Canopy and understory nitrogen addition increase the xylem tracheid size of dominant broadleaf species in a subtropical forest of China

Resource-acquisitive species have greater plasticity in leaf functional traits than resource-conservative species in response to nitrogen addition in subtropical China

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.

Responses of plant carbon and nitrogen assimilations to nitrogen addition in a subtropical forest: Canopy addition vs. understory addition

The global atmospheric nitrogen (N) deposition has intensified in recent years, resulting in a complex impact on forest ecosystems. This study investigated the effects of canopy (CAN) and understory additions of N (UAN) on leaf carbon (C) and N assimilations, as well as growth parameters of representative woody plant species in an evergreen broad-leaved forest, i.e. Castanea henryi, Schefflera heptaphylla, Blastus cochinchinensis, and Lasianthus chinensis. The results showed that leaf N assimilation key enzyme nitrate reductase (NR) activities of B. cochinchinensis and S. heptaphylla were significantly decreased by UAN, and were significantly decreased by CAN for C. henryi. CAN significantly decreased the nitrite reductase activity of C. henryi, while significantly increased that of L. chinensis. However, the Amax values of each woody species were not significantly different among control (CK), CAN, and UAN. Community surveys demonstrated that CAN and UAN inhibited the growth (diameter at breast height, height, or crown width) of the representative large tree, C. henryi, while promoting the growths of understory woody species (B. cochinchinensis and L. chinensis). Overall, N addition was found to change the physiological processes of N and C metabolisms of the dominant woody species in an evergreen broad-leaved forest. The community of subtropical evergreen broad-leaved forests may further decline and its C fixation capacity may be detrimentally changed under N deposition in the future.

The carbon and nitrogen metabolisms of Ardisia quinquegona were altered in different degrees by canopy and understory nitrogen addition in a subtropical forest

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.

Effects of Simulated Nitrogen Deposition and Micro-Environment on the Functional Traits of Two Rare and Endangered Fern Species in a Subtropical Forest

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.

Nitrogen deposition increases xylem hydraulic sensitivity but decreases stomatal sensitivity to water potential in two temperate deciduous tree species

Although the effects of nitrogen deposition on tree water relations are studied extensively, its impact on the relative sensitivities of stomatal and xylem hydraulic conductance to vapor pressure deficit and water potential is still poorly understood. This study investigated the effects of a 7-year N deposition treatment on the responses of leaf water relations and sensitivity of canopy stomatal conductance to vapor pressure deficit (VPD) and water potential, as well as the sensitivity of branch hydraulic conductance to water potential in a dominant tree species (Quercus wutaishanica) and an associated tree species (Acer mono) in a temperate forest. It was found that the N deposition increased stomatal sensitivity to VPD, decreased stomatal sensitivity to water potential, and increased the vulnerability of the hydraulic system to cavitation in both species. The standardized stomatal sensitivity to VPD, however, was not affected by the N deposition, indicating that the stomata maintained the ability to regulate the water balance under nitrogen deposition condition. Although the increased stomatal sensitivity to VPD could compensate the decreased stomatal sensitivity to water potential to some extent, the combined response would increase the percentage loss of hydraulic conductivity (PLC) when 50 % loss in stomatal conductance occurred, particularly in the dominant species Q. wutaishanica. The result indicates that N deposition would increase the risk of hydraulic failure in those species if the soil and/or air becomes drier under future climate change scenarios. The results of the study can have significant implications on the modelling of ecosystem vulnerability to drought under the scenario of atmospheric nitrogen deposition.

Soil bacterial community is more sensitive than fungal community to canopy nitrogen deposition and understory removal in a Chinese fir plantation

