Effect of in situ light x soil N resource interaction on Quercus petraea seedlings mixed-grown with Molina caerulea

Availability of aerial and soil resources in oak forests is likely to change in the near future: wood harvest will increase to fulfill biomass demand and atmospheric N deposition is still high. Accordingly, oak seedlings will benefit from a larger availability of both incoming irradiance and soil inorganic N (Ni) amounts. Such changes may also favor graminoids, jeopardizing oak regeneration. Methods Along an in situ light gradient, 60 1-year old oak seedlings were planted. Effects of M. caerulea and N fertilization were tested by hand-weeding and applying 90 kg inorganic N.ha-1. Seedling growth as well as tracking of 15N fate was monitored and seedlings were harvested in autumn 2015 after one growing season. Results Tree growth increases along light gradient. Neither presence of M. caerulea nor N fertilization has any effect on oak growth. Under high irradiance, 15N was preferentially allocated to leaves instead of stem. Eventually, only growth and N allocation were larger in fine roots when no N fertilization was applied or in mixture. Discussion Growth seedlings was positively responsive to light but not to changes in soil inorganic N resources due to presence of M. caerulea and/or N fertilization. Better growth might result from an optimized N allocation toward leaves. Fine roots biomass positively responded to N soil limitation, supporting foraging strategy. Nevertheless, lack of N resource due to biotic competition would promote graminoids uprooting in forest management practice to ensure a high success of regeneration.