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In contrast to N soil availability or carbon (C) reserves, the role of N reserves in plant interactions has been under-researched. The N-loading of tree seedlings in nurseries before plantation within a forest community to increase their survival and early growth is still controversial. We hypothesized that a larger N-loaded pool in oak seedlings during year n - 1 might favor growth and function during yearn through higher N remobilization, and thus lessen the impact of competition from Deschampsia cespitosa (tussock grass).

In 2015, we cultivated two sets of one-year oak seedlings in 20 L pots with two nitrogen supply levels: no supply (N-unloaded) and 87 kg ha(-1) (N-loaded), without competition and without water limitation. A greenhouse experiment was then undertaken in 2016 with the same two oak batches, but with 0, 1, 2 or 3 tillers of D. cespitosa in both N-loaded and N-unloaded batches. Half the pots were subjected to drought for one month (August 2016).

Soil water availability was the prime factor driving plant growth and photosynthetic responses in oak, both with and without competition. Under well-watered conditions, a greater internal N pool was correlated with higher oak photosynthesis. A high internal N pool was also beneficial to oak seedlings under low competitive pressure from D. cespitosa. However, when grass density increased, intraspecific competition outweighed interspecific competition, and attenuated the negative effect of D. cespitosa on oak seedlings.

A carry-over effect of N-loading thus had a positive effect on growth and function in oak seedlings when no water stress occurred under low competition by D. cespitosa: this effect may derive from more N storage in perennial tissues, resulting in more N mobilized during vegetative growth. Our results underline the importance of the internal N pool in plant competition, and are evidence that a higher potential regenerative success rate of oak seedlings is related to abiotic conditions, competitive pressure and internal N pool.