Soil microorganisms are key regulators for plant growth and ecosystem health of forest ecosystem. Although previous research has demonstrated that soil microorganisms are greatly affected by understory nitrogen (N) addition, little is known about the effects of canopy N addition (CNA) and understory management on soil microorganisms in forests. In this study, we conducted a full designed field experiment with four treatments: CNA (25 kg N ha-1 year-1), understory removal (UR), canopy N addition, and understory removal (CNAUR) (25 kg N ha-1 year-1), and control in a Chinese fir plantation. High-throughput sequencing and qPCR techniques were used to determine the abundance, diversity, and composition of bacterial and fungal communities in three soil layers. Our results showed that CNA increased bacterial diversity in the 10-20 cm soil layer but decreased bacterial abundance in the 20-40 cm soil layer and fungal diversity in the 0-10 cm soil layer. UR increased bacterial abundance only in the 20-40 cm soil layer. CNA, not UR significantly altered the compositions of soil bacterial and fungal community compositions, especially in the 0-20 cm soil layer. CNA sharply reduced the relative abundance of copiotrophic taxa (i.e., taxa in the bacterial phylum Proteobacteria and the orders Eurotiales and Helotiales in the fungal phylum Ascomycota) but increased the relative abundance of oligotrophic taxa (i.e., in the bacterial phylum Verrucomicrobia). RDA analysis revealed that soil pH, DON, and DOC were the main factors associated with the variation in bacterial and fungal communities. Our findings suggest that short-term CNA changes both soil bacterial and fungal communities, with stronger responses in the surface and middle soil than in the deep soil layer, and that UR may enhance this effect on the soil bacterial abundance. This study improves our understanding of soil microorganisms in plantations managed with understory removal and that experience increases in N deposition.

Contrasting responses in growth, photosynthesis and hydraulics of two subtropical tree species to cadmium contamination as affected by elevated CO2 and nitrogen addition

Plant growth, photosynthesis, and hydraulics are affected by heavy metals but also by elevated atmospheric CO₂ concentration (e[CO₂]) and nitrogen (N) deposition. However, few studies have investigated the response of woody species to the combined effects of these three factors. We conducted an open-top chamber experiment with two common subtropical trees (Acacia auriculiformis and Syzygium hainanense) to explore the effects of cadmium (Cd)-contamination, e[CO₂], and N addition on plant eco-physiological traits. We found that the growth of A. auriculiformis was insensitive to the treatments, indicating that it is a Cd-tolerant and useful afforestation species. For S. hainanense, in contrast, e[CO₂] and/or N addition offset the detrimental effects of Cd addition by greatly increasing plant biomass and reducing the leaf Cd concentration. We then found that e[CO₂] and/or N addition offset the detrimental Cd effects on S. hainanense biomass by increasing its photosynthetic rate, its N concentration, and the efficiency of its stem water transport network. These offsetting effects of e[CO₂] and/or N addition, however, came at the expense of reduced xylem hydraulic safety resulting from wider vessels, thinner vessel walls, and therefore weaker vessel reinforcement. Our study suggests that, given future increases in global CO₂ concentration and N deposition, the growth of Cd-tolerant tree species (like A. auriculiformis) will be probably stable while the growth of Cd-sensitive tree species (like S. hainanense) might be enhanced despite reduced hydraulic safety. This also suggests that both species will be useful for afforestation of Cd-contaminated soils given future global change scenarios.

Differential effects of nitrogen vs. phosphorus limitation on terrestrial carbon storage in two subtropical forests: A Bayesian approach

Nitrogen (N) and phosphorus (P) have been demonstrated to limit terrestrial carbon (C) storage in terrestrial ecosystems. However, the reliable indicator to infer N and P limitation are still lacking, especially in subtropical forests. Here we used a terrestrial ecosystem (TECO) model framework in combination with a Bayesian approach to evaluate effects of nutrient limitation from added N/P processes and data sets on C storage capacities in two subtropical forests (Tiantong and Qianyanzhou [QYZ]). Three of the six simulation experiments were developed with assimilating data (TECO C model with C data [C-C], TECO C-N coupling model with C and N data [CN-CN], and TECO C-N-P model with C, N, and P data [CNP-CNP]), and the other three ones were simulated without assimilating data (C-only, CN-only, and CNP-only). We found that P dominantly constrained C storage capacities in Tiantong (42%) whereas N limitation decreased C storage projections in QYZ (44%). Our analysis indicated that the stoichiometry of wood biomass and soil microbe (e.g., N:P ratio) were more sensitive indicators of N or P limitation than that of other pools. Furthermore, effects of P-induced limitation were mainly on root biomass by additional P data and on both metabolic litter and soil organic carbon (SOC) by added P processes. N-induced effects were mainly from added N data that limited plant non-photosynthetic tissues (e.g., woody biomass and litter). The different effects of N and P modules on C storage projections reflected the diverse nutrient acquisition strategies associated with stand ages and plant species under nutrient stressed environment. These findings suggest that the interaction between plants and microorganisms regulate effects of nutrient availability on ecosystem C storage, and stoichiometric flexibility of N and P in plant and soil C pools could improve the representation of N and P limitation in terrestrial ecosystem models.

