Seasonal variations in plant nitrogen acquisition in an ectomycorrhizal alpine forest on the eastern Tibetan Plateau, China

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.