Qiongzhuea tumidinoda was one of the two bamboo species listed in the first edition of the “Chinese Rare and Endangered Plants Protection List” published in 1984, designated as a nationally protected plant at the third level. It is indigenous to the southwestern region of China, with its natural distribution confined to a narrow strip along the lower reaches of the Jinsha River in the provinces of Sichuan and Yunnan. There exists a total area of 13900 hectares of natural Q. tumidinoda resources in Daguan County. This area accounts for 59% of the global total area of natural Q. tumidinoda, which amounts to 23560 hectares (Dong, 2019; Wu et al., 2023b). The highly raised nodes on the culms of Q. tumidinoda make it an excellent material for crafting walking sticks, bamboo handicrafts, and round bamboo furniture (Yiyuan et al., 2020; Li et al., 2021). Bamboo shoots from Q. tumidinoda are renowned for their exquisite taste, crisp and tender texture, and delightful sweetness, while also boasting a rich nutritional profile. Consequently, over 90% of products derived from Q. tumidinoda, such as fresh bamboo shoots, dried bamboo shoots and salted bamboo shoots, have consistently enjoyed robust sales in Japan and the Greater China region, encompassing Hong Kong, Macau, and Taiwan (Dong, 2019; Li et al., 2021).

Q. tumidinoda demonstrates rapid growth, taking just around 50 days from the emergence of bamboo shoots to reaching full height and diameter, enabling the proliferation of numerous new individuals within a span of two months and consuming substantial nutrients (Dong et al., 2002; Wu et al., 2022). Q. tumidinoda forests are primarily managed for bamboo timber and bamboo shoots. With the rapid development of the bamboo industry, the substantial annual harvest of bamboo timber and bamboo shoot biomass inevitably leads to direct removal of a significant amount of nutrients, resulting in the depletion of soil nutrients in bamboo forests (Yang et al., 2012; Li et al., 2024). Furthermore, the slow decomposition of residual rhizomes and stumps left after harvesting in bamboo forests results in a low nutrient return rate (Jiang, 2007; Zheng et al., 2022). Therefore, achieving sustainable high yields in bamboo forests requires nutrient supplementation through fertilization. Among these, nitrogen fertilizer stands as the primary nutrient factor enhancing bamboo forest productivity (Sardar et al., 2023; Zhao and Cai, 2023; Zou et al., 2023). In bamboo forest ecosystems, nitrogen allocation directly influences the growth of bamboo shoots, the development of bamboo culms, and the overall productivity of the stand (Wu et al., 2023a; Zuo et al., 2024).

However, overreliance solely on nitrogen fertilizers can lead to a series of issues such as soil compaction and groundwater contamination, particularly pronounced in monoculture bamboo forests (Tariq et al., 2015; Wang et al., 2023a). To tackle this challenge, strategies involving intercropping broadleaf forests with bamboo stands are frequently employed (Peng et al., 2021). Research indicated that compared to pure forests, mixed forests may have had higher species diversity, leading to potentially more diverse root exudates and leaf litter, further enhancing soil chemical properties such as total nitrogen (Gillespie et al., 2021; Liang et al., 2022). Additionally, studies found differences in microbial diversity and composition between pure and mixed forests, resulting in distinct nitrogen utilization patterns possibly indirectly influenced by pH and differing litter qualities (Wen et al., 2014; Bai et al., 2023). These variances possibly led to higher rates of soil nitrogen mineralization and nitrification in broadleaf trees in mixed forests compared to pure ones, consequently elevating nitrogen concentrations in the soil (Yan et al., 2008; Kong et al., 2020; Yan et al., 2022). Furthermore, broadleaf forests aid in soil moisture retention, providing compensatory ecosystem services to address the limitations of pure bamboo forest ecosystems (Bauhus et al., 2017; Gong et al., 2022). Research has shown that this difference was determined by specific hybrid tree species and hybrid ratios. For instance, Weih et al (Weih et al., 2021). studied the nitrogen utilization patterns of four hybrid willow forests and found that individual species’ functionalities played a determining role. Recent studies on nitrogen in monoculture and mixed forests focused mostly on the distribution patterns of nitrogen in plants, soil or systems, but there was limited research on the fate of nitrogen in monoculture and mixed forests (Voigtlaender et al., 2019; Masuda et al., 2022).

