Global-scale impacts of nitrogen deposition on tree carbon sequestration in tropical, temperate, and boreal forests: A meta-analysis

Complex imprint of air pollution in the basal area increments of three European tree species

The impact of atmospheric pollution on the growth of European forest tree species, particularly European beech, Silver fir and Norway spruce, is examined in five mesic forests in the Czech Republic. Analyzing of basal area increment (BAI) patterns using linear mixed effect models reveals a complex interplay between atmospheric nitrogen (N) and sulphur (S) deposition, climatic variables and changing CO2 concentrations. Beech BAI responds positively to N deposition (in tandem with air CO2 concentration), with soil phosphorus (P) availability emerging as a significant factor influencing overall growth rates. Fir BAI, on the other hand, was particularly negatively influenced by S deposition, although recent growth acceleration suggests growth resilience in post-pollution period. This fir growth surge likely coincides with stimulation of P acquisition following the decline of acidic pollution. The consequence is the current highest productivity among the studied tree species. The growth dynamics of both conifers were closely linked to the stoichiometric imbalance of phosphorus in needles, indicating the possible sensitivity of exogenous controls on nutrient uptake. Furthermore, spruce BAI was positively linked to calcium availability across sites. Despite enhanced water-use efficiency under elevated CO2, spruce growth is constrained by precipitation deficit and demonstrates weakening resilience to increasing growing season air temperatures. Overall, these findings underscore the intricate relationships between atmospheric pollution, nutrient availability, and climatic factors in shaping the growth dynamics of European forest ecosystems. Thus, incorporating biogeochemical context of nutrient availability is essential for realistic modelling of tree growth in a changing climate.

Concurrent and legacy effects of sheep trampling on soil organic carbon stocks in a typical steppe, China

Grazing plays a key role in ecosystem biogeochemistry, particularly soil carbon (C) pools. The non-trophic interactions between herbivores and soil processes through herbivore trampling have recently attracted extensive attention. However, their concurrent and legacy effects on the ecosystem properties and processes are still not clear, due to their effects being hard to separate via field experiments. In this study, we conducted a 2-year simulated-sheep-trampling experiment with four trampling intensity treatments (i.e., T0, T40, T80, and T120 for 0, 40, 80, and 120 hoofprints m-2, respectively) in a typical steppe to explore the concurrent and legacy effects of trampling on grassland ecosystem properties and processing. In 2017 (trampling treatment year), we found that trampling decreased aboveground biomass (AGB) of plant community and community-weighted mean shoot C concentration (CWM C), soil available nitrogen (N) and available phosphorus (P), but did not affect plant species diversity and belowground biomass (BGB). We show that compared with T0, trampling increased soil bulk density (BD) at T80, and decreased soil organic carbon (SOC) stocks. After the cessation of trampling for two years (i.e., in 2019), previous trampling increased plant diversity and BGB, reaching the highest values at T80, but decreased soil available N and available P. Compared with T0, previous trampling significantly increased soil BD at T120, while significantly decreased CWM C at T80 and T120, and reduced SOC stocks at T80. Compared with 2017, the trampling negative legacy effects amplified at T80 but weakened at T40 and T120. We also show that trampling-induced decreases in soil available N, AGB of Fabaceae and CWM C were the main predictors of decreasing SOC stocks in 2017, while previous trampling-induced legacy effects on soil available P, AGB of Poaceae and CWM C contributed to the variations of SOC stocks in 2019. Taken together, short-term trampling with low intensity could maintain most plant functions, while previous trampling with low intensity was beneficial to most plant and soil functions. The results of this study show that T40 caused by sheep managed at a stocking rate of 2.7 sheep ha-1 is most suitable for grassland adaptive management in the typical steppe. The ecosystem functions can be maintained under a high stocking rate through the process of providing enough time to rebuild sufficient vegetation cover and restore soil through measures such as regional rotational grazing and seasonal grazing.

