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Conceptual model of responses of major structure and functions of forest ecosystem to CAN and UAN. The magnitude of the N fluxes, integrated over the whole forest ecosystem was adapted from the observations of canopy retention N investigation (referred to the text), suggesting that the canopies might retain the deposited N from the atmosphere. “?” indicated that the amount of redistributed N need further verification. Hypotheses on the responses of plants and soil food web to N solution via CAN or UAN approach please refer to the following text. The figure was created using Adobe Illustrator CS 5.0 and Adobe Photoshop CS 5.0 software. Figure drawing by X.Q. Rao.
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1 Increasing atmospheric nitrogen (N) deposition could profoundly impact community structure and ecosystem functions in forests. However, conventional experiments with understory addition of N (UAN) largely neglect canopy-associated biota and processes and therefore may not realistically simulate atmospheric N deposition to generate reliable impact...
Contexts in source publication
Context 1
... leaf growth and more leaf litter. Greater litterfall mass could favor detritus food web, and thus fungal channel pathway dominates [56][57][58] . By contrast, UAN might favor the understory plant species and leads to greater root ingrowths and faster root turnover, which favors rhizosphere food web and bacterial channel pathway dominates 59 (Fig. 4), and iv) when excess dose of N solution is used: CAN could damage the canopy tree species and other biota due to osmotic effects of salt, and consequently reducing growth of canopy tree species and other biota. However, the N in throughfall might be suitable for understory plant species and favors rhizosphere food web and bacterial ...
Context 2
... rhizosphere food web and bacterial channel pathway 56,57,59 . On the contrary, UAN could cause detrimental effects on understory plants and soil food web due to serious stress of high N input, resulting in reduced understory plant growth and damaged soil food web. Fungal channel pathway could dominate in response to greater litterfall inputs 58 (Fig. ...
Context 3
... to UAN, CAN can be used to better illustrate the fate of the whole-forest ecosystem in response to elevated N deposition and to identify the major pathways and processes of N cycling (Fig. 4). CAN enables us to estimate the complete N budget by quantifying N inputs and outputs in forest eco- systems, where we need to explicitly include not only N added onto forest canopy but also N retained in the canopy, throughfall, stem-flow and leached out as soil solution. Most of these processes are neglected with UAN, which applies N ...
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... The study site is situated in the Jigongshan (JGS) National Nature Reserve ( Carruth., Quercus variabilis Bl., and Liquidambar formosana Hance as the predominant canopy tree species. The region features yellow-brown sandy-loam soil [47,48]. ...
... A comparative experiment with CN and UN at addition rates of 0, 25, and 50 kg N ha −1 yr −1 was conducted. The experiment employed a completely randomized block design with four blocks (representing four replicates) [47,49,50]. Each block included the following five treatments: (1) CN at 25 kg N ha −1 yr −1 (CN25); (2) CN at 50 kg N ha −1 yr −1 (CN50); (3) UN at 25 kg N ha −1 yr −1 (UN25); (4) UN at 50 kg N ha −1 yr −1 (UN50); and (5) a control (CK, without N addition). ...
... acutissima Carruth., Quercus variabilis Bl., and Liquidambar formosana Hance as the predominant canopy tree species. The region features yellow-brown sandy-loam soil [47,48]. ...
Significant influences on tree growth and forest functionality are attributed to nitrogen (N) addition. However, limited research has been conducted on the effects of N addition on forest spatial structure. In this study, we examined the effects of different N addition methods and concentrations on the stand spatial structure of a deciduous broad-leaved forest over the period 2012 to 2017. Five N addition treatments were implemented: CK (control group without N addition), CN25 (low N concentration added to the canopy), CN50 (high N concentration added to the canopy), UN25 (low N concentration added to the understory), and UN50 (high N concentration added to the understory). The results showed a moderate influence of N addition (CN25, CN50, UN25, UN50) on optimizing the stand spatial structure. CN25, CN50, and UN25 increased the mean values of the mingling degree (M) and neighborhood comparison (U), while decreasing the mean value of the uniform angle index (W), although these effects were not significant. Enhancements in the average value of the crowding degree (C) and comprehensive spatial structure index (CSSI) between 2012 and 2017 were found in all five treatments, demonstrating statistical significance. Assessing the distribution of the stand spatial structure index, CN25, CN50, and UN25 increased the proportion of M at an intensity (M = 0.75) and extreme intensity (M = 1), while decreasing the proportion at zero intensity (M = 0), weak intensity (M = 0.25), and moderate intensity (M = 0.5). A decrease in the proportion of trees was noted when U = 0 (excluding UN50), with no discernible pattern found in the frequency distribution of other values. CN50 and UN25 increased the proportion of W at a moderate level (W = 0.5), while CN25 and UN50 reduced it. No clear pattern was detected in the frequency distributions of other values. All five treatments increased the proportion of C at the maximum level (C = 1), while decreasing the proportions at levels of 0, 0.25, and 0.5 in 2017. Intriguingly, nitrogen addition treatments appeared to optimize the stand spatial structure to some extent and stimulated the growth of trees with larger diameters. Nevertheless, the short duration of the data collection period, spanning only five years, may have influenced the significance of the outcomes, underlining the requirement for extended studies. Conclusively, N deposition adjusted and enhanced the stand spatial structure to various degrees within the research region, providing valuable insights for further optimization of forest management.
