Responses of root morphology and seedling growth in three tree species to heterogeneous supplies of ammonium and nitrate

Distribution of absorbable N forms (NH4+ and NO3-) in soil is subject to high spatial heterogeneity, and NH4+ and NO3- deficiency is one of the major factors limiting the productivity of forest plantations. Tree species may have developed different N preferences during their evolution. Here, we investigated the N uptake preference and root growth characteristics in three major tree species (Cunninghamia lanceolata, Pinus massoniana, and Schima superba) in southern China under homogeneous and heterogeneous environments of N forms. Seven N -supply proportions were created with two N forms to establish four levels of heterogeneity, with the NH4+ to NO3- ratio being either 10:0 or 0:10 as high hetero-N, either 8:2 or 2:8 as medium hetero-N, either 6:4 or 4:6 as low hetero-N, and 5:5 as homo-N, respectively. Each seedling was cultured in a specially designed container with two tray halves supplied with equal amounts of total nitrogen but reversed ratios of ammonium and nitrate. Root morphology, root and seedling growth, root-to-shoot ratio, N accumulation and N use efficiency of seedlings were quantified. For all the three species, the root traits were either symmetrical or asymmetrical between the paired patches with the homogenous and heterogeneous supplies of NH4+ and NO3-, respectively. Roots of C. lanceolata in the patch of higher NO3- tend to grow thinner and longer than their counterparts with higher NH4+ supply, whereas roots for P. massoniana and S. superba tend to grow thinner and longer in the higher NH4+ supply patch. The belowground growth increased with the increase in heterogeneity level, but the aboveground growth decreased. The N heterogeneity magnitude was negatively correlated with specific root length and total N accumulation, and positively correlated with root-to-shoot ratio and N use efficiency. The change of root growth pattern might be a strategy to adapt to heterogeneous environments of N forms in soil, and the increase in root-to-shoot ratio would optimize nutrient partitioning and improve N-nutrient absorption in hetero-N soils. Among the three tree species, S. superba had higher responsiveness to the level of N heterogeneity than C. lanceolata and P. massoniana. Our results suggested that C. lanceolata prefers NO3- over NH4+, whereas P. massoniana and S. superba prefer NH4+ over NO3-. The findings confirmed a species-specific preference for different N forms and could provide a reference for the nitrogen management of different species in silviculture practice.