Topography-driven divergence of plant nitrogen acquisition strategies in forests

Soil nitrogen (N) supply and plant N acquisition strategies are central to species coexistence and ecosystem functioning, but how topography regulates plant N acquisition strategies remains poorly understood. Here, we used ¹⁵N natural abundance approach to quantify plant N uptake proportions along a valley to slope gradient in Southwest China. We further measured leaf and root functional traits with soil N transformation rates to explore topography controls of plant N acquisition strategies.

The results showed that soil nitrate (NO₃^–), decreased significantly from valley to slope, whereas soil ammonium (NH₄⁺) and extractable organic N (EON) increased significantly. These differences were attributed to distinct soil N transformation pathways, with markedly higher soil N mineralization and nitrification rates in valley soils. Consistent with the shifts of soil N availability, plants predominantly utilized NO₃^– (83.1%) in the valley, but markedly increased the uptake proportions of organic N and NH₄⁺ at slope. In addition, leaf functional traits shifted from an acquisitive strategy in valley plants, characterized by high leaf N concentrations, to a conservative strategy in slope plants with higher leaf carbon (C) to N ratios and increased leaf thickness. In contrast, root functional traits changed from “amount” strategies in the valley, indicated by high specific root length, to “efficiency” strategies reflected by high root N uptake rates at slope. Our structural modeling indicated that topography-driven shifts in plant biomass, leaf and root C: N, soil physicochemical properties, and soil enzyme activity constrain soil N mineralization and nitrification rates on slopes, thereby increasing the relative contribution of EON and NH₄⁺ in soils and driving a corresponding shift in plant N acquisition strategies.

Together, these findings highlight that plants adapt to topography-driven variations in soil N supply by coordinating above and belowground functional traits. Our results provide a scientific basis for future forest restoration and species selection across topographic gradients.