樟树(Cinnamomum camphora)幼苗细根形态对氮磷添加和幼苗密度的响应

全球氮沉降对森林生态系统结构和功能的影响已成为现代生态学研究热点之一,我国华南地区氮沉降的增长引起了土壤酸化和磷限制加剧等一系列生态问题。密度制约着植物个体对环境资源的吸收利用,是自然界中十分重要的选择压力之一。因此研究樟树(Cinnamomum camphora)幼苗的细根形态对氮磷添加和密度的响应,有利于了解亚热带树木根系对氮沉降和磷添加与林分密度的响应过程和机制,并为全球变化背景下樟树林生态系统的管理提供依据。本研究以1年生樟树幼苗为试验材料,选择氯化铵(NH_4Cl)作为氮肥以模拟大气氮沉降,并且以二水合磷酸二氢钠(NaH_2PO_4·2H_2O)模拟磷添加,氮磷处理设置4个水平,即对照、施N、施P和施N+P;种植密度设置10、20、40和80株/m~2 4个水平。测定各处理樟树幼苗细根的根长、表面积、体积和根尖数,分析氮磷添加、密度和两者交互作用对樟树幼苗细根的影响。研究结果表明,与对照处理相比,N、P和N+P处理促进了幼苗细根长度、表面积、体积以及根尖数的增加。低密度条件下的N添加对幼苗根系形态的促进效果强于P添加。N+P处理对10、20、40株/m~2幼苗根系形态的促进效果最佳,而各处理对80株/m~2幼苗根系形态的促进效果均无显著性差异。随着种植密度的增大,幼苗细根长度、表面积、体积和根尖数均减少。樟树幼苗的细根长度、表面积、体积和根尖数在各密度间和不同氮磷添加处理间均有显著性差异,密度和氮磷处理间的交互作用对根系形态各指标均无显著影响。

Responses of fine root morphology of Cinnamomum camphora seedlings to nitrogen and phosphorus additions and planting density

With the continuing deposition of atmospheric N and wide application of P fertilizer, N and P have become a major factor affecting the ecosystem C balance. Fine root morphological characteristics, such as the root length, surface area, volume, and tips, play a critical role in many functions, including nutrient absorption. Global N deposition and P addition induce changes in root morphology. Seedling density is an important factor affecting root morphology because of the enhanced resource competition among high density seedlings, which reflects the allocation ratio of photosynthetic C in the roots, and ultimately affects the overall plant C allocation balance. Understanding the effects of N and P additions and planting density on the fine root morphology is important to be able to improve fine root health. However, knowledge regarding the effects of N and P additions on the root morphology is limited, and it is still unclear how N and P additions and seedling density alter the fine root morphology of Cinnamomum camphora seedlings. In this study, the root morphology of C. camphora seedlings planted at different densities was evaluated, along with the root length, surface area, volume, and tips, in order to reveal the effects of N and P additions and plant density on the fine root morphological characteristics of this species. One-year-old C. camphora seedlings were used as test materials, and NH₄Cl and NaH₂PO₄·2H₂O were selected to simulate N deposition and P addition, respectively. N and P additions were performed with four different levels of treatment (control, N, P, and N+P), and seedlings were planted at four different densities (10, 20, 40, and 80 seedlings/m²). The results showed that the N, P, and N+P treatments increased the root length, surface area, volume, and tips of seedlings compared to those of the control treatment. The effect of N addition on root morphology was stronger than that of P addition under low planting densities. The N+P treatment had the greatest effect on the fine root morphology at densities of 10, 20, and 40 seedlings/m², while there was no significant difference in the effect of each treatment at 80 seedlings/m². With increased planting density, the length, surface area, volume, and tips of the seedling roots tended to decrease. The effects of different densities or treatments on the fine root length, surface area, volume, and tips of the seedlings were significantly different, but the interactive effect of density and N and P additions on the fine root morphology of the seedlings was not significant.