Sequestration of soil nitrogen as tannin–protein complexes may improve the competitive ability of sheep laurel ( Kalmia angustifolia ) relative to black spruce ( Picea mariana )

Summary•  The role of litter tannins in controlling soil nitrogen (N) cycling may explain the competitive ability of Kalmia relative to black spruce ( Picea mariana ), although this has not been demonstrated experimentally. •  Here, the protein‐precipitation capacities of purified tannins and leaf extracts from Kalmia and black spruce were compared. The resistance to degradation of tannin–protein precipitates from both species were compared by monitoring carbon (C) and N dynamics in humus amended with protein, purified tannins or protein–tannin precipitates. The purity of the precipitates was verified using solid‐state 13 C nuclear magnetic resonance (NMR) spectra. The ability of mycorrhizal fungi associated with both species to grow on media amended with tannin–protein complexes as the principal N source was also compared. •  The protein precipitation capacity of Kalmia tannins was superior to those of black spruce. Humus amended with protein increased both mineral and microbial N, whereas humus amended with tannin–protein precipitates increased dissolved organic N. Mycorrhizal fungi associated with Kalmia showed better growth than those associated with black spruce when N was provided as tannin–protein precipitates. •  These data suggest that Kalmia litter increases the amount of soil N sequestered as tannin–protein complexes, which may improve the competitive ability of Kalmia relative to black spruce by favouring N uptake by mycorrhizas associated with the former.