Abstract
Plant nitrogen (N) acquisition plays an important role in regulating plant growth and ecosystem functions. However, the seasonal variations in the relative contributions of different N sources to plant N uptake and how plants modify their N absorption preferences, especially in ectomycorrhizal forests, are not well understood. We used the in situ 15N-labeling method to quantitatively estimate the relative uptake contributions of plants for three different soil N sources (nitrate, ammonium and amino acids) and plant N acquisition preferences in an ectomycorrhizal alpine forest (a 70-year-old spruce plantation on the eastern Tibetan Plateau, China) during the growing season and the nongrowing season. Across the two seasons, plants in the spruce plantation showed a greater preference for acquiring soil NH4+-N, with soil NH4+ contributing more than 50% to the total N uptake of plants (57.88% during the growing season and 52.72% during the non-growing season). Moreover, amino acids exhibited a considerable contribution to the total plant N uptake, and their contribution was significantly higher during the non-growing season (33.47%) than that during the growing season (9.86%). Accordingly, plants showed a greater preference for taking up amino acids over NO3− -N in the soil as the season changed from the growing season to the non-growing season. Collectively, our data demonstrate that soil inorganic N is the predominant N source for plants in alpine forests, irrespective of seasonal variations. However, soil amino acids could also be an important supplementary N source for the plant N economy, especially during the non-growing season, when inorganic N availability is constrained. Our findings also suggest that plants in ectomycorrhizal alpine forests modify their nutrient absorption preference in response to seasonal changes.
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