Nitrogen (N) is one of the most important and limiting elements in forest ecosystems (LeBauer and Treseder, 2008). Plants utilize nitrogen in various forms, including ammonium (NH₄ ⁺–N), nitrate (NO₃ ⁻–N), and organic nitrogen, and the absorbed nitrogen contributes to their productivity, biodiversity, and ecosystem functions (Kahmen et al., 2008; Andersen and Turner, 2013).

The N-acquisition strategies of plants are influenced by abiotic and biotic factors. The availability of soil N can affect root N uptake and substrate preference. For example, plants in boreal forests (Nordin et al., 2001), temperate forests (Zhou et al., 2019), subtropical plantations (Liu et al., 2020), and tropical forests (Liu et al., 2017) preferred to absorb ammonium where the ammonium content was higher than the nitrate level in the soil. Similarly, in areas where soil nitrate was dominant, temperate Fagus sylvatica (Jacob and Leuschner, 2015), Schima superba, and Liquidambar formosana (Gessler et al., 1998) preferred to absorb nitrate. Some evidence suggests that plants may prefer organic N over inorganic N, or absorb both equally, especially in low-temperature, N-restricted polar, alpine, and boreal ecosystems (Chapin et al., 1993; Näsholm et al., 1998; Öhlund and Näsholm, 2001; Simon et al., 2021). However, some studies have reported that the plant N uptake preference was in contrast to the predominant form of N in the soil. For example, Fagus grandifolia seedlings (Templer and Dawson, 2004), Larix gmelinii, and Betula platyphylla (Gao et al., 2020) preferred to absorb nitrate, although the content of ammonium in the soil was 2–14 times higher than that of nitrate. This indicates that the plant uptake preference for different forms of N is not only related to the availability of N in the soil but also to internal factors specific to the species. For instance, the inorganic N uptake rate of arbuscular mycorrhizal species was higher than that of ectomycorrhizal species (Liese et al., 2018). In recent years, negative correlations (Liu et al., 2020; Yi et al., 2023), positive correlations (Hong et al., 2018), and no correlation (Ma et al., 2018) have been reported regarding the linear relationship between physiological N uptake rate and the morphology of specific root length (SRL). Therefore, the root N uptake rate and preference are complex phenomena due to the combined influence of abiotic and biotic factors. In addition, the relationship between the root N uptake rate and the structural traits of the same tree species based on age is an aspect that has yet to be examined.

With the increase in tree age, the biotic and abiotic factors that affect root N absorption often change, which in turn will affect root N absorption. However, the existing research results have shown large variations in the N uptake with tree age due to differences in the study sites and tree species. Tropical Hevea brasiliensis aged 7–49 years showed a preference for ammonium. The ammonium uptake rate increased initially and then decreased sharply, and the glycine uptake decreased initially and then increased sharply with age (Liu et al., 2018). Subtropical Cunninghamia lanceolata aged 5–30 years preferred to absorb ammonium, and the ammonium uptake rates for 30- and 5-year-old trees were similar and were higher than those of 13-year-old trees (Li et al., 2016). Temperate Pinus koraiensis aged 14–217 years preferred ammonium, and the ammonium uptake rate decreased with tree age (Ren et al., 2021). F. sylvatica aged 5–130 years preferred organic glutamine and arginine, and the inorganic and glutamine uptake rates did not differ among age classes (Simon et al., 2021). Therefore, given the uncertainties in the relationship between N uptake preferences (or the uptake rates) and stand age, conducting studies in different geographic regions and forest types will contribute to a more comprehensive understanding of the N acquisition strategies during forest succession.

