Introduction

Carbon dioxide (CO2) is enriched in the atmosphere due to anthropogenic activities, mainly the burning of fossil fuels, deforestation, industrial processes, changing lifestyles, etc., leading to global climate change (Singh and Kumar 2022). Atmospheric CO2 concentration has increased since the pre-industrial era, which is expected to reach 720-1000 ppm and will contribute to a rise in global air temperatures from 2.6 to 5.4°C by the end of the 21st century (Sharma et al. 2021).The rise in atmospheric CO2 concentration is anticipated to influence the vegetation system's structure, function, and overall productivity (Prakash et al. 2022).

Elevated atmospheric CO2 generally enhances plant growth by stimulating photosynthesis mechanisms (Singh et al. 2018). This phenomenon leads to improved biomass production and higher yields, especially when sufficient resources such as soil nitrogen (N), water, and favorable temperatures are available (Chen et al. 2019; Dhyani et al. 2021; Kumar et al. 2021). Nevertheless, in the face of changing climatic conditions, studies indicated that soil nitrogen may be depleted due to higher plant growth rates under rising atmospheric CO2 concentrations (Stitt and Krapp 1999; Xu et al. 2022). The scenario of diminishing soil nitrogen under changing climatic conditions may result in reduced CO2 fertilization effects (Stitt and Krapp 1999; Yadav et al. 2019a, b; Nirmal et al. 2021). Hence, supplementing reactive nitrogen to the soil system is essential in such a scenario to meet the plant nitrogen requirements, ensuring optimal plant growth, development, and carbon sequestration (Singh et al. 2014).

Low soil nitrogen affects tree nitrogen levels, including proteins and enzymes that can alter metabolic mechanisms, including physiological processes (Singh et al. 2014; Liu et al.2018; Liao et al. 2019; Ata-Ul-Karim et al. 2022; Xu et al. 2022). Moreover, nitrogen deficient soil alters the tree phenology and insects, including pollinators, thereby disrupting the entire forest system/ecosystem (Huang et al. 2018; Wang and Tang 2019; Ata-Ul -Karim et al. 2022). Liu et al. (2018) reported increased plant biomass by 30.77% and 31.37% at low (4 mg/L) and high (6 mg/L) application, respectively, under elevated CO2 conditions (700 µmol mol−1). Xu et al. (2022) found that elevated CO2 (800 ± 20 µmol mol−1) in conjunction with nitrogen application strongly increased shoot and root biomass and the nitrogen and protein concentrations of Agropyron mongolicum. Stitt and Krapp (1999) reported that plants growing in a CO2-enriched atmosphere require higher nitrogen fertilizers for proper physiological function and other metabolic processes. Wei et al. (2018) found higher tomato yields with improved quality, increased CO2 concentration, and higher nitrogen application to maintain tomato yield and quality in the future with changing climate scenario.

Therefore, soil deficiency in climate change scenarios would profoundly impact various tree species, particularly in non-agricultural settings, such as urban plantations, roadside plantings, and similar environments where nitrogen application is not a common practice.(Gómez-Guerrero and Doane 2018). This phenomenon may significantly diminish urban trees' CO2 absorption potential and productivity, resulting in a decline in tree species' adaptation and mitigation potential. This scenario may be a major problem in urban areas where limited resources and elevated atmospheric CO2, temperature, air pollutants, etc, are now becoming common (Singh et al. 2018; Sharma et al. 2018; Sharma and Singh 2021).