... Previous studies have primarily relied on understory N and water addition methods, disregarding processes such as N or water absorption and interception by forest canopy layers, which cannot accurately simulate the natural process of atmospheric N deposition and precipitation [24,25] . Research has demonstrated that more than 40% of N can be absorbed and intercepted by forest canopy layers [26] . ...
... However, this experiment suggests that the absorption rates of N solution or water attached to the surface are extremely low for most dominant species are extremely low for most dominant species, making it difficult to affect their leaves. In addition, the interception of forest canopies can weaken the soil entry effect, further reducing the treatment effect [24,36] . The results also indicate that the canopy addition of N did not significantly increase the available N of the soil (p < 0.05; Table 4). ...
Plant functional traits are indicative of plant responses to environmental changes, influencing ecosystem functions. Leaves, as a key focus in studying plant functional traits, present an area where the impact of nitrogen deposition and altered rainfall patterns on functional diversity remains ambiguous. To elucidate plant response mechanisms to environmental factors, we employed a canopy-based platform to add nitrogen, water, and their combination. We assessed the functional traits and community-weighted mean of the leaves of three dominant trees and three dominant shrubs. The results showed that nitrogen addition to the canopy significantly increased the leaf dry matter content of the Celtis sinensis Pers, but markedly decreased the specific leaf area of the Liquidambar formosana Hance. The nitrogen-water interaction did not notably affect the specific leaf area and equivalent water thickness of leaves. Canopy addition of nitrogen, water, and their combined interaction substantially lowered leaf nitrogen content and markedly increased leaf C/N. The structural equation model demonstrated a significant negative correlation between leaf dry matter content, equivalent water thickness, and leaf nitrogen content, as well as between equivalent water thickness and leaf phosphorus content. Our results provide evidence for the adaptation of plants to the environment and different strategies for resource and energy utilization.
... Notably, the aforementioned study applied a direct spray of N solution onto the forest floor to simulate the increased deposition of atmospheric N in forest ecosystems without considering a series of interception processes, such as N absorption, adsorption, and transformation by the tree canopy [36]. In reality, a large proportion of N is initially absorbed by leaves and subsequently stored in branches and stems [37]; thus, ignoring the forest canopy could introduce several uncertainties in the findings of this study [38][39][40][41]. ...
... This platform enables a more precise simulation of atmospheric N deposition processes. The northern subtropical forest ecosystem (adjacent to the warm temperate climate zone) was chosen as the research site for this study because of the unique convergence of plants from both the northern and southern regions, rendering the forest ecosystem's structure and function highly responsive to environmental changes [36,42,43]. In this mixed zone Forests 2024, 15, 416 4 of 18 Forests 2024, 15, x FOR PEER REVIEW 4 of 18 ...
... The availability of soil P is influenced by factors such as soil mineral composition, pH, moisture content, temperature variation, vegetation type, land management practices, between the northern subtropical and warm temperate regions, southern plant species, such as Pinus massoniana, Castanea mollissima, Pteroceltis tatarinowii, and Lindera glauca, cohabit with northern plant species, such as Quercus variabilis, Quercus acutissima, and Quercus aliena. Consequently, it serves as a natural laboratory with excellent representativeness and potential for investigating the response and feedback mechanisms of nutrient availability to global change [36]. Understanding the P transformation process and plant P demand in northern subtropical forest soils under the backdrop of long-term N deposition is important for evaluating the impact of N deposition on forest ecosystem structure and function. ...
Soil phosphorus (P) is a critical factor that limits plant productivity. Enhanced nitrogen (N) deposition has the potential to modify P transformation and availability, thereby potentially affecting the long-term productivity of forests. Here, we conducted an 11-year-long field experiment to simulate N deposition by adding N to the forest canopy in a N-limited northern subtropical forest in central China and assessed the changes in soil organic P mineralization, P fractions, microbial biomass P content, phosphatase activity, and plant P content under N deposition. Our objective was to establish a theoretical framework for addressing the P supply and sustaining plant productivity in soils with low P availability, particularly in a changing global setting. The results demonstrated a substantial reduction in the levels of total, organic, and available P owing to the canopy addition of N. Furthermore, there was a marked decrease in the proportion of organic P in the total P pool. However, no substantial changes were observed in the soil inorganic P content or the proportion of inorganic P within the total P across different treatments. Canopy N addition significantly enhanced the microbial biomass P content, phosphatase activity, and organic P mineralization rate, suggesting that in soils with limited P availability, the primary source of P was derived from the mineralization of organic P. Canopy N addition substantially increased the P content in leaves and fine roots while concurrently causing a considerable decrease in the N:P ratio. This indicates that N deposition increases P demand in plants. Correlation analysis revealed a significant negative association among the total, organic, and available P levels in the soil and plant P concentrations (p < 0.05). This suggests that the primary cause of the reduced fractions of P was plant uptake following canopy N addition. Various studies have demonstrated that N deposition induces an augmented P demand in plants and expedites the utilization of available P. A substantial reduction in potentially accessible soil P caused by N deposition is likely to exacerbate regional P depletion, thereby exerting adverse impacts on forest ecosystem productivity.
