Abstract: Co-existing forest tree species associated with ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM) fungi may have different nitrogen (N) acquisition strategies for various soil nitrogen (N) forms which can be difficult to determine under field conditions. Paired (ammonium/nitrate) ¹⁵N labeling to soils demonstrated that common conifers in Northeast China associated with ECM fungi took up more nitrate than ammonium. The N uptake strategy of co-existing broadleaved species, associated with either AM fungi or ECM fungi, remains to be determined. We conducted paired ¹⁵N labeling on seedlings of six common broadleaved species and four conifer species to reveal more nitrate uptake than ammonium by all ten species. Nitrate uptake contributed 49~83% (average 69%) to N uptake by these species, ranking in the order of AM broadleaved species (average 79%), ECM broadleaved species (average 70%) and ECM conifer species (average 60%). Random forest analysis suggested plant functional groups, mycorrhizal types and fine root to shoot ratios as important factors supporting the higher nitrate uptake by these species. Our results provided convincing evidence of substantial nitrate assimilation to different extents by common conifer and broadleaved tree species in Northeast China.

Key words: nitrogen uptake strategy, ¹⁵N tracer, species coexistence, broadleaved species, mycorrhizal type, fine root to shoot ratio