposition. If root nitrogen resorption is ignored, models may underestimate how trees sustain growth and regulate carbon storage over time.

The study reframes tree nitrogen use as an age-dependent system rather than a fixed balance between soil uptake and leaf recycling. It also points to a broader challenge for forest ecology: future models may need to move beyond trait indicators and develop flux-based approaches that can quantify how much each nitrogen pathway contributes.