Root traits can reflect the underground resource strategies of tree species. Root trait variation was initially assumed to be a one-dimensional root economic spectrum with a trade-off between resource acquisition and conservation (Comas and Eissenstat, 2004; Roumet et al., 2016). The roots in nutrient- and water-rich environments were characterized by their smaller diameter, higher SRL, higher N content, lower root tissue density (RTD), shorter root life span, and lower mycorrhizal colonization rate. As such roots have high nutrient and water absorption capacity, this indicated a resource-acquisitive strategy (Roumet et al., 2006, 2016; Pinno and Wilson, 2013; McCormack et al., 2015; Fort et al., 2016; Li et al., 2017). When the water and fertilizer contents are low, plant roots show the opposite pattern, a resource-conservative strategy. However, an increasing number of studies have demonstrated that variations in root traits are multidimensional rather than reflecting a single axis related to resource economics (Kong et al., 2014; Wang et al., 2018; Kong et al., 2019; Bergmann et al., 2020; Ding et al., 2020; Weigelt et al., 2021; Yan et al., 2022). For example, Bergmann et al. (2020) analyzed the root traits of 1,810 species and proposed a two-dimensional root economic space with conservation and collaboration gradients. In addition, root exudation traits (Wen et al., 2022), root respiration (Han and Zhu, 2021), and N uptake rate (Yi et al., 2023) were integrated into the current theoretical framework of root economic space. The above studies reflect the trade-off dimensions of the root morphological, architectural, anatomical, chemical, and physiological traits across species. However, there are few studies on root economic space based on the age gradient of the same tree species.

In this study, root physiology (N uptake rate), morphology (diameter, SRL, and RTD), architecture (branching ratio and intensity), chemistry (N and C contents), and mycorrhizal colonization rate in the L. principis-rupprechtii chronosequence (aged 18, 27, 37, 46, and 57 years) in the Saihanba Mechanical Forest Farm in Northern China were analyzed. We aimed to verify 1) how the N uptake rates and substrate preferences of L. principis-rupprechtii plantations change with tree age and 2) how the economic space based on root functional traits changes along the age gradient in L. principis-rupprechtii plantations. Here, we hypothesized that 1) with the increase in tree age, the N uptake rate of L. principis-rupprechtii roots would decrease while the nitrogen preference would not vary (Simon et al., 2021; Ren et al., 2023) and 2) trees with different ages would show different N strategies, indicating two dimensions of collaborative and conservative root functional traits along the age gradient. These hypotheses were formed following the root economic space theory for multi-tree species (Bergmann et al., 2020).

In the long-term evolutionary process, plants have formed absorption mechanisms for different forms of nitrogen, generally showing a preference for a specific N form (Bueno et al., 2019). In this study, L. principis-rupprechtii preferred to absorb ammonium and glycine, and the contribution of ammonium increased while that of glycine decreased with tree age. Our study showed that the preference for ammonium uptake did not change with age, supporting our first hypothesis. This is consistent with previous studies demonstrating that trees (e.g., C. lanceolata, H. brasiliensis, and P. koraiensis) of different ages showed a preference for ammonium (Li et al., 2016; Liu et al., 2018; Ren et al., 2021). Firstly, the nitrate in plants needs to be converted into ammonium, then glutamate, and then further utilized by plants (Wang and Macko, 2011). Therefore, the energy required for plants to utilize ammonium is less than that for nitrate; this may be one of the reasons for plants preferring ammonium. Another reason could be that ammonium is abundant in soils, and the content of ammonium increased with increasing forest age in this study ( Table 2 ). Other studies have found that tree species prefer to absorb ammonium, consistent with the predominance of ammonium in soils (Finzi and Berthrong, 2005; Liu et al., 2017; Zhou et al., 2019). Moreover, Zhang et al. (2018) found that the N uptake preference of P. asperata in the Tibetan Plateau changed from nitrate to ammonium nitrogen with increasing age and speculated that this was related to the change in the dominant N form in the soil.

