Soil diffusive fluxes constitute the bottleneck to tree nitrogen nutrition in a Scots pine forest
PLANT AND SOIL, 2016, 399 (1-2):109-120

Oyewole OA, Jamtgard S, Gruffman L, Inselsbacher E, Nasholm T

Abstract
Background and aims
In nutrient poor environments, plant nitrogen (N) acquisition is governed by soil diffusive fluxes and root uptake capacities. However, the relationship between these two processes is not well understood. We explored a way of comparing the processes, enabling identification of the limiting factor for tree N acquisition.

Methods
The study comprised N-fertilized and N-limited Scots pine stands, and measurements of uptake capacities of detached tree roots and of induced soil diffusive fluxes (through in-situ microdialysis) done at the onset and the end of the growing season.

Results
Soil N fluxes were higher at the onset than at the end of the growing season and amino acids comprised a larger fraction of N than inorganic N. N fertilization reduced root uptake capacities of NH4 +, glycine and NO3 − but not of arginine. For all N compounds except NO3 −, diffusive fluxes were significantly lower than root N uptake capacities.

Conclusions
Our results suggest that soil N supply in both, N-fertilized and N-limited forest stands, is dominated by amino acids, thus being the major component of plant-available N. Uptake of N appears more constrained by the diffusive fluxes of N compounds rather than root uptake capacity, except for NO3 −.

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