Nitrogen addition favors terrestrial ecosystem carbon sink: A global meta-analysis

Long-term atmospheric nitrogen (N) deposition has been known to enhance plant productivity by increasing available N in terrestrial ecosystems. However, the response of carbon process to N deposition in terrestrial ecosystems remains unclear, particularly about different climate regions and biomes. In this study, we synthesized 1281 pairwise observations from 218 published articles on experimental N addition globally, aiming to quantify the responses of the carbon process and its mechanisms to N addition. Our results revealed a significant overall increase in net ecosystem productivity (NEP) by 31.42 % following N addition treatment, owing to increased aboveground net primary productivity (ANPP, 16.46 %), belowground net primary productivity (BNPP, 27.74 %), and reduced soil respiration (Rs, -2.56 %), soil heterotrophic respiration (Rh, -6.24 %). Furthermore, the effects of N addition on NEP varied with biomes and climate regions. The positive effect of N addition on NEP was more pronounced in arid regions (28.67 %) compared to humid regions (21.92 %), primarily driven by a higher increase in vegetation productivity. Additionally, N addition exhibited a higher increase in NEP (33.95 %) in forest compared to grassland (31.80 %), resulting from a more reduction in respiratory processes. However, the positive effects of N addition on NEP diminished with increasing experimental duration. Furthermore, ANPP and BNPP displayed a convex relationship with N addition rates, with the optimum BNPP addition rate exceeding that of ANPP. In contrast, Rs exhibited a concave response to addition rates. These findings suggest that carbon sink in terrestrial ecosystems could be enhanced under future atmospheric N deposition, especially in arid regions and forest ecosystems. Our study provided insight for predicting how N deposition influences terrestrial ecosystem carbon process.

Patterns and drivers of atmospheric nitrogen deposition retention in global forests

Forests are the largest carbon sink in terrestrial ecosystems, and the impact of nitrogen (N) deposition on this carbon sink depends on the fate of external N inputs. However, the patterns and driving factors of N retention in different forest compartments remain elusive. In this study, we synthesized 408 observations from global forest 15N tracer experiments to reveal the variation and underlying mechanisms of 15N retention in plants and soils. The results showed that the average total ecosystem 15N retention in global forests was 63.04 ± 1.23%, with the soil pool being the main N sink (45.76 ± 1.29%). Plants absorbed 17.28 ± 0.83% of 15N, with more allocated to leaves (5.83 ± 0.63%) and roots (5.84 ± 0.44%). In subtropical and tropical forests, 15N was mainly absorbed by plants and mineral soils, while the organic soil layer in temperate forests retained more 15N. Additionally, forests retained moreN 15 H 4 + $$ {}^{15}\mathrm{N}{\mathrm{H}}_4^{+} $$ thanN 15 O 3 - $$ {}^{15}\mathrm{N}{\mathrm{O}}_3^{-} $$ , primarily due to the stronger capacity of the organic soil layer to retainN 15 H 4 + $$ {}^{15}\mathrm{N}{\mathrm{H}}_4^{+} $$ . The mechanisms of 15N retention varied among ecosystem compartments, with total ecosystem 15N retention affected by N deposition. Plant 15N retention was influenced by vegetative and microbial nutrient demands, while soil 15N retention was regulated by climate factors and soil nutrient supply. Overall, this study emphasizes the importance of climate and nutrient supply and demand in regulating forest N retention and provides data to further explore the impacts of N deposition on forest carbon sequestration.

Evidence and causes of recent decreases in nitrogen deposition in temperate forests in Northeast China

High reactive nitrogen (N) emissions due to anthropogenic activities in China have led to an increase in N deposition and ecosystem degradation. The Chinese government has strictly regulated reactive N emissions since 2010, however, determining whether N deposition has reduced requires long-term monitoring. Here, we report the patterns of N deposition at a rural forest site (Qingyuan) in northeastern China over the last decade. We collected 456 daily precipitation samples from 2014 to 2022 and analysed the temporal dynamics of N deposition. NH4+-N, NO3--N, and total inorganic N (TIN) deposition ranged from 10.5 ± 3.5 (mean ± SD), 6.1 ± 1.6, and 16.6 ± 4.7 kg N ha-1 year-1, respectively. Over the measurement period, TIN deposition at Qingyuan decreased by 55 %, whereas that in comparable sites in East Asia declined by 14-34 %. We used a random forest model to determine factors influencing the deposition of NH4+-N, NO3--N, and TIN during the study period. NH4+-N deposition decreased by 60 % because of decreased agricultural NH3 emissions. Furthermore, NO3--N deposition decreased by 42 %, due to reduced NOx emissions from agricultural soil and fossil fuel combustion. The steep decline in N deposition in northeastern China was attributed to reduced coal consumption, improved emission controls on automobiles, and shifts in agricultural practices. Long-term monitoring is needed to assess regional air quality and the impact of N emission control regulations.

