Where there is nitrogen, there is growth: Willows' fast uptake and transport abilities facilitate their thriving in extremely low-nitrogen environments

Willows ( Salix spp.) are recognized as promising renewable bioenergy sources for their rapid growth and high biomass. However, their cultivation often requires significant nitrogen (N) fertilizer inputs, prompting questions about whether they truly qualify as low-N-input species. Additionally, the mechanisms governing N uptake and transport within willows remain unclear. This study demonstrated that willow ecotypes can thrive in tap water with extremely limited nutrient availability over extended periods and even minimal N supplementation (10 μM) can stimulate their growth. The concentrations of ammonium and nitrate in soils naturally inhabited by willows ranged from 3.6 mg/kg to 24.7 mg/kg and 0.9 mg/kg to 37.3 mg/kg, respectively, confirming their adaptability to a wide spectrum of N availability. Through a series of 15 N-labeling assays, we observed that willows efficiently acquired, transported, and recycled N from source to sink tissues. Notably, N uptake occurred rapidly, with 56 % of absorbed N transported to shoots via the xylem within one minute. RT-qPCR analysis further identified the swift upregulation of key N-associated transporters and enzymes, including SsAMT , SsGS , and SsLHT , underscoring the molecular mechanisms underlying their N use efficiency. Within 24 h of NH 4 + exposure, 7 % of N was recycled back to roots via the phloem. These findings together elucidate the dynamic processes of N uptake and transport in willows, emphasizing their suitability for low-input agricultural systems and advancing strategies for sustainable biomass production. • A minimal nitrogen supply (10 μM NH 4 + ) enhances the growth of Salix suchowensis . • Within 1 min of NH 4 + exposure, 56 % of absorbed N transported to shoots. • Within 24 h of NH 4 + exposure, 7 % of N was recycled back to roots via the phloem. • Key nitrogen-associated genes ( AMT , GS , and LHT ) were upregulated rapidly by NH 4 + .