Q. tumidinoda, a small to medium-sized bamboo species, thrives in temperate and humid environments. Lots of research have indicated that mixed forests of Q. tumidinoda with broadleaf trees exhibit superior productivity (Zhang et al., 2020b; Chen et al., 2022), soil quality (Xia et al., 2022; Yuan et al., 2022), species diversity (Yang et al., 2012) and water conservation (Zhong et al., 2020) compared to pure Q. tumidinoda forest. Studies on nitrogen in Q. tumidinoda have mainly focused on soil nitrogen concentration related to its growth (Zhang et al., 2020b; Zhong et al., 2023), whereas research on nitrogen allocation patterns in bamboo and its utilization in different forest types remains unexplored. Simultaneously, to effectively utilize nitrogen, the ¹⁵N tracing technique has been widely employed to quantify nitrogen fertilizer uptake, residual amounts, and losses in research. However, few studies have existed regarding the distribution and translocation of urea-¹⁵N in different forest types within Q. tumidinoda ecosystems. This study focuses on bamboo and soil from Q. tumidinoda forests and mixed forests of Q. tumidinoda with P. chinense. Utilizing ¹⁵N tracing techniques, the objectives are: (1) to compare the partitioning efficiency of applied nitrogen in different organs at various ages between pure forests and mixed forests; (2) to compare the nitrogen recovery and residue rates in bamboo ecosystems between pure forests and mixed forests to determine which type exhibits higher rates.

Bamboo and broadleaved mixed forest is an excellent agricultural forestry model featuring bamboo. Research indicates that in competition with broadleaved trees, bamboo exhibits a greater advantage, possibly owing to its enhanced plasticity and environmental adaptability (Zheng and Lv, 2023). For instance, an experiment on nitrogen uptake conducted with Castanopsis fargesii and moso bamboo revealed that moso bamboo maintained dominance due to its higher tolerance threshold to ammonium nitrogen (Zou et al., 2020). Numerous studies indicated that a beneficial competition was established when bamboo was mixed with an appropriate proportion of broadleaved trees (Cao, 2001; Cheng et al., 2015; Yan et al., 2018). For example, research indicated that when the intercropping ratio of moso bamboo and broadleaved trees was in the range of 20–30%, optimal soil nutrients were achieved, leading to the best growth performance of moso bamboo (Zhang et al., 2020a). Similarly, The intercropping of Q. tumidinoda with other tree species had a certain impact on the growth of the bamboo forest, and the extent of this impact depended on the choice of tree species (Zhou et al., 2016).

Previous research indicated that P. chinense was one of the excellent native tree species for establishing Q. tumidinoda mixed forests. In the mixed forests of P. chinense and Q. tumidinoda, the diameter at breast height, height, and biomass of bamboo were significantly higher than those in pure Q. tumidinoda forests (Zhang et al., 2020b; Chen et al., 2022). This study also confirmed these findings, where the biomass of various parts of Q-P type’s Q. tumidinoda was significantly higher compared to Q type (except for rhizome and rhizome root). However, the difference in underground rhizome-root system (rhizome and rhizome root) between Q-P and Q types was not significant, possibly due to the occupation of certain underground spaces by p. chinense root. In the fertilized treatment, there was a significant difference in the underground rhizome-root system of Q-P type, indicating that compared to Q type, Q-P type’s rhizome-root system absorbed more nitrogen, consequently accumulating more biomass. Furthermore, compared to the unfertilized treatment, the biomass of various bamboo organs in both forest types increased under fertilization, but only the rhizome and rhizome roots reached significant levels, This could be attributed to the relatively high soil temperature and abundant rainfall during this period, which prompts bamboo to primarily focus its growth on the underground parts, accumulating a significant amount of nutrients in its rhizomes and shoots (Chen and Yang, 2003; Zhu et al., 2023).

Due to age structure and individual size ( Figure 11 ), the biomass of various organs of the four-year-old bamboo was significantly lower than in other age groups. The biomass of culm was the highest, consistent with previous research findings, accounting for 42.72% of the total biomass as revealed by this study (Chen et al., 2022).

Following fertilization, both the nitrogen concentration and uptake in various organs of Q-P type were significantly higher than in Q type, further emphasizing Q-P type’s greater nitrogen absorption. The leaves exhibited the highest nitrogen concentration and uptake, likely attributed to their photosynthetic activity. Despite the culms having the largest biomass, their nitrogen absorption was lower due to their comparatively lower nitrogen concentration.