In regions like India, where effective nutrient management practices in plantation forestry are not widely implemented, challenges such as limited soil nitrogen availability and changing climatic conditions would substantially impact the productivity of tree species. Under these circumstances, the ability to adapt to and effectively mitigate the consequences of climate change through the forestry system becomes increasingly challenging. Hence, there is a pressing need to enhance the mitigation potential of forestry and tree species in terms of carbon sequestration. This could serve as a potent mechanism to counteract global climate change through strategic forestry interventions.Optimization of nitrogen use presents a valuable approach to enhance the carbon sequestration rate and productivity in nitrogen-deprived soils. Species-specific application of nitrogen to the soil system may effectively boost tree species' carbon sequestration capacity, contributing towards achieving Sustainable Development Goals' targets via adapting and mitigating climate change by forestry system.There is a lack of scientific understanding regarding the influence of soil nitrogen availability on the growth, development, and physiological response of tree species under elevated CO2 concentrations. Therefore, the present study aims to elucidate the effects of nitrogen applications on the biophysical, growth, and physiological responses of Neolamarckia cadamba grown under elevated CO2 concentration.

Material and methods

Experimental setup

The study was conducted in the automated open chambers (OTCs) facility existing at the Forest Research Institute, Dehradun, Uttarakhand (32°20′ 44.2172″ N, 78°0′ 41.6185″ E, and 668 m.a.s.l.). This system consists of three components, namely open-top chambers (OTCs), CO2 distribution system, and the controller with the data logger. The size of each OTC was 3.0 × 3.0 ×4.0 m (width × length × height). The experiment was set up in a split-plot design with three replications of nitrogen and CO2 concentration. A set of three-month-old and uniform seedlings (n=6 seedlings) of N. cadamba were exposed to ambient CO2 concentration (aCO2; 400 ± 14 µmol CO2 mol−1) and elevated CO2 concentration (eCO2, 800 ± 20 µmol mol-1) in automated OTCs conditions. In addition, the potted seedlings were supplied with three nitrogen regimes (Low nitrogen-N200 kg N ha−1, medium nitrogen-N300 kg N ha−1, and high nitrogen-N500 kg N ha−1) under the above conditions. CO2 concentration and nitrogen application were considered the main and subplot treatments, respectively.

Analysis of biophysical and growth traits

Biophysical and growth traits, mainly plant height (cm), collar diameter (mm), leaves, branches, and leaf area (cm2 leaf−1) were measured from the plants growing in the treatments. Plant height and collar diameter were measured using a meter scale and digital vernier calliper, respectively (Sharma et al. 2018). Leaf area was computed using the graph paper method (Singh et al. 2018). Leaves were detached carefully from the plants and spread over the graph paper (millimeter scale). The area of the graph paper covered with leaves was then counted to estimate the leaf area per leaf (Singh et al. 2018).

Analysis of physiological functional traits

The response of physiological functional traits determines plant species' performance, adaptation, and productivity were measured. The critical physiological functional characteristics such as CO2 assimilation rate (A, μmol CO2 m−2 s−1), transpiration rate (E, mmol H2O m−2 s−1), stomatal conductance (Gs, mol H2O m−2 s−1), and water use efficiency (WUE) were investigated using portable photosynthetic system (Model 6400 XT- LICOR, Incl, USA) (Singh et al. 2010). The three youngest and fully expanded leaves from each plant were selected to monitor physiological functional traits. Hence, eighteen leaves (n=18) from a set of six seedlings were monitored for physiological parameters in each treatment. All these traits were observed between 11:30 a.m. and 12:30 p.m. under clear skies to avoid the photoinhibition effects. Water use efficiency (WUE) was calculated as the ratio of CO2 assimilation (A) and transpiration (E) of the leaf (Singh et al. 2018).

Statistical analysis

The experiment was set up in a split plot design with three replicates. The biophysical, growth, and physiological traits were subjected to ANOVA (=0.05) using STATISTICA 7.0 software to understand the effects of treatments on the plant traits. ANOVA (á=0.05) was used to understand whether the treatment means differed significantly. Pearson correlation was performed with R-Studio software to understand the relationship between the selected tree species' biophysical, growth, and physiological functional traits.

