Acquisition of nitrogen from tannin protein complexes in ectomycorrhizal pine seedlings

Ectomycorrhizal and ericoid fungi are increasingly recognized for their capacity to break down soil organic matter and getting access to organic nitrogen (N). In many forest ecosystems proteins complexed by plant-produced tannins constitute a substantial amount of organic N. Yet, it is currently unknown to what extent these N sources are accessible to ectomycorrhizae and their associated plants. In a pot experiment, we provided ectomycorrhizal pine (Pinus pinea) seedlings with exactly the same amount of N either in mineral form, as protein, or as two different tannin-protein complexes, formed with commercially available tannins or tannins isolated from pine leaf litter. Over a period of 18 months, pine seedlings grew at least two times bigger in all N-treatments. However, growth and N acquisition were roughly two and three times larger, respectively, with mineral N or protein supply compared to tannin-protein complexes as the sole N source (no difference between the two different complexes). With tannin-protein complexes, pine seedlings allocated relatively more biomass into fine roots. In addition, the decrease in tissue N concentration in the tannin-protein complex treatments was lower in fine roots (30%) than in needles (53%). Our results showed that tannins strongly influence N nutrition in ectomycorrhizal pine seedlings. Tannin-protein complexes are an accessible N source for ectomycorrhizal pine seedlings although they grew better with easier accessible N. The respective contributions of ectomycorrhizal fungi and other free or root-associated microorganisms in the breakdown of tannin protein complexes recquires further exploration. Tannin-protein complexes may be an important N source in ecosystems where organic nitrogen dominates, but the respective contributions of ectomycorrhizal fungi and other free or root-associated microorganisms in the breakdown of tannin protein complexes recquires further exploration.