In bamboo forest ecosystems, nitrogen fertilizer is primarily utilized in three ways: uptake by bamboo, retention within the soil, or loss from the bamboo-soil system (Chalk et al., 2015; Wang et al., 2023b). Currently, the use of ¹⁵N isotope tracing technology is considered the optimal method for studying nitrogen fertilizer utilization efficiency and nitrogen balance in bamboo forest ecosystems (Haque et al., 2022). This technique, employing labeled ¹⁵N fertilizers, enables direct or indirect determination of nitrogen recovery by bamboo, residual fertilizer levels in the soil, and nitrogen loss rates (Raymond et al., 2016; Su et al., 2019). In this study, ¹⁵N distribution varied among different types of Q. tumidinoda forests, yet the overall allocation pattern remained largely consistent. A range of 32.72% to 38.53% of the total ¹⁵N absorption was allocated to the leaves, representing the highest proportion. Following this, the branches accounted for 19.06% to 23.89% of the total ¹⁵N absorption. This alignment with the distribution of 26.90% to 37.21% of total ¹⁵N absorption in the leaves in Moso bamboo forests. However, discrepancies in the overall ¹⁵N absorption distribution were evident among different Q. tumidinoda forests. For instance, in Moso bamboo forests, bamboo stump ranked second (Su et al., 2019), indicating variations possibly attributed to different bamboo species. Based on previous studies, the residual amount and downward movement of nitrogen fertilizer can be reflected by the concentration of ¹⁵N in the soil layers (Wang et al., 2011; Jing et al., 2020). The average residual amount of ¹⁵N-labeled urea in the 0–60 cm soil layer of Q-P type was 567.73 g ha^-1, accounting for 23.46% of the total applied ¹⁵N. It was significantly higher in all soil layers compared to Q type, correlating with the soil organic matter content. Research has shown that nitrogen becomes immobilized within soil organic matter (Mostafa et al., 2020). In this study, the participation of p. chinense ‘s litter in decomposition led to higher organic matter content in all soil layers of Q-P type compared to Q type, consequently immobilizing more nitrogen. The residual ¹⁵N in both forest types exhibited a decreasing trend with increasing soil depth, consistent with previous reports (Ru et al., 2020; Nguyen et al., 2021; Effah et al., 2022).

It is well-known that ammonia volatilization, nitrification-denitrification, runoff, and leaching are the primary pathways for nitrogen loss (Räbiger et al., 2020; Shi et al., 2020; Lan et al., 2022). In this study, it was found that the nitrogen recovery efficiency and soil nitrogen residual efficiency of the mixed forest (Q-P) were significantly higher than those of the pure forest (Q), which may be attributed to the differences in nitrogen loss between them. All types of nitrogen loss pathways in the Q-P type (including ammonia volatilization, runoff, leaching, and nitrification-denitrification) were significantly higher than those in the Q type ( Figure 12 ). This conclusion can be inferred from previous studies. For instance, research has indicated that compared to pure bamboo forests, the canopy of mixed bamboo and broadleaf tree forests provides effective shading, thus reducing ammonia volatilization (Cheng et al., 2015; Zhang et al., 2020a). Additionally, the canopy interception by broadleaf trees in mixed forests, along with the impact of their litter and root systems on soil and water conservation (Zhang et al., 2020a; Nainar et al., 2021; Ding et al., 2023), results in lower nitrogen losses from runoff and leaching compared to pure forests. Apart from differences in nitrogen loss, the Q-P type exhibited two distinct nitrogen recovery pathways: the recovery rate of ¹⁵N from P. chinense (5.37%, Table 4 ) and the recovery rate of ¹⁵N from bamboo (40.43%). Compared to the single forest type of Q type, Q-P type more efficiently utilized the abundant nitrogen resources.

In this study, the mean nitrogen recovery rate for both bamboo forest systems was 37.21%, significantly higher than the nitrogen recovery rate determined by Su et al (Su et al., 2019). in fertilized Moso bamboo forests (28.98%). These findings differed from the recovery rate of fertilization in Moso bamboo forests reported by Mao et al (Mao et al., 2016). (13.96%). This discrepancy could potentially stem from variations in bamboo biological characteristics, timing, and dosage of fertilizer applications. Therefore, further experiments are needed to validate the specific reasons.

In this study, it was clearly indicated that there are significant differences in the fate and distribution ratios of nitrogen labeled urea applied in different types of bamboo forest ecosystems. The bamboo forest with mixed P. chinense and Q. tumidinoda exhibited notably higher nitrogen recovery and soil residue compared to the pure Q. tumidinoda forest, while showing an opposite trend in nitrogen loss rate. In these two types of bamboo forests, despite the largest biomass being in the bamboo culms, the leaves exhibited the highest nitrogen absorption and content. The residual ¹⁵N was primarily concentrated in the fertilized layer. These studies indicate that the proportion of trees to bamboo in this experimental design may fall within an appropriate range of mixed cropping ratios, thereby enhancing the nitrogen recovery efficiency of bamboo and reducing nitrogen loss efficiency. However, the competitive relationship between trees and bamboo cannot be ignored. Therefore, we hypothesize that increasing or decreasing the proportion of trees in bamboo forests may have similar or opposite effects on the nitrogen cycle of bamboo, which requires further experimental support. Additionally, the impact of different types of mixed tree species on bamboo may vary, especially the intercropping of nitrogen-fixing tree species with bamboo, which will be the focus of future research.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

YW: Formal analysis, Investigation, Writing – original draft, Writing – review & editing, Conceptualization, Methodology. WD: Conceptualization, Funding acquisition, Supervision, Writing – review & editing. HZ: Methodology, Writing – review & editing. JD: Data curation, Investigation, Writing – review & editing. WL: Data curation, Investigation, Writing – original draft. CP: Formal analysis, Writing – original draft. XL: Formal analysis, Writing – original draft. ZX: Project administration, Resources, Writing – review & editing.