Uptake of nitrogen forms by diploid and triploid white poplar depends on seasonal carbon use strategy and elevated summer ozone.

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The ability of plants to acquire soil nitrogen (N) sources is plastic in response to abiotic and biotic factors. However, information about how plant preferences among N forms changes in response to internal plant N demand through growth phases, or to environmental stress such as ozone (O3), is scarce. Diploid and triploid Chinese white poplar were used to investigate N form preferences at two key developmental periods (spring, summer) and in response to summer O3 (ambient, 60 ppb above ambient). We used stable isotopes to quantify NH4+, NO3- and glycine N-uptake rates. Carbon acquisition was recorded simultaneously. Both ploidy levels differed in growth, N form preferences, and N and C use strategies. Diploid white poplars grew faster in spring but slower in summer compared with triploids. Diploid white poplars also showed plasticity among N form preferences through the season, with no preferences in spring, and NO3- preferred in summer, while triploids showed an overall preference for NO3-. Carbon acquisition and NO3- uptake were inhibited in both ploidy levels of poplar at elevated O3, which also reduced diploid total N uptake. However, triploid white poplars alleviated N uptake reduction, switching to similar preferences among N forms. We conclude that N form preferences by white poplar are driven by internal C and N use in response to nutrient demands, and external factors such as O3.