Results and discussion

Response of biophysical and growth traits of plant species

The response of the biophysical and growth traits of the N. cadamba plant with increasing atmospheric CO2 concentration and nitrogen regimes is shown in Figs. 1 and 2. The study showed a significant increase in plant height grown at medium nitrogen availability (N300) compared to low (N200) and high nitrogen levels (N500) under aCO2 and eCO2 (Fig. 1). With all nitrogen and CO2 treatments, the plants grown in medium nitrogen (N300) and eCO2 (800 ± 20 mol mol−1) showed maximum plant height (189.16 ± 2.63 cm) than medium nitrogen (N300) under aCO2 (151.41 ± 2.02 cm). This increase in plant height was approximately 25% more than the counterparts (Fig. 1). A carbon dioxide-enriched environment and soil nitrogen availability may synergistically affect plant growth, development, and productivity. Sufficient availability of critical resources such as CO2 and nitrogen work synergistically and significantly enhance the photosynthesis mechanism, resulting in more carbohydrate and biomass production than either factor alone. Wei et al (2018) found that available soil N mediates the growth and development of tree species in response to increasing atmospheric CO2 concentration. Further, N. cadamba growth was found to be decreased under nitrogen-poor soils (Lu et al. 2021).

Fig. 1
figure 1

Effect of nitrogen application (low-N200 Kg N ha−1, medium-N300 Kg N ha−1, and high-N500 Kg N ha−1) on biophysical and growth traits of N. cadamba grown under CO2 concentration (Ambient (aCO2); 400 ± 14 µmol CO2 mol−1; Elevated (eCO2); 800 ± 20 µmol CO2 mol–1). Error bars indicate the standard error of the mean. Results are mean of six replications (N=6)

Full size image
Fig. 2
figure 2

Effect of nitrogen application (Low-N200 Kg N ha−1, medium-N300 Kg N ha−1, and high-N500 Kg N ha−1) on leaf area of N. cadamba grown under CO2 concentration (Ambient (aCO2); 400 ± 14 µmol CO2 mol−1; Elevated (eCO2); 800 ± 20 µmol CO2 mol−1). Error bars indicate the standard error of the mean. Results are mean of six replications (N=6)

Full size image

The higher collar diameter was reported at medium soil nitrogen (N300) compared to low (N200) and high nitrogen (N500) (Fig. 1). However, collar diameter declined under high nitrogen (N500) with eCO2 (19.22 ± 1.37 mm). The plant attained higher collar diameter in medium nitrogen (N300) under eCO2 (26.16 ± 0.75 mm) than aCO2 (9.57 ± 0.52 mm). This suggested that limited nitrogen availability might impede plant growth under changing climatic variability, particularly under future atmospheric CO2 concentrations (Medina 2022).

The plants produced more leaves (29.57 ± 2.25) under eCO2 and medium nitrogen (N300) than aCO2 (26.42 ± 3.12) (Fig. 1). Further, plants grown in eCO2 with N500 demonstrated less leaves (20.42 ± 0.31) compared to low (N200 Kg N ha-1) and medium nitrogen availability (N300 Kg N ha−1). Wei et al. (2018) reported soil N depletion due to higher plant growth rates under elevated CO2 concentration. In such circumstances, nitrogen fertilizers have been applied additionally to take advantage of rising CO2 concentration to improve the CO2 fertilization effect (Chen et al. 2019).

Branches per plant were found to be more in N300 and N500 than N200 nitrogen, with the maximum branches (6.58 ± 0.32) at N500 under aCO2 (Fig. 1). Higher leaf area (340.30 ± 7.81 cm2 leaf−1) was at medium nitrogen (N300) under eCO2 than aCO2 (325 ± 8.38 cm2 leaf−1) (Fig. 2). The lower soil available N in the face of climate change declines the plant nitrogen content and protein and, in turn, affects the activities of various enzymes involved in the photosynthesis mechanisms, which is the crucial process that decides plant growth performance (Sharma et al. 2017). It has been reported that soil nitrogen availability is the critical constraint of plant growth and development and obtaining beneficial impacts of CO2-enriched environmental conditions (Schleppi et al. 2019). Hence, understanding how elevated CO2 and nitrogen affect plant growth dynamics and productivity is essential for accurately predicting the impacts of climate change on the growth dynamics and productivity of tree species at the individual or ecosystem level.

