Abstract:

Objective: NH₄⁺-N and NO₃^--N are the main inorganic N forms absorbed and utilized by trees. As distribution of absorbable N forms in soil is subject to great spatial heterogeneity, different tree species tend to develop different preferences for nutrients and absorption strategies. This study investigated the preference behavior of N, root morphological traits, and seedlings growth in Cunninghamia lanceolata, Pinus massoniana, and Schima superba under N heterogeneous environments. The results should reveal the mechanisms of nutrient preference and N foraging strategy in response to N heterogeneous environment, and provide a scientific theoretical basis for different tree species configuration of the mixed broadleaf-conifer forest plantations. It may also contribute to silviculture practice to improve the N use efficiency and productivity of the plantations. Method: In this study, four N treatments with different N forms (NH₄⁺-N and NO₃^--N) and proportion were constructed with the self-designed root box culture system. The NH₄⁺:NO₃^- was 10:0 and 0:10 for the high hetero-N, 8:2 and 2:8 for the medium hetero-N, 6:4 and 4:6 for the low hetero-N, and 5:5 for the homo-N, respectively. The seedlings of C. lanceolata, P. massoniana and S. superba were subjected to the four N treatments in order to investigate the root growth and morphological traits, and seedlings growth after 180 days treatment. Results: (1) under the heterogeneous N supply treatments, the root length, surface area and biomass of C. lanceolata in NO₃^--N patches were greater than in the NH₄⁺-N patches, but the root length, surface area and biomass of P. massonina and S. superba in NH₄⁺-N patches were greater than in the NO₃^--N patches. There was no significant difference in root morphological traits between NO₃^--N and NH₄⁺-N nutrient patches under the homogeneous N supply. The total root length, surface area and biomass of the three species were as the follow:homogeneity < low heterogeneity < medium heterogeneity < high heterogeneity. For example, the root biomass under the treatment of high heterogeneity was 7.96%-20.15%, 3.47%-19.07%, and 4.49%-9.08% higher than that with the other three treatments, respectively. (2) The height, diameter and total biomass of the three tree species were as the follow:low heterogeneity > homogeneity > medium heterogeneity > high heterogeneity. For example, the biomass under the treatment of low heterogeneity was 5.40%-33.67%, 7.61%-31.24%, and 11.32%-36.61% higher than that of the other three treatments, respectively. The root to shoot ratio of the three species under the high heterogeneity treatment increased by 58.47%-92.68% compared with the treatment of homogeneous N supply Conclusion: Under the heterogeneous N supply, C. lanceolata tends to grow more and slender roots in NO₃^--N patches, but P. massoniana and S. superba tends to grow more and slender roots in NH₄⁺-N patches. The root length, surface area and biomass of the three tree species increased with the increase of heterogeneity, but the average diameter was smaller. The height, diameter and biomass of the three tree species were greatest in the environment of lower heterogeneity and homogeneous N supply. It is concluded that the C. lanceolata prefers to absorb and utilize NO₃^--N, while P. massoniana and S. superba prefer to absorb and utilize the NH₄⁺-N under the environment of heterogeneous N supply.

Key words: root morphology, nitrogen uptake preference, foraging strategy for nitrogen, Cunninghamia lanceolata, Pinus massoniana, Schima superba