... Leaf is an important organ for characterizing plant responses to environmental change and has a high degree of plasticity during the long-term evolution of plants , serving as energy converter for primary producers in ecosystems (Wright et al., 2005). Thus, it is very convincing to reflect the influence of environmental factors through changes in leaf characteristics (Sakschewski et al., 2015;Zhang et al., 2015a,Zhang et al., 2015b. Leaf functional traits (LFTs) are known as the predictors of individual species performance and are commonly used to assess plant-environment interactions or to quantify specific responses of plants to ecosystem processes (Damián et al., 2018;Jones et al., 2013). ...
... Specially, N was sprayed onto the floor, which bypassed canopy effects (i.e., canopy retention and foliage uptake) and could not actually reflect the effects of N deposition in natural ecosystem. Previous researches have demonstrated that, at least for short time scales, understory N addition would overestimate the effects of N deposition on forest soil characteristics and underestimate the impacts of N deposition on leaf (Shi et al., 2016;Zhang et al., 2015a, Zhang et al., 2015b. Additionally, only 1-5 % of N was absorbed by leaves and twigs in an area with high N deposition (30 kg N ha − 1 yr − 1 ) (Adriaenssens et al., 2012), whereas our study location is already above that. ...
... The difference between the two methods of N deposition must therefore be quantified by further experiments on canopy N addition. Different forms of N addition research have received concern in recent years (Zhang et al., 2015a,Zhang et al., 2015b. Some studies have showed that plants can either directly take in and use urea or they can take up urea-derived N in the form of NH 4 + after urea has been hydrolyzed by urease (Bai et al., 2021). ...
The evergreen broad-leaf forest is subtropical zonal vegetation in China, and its species diversity and stability are crucial for maintaining forest ecosystem functions. The region is generally affected by global changes such as high levels of nitrogen deposition. Therefore, it is critical to determine the adaptation strategies of subtropical dominant species under nitrogen addition. Here, we conducted two-year field experiments with nitrogen addition levels as 0 kg N ha-1 yr-1 (CK), 50 kg N ha-1 yr-1 (LN) and 100 kg N ha-1 yr-1 (HN). We investigated the effects of nitrogen addition on leaf functional traits (including nutrition, structural and physiological characteristics) of five dominant species in subtropical evergreen broad-leaf forest. Results suggested that the effect of nitrogen addition on leaf functional traits was species-specific. Contrary to Rhododendron delavayi and Eurya muricata, Quercus glauca, Schima superba and Castanopsis eyrei all responded more to the HN treatment than LN treatment. Compared to other leaf functional traits, leaf anatomical structure traits had the highest average plasticity (0.246), and the relative effect of leaf photosynthetic property was highest (7.785) under N addition. Among the five species, S. superba was highest in terms of the index of plasticity for leaf functional traits under nitrogen addition, followed by Q. glauca, E. muricata, C. eyrei and R. delavayi. The major leaf functional traits representing the economic spectrum of leaves (LES) showed resource acquisitive strategy (high SLA, LNC, LPC, Pn) and conservative strategy (high LTD, LDMC, C/N) clustering on the opposite ends of the PCA axis. The PCA analysis indicated that species with high leaf plasticity adopt resource acquisitive strategy (S. superba and Q. glauca), whereas species with low leaf plasticity adopt resource conservative strategy (E. muricata, C. eyrei and R. delavayi). In aggregate, resource-acquisitive species benefit from nitrogen addition more than resource-conservative species, suggesting that S. superba and Q. glauca will occupy the dominant position in community succession under persistently elevated nitrogen deposition.
... Under nitrogen deposition, the leaves of the canopy retain the most abundant resources and have the greatest potential for carbon assimilation. The metabolic activities of the canopy greatly affect the carbon sequestration capacity of the entire forest ecosystem (Zhang et al., 2015). Due to the difficulty of conducting manipulative experiments in forest ecosystems, many studies have applied nitrogen fertilizer under the forest canopy to investigate the impact of forest nitrogen deposition (Bobbink et al., 2010;Lu et al., 2018). ...
... This area has a subtropical monsoon climate, with alternating dry and wet seasons throughout the year; April through October is the wet season and November through March is the dry season. The average annual rainfall is 2364 mm, and the average annual temperature is 20.8 • C. The main soil type is lateritic red soil (Zhang et al., 2015). The regional vegetation in this area is subtropical evergreen broad-leaved forest, with a mean canopy height of about 26 m and a mean canopy closure of about 92 %. ...