The morphological and physiological function is regulated by nitrogen availability. Nitrogen enrichment promotes plant fitness, tissue nutrition, and shoots and root growth under increased carbon dioxide levels. Increased carbon dioxide and nitrogen interact synergistically to affect plant performance, particularly in relation to plant size, showing that nitrogen effects can be aggregated by increased carbon dioxide (Apurva et al. 2017; Kumari and Singh 2018; Guo et al. 2022). Cao et al. (2008) discovered that an excessive application of nitrogen can have a contrary effect, potentially resulting in a decline in growth and adversely impacting both plant morphology and developmental processes.

Response of physiological functional traits of plant species

The physiological response of N. cadamba was significantly affected by increased atmospheric CO2 concentration and nitrogen regime (Fig. 3). Leaf CO2 assimilation rate was significantly improved under eCO2 with medium nitrogen (N300) compared to low (N200) and high nitrogen (N500). Leaf CO2 assimilation rate was highest (9.00 ± 0.42 µmol CO2 m−2 s−1) and increased by 16% in N300 and eCO2 than aCO2 (7.73 ± 0.33 µmol CO2 m−2 s−1) (Fig 3). Riberio et al. (2021) reported an enhanced leaf CO2 assimilation rate under increased CO2 concentration with sufficient nitrogen availability. Reduced photosynthesis has been reported at low soil N and ambient CO2 concentrations (Domiciano et al. 2020). It has been reported that sufficient soil nitrogen combined with higher CO2 concentrations induces carboxylation, resulting in an improved CO2 assimilation rate (Bassi et al. 2018). It is well acknowledged that nitrogen is a limiting factor facilitating the photosynthesis process (Singh et al. 2010), and sufficient nitrogen can improve the Rubisco content and its activity together with chlorophyll content, resulting in enhanced carbon assimilation rate and plant productivity under elevated CO2 conditions (Evans 1989; Shangguan et al. 2000; Yang et al. 2022).

Fig. 3
figure 3

Effect of nitrogen application (low-N200 Kg N ha−1, medium-N300 Kg N ha−1, and high-N500 Kg N ha−1) on physiological traits of N. cadamba grown under CO2 concentration (Ambient (aCO2); 400 ± 14 µmol CO2 mol−1; Elevated (eCO2); 800 ± 20 µmol CO2 mol−1). Error bars indicates the standard error of the mean. Results are mean of six replications (N=18)

Full size image

The plants under aCO2 and N200 had expressed lower transpiration rate (4.46 ± 0.19 mmol H2O m−2 s−1) by 39% than eCO2 (6.21 ± 0.56 mmol H2O m−2 s−1) (Fig 3). Stomatal conductance was found to be maximum (0.44 ± 0.03 mol H2O m−2 s−1) under eCO2 than aCO2 (0.28 ± 0.02 mol H2O m−2 s−1) in N200 (Fig 3). Polley et al. (1999) reported a relationship between transpiration and soil N, noting that soil N availability and CO2 concentration affect transpiration rate by altering plant nitrogen content. The present study showed that the combination of nitrogen application and increased CO2 concentration significantly increased water loss through leaf transpiration (Fig. 3). The higher release of water molecules from the foliage could increase CO2 gas exchange. Thus, due to the increased photosynthesis, more CO2 is available to produce higher carbohydrates. These results were supported by gas exchange observations in cucumber plants exposed to similar conditions (Pinero et al. 2021). The increased gas exchange could be due to increased NH4+ in the leaves, which can acidify the cytoplasm and increase stomatal conductance (Hachiya and Sakakibara 2016). The acidification process can increase plasma membrane H+-ATPase activity, increasing leaf transpiration (Hedrich et al. 2001).

Water use efficiency (WUE) was increased by 16% under ambient CO2 concentration (1.80 ± 0.23) compared to elevated CO2 concentration (1.46 ± 0.10) in the high nitrogen application (N300). However, nitrogen application has been reported to stimulate water use efficiency by N. cadamba when the CO2 concentration in the environment increases (Fig. 3). Cruz et al. (2014) documented similar results with increased CO2 concentration and N application. Torralbo et al. (2019) reported the opposite effect on the carbon assimilation rate in durum wheat, although similar responses were observed on water use efficiency.