Canopy and understory additions of nitrogen change the chemical composition, construction cost, and payback time of dominant woody species in an evergreen broadleaved forest

Simulated nitrogen deposition experiments in forests have mainly used understory nitrogen application, i.e., they failed to consider how canopy interception may alter the effects of nitrogen deposition on forest plants. This study used canopy addition of nitrogen, understory addition of nitrogen, and no-nitrogen addition control to study the effect of nitrogen deposition on the allocation of carbon assimilation products of representative woody species in an evergreen broad-leaved forest. Results showed that the maximum photosynthetic rate (A_sat) of Blastus cochinchinensis (a shrub), Ardisia quinquegona (a small tree), and Schefflera octophylla (a small tree) were significantly higher, but A_sat of Schima superba (a large tree) was significantly lower under canopy addition of nitrogen than under the control. Canopy and understory additions of nitrogen did not change A_sat of Lasianthus chinensis (a shrub). Compared with the control, leaf chemical compositions of these plants were differentially changed by canopy and understory additions of nitrogen. These changes were accompanied by a significant increase in construction cost of A. quinquegona, S. octophylla, and S. superba under canopy addition of nitrogen and of L. chinensis, A. quinquegona, and S. superba under understory addition of nitrogen. The payback time was significantly shorter for B. cochinchinensis, A. quinquegona, and S. octophylla but was significantly longer for S. superba under canopy addition of nitrogen than under the control. In contrast, the payback time was significantly shorter for B. cochinchinensis and A. quinquegona under understory addition of nitrogen than under the control. Correlation analyses showed that the changes in protein and structural carbohydrate contents helped explain the changes in payback time. In summary, nitrogen deposition may increase carbon assimilation and allocation in shrubs and small trees, and large trees may require a longer period to increase carbohydrates, which may help explain the ongoing transformation of evergreen broad-leaved forests into shrublands.

Understory and canopy additions of nitrogen differentially affect carbon and nitrogen metabolism of Psychotria rubra in an evergreen broad-leaved forest

To assess the effects of nitrogen deposition on forest plants, researchers have conducted many nitrogen-addition experiments. Most of these experiments, however, failed to fully simulate atmospheric nitrogen deposition because they failed to assess interception of nitrogen deposition by the forest canopy. Here, we used transcriptomics, proteomics, and metabolomics to compare the effects of understory nitrogen addition (UAN), canopy nitrogen addition (CAN), and a control (no nitrogen addition) on carbon and nitrogen metabolism in leaves of Psychotria rubra, a dominant subtropical understory plant species in evergreen broad-leaved forests in South China. We first established a reference P. rubra transcriptome via RNA-seq and obtained a total of 93,986 unigenes from de novo assembly. Next, we quantitatively investigated the proteome and metabolome in leaves, and identified 4021 proteins and 562 metabolites. Under the CAN treatment and relative to the control, 36 genes were up-regulated and 23 were down-regulated, and we identified 46 up-regulated proteins and 49 down-regulated proteins. Under the UAN treatment and relative to the control, 1525 genes were up-regulated and 224 genes down-regulated, and we identified 35 up-regulated proteins and 71 down-regulated proteins. These differentially expressed genes and proteins were related to photosynthesis, amino acid metabolism, and flavonoid biosynthesis. Accordingly, 15 differentially accumulated metabolites in response to CAN and 20 in response to UAN were found; these differentially accumulated metabolites included 4 amino acids and 3 flavonoids. Taken together, our results show that the UAN treatment had a greater effect on photosynthesis, amino acid metabolism, and flavonoid biosynthesis than the CAN treatment. Most importantly, the results indicate that understory application of nitrogen in experiments may incorrectly estimate the effects of nitrogen deposition on nitrogen and carbon utilization by P. rubra and perhaps by other understory woody plants in the evergreen broad-leaved forests in South China as well.