... The experiment had a full factorial design with three levels of nitrogen application, including a control group without nitrogen addition (CT), a forest canopy nitrogen addition treatment at 25 kg N ha − 1 y − 1 (CN25), and a forest canopy nitrogen addition treatment at 50 kg N ha − 1 y − 1 (CN50). Based on a preliminary investigation, it was found that the average nitrogen deposition in rainfall in the Shimentai National Nature Reserve was 34.1 kg N ha − 1 y − 1 (Zhang et al., 2015). Thus, CN25 was designed as the medium level, while CN50 was designed as the high level of manipulated nitrogen deposition treatment. ...
Many studies have focused on the impact of nitrogen deposition on plants, but due to technical limitations, research on the responses of forest canopy to manipulated nitrogen deposition is relatively scarce. Based on a canopy nitrogen addition (CN) platform, this study used laboratory analysis and unmanned aerial vehicle (UAV) observations to assess the impact of CN on the canopy traits of dominant tree species (Engelhardia roxburghiana, Schima superba, and Castanea henryi) in an evergreen broad-leaved forest in China. The results showed that nitrogen application at 25 kg N ha-1 y-1 (CN25) and 50 kg N ha-1 y-1 (CN50) significantly increased the actual net photosynthetic rate (An) of all the three tree species. CN25 significantly increased superoxide dismutase (SOD), catalase (CAT), and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities in C. henryi. CN50 significantly increased the leaf area of all the three tree species and significantly reduced the leaf thickness of C. henryi, and significantly increased the POD and Rubisco activities in S. superba and C. henryi. CN significantly changed the number of forest gaps, but did not significantly change the area of forest gaps within the sample plots. CN25 significantly decreased the vertical projection area but increased the canopy flowering coverage of S. superba in dominant directions. CN25 and CN50 significantly increased the flowering coverage of C. henryi in favorable directions. It is found that under long-term (10-year) nitrogen addition, the balance between carbon fixation and antioxidant defense functions of E. roxburghiana may be broken down, but the carbon assimilation, antioxidant capacity and reproduction potential of S. superba and C. henryi may be well coordinated, which will have a potential impact on the species composition and ecological functions of the evergreen broad-leaved forest. This study may also provide scientific basis for forest management in the context of enhanced atmospheric nitrogen deposition.
... Although numerous studies have evaluated the effects of N and W addition on the fine root biomass and morphology of plants in forest ecosystems, most previous experimental work has focused solely on the addition of N and W to the soil or leaf litter, failing to consider canopy-associated ecological processes, such as evaporation, absorption by leaves, and microbial activity (Zhang et al., 2015;Shi et al., 2017). Therefore, little is known about the impact of N deposition and increased precipitation on the forest canopy and the structure of the forest ecosystem. ...
... Therefore, little is known about the impact of N deposition and increased precipitation on the forest canopy and the structure of the forest ecosystem. For instance, foliar uptake of N and W may substitute or supplement root foraging, leading plants to alter their investment in fine roots (Zhang et al., 2015). In this study, we experimented in an area of broad-leaved mixed forest located within the Jigongshan (JGS) Nature Reserve, Central China. ...
... This was not consistent with our first hypothesis, which we believe could be attributed to how N was applied. Compared with the traditional method of N addition to soils, we applied N at the level of the forest canopy, during which it can be assumed that part of the applied N was lost via natural processes such as evaporation, absorption by leaves and microorganisms, and adsorption by bark (Zhang et al., 2015). Our previous study demonstrated that 44-52% of N and W addition was intercepted by the canopy . ...
Introduction
Increasing atmospheric N deposition and changes in precipitation patterns could profoundly impact forest community structure and ecosystem functions. However, most N and water (W) addition experiments have focused on direct N application to leaf litter or soil, neglecting canopy processes such as leaf evaporation and absorption.
Methods
In this study, we aimed to assess the effects of atmospheric N deposition and increased precipitation on the fine root biomass and morphology of plants in a temperate deciduous forest. To achieve this, we applied N and W above the forest canopy and quantified the seasonal dynamics (January, July, and October) of fine root biomass and morphology.
Results
Our results revealed that only canopy W addition significantly increased the biomass of fine roots in January compared to that in other seasons (p < 0.05). We observed no significant interaction effect of N and W on fine root biomass. However, we found that the different growth seasons had a significant impact on the fine root biomass (p < 0.001). The combined application of N and W significantly affected the root tip density (p = 0.002). Although canopy N addition was significantly positively correlated with available soil N (p < 0.05), we detected no significant association with fine root biomass or morphology.
Discussion
The findings of this study indicated that fine root biomass and morphology, are affected to a greater extent by the provision of W than by N application. These findings provide a new perspective and a more precise understanding of the effects of the actual N deposition and precipitation on the dynamics of plant fine roots in forest ecosystems.
