Soil nitrogen assimilation of 1-year oak seedlings: Implication for forest fertilization and management

Reserves in cotyledons and nutrients in soil are essential for the early development of seedlings of oaks. However, much less is known to what extent the reserves in seeds and/or soil nutrients contribute to seedling development and growth at early stages, especially at multi-species level. We carried out an indoor experiment in which the soil was enriched in 15N to quantify N uptake and its distribution in the seedlings of five red and white oak species. Dry masses of seedlings did not vary significantly with N fertilization rates, reflecting the low morphological and physiological plasticity of oak seedlings to soil fertility. However, dry masses of roots of white oak species were much higher than those of red oak species. Soil-derived N incorporated into the seedlings of the five oak species increased with seedling growth and enhanced 15N fertilization rates. Soil-derived N in the seedlings of white oak species was much higher than those of red oak species. Acorn-derived N in seedlings decreased with N fertilization rates, implying that acorn N mobilization can be accelerated under low soil nutrient level. We found higher soil N/acorn N ratio in the seedlings of white oak species than those of red oak species, indicating the rapid uptake of soil N in the seedlings of white oak species. Acorn N rather than soil N played a crucial role in supporting oak seedling development and growth at the early stage. Our results provide insight into the role of acorn N and soil fertility to 1-year oak seedlings and have great implication for forest fertilization and management.