Global response patterns of plant photosynthesis to nitrogen addition: A meta-analysis

A mechanistic understanding of plant photosynthetic response is needed to reliably predict changes in terrestrial carbon (C) gain under conditions of chronically elevated atmospheric nitrogen (N) deposition. Here, using 2,683 observations from 240 journal articles, we conducted a global meta-analysis to reveal effects of N addition on 14 photosynthesis-related traits and affecting moderators. We found that across 320 terrestrial plant species, leaf N was enhanced comparably on mass basis (N_mass , +18.4%) and area basis (N_area , +14.3%), with no changes in specific leaf area or leaf mass per area. Total leaf area (TLA) was increased significantly, as indicated by the increases in total leaf biomass (+46.5%), leaf area per plant (+29.7%), and leaf area index (LAI, +24.4%). To a lesser extent than for TLA, N addition significantly enhanced leaf photosynthetic rate per area (A_area , +12.6%), stomatal conductance (g_s , +7.5%), and transpiration rate (E, +10.5%). The responses of A_area were positively related with that of g_s , with no changes in instantaneous water-use efficiency and only slight increases in long-term water-use efficiency (+2.5%) inferred from ¹³ C composition. The responses of traits depended on biological, experimental, and environmental moderators. As experimental duration and N load increased, the responses of LAI and A_area diminished while that of E increased significantly. The observed patterns of increases in both TLA and E indicate that N deposition will increase the amount of water used by plants. Taken together, N deposition will enhance gross photosynthetic C gain of the terrestrial plants while increasing their water loss to the atmosphere, but the effects on C gain might diminish over time and that on plant water use would be amplified if N deposition persists.

Canopy and understory nitrogen additions did not significantly change the community structure of soil fauna under a mature subtropical forest

Increasing nitrogen (N) deposition has seriously harmed the structure and function of ecosystems throughout the world and this problem has been increasing. How N deposition affects soil faunal communities is poorly understood, compared to plant and microbial communities. Canopy and understory N additions of 25 and 50 kg ha^-1 year^-1 were employed to determine whether the effects of N addition on the soil fauna differ between N released to the canopy or to the understory. Specifically, we examined how the soil fauna survives when N additions produce desynchronized and complex impacts on the soil, microbes and litter quantity under mature subtropical forest ecosystems. Our results showed that no significant differences were observed between the soil faunal communities receiving canopy and understory N additions. This is consistent with our observation that the concentrations of ammonia nitrogen and nitrate nitrogen in the soil did not significantly differ under the two different methods of applying N. There were no observed effects on the litter quantity, soil microbial Phospholipid Fatty Acids or soil physical-chemical properties; therefore, it is not surprising that N treatments for 4 years did not significantly alter the community structure of soil fauna under the mature subtropical forest sites. However, the shifts in seasonal differences in the microbial communities under the N treatments had a positive effect on soil microbial development compared to control, which might also produce a time-delay influence on the relative development of the soil fauna under mature subtropical forest in the future. Further dynamic monitoring is needed to illustrate the possible effects and mechanisms by which increasing N deposition may alter soil faunal development in the future.

Specific responses of sap flux and leaf functional traits to simulated canopy and understory nitrogen additions in a deciduous broadleaf forest

To investigate the effects of atmospheric nitrogen (N) deposition on water use characteristics and leaf traits of trees, we performed canopy (C50) and understory (U50) N additions as NH4NO3 of 50 kg N ha-1 year-1 in a deciduous broadleaf forest of central China. We measured xylem sap flux, crown area:sapwood area ratio (Ca:As), specific leaf area (SLA), mass-based leaf nitrogen content (Nmass) and leaf carbon isotope ratio (δ13C) of Liquidambar formosana Hance, Quercus acutissima Carruth. and Quercus variabilis Blume. Functional traits under different N addition treatments and their responses among tree species were compared and the relationship between xylem sap flux and leaf functional traits under N additions were explored. Results showed that under U50 sap-flux density of xylem significantly decreased for three tree species. But the effect of C50 on sap flux was species-specific. The decrease of sap-flux density with N additions might be caused by the increased Ca/As. δ13C remained constant among different N addition treatments. The responses of SLA and Nmass to N additions were species- and N addition approaches-specific. The correlation of xylem sap flux with leaf traits was not found. Our findings indicate that the effects of canopy N addition on xylem sap flux and leaf functional traits were species-specific and it is necessary to employ canopy N addition for exploring the real responses of forest ecosystems to climate changes in the future researches.