... Previous studies have promoted the understanding of N deposition on the impact of forest structure and function; however, most of the research studies were carried out using simulated N deposition experiments under the canopy by spraying or applying N solution to the soil (Ferretti et al. 2014;Ibáñez et al. 2018). This approach omitted N interception and absorption effects by forest canopy and underestimated the physiological metabolism changes in leaves influenced by N deposition (Zhang et al. 2015;Wang et al. 2021), which could not truly reflect the response of forest ecosystem to N deposition. Moreover, it should be noted that genes cause the changes in metabolites, and the physiological and phenotypic changes of plants under environmental stress are reflected extensively at the molecular level ). ...
... The background N deposition at the Shimentai National Nature Reserve was 34.1 kg N ha −1 year −1 and was predicted to be 50 kg N ha −1 year −1 by 2030 (Zhang et al. 2015). Accordingly, an experimental design consisting of the following treatments was used: CK = the control (without N addition), CAN = canopy addition of N at 50 kg N ha −1 year −1 , and UAN = understory addition of N at 50 kg N ha −1 year −1 to explore the influence of excessive N addition on plant growth and development at both the physiological and the molecular levels. ...
Although effects of atmospheric nitrogen (N) deposition on forest plants have been widely investigated, N interception and absorption effects by forest canopy should not be neglected. Moreover, how N deposition change the molecular biological process of understory dominant plants, which was easily influenced by canopy interception so as to further change physiological performance, remains poorly understood. To assess the effects of N deposition on forest plants, we investigated the effects of understory (UAN) and canopy N addition (CAN) on the transcriptome and physiological properties of Ardisia quinquegona, a dominant subtropical understory plant species in an evergreen broad-leaved forest in China. We identified a total of 7394 differentially expressed genes (DEGs). Three of these genes were found to be co-upregulated in CAN as compared to control (CK) after 3 and 6 h of N addition treatment, while 133 and 3 genes were respectively found to be co-upregulated and co-downregulated in UAN as compared to CK. In addition, highly expressed genes including GP1 (a gene involved in cell wall biosynthesis) and STP9 (sugar transport protein 9) were detected in CAN, which led to elevated photosynthetic capacity and accumulation of protein and amino acid as well as decrease in glucose, sucrose, and starch contents. On the other hand, genes associated with transport, carbon and N metabolism, redox response, protein phosphorylation, cell integrity, and epigenetic regulation mechanism were affected by UAN, resulting in enhanced photosynthetic capacity and carbohydrates and accumulation of protein and amino acid. In conclusion, our results showed that the CAN compared to UAN treatment had less effects on gene regulation and carbon and N metabolism. Canopy interception of N should be considered through CAN treatment to simulate N deposition in nature.
... Our research was carried out in an evergreen broadleaved forest (24 • [70], which is twice the maximum empirical critical loads (17 kg N ha −1 yr −1 ) for N deposition on territorial ecosystems [71,72]. ...
... The UAN was also used as a reference for the CAN effect. From 2013 to 2021, the CAN and UAN plots were sprayed with an NH4NO3 solution once a month from April to October, as described, in detail, by Zhang et al. [70]. In brief, the CAN was applied via a 35 m high tower (5-8 m above the canopy, with 4 sprinklers) built in the center of each CAN plot, whereas the UAN was applied using five sprinklers that were evenly distributed 1.5 m above the ground in each UAN plot. ...
... The UAN was also used as a reference for the CAN effect. From 2013 to 2021, the CAN and UAN plots were sprayed with an NH 4 NO 3 solution once a month from April to October, as described, in detail, by Zhang et al. [70]. In brief, the CAN was applied via a 35 m high tower (5-8 m above the canopy, with 4 sprinklers) built in the center of each CAN plot, whereas the UAN was applied using five sprinklers that were evenly distributed 1.5 m above the ground in each UAN plot. ...
Although the effects of N deposition on forest plants have been widely reported, few studies have focused on rare and endangered fern species (REFs). Information is also lacking on the effects of micro-environments on REFs. We investigated the effects of N addition (canopy and understory N addition, CAN, and UAN) and micro-environments (soil and canopy conditions) on the functional traits (growth, defense, and reproduction; 19 traits in total) of two REFs—Alsophila podophylla and Cibotium baromet—in a subtropical forest in South China. We found that, compared to controls, CAN or UAN decreased the growth traits (e.g., plant height, H) of C. baromet, increased its defense traits (e.g., leaf organic acid concentrations, OA), delayed its reproductive event (all-spore release date), and prolonged its reproductive duration. In contrast, A. podophylla showed increased growth traits (e.g., H), decreased defense traits (e.g., OA), and advanced reproductive events (e.g., the all-spore emergence date) under CAN or UAN. Meanwhile, the negative effects on the C. baromet growth traits and A. podophylla defense traits were stronger for CAN than for UAN. In addition, the soil chemical properties always explained more of the variations in the growth and reproductive traits of the two REFs than the N addition. Our study indicates that, under simulated N deposition, C. baromet increases its investment in defense, whereas A. podophylla increases its investment in growth and reproduction; this may cause an increasing A. podophylla population and decreasing C. baromet population in subtropical forests. Our study also highlights the importance of considering micro-environments and the N-addition approach when predicting N deposition impact on subtropical forest REFs.