Soil microbes deal with the nitrogen deposition enhanced phosphorus limitation by shifting community structure in an old-growth subtropical forest

Elevated atmospheric nitrogen (N) deposition potentially enhances the degree of phosphorus (P) limitation in tropical and subtropical forests. However, it remains elusive that how soil microorganisms deal with the N deposition-enhanced P limitation. We collected soils experienced 9 years of manipulative N input at various rates (0, 40, and 80 kg N ha-1 y-1) in an old-growth subtropical natural forest. We measured soil total and available carbon (C), N and P, microbial biomass C, N and P, enzyme activities involved in C, N and P acquisition, microbial community structure, as well as net N and P mineralization. Additionally, we calculated element use efficiency and evaluated microbial homeostasis index. Our findings revealed that N input increased microbial biomass C:P (MBC:P) and N:P (MBN:P) ratios. The homeostasis indexes of MBC:P and MBN:P were 0.68 and 0.75, respectively, indicating stoichiometric flexibility. Interestingly, MBC:P and MBN:P correlated significantly with the fungi:bacteria ratio (F:B), not with N and P use efficiencies, net N and P mineralization, and enzyme C:P (EEAC:P) and N:P (EEAN:P) ratios. Furthermore, EEAC:P and EEAN:P correlated positively with F:B but did not negatively correlate with the C:P and N:P ratios of available resources and microbial biomass. The effects of N deposition on MBC:P, MBN:P and EEAN:P became insignificant when including F:B as a covariate. These findings suggest that microbes flexibly adapted to the N deposition enhanced P limitation by changing microbial community structure, which not only alter microbial biomass C:N:P stoichiometry, but also the enzyme production strategy. In summary, our research advances our understanding of how soil microorganisms deal with the N deposition-enhanced soil P limitation in subtropical forests.

Species-specific responses of C and N allocation to N addition: evidence from dual 13C and 15N labeling in three tree species

Soil nitrogen (N) availability affects plant carbon (C) utilization. However, it is unclear how various tree functional types respond to N addition in terms of C assimilation, allocation, and storage. Here, a microcosm experiment with dual 13C and 15N labeling was conducted to study the effects of N addition (i.e., control, 0 g N kg-1; moderate N addition, 1.68 g N kg-1; and high N addition, 3.36 g N kg-1 soil) on morphological traits, on changes in nonstructural carbohydrates (NSC) in different organs, as well as on C and N uptake and allocation in three European temperate forest tree species (i.e., Acer pseudoplatanus, Picea abies and Abies alba). Our results demonstrated that root N uptake rates of the three tree species increased by N addition. In A. pseudoplatanus, N uptake by roots, N allocation to aboveground organs, and aboveground biomass allocation significantly improved by moderate and high N addition. In A. alba, only the high N addition treatment considerably raised aboveground N and C allocation. In contrast, biomass as well as C and N allocation between above and belowground tissues were not altered by N addition in P. abies. Meanwhile, NSC content as well as C and N coupling (represented by the ratio of relative 13C and 15N allocation rates in organs) were affected by N addition in A. pseudoplantanus and P. abies but not in A. alba. Overall, A. pseudoplatanus displayed the highest sensitivity to N addition and the highest N requirement among the three species, while P. abies had a lower N demand than A. alba. Our findings highlight that the responses of C and N allocation to soil N availability are species-specific and vary with the amount of N addition.

Nitrogen deposition caused higher increases in plant-derived organic carbon than microbial-derived organic carbon in forest soils

Plant- and microbial-derived organic carbon, two components of the soil organic carbon (SOC) pool in terrestrial ecosystems, are regulated by increased atmospheric nitrogen (N) deposition. However, the spatial patterns and driving factors of the responses of plant- and microbial-derived SOC to N deposition in forests are not clear, which hinders our understanding of SOC sequestration. In this study, we explored the spatial patterns of plant- and microbial-derived SOC, and their responses to N addition and elucidated their underlying mechanisms in forest soils receiving N addition at four sites with various soil and climate conditions. Plant- and microbial-derived SOC were quantified using lignin phenols and amino sugars, respectively. N addition increased the total microbial residues by 20.5% on average ranging from 9.4% to 34.0% in temperate forests but not in tropical forests, and the increase was mainly derived from fungal residues. Lignin phenols increased more in temperate forests (average of 63.8%) than in tropical forests (average of 15.7%) following N addition. The ratio of total amino sugars to lignin phenols was higher in temperate forests than in tropical forests and decreased with N addition in temperate forests. N addition mainly regulated soil microbial residues by affecting pH, SOC, exchangeable Ca2+, gram-negative bacteria biomass, and the C:N ratio, while it mainly had indirect effects on lignin phenols by altering SOC, soil C:N ratio, and gram-negative bacteria biomass. Overall, our findings suggested that N deposition caused a greater increase in plant-derived SOC than in microbial-derived SOC and that plant-derived SOC would have a more important role in sequestering SOC under increasing N deposition in forest ecosystems, particularly in temperate forests.

