Root Development in Cunninghamia lanceolata and Schima superba Seedlings Expresses Contrasting Preferences to Nitrogen Forms

The inorganic nitrogen (N) that can be absorbed and utilized by plants is mainly ammonium N (NH4+-N) and nitrate N (NO3−-N), which may affect seedlings’ root morphology and growth through its heterogeneous distribution. Root morphology and seedling growth were investigated in a subtropical major conifer (Cunninghamia lanceolata) and a broadleaf tree species (Schima superba) under five different NH4+-N to NO3−-N ratios (10:0, 0:10, 7:3, 3:7, 5:5). Results: (1) While both species developed thinner roots under the treatment with a high NO3−-N concentration, the roots of C. lanceolata were longer than those of S. superba. In contrast, the roots of both species were thicker under the treatment with a high NH4+-N concentration, with those in S. superba being much longer than those in C. lanceolata. (2) The mixed NH4+-N and NO3−-N treatments were more conducive to the aboveground growth and biomass accumulation of both tree species and the underground growth of S. superba. N sources with high NO3−-N concentrations were more suitable for underground growth in C. lanceolata seedlings and aboveground growth in S. superba seedlings. Under the N sources with high NH4+-N concentrations, C. lanceolata tended to develop aboveground parts and S. superba tended to develop underground parts. (3) The roots of the two tree species adopted the expansion strategy of increasing the specific root length and reducing the root tissue density under the N sources with high NO3−-N concentrations but the opposite with high NH4+-N concentrations. The root-to-shoot ratio of C. lanceolata increased under high NO3−-N concentrations, while that of S. superba increased under high NO3−-N concentrations. These results indicate that the responses of root morphology to different N forms are species-specific. Furthermore, according to the soil’s N status, NH4+-N can be appropriately applied to C. lanceolata and NO3−-N to S. superba for cultivating seedlings.