... Finally, the use of modeled N deposition data neglects the role of retention and transformation processes in the forest canopy, which constitute an important part of the N cycle in forest ecosystems (Bortolazzi et al., 2021;Zhang et al., 2015). Tree crowns can intercept and retain a large proportion of N deposition (Bortolazzi et al., 2021;Sievering et al., 2007;Wortman et al., 2012). ...
Hintergrund
Seit der Industriellen Revolution und der Erfindung des Haber-Bosch-Verfahrens zur industriellen Ammoniaksynthese hat die atmosphärische Deposition von reaktivem Stickstoff (N) erheblich zugenommen. In der Folge änderte sich die Rolle von N in Mitteleuropa grundlegend, von einem limitierenden Nährstoff zu einem Belastungsfaktor, der Eutrophierung und eine Vielzahl anderer Umweltprobleme verursacht. Zu diesen Problemen gehören die Homogenisierung der Vegetation, der Verlust oligotropher Arten, die Bodenversauerung, die Entwicklung von Nährstoffungleichgewichten und die Grundwasserbelastung durch Nitratauswaschung. In mitteleuropäischen Wäldern kam es zeitgleich mit dem Anstieg der N-Deposition zu einer Abnahme der Waldnutzungsintensität und Holzernte, wodurch sich der Nährstoffentzug aus den Waldökosystemen verringerte.
Oftmals ist es schwierig zu ermitteln, inwiefern die N-Deposition zu Eutrophierungs-erscheinungen in der Krautschicht von Wäldern beiträgt. Die vorliegenden Arten-gemeinschaften, aber auch abiotische Faktoren (wie das Klima, Bodeneigenschaften oder die Wasser- und Nährstoffverfügbarkeit) beeinflussen die Empfindlichkeit verschiedener Waldgesellschaften gegenüber N-Einträgen. Für den Naturschutz ist es von großer Bedeutung zu wissen, in welchen Waldtypen und unter welchen Boden- und Umweltbedingungen Auswirkungen der N-Deposition auf die Waldbodenvegetation zu erwarten sind. Ein wichtiger Schritt zum Verständnis der standortsabhängigen Reaktion der Vegetation auf die N-Deposition ist die Bewertung, inwiefern das Vorkommen oligotropher und nitrophiler Arten durch die N-Konzentration im Boden im Vergleich zu anderen Boden- und Standortsfaktoren bestimmt wird.
Forschungsziele
Das übergeordnete Ziel dieser Arbeit war es, den derzeitigen Wissensstand zu den standortsabhängigen Auswirkungen der N-Deposition in verschiedenen Waldtypen zu verbessern. Ein zweites zentrales Ziel war es zu untersuchen, ob N die Hauptursache für Eutrophierungserscheinungen in den einzelnen Waldtypen ist, und zu ermitteln, welche anderen bodenchemischen und ökologischen Variablen diese Erscheinungen bedingen oder beeinflussen. Im Fokus standen insbesondere oligo-mesotrophe, potenziell N-empfindliche, naturschutzfachlich wertvolle Waldtypen der gemäßigten Breiten.
Die wichtigsten Forschungsziele waren:
(1) Die Bewertung des Beitrages der atmosphärischen N-Deposition zu Eutrophierungs-gradienten in der Bodenvegetation verschiedener Waldtypen anhand von modellierten N-Depositionskarten.
(2) Die Analyse, inwieweit Blatt-N-Konzentrationen verschiedener Pflanzenarten in der Krautschicht der Wälder die Höhe atmosphärischer N-Deposition und die N-Konzentrationen im Boden widerspiegeln.
(3) Die Untersuchung, ob die Artengemeinschaften der Waldbodenvegetation in den untersuchten Waldtypen als Folge der N-Deposition und geänderter Waldnutzung heute weniger oligotroph sind als in den 1950er - 1970er Jahren.
(4) Die Ermittlung der Rolle von N-Konzentrationen im Boden sowie von anderen Nährstoffen und Umweltfaktoren beim Rückgang oligotropher Arten in der Waldbodenvegetation. Ein Schwerpunkt lag dabei auf der relativen Bedeutung verschiedener Faktoren in Waldtypen auf sauren und karbonatischen Böden.
(5) Die Untersuchung, inwiefern die N-bedingte Eutrophierung der Waldbodenvegetation in Wäldern auf sauren Böden durch die Verfügbarkeit anderer Nährstoffe beeinflusst wird. Hierbei lag ein Fokus auf Mangan, das eine wesentliche Rolle bei der pilzlichen Zersetzung von ligninhaltigem organischem Material spielt.
