Comparative study of urea-15N fate in pure bamboo and bamboo-broadleaf mixed forests

Abstract

Objectives: Bamboo is a globally significant plant with ecological, environmental, and economic bene-fits. Choosing suitable native tree species for mixed planting in bamboo forests is an effective measure for achieving both ecological and economic benefits of bamboo forests. However, little is currently known about the impact of bamboo forests on nitrogen cycling and utilization efficiency after mixing with other tree species. Therefore, our study aims to compare the nitrogen cycling in pure bamboo forests with that in mixed forests.

Methods: Through field experiments, we investigated pure Qiongzhuea tumidinoda forests and Q. tumidinoda-Phellodendron chinense mixed forests, and utilized ¹⁵N tracing technology to explore the fertilization effects and fate of urea-¹⁵N in different forest stands.

Results: The results demonstrated the following: 1) in both forest stands, bamboo culms account for the highest biomass percentage (42.99%-51.86%), while the leaves exhibited the highest nitrogen concentration and total nitrogen uptake (39.25%-44.52%/29.51%-33.21%, respectively) Additionally, the average nitrogen uptake rate of one-year-old bamboo is higher (0.25 mg kg^-1 a^-1) compared to other age groups. 2) the urea-¹⁵N absorption in mixed forests (1066.51-1141.61 g ha^-1, including 949.65-1000.07 g ha^-1 for bamboo and 116.86-141.54 g ha^-1 for trees) was significantly higher than that in pure forests (663.93-727.62 g ha^-1, P<0.05). Additionally, the ¹⁵N recovery efficiency of culms, branches, leaves, stumps, and stump roots in mixed forests was significantly higher than that in pure forests, with increases of 43.14%, 69.09%, 36.84%, 51.63%, 69.18%, 34.60%, and 26.89%, respectively. 3) the recovery efficiency of urea-¹⁵N in mixed forests (45.81%, comprising 40.43% for bamboo and 5.38% for trees) and the residual urea-¹⁵N recovery rate in the 0-60 cm soil layer (23.46%) are significantly higher compared to those in pure forests (28.61%/18.89%). This could be attributed to the nitrogen losses in mixed forests (30.73%, including losses from ammonia volatilization, runoff, leaching, and nitrification-denitrification) being significantly lower than those in pure forests (52.50%).

Conclusion: These findings suggest that compared to pure bamboo forests, bamboo in mixed forests exhibits higher nitrogen recovery efficiency, particularly with one-year-old bamboo playing a crucial role.

Keywords: 15N tracing technology; N recovery efficiency; Qiongzhuea tumidinoda; bamboo-broadleaf mixed forests; biomass.

Figure 1

Geographical location of the research area. (A) China; (B) Daguan County, Zhaotong city; (C, D) Q and Q-P represent a mixed forest of pure Q. tumidinoda and Q. tumidinoda-Phellodendron chinense, respectively.

Figure 2

Organ biomass characteristics of Qiongzhuea tumidinoda forests across four types of plots (Mean ± SD). In the first five figures, different uppercase letters in the same sampling site represent significant distinctions between different ages (P< 0.05), and different lowercase letters in the same age indicate significant differences between sampling sites (P< 0.05). In the last figure, different lowercase letters within the same organ indicate significant differences between different plots (P< 0.05).

Figure 3

Organ nitrogen concentration of Qiongzhuea tumidinoda forests across four types of plots (Mean ± SD). In the first five figures, different uppercase letters in the same sampling site represent significant distinctions between different ages (P< 0.05), and different lowercase letters in the same age indicate significant differences between sampling sites (P< 0.05). In the last figure, different lowercase letters within the same organ indicate significant differences between different sampling plots (P< 0.05).

Figure 4

Organ nitrogen uptake of Qiongzhuea tumidinoda forests across four types of plots (Mean ± SD). In the first five figures, different uppercase letters in the same plot represent significant distinctions between different ages (P< 0.05), and different lowercase letters in the same age indicate significant differences between plots (P< 0.05). In the last figure, different lowercase letters within the same organ indicate significant differences between different plots (P< 0.05).

Figure 5

¹⁵N uptake of Qiongzhuea tumidinoda forests across four types of plots (Mean ± SD). In the first five figures, different uppercase letters in the same plot represent significant distinctions between different ages (P< 0.05), and different lowercase letters in the same age indicate significant differences between plots (P< 0.05). In the last figure, different lowercase letters within the same organ indicate significant differences between different plots (P< 0.05).

Figure 6

Total absorption efficiency of different ages in Qiongzhuea tumidinoda forests. Different lowercase letters represent significant differences between different ages (P< 0.05).

Figure 7

The distribution of residual urea-¹⁵N in the soil. Different lowercase letters of the same soil layer indicate significant differences among different sampling sites at the P< 0.05 level.

Figure 8

Effects of various forest types on ¹⁵N recovery efficiency in Qiongzhuea tumidinoda forests. Different lowercase letters within the same organ indicate significant differences between different sampling sites (P< 0.05).

Figure 9

Effects of various forest types on the N recovery efficiency of different ages in Qiongzhuea tumidinoda forests. Different lowercase letters within the same forest type indicate significant differences in nitrogen recycling efficiency among different ages (P< 0.05).

Figure 10

Effects of various forest types on the fate of urea-¹⁵N in Qiongzhuea tumidinoda forests.

Figure 11

Comparison of Morphological Characteristics of Qiongzhuea tumidinoda in different forest types. The different lowercase letters of columns of the same color representing the height and diameter at breast height at different ages show significant differences (P< 0.05), respectively.

Figure 12

Comparison of nitrogen loss rates through various pathways of Qiongzhuea tumidinoda in different forest types. Different lowercase letters within the same forest type indicate significant differences in nitrogen loss rates through different nitrogen loss pathways (P< 0.05), respectively. Avo, ammonia volatilization; Nru, nitrogen runoff; Nle, nitrogen leaching; Nde, nitrification-denitrification.