It is widely recognized that different plant species exhibit diverse responses to increasing atmospheric CO2 levels and the availability of soil nitrogen. These responses are reliant upon the unique and species-specific mechanisms governing photosynthesis and carbon exchange, as well as the plants' capacity to access and acquire nitrogen from the soil. In a changing climate, these plants may demonstrate contrasting nitrogen use efficiency and the allocation of nitrogen, along with other essential macro and micronutrients, among various plant parts, depending on soil nitrogen availability.

In certain situations, soil resources can lead to competition among different plant species for carbon and nitrogen resources. Consequently, plant species may respond differently to elevated CO2 concentrations and nitrogen availability based on their specific photosynthetic pathways and nitrogen-fixing capabilities. This study investigated the impact of increasing atmospheric CO2 levels and nitrogen applications, opening up a new opportunity to predict the responses of various plantation species under future climate changes and nitrogen limited conditions.

Therefore, the study recommends conducting long-term and systematic research to gain a comprehensive understanding of how plantation species respond to these factors and how their nitrogen requirements can be optimized to maximize the CO2 fertilization effects. This is particularly pertinent in regions where current nitrogen management practices are inadequate. The data generated from such studies holds the potential to be instrumental in forecasting the likely effects of elevated CO2 concentrations on plant species under varying nitrogen availability, as required by process-based dynamic global vegetation models. This knowledge is indispensable for advancing sustainable land management practices and enhancing scientific understanding of ecosystem dynamics in relation to climate change and nutrient availability.

Interlinking between biophysical, growth, and physiological functional traits

The interlinking between plant functional traits is depicted in Fig. 4. The analysis revealed significant correlations among biophysical, growth, and physiological plant traits. Among biophysical attributes, it was found that plant height exhibited strong correlations with the leaf CO2 assimilation rate, stomatal conductance, water use efficiency, and transpiration rate (Fig. 4). Moreover, leaf area was identified as strongly correlated with leaf CO2 assimilation rate, stomatal conductance, water use efficiency, and transpiration rate (Fig. 4). Understanding the interlinking between morphological, growth, and physiological parameters is essential in plant biology, from agriculture and forestry to ecological and conservation research. This understanding helps researchers and land managers make informed decisions about plant species and their resource requirements in diverse environments, which aid in developing strategies to adapt and mitigate climate change impacts and achieving sustainability targets.

Fig. 4
figure 4

Correlation between plant functional traits of N. cadamba. The symbols are denoted as follows: PT, plant height; CD, collar diameter; NL, numbers of leaves; LA, leaf area; NB, number of branches; A, leaf CO2 assimilation rate; E, leaf transpiration rate; Gs, stomatal conductance; and WUE, water use efficiency

Full size image

Conclusion

This study has provided invaluable insights into the crucial role of soil nitrogen availability in shaping the effect of CO2 fertilization on N. cadamba, an important urban plantation tree species. It is concluded that the magnitude of CO2-induced growth enhancement and improved physiological responses is intricately linked to the nitrogen status of the soil. These findings stressed the significance of considering soil nutrient availability while planning urban tree planting initiatives to enhance urban green spaces (UGS) and mitigate the impacts of climate change. Moreover, the study suggested the imperative need for sustainable urban forestry practices, especially soil nutrient management, i.e., nitrogen supplementation, to optimize the benefits of elevated atmospheric CO2 on tree growth and carbon sequestration in urban environments. The study may provide valuable guidance to urban planners and managers, enabling them to design and implement urban forestry strategies that foster healthier and more resilient urban ecosystems. Furthermore, this study contributes to global efforts to combat climate change by recognizing the complex interaction between soil nitrogen availability and CO2 responsiveness in urban environments, aligning with the objectives of Sustainable Development Goals (SDGs).