Untersuchungsstandorte und Studiendesign
In einem empirischen Ansatz wurden 153 Waldbestände aus zehn verschiedenen oligo-mesotrophen Waldgesellschaften im Untersuchungsgebiet Baden-Württemberg untersucht. In jedem Waldbestand wurden zeitgleich Vegetations- und Bodendaten über vegetationsbasierte Eutrophierungsgradienten hinweg erhoben. Mit dieser soliden Datengrundlage wurden multivariate Analysen zu Zusammenhängen zwischen der Waldbodenvegetation und der N-Deposition, N-Konzentrationen im Boden und einer Vielzahl anderer Boden- und Umweltvariablen durchgeführt. Die Datenerhebung umfasste Vegetationsaufnahmen (einschließlich Bodenkryptogamen), Messungen von Baumhöhen und Brusthöhen-durchmessern sowie umfassende bodenchemische Analysen auf je drei Wiederholungsflächen in jedem Waldbestand. Daten zur modellierten N-Deposition, zum Klima und zu anderen Umweltvariablen wurden aus Karten- und Rasterdaten ermittelt. In einem Teil der Untersuchungsflächen in bodensauren Wäldern wurden zudem die Blatt-N-Konzentrationen ausgewählter Arten der Krautschicht gemessen. Auf 18 dieser Standorte wurde der Datensatz außerdem durch fortlaufende chemische Analysen der Bodenlösung ergänzt, die mittels Saugkerzenanlagen gewonnen wurde. Die aktuellen Vegetationsaufnahmen aus sechs Waldgesellschaften wurden mit historischen Vegetationsaufnahmen des jeweiligen Waldtyps verglichen, die zwischen 1950 und 1976 im selben Untersuchungsgebiet durchgeführt wurden. Dieser Vergleich ermöglichte die Untersuchung von Vegetationsveränderungen, die zeitgleich zu Veränderungen der N-Deposition und der Waldbewirtschaftung auftraten.
Ergebnisse und Diskussion
Die N-Deposition war in den zehn untersuchten oligo-mesotrophen temperaten Waldgesellschaften nur ein untergeordneter Treiber der Eutrophierungserscheinungen in der Waldbodenvegetation (Ziel 1). Andere Standortsfaktoren, insbesondere das C/N-Verhältnis (Kohlenstoff/N-Verhältnis) und der pH-Wert des Bodens sowie der Deckungsgrad des Kronendachs, spielten eine größere Rolle bei der Erklärung der Vegetationsgradienten als die N-Deposition. Dies galt für Gradienten in der Artenzusammensetzung (ausgedrückt durch mittlere Ellenberg N-Zeigerwerte und das Auftreten von Nitrophyten und N-sensitiven Charakterarten) als auch für Gradienten in den N-Gehalten ausgewählter Arten der Krautschicht (Ziel 2). Der Vergleich der aktuellen Vegetationserhebungen mit historischen Aufnahmen offenbarte eine Zunahme der Anzahl nitrophiler Arten in fast allen Waldtypen. Zudem zeigte sich in den besonders sauren und oligotrophen Beerstrauch-Tannenwäldern (Vaccinio-Abieteten) und Hainsimsen-Traubeneichenwäldern (Luzulo-Querceten) eine grundlegende Veränderung der Artenzusammensetzung (Ziel 3). Somit wurde ein allgemeiner Eutrophierungstrend über die Zeit beobachtet, während die N-Deposition in den aktuellen Vegetationserhebungen nur selten als Ursache für die Eutrophierungsgradienten der Vegetation identifiziert wurde.
Multivariate Analysen in den einzelnen Waldtypen zeigten, dass mehrere sich gegenseitig beeinflussende Faktoren die Eutrophierungsgradienten der Vegetation bestimmen (Ziel 4). Die Verfügbarkeit von N und Phosphor (P), der pH-Wert des Bodens und die Basensättigung waren die wichtigsten Prädiktoren für die Artenzusammensetzung. In einigen Fällen waren jedoch auch der Kronenschlussgrad und die Baumhöhe eng mit Gradienten in der Bodenvegetation korreliert. Die Wichtigkeit der verschiedenen Einflussfaktoren variierte zwischen verschiedenen Waldtypen stark: In Laubwäldern auf Karbonatböden standen Eutrophierungs-erscheinungen der Waldbodenvegetation in engem Zusammenhang mit einem hohen Kronenschlussgrad und niedrigen C/P-Verhältnissen im Boden. In Nadelwäldern auf sauren Böden wurden Eutrophierungssignale in erster Linie durch höhere pH-Werte des Bodens, eine abnehmende Humusauflage und niedrige C/N-, C/P- und N/P-Verhältnisse bestimmt. In bodensauren Laubwäldern waren die Basensättigung, Kationenkonzentrationen, das C/N- und C/P-Verhältnis und der anorganische Stickstoff im Boden die wichtigsten Prädiktoren für die Eutrophierungsgradienten. Kontinuierliche Bodenwasseranalysen in einer Teilmenge von 18 Probeflächen in bodensauren Laubwäldern bestätigten die wichtige Rolle des C/N-Verhältnisses und der Basensättigung für die Artenzusammensetzung. Die Ergebnisse zeigten jedoch auch die bedeutende Rolle von Mangan, dem als Kofaktor bei der pilzlichen Zersetzung von lignifiziertem organischem Material eine große Bedeutung zukommt (Ziel 5).
