Inorganic Nitrogen Sources Alter the Root Morphology of Neotropical Tree Seedlings from Different Successional Groups

Root morphological responses of neotropical tree seedlings to nitrate and ammonium nutrition vary according to the successional group, but may also depend on each species. Neotropical tree seedlings from contrasting successional groups have different nitrogen (N) use strategies, as well as they exhibit striking differences in root morphology. Here, we assessed the effects of hydroponic cultivation with nitrate or ammonium on the growth and root morphological traits of seedlings of pioneer (Cecropia pachystachya, Croton floribundus, Schinus terebinthifolius, Solanum mauritianum, and Trema micrantha) and non-pioneer (Cabralea canjerana, Campomanesia xanthocarpa, Cariniana estrellensis, Guarea kunthiana, and Poecilanthe parviflora) tree species native to Brazilian Atlantic Forest. Nitrate-grown seedlings of pioneer species showed higher shoot and root dry matters and total root length compared to those cultivated with ammonium, while non-pioneer trees showed smaller or no difference in these traits when comparing N sources. These results agree with the decrease of nitrate availability in the soil relative to that of ammonium over an Atlantic Forest succession. Regardless of the successional group, nitrate treatment resulted in longer main and lateral roots in most species. The effects of nitrate and ammonium on specific root length, lateral-root density, and fine-root diameter could be associated with successional groups and N source preference. However, root hair responses varied according to each species. Cecropia pachystachya and C. canjerana roots had morphological responses to N source intermediaries between the successional groups, which are coherent with their ecological niches. These results demonstrate that the effects of N source on the growth and root morphology of neotropical tree seedlings could be associated with their successional group, although species-specific responses were also observed.