The ammonium uptake rate of L. principis-rupprechtii roots significantly decreased and then increased, while the glycine and nitrate uptake rates decreased with increasing tree age, partly supporting our first hypothesis. This is in contrast to previous studies. Liu et al. (2018) found that the ammonium uptake rate of H. brasiliensis increased initially and then decreased sharply, while the glycine uptake decreased initially and then increased sharply with age. The ammonium uptake rate of P. koraiensis decreased with tree age (Ren et al., 2021), while that of F. sylvatica did not differ (Simon et al., 2021) with tree age. The reason for the contradictory findings of nitrogen absorption with age may be related to the differences in tree species and the availability of soil nitrogen. In this study, the nitrate and glycine uptake rates were positively linearly correlated with the nitrate and glycine contents in soils across the age gradient. Another reason for the changes in N uptake by plants with age may be related to the root traits of the species. The contribution of ammonium to the N uptake increased with tree age as a result of the decreased nitrate and glycine uptake in older L. principis-rupprechtii. Compared to nitrate, the diffusion rate of ammonium ions in the soil is much lower (Barber, 1995), suggesting that, in the case of ammonium, the roots should grow closer (and show increased branching) to the N source for more efficient uptake. Moreover, localized ammonium increases lateral root branching, while nitrate induces lateral root elongation (Remans et al., 2006; Lima et al., 2010). In our study, with the increase in forest age, the decrease in soil nitrate content was accompanied by a decrease in SRA. The decreased nitrate uptake in older trees could be caused by the ECM increasing with tree age ( Figure 2 ), with the ability of the colonized root tips to take up nutrients directly disappearing as a result.

In this study, the total N uptake rate decreased significantly with increasing tree age, indicating that the nitrogen demand of older trees was relatively reduced compared to that of younger trees. This is consistent with previous studies showing that the root total N uptake rate of rubber trees and Korean pine decreased gradually with the increase in tree age (Liu et al., 2018; Ren et al., 2021). One possible reason is that younger trees grow at a faster rate and have higher nutrient requirements for adding biomass than older trees (Borchert, 1975; Ryan et al., 1997). As trees age, their growth rates gradually reach a maximum, and the available N in the soil tends to decrease (Gower et al., 1996; Tang et al., 2014). A second reason for the decrease in the N uptake rate of roots with increasing tree age may be related to leaf N resorption. Significant positive correlations between leaf N resorption efficiency and tree age have been observed in L. principis-rupprechtii plantations (Sun et al., 2016). As a mature forest tree, the older larch is a stronger N reservoir and has a greater N retention capacity than the younger tree (Simon et al., 2011; Sun et al., 2016); therefore, its growth may depend more on the nutrient recovery of its own organs than on the available nutrients in the soil. Although the soil types and stand conditions of our five age plots were similar, we still cannot rule out the effect of original soil differences on the N uptake rate.

The SRL and specific root surface area of L. principis-rupprechtii decreased with the increase in tree age as a result of increased root diameter and tissue density, similar to previous results. For example, Rosenvald et al. (2013) confirmed that the diameter and RTD of the first-order roots of Betula pendula gradually increased, while the SRL and SRA gradually decreased with the increase in forest age, ranging from 3 to 60 years. Moreover, for fine roots less than 2 mm in diameter, younger trees having higher values of SRL and SRA than older trees were reported in Cryptomeria japonica (Fujimaki et al., 2007), P. sylvestris (Jagodzinski and Kalucka, 2010), B. pendula (Kalliokoski et al., 2010), F. sylvatica, Quercus robur, Alnus glutinosa (Jagodzinski et al., 2016), Fraxinus velutina (Li et al., 2020), and Fraxinus mandshurica (Li et al., 2021). Considering the lower RC in younger L. principis-rupprechtii, the higher values of SRA at a young age indicate rapid growth and low construction costs, constituting a cost-saving method for acquiring soil resources. Compared to older trees, younger trees may need to develop more effective root systems to cope with survival pressures such as limited sunlight and soil resources. In this study, the root BR (the ratio of the tips of first-order roots to second-order roots) and BI (tips of first-order roots per length of second-order roots) were higher in young larch than in older larch. Previous studies have found that the root branching frequency (measured as tips per root dry mass) was higher in younger trees (Rosenvald et al., 2013). However, Børja et al. (2008) found that stand age had no effect on the root branching frequency of Norway spruce plantations. Normally, roots with higher SRL, SRA, and branches, but lower RTD and diameter, indicate a greater absorption capacity for soil resources (Wang et al., 2016; Weemstra et al., 2016). In our study, roots with lower diameter and RTD and higher SRL, SRA, BR, and BI exhibited higher nitrate, glycine, and total N uptake rates. Therefore, younger trees tended to develop roots with more morphologically and physiologically efficient nutrient uptake capacity to compensate for the smaller size of the root system.