Die Ergebnisse spiegeln die komplexe Situation in Wäldern wider, in denen die N-Verfügbarkeit nicht nur durch Deposition, sondern auch durch Zersetzung und Mineralisierung von organischem Material und Retention beeinflusst wird. In Wäldern auf sauren Böden erklärten vor allem das C/N-Verhältnis, die Basensättigung, der pH-Wert und die Mangankonzentration im Boden die Trophiegradienten. Diese Variablen haben allesamt einen wesentlichen Einfluss auf Transformationsprozesse im N-Kreislauf des Waldes. Die gemeinsame Limitierung durch P und N in Wäldern auf Karbonatböden und Nadelwäldern auf sauren Böden verdeutlicht, dass neben N auch andere Variablen das Vorkommen nitrophiler Arten beeinflussen können. Im Untersuchungsgebiet Baden-Württemberg tragen die ausgeprägten klimatischen und physiographischen Unterschiede, im Gegensatz zum moderaten N-Depositionsgradienten, dazu bei, dass die N-Deposition bei der Erklärung von Vegetationsgradienten nur eine untergeordnete Rolle spielt.
Schlussfolgerungen
Zahlreiche voneinander abhängige biotische und abiotische Variablen beeinflussen das Auftreten von Eutrophierungserscheinungen im Wald und die Auswirkungen atmosphärischer N-Einträge auf die Waldbodenvegetation. Zu diesen Faktoren gehören die natürliche Nährstoff- und Wasserverfügbarkeit, die Lichtverhältnisse, das Klima, der Säuregrad des Bodens sowie die frühere und heutige Waldnutzung. Die vorliegende Arbeit trägt zu einem besseren Verständnis dieser Standortsabhängigkeit bei und liefert Erklärungsansätze dafür, warum die andauernde N-Deposition nicht in allen Waldtypen zu einheitlichen Veränderungen der Vegetationszusammensetzung führt. Die Ergebnisse verdeutlichen, wie wichtig es ist, Unterschiede zwischen verschiedenen Waldlebensraumtypen sowie standortspezifische Eigenschaften zu berücksichtigen, wenn potenzielle Auswirkungen der N-Deposition auf die Vegetation evaluiert werden. Ein stärkeres Augenmerk muss auf zugrundeliegende Mechanismen gerichtet werden, die Prozesse des Stickstoffkreislaufs im Wald bestimmen, wie zum Beispiel die Zersetzung und Mineralisierung von organischem Material. Die Berücksichtigung der erheblichen Standortsabhängigkeit der Auswirkungen atmosphärischer N-Einträge kann dazu beitragen, die Eutrophierung und den Verlust gefährdeter oligotropher Arten zu vermeiden, indem Bewirtschaftungsentscheidungen unterstützt und kritische Belastungsgrenzen präzisiert werden.
... Third, we applied an understory N deposition in this study, which ignored some critical ecological processes comparing realistic atmospheric N deposition, e.g., N retention, interception, absorption, and transformation of atmospherically deposited N [85]. Thus, simulating the canopy N deposition would be more realistic to represent the true effects of N deposition on belowground carbon cycling. ...
Elevated nitrogen (N) and phosphorus (P) depositions have greatly affected belowground carbon processes in forest ecosystems. However, open questions still remained on the effects of N and P depositions on belowground carbon processes, including soil respiration (RS), its source components—autotrophic respiration (RA) and heterotrophic respiration (RH), and total belowground carbon allocation (TBCA) in Moso bamboo forests—one of the most important forest types with wide distributions in subtropical China. To fill this knowledge gap, a two-year N, P, and NP experiment was conducted in Moso bamboo forests. Results showed that RS, RA, and RH had a strong seasonal variability and were exponentially correlated with soil temperature. N and P depositions did not change RS and RA. However, P deposition increased RH due to the stimulation of microbial activities, indicating a significant soil carbon loss under P deposition. N and P depositions did not affect TBCA. However, NP deposition significantly increased root carbon-use efficiency. Net ecosystem production (NEP) varied from 198 ± 104 to 529 ± 225 g C m−2 year−1, indicating that Moso bamboo is an important carbon sink. P deposition marginally decreased NEP, while N and NP depositions did not affect NEP, which indicates that N deposition alleviated the suppression of P deposition on NEP. These findings highlight the inconsistent responses of RA, RH, and NEP to N, P, and NP depositions, which should be differently considered to increase the accuracy of predicting belowground carbon dynamics.
