Nitrogen loading increases the ozone sensitivity of larch seedlings with higher sensitivity to nitrogen loading.

Larch (Larix sp.) tree is a critical species for the future afforestation in Northeast Asia. The impacts of elevated concentrations of ground-level ozone (O) and nitrogen (N) deposition are raising concerns. However, knowledge of the combined effects of elevated O and N loading are still limited. We investigated whether nitrogen loading mitigates the negative impacts of ozone on two larch species: the Japanese larch (L. kaempferi) and its hybrid larch F (L. gmelinii var. japonica × L. kaempferi) or not. We used open-top cambers and compared responses of the larch seedlings. Results showed the N loading mitigated the negative effects of O on Japanese larch. However, in hybrid larch F, N loading did not mitigate O-induced inhibition of growth and photosynthetic capacity. Mitigation effect of N loading on negative O impacts may vary between the two Larix spp., Hybrid larch F could be more affected by the combined effects of O and N loading due to its higher growth response to N loading. Elevated O also reduced leaf nitrogen/phosphorus (N/P) ratio by elevated O, with significant effects in hybrid larch F, particularly under N loading. In the present study, leaf N/P ratio was utilized to validate the hypothesis that a positive effect of N loading may be observed if O does not induce P limitation in Larix spp. We demonstrated a potential leaf N/P ratio function, which could reflect responses to O and N loading in hybrid larch F.