Mycorrhizae and the physiological N uptake rates are important factors in root resource acquisition, but they have seldom been investigated from the standpoint of the theory of root economic space under intraspecies changes across tree ages. The PCA showed that the root functional traits (i.e., morphology, architecture, chemistry, mycorrhizal symbioses, and physiology) loaded onto two predominant axes comprising “collaboration” and “conservation” gradients in belowground resource acquisition in the L. principis-rupprechtii chronosequence ( Figures 4 , 6 ). This result supported our second hypothesis that trees with different ages show different N strategies, appearing to shift from an acquisitive strategy to a conservative strategy associated with increasing tree age. Along the collaboration gradient, younger L. principis-rupprechtii tended to invest more carbon to develop the root system itself for resource exploration—a “do-it-yourself” strategy (Bergmann et al., 2020) that involved features such as increased SRL and SRA, decreased RTD and diameter (morphology), and increased uptake rates of nitrate and glycine (physiology). Conversely, older L. principis-rupprechtii invested more carbon in acquiring mycorrhizal partners—an “outsourcing” strategy (Bergmann et al., 2020). This variation in root functional traits with age on the “collaboration” axis is consistent with the study of Ren et al. (2023) confirming a “collaboration” gradient of root economic space identified for P. koraiensis, Picea koraiensis, and Abies nephrolepis in three age classes. In contrast, the “conservative” axis of root chemical and architectural traits was orthogonal to the “collaboration” axis, indicating the multidimensional aspect of root economic space. The conservative–acquisitive gradient represents the fast–slow trade-offs between traits associated with high metabolic activity (e.g., root nitrogen and respiration rate) and root construction costs (e.g., root carbon and RTD) (McCormack et al., 2015; Roumet et al., 2016; Freschet and Roumet, 2017). The relatively efficient roots associated with higher N content and BI and lower C content of younger L. principis-rupprechtii indicated a fast, resource-acquisitive strategy. Older trees with thicker roots exhibited a slower, resource-conserving strategy.

The uptake rates of nitrate, glycine, and total N of L. principis-rupprechtii decreased with increasing tree age, while the uptake rates of ammonium decreased initially and then increased with tree age. Across all tree ages, L. principis-rupprechtii preferred to absorb ammonium (45%–90%), followed by glycine (5%–40%) and nitrate (4%–11%). The glycine and nitrate uptake rates were positively correlated with the soil glycine and nitrate contents. The AD, tissue density, and ECM of the roots increased, while the SRL, SRA, BR, and BI decreased with increasing tree age in L. principis-rupprechtii plantations. We found that the strategies of resource acquisition along the age gradient of the same tree species showed a two-dimensional root economic space, and the root functional traits varied along the conservation and collaboration gradients. In L. principis-rupprechtii plantations, the root resource acquisition strategy appears to shift from an acquisitive strategy to a conservative strategy associated with increasing tree age. Along the collaboration gradient, younger trees relied more on their own root morphology and physiology to acquire soil resources—a “do-it-yourself” strategy, as reflected by the increased SRL and SRA and the increased uptake rate. Conversely, older L. principis-rupprechtii depended more on mycorrhizal partners—an “outsourcing” strategy.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding authors.

QL: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Writing – original draft, Writing – review & editing. YXC: Investigation, Writing – review & editing. YMC: Conceptualization, Writing – original draft, Writing – review & editing.