Effects of Different Ammonium to Nitrate Ratios on Nitrogen Uptake Preference and Traits of Absorptive Roots of Subtropical Trees Seedlings

【Objective】This study aims to clarify the effects of different ammonium to nitrate ratios on nitrogen (N) uptake preference and traits of absorptive roots of coniferous and broadleaf seedlings, so as to provide scientific basis for understanding nutrient acquisition and soil N cycle in subtropical plantations.【Method】Two-year-old seedlings of common subtropical tree species, including Pinus elliottii, Pinus massoniana, Michelia maudiae, and Liquidambar formosana, were cultured in sands with addition of three different ammonium to nitrate ratios (15∶1, 8∶8, and 1∶15). The inorganic N uptake rates of plants were measured by using the 15N isotope tracer technique, and the correlations between the inorganic N uptake rates and the morphological and architectural traits of absorptive roots were analyzed. 【 Result】 1) The inorganic N uptake rates of the seedlings of the four species varied under environments with different ammonium to nitrate ratios. The four species preferred to absorb NH4+-N when ammonium was dominant in the sand culture, but preferred to absorb NO3−-N when nitrate was dominant. However, species showed different N uptake when the ratio of ammonium to nitrate was 8∶8. The coniferous species absorbed NO3−-N at a rate more than twice that of NH4+-N, but there was no significant difference in the uptake of the two forms of nitrogen by broadleaved species. 2) Ammonium-nitrate ratio had a significant impact on root specific length and specific surface area, but had no effect on diameter, tissue density, branching ratio and branching intensity. 3) The inorganic N uptake rate of the fourspeciesseedlingswassignificantlypositivelycorrelatedwithrootbranches,andsignificantlynegativelycorrelatedwithorganbiomass. 【Conclusion】The N uptake rate of seedlings of four tree species depends on substrate N availability and species specificity in response to contrasting edaphic environment. The two coniferous species obtain nutrients through the trade off between physiology (decreased N uptake rate) and morphology (increased specific root length and decreased diameter). However, the two broadleaved species obtain nutrients by simultaneously strengthening the morphological (increased specific root length and surface area), architectural (increased branching ratio), and physiological (increased NH4+-N uptake rate) traits of absorptive roots. These various nutrient acquisition strategies enable tree species to better adapt to changing environments. © 2023 Chinese Society of Forestry. All rights reserved.