Characteristics and drivers of vegetation productivity sensitivity to increasing CO2 at Northern Middle and High Latitudes

Understanding and accurately predicting how the sensitivity of terrestrial vegetation productivity to rising atmospheric CO2 concentration (β) is crucial for assessing carbon sink dynamics. However, the temporal characteristics and driving mechanisms of β remain uncertain. Here, observational and CMIP6 modeling evidence suggest a decreasing trend in β at the Northern Middle and High Latitudes during the historical period of 1982-2015 (-0.082 ± 0.005% 100 ppm-1 year-1). This decreasing trend is projected to persist until the end of the 21st century (-0.082 ± 0.005% 100 ppm-1 year-1 under SSP370 and -0.166 ± 0.006% 100 ppm-1 year-1 under SSP585). The declining β indicates a weakening capacity of vegetation to mitigate warming climates, posing challenges for achieving the temperature goals of the Paris Agreement. The rise in vapor pressure deficit (VPD), that triggers stomata closure and weakens photosynthesis, is considered as the dominated factor contributing to the historical and future decline in β, accounting for 62.3%-75.2% of the effect. Nutrient availability and water availability contribute 15.7%-21.4% and 8.5%-16.3%, respectively. These findings underscore the significant role of VPD in shaping terrestrial carbon sink dynamics, an aspect that is currently insufficiently considered in many climate and ecological models.

Fine root production, mortality, and turnover in response to simulated nitrogen deposition in the subtropical Abies georgei (Orr) forest

Increased nitrogen deposition has important effects on below-ground ecological processes. Fine roots are the most active part of the root system in terms of physiological activity and the main organs for nutrient and water uptake by plants. However, there is still a limited understanding of how nitrogen deposition affects the fine root dynamics in subtropical Abies georgei (Orr) forests. Consequently, a three-year field experiment was conducted to quantify the effects of three forms of nitrogen sources ((NH4)2SO4, NaNO3, and NH4NO3) at four levels (0, 5, 15, and 30 kg N·ha-1·yr-1) on the fine root dynamics in Abies georgei forests using a randomized block-group experimental design and minirhizotron technique. The first year of nitrogen addition did not affect the first-class fine roots (FR1, 0 < diameter < 0.5 mm) and second-class fine roots (FR2, 0.5 < diameter < 1.0 mm). The next two years of nitrogen addition significantly increased the production, mortality, and turnover of FR1 and FR2; the three year of nitrogen addition did not affect the dynamics of the third- class fine roots (FR3, 1.0 < diameter < 1.5 mm) and fourth- class fine roots (FR4,1.5 < diameter < 2.0 mm). Nitrogen addition positively affected the dynamics of FR1, FR2, FR3 and FR4 by positively affecting the carbon, nitrogen, and phosphorus contents of fine roots and indirectly affecting the soil pH. Increased carbon allocation to FR1 and FR2 may represent a phosphorus acquisition strategy when nitrogen is not the limiting factor. The nitrogen addition forms and levels affected the fine root dynamics in the following orde: NH4NO3 > (NH4)2SO4 > NaNO3 and high nitrogen > medium nitrogen > low nitrogen. The results suggest that the different-diameter fine root dynamics respond differently to different nitrogen addition forms and levels, and linking the different-diameter fine roots to nitrogen deposition is crucial.

Eastern Canadian boreal forest soil and foliar chemistry show evidence of resilience to long-term nitrogen addition

The boreal forest is one of the world's largest terrestrial biome and plays crucial roles in global biogeochemical cycles, such as carbon (C) sequestration in vegetation and soil. However, the impacts of decades of N deposition on N-limited ecosystems, like the eastern Canadian boreal forest, remain unclear. For 13 years, N deposition was simulated by periodically adding ammonium nitrate on soils of two boreal coniferous forests (i.e., balsam fir and black spruce) of eastern Canada, at low (LN) and high (HN) rates, corresponding to 3 and 10 times the ambient N deposition, respectively. We show that more than a decade of N addition had no strong effects on mineral soil C, N, P, and cation concentrations and on foliar total Ca, K, Mg, and Mn concentrations. In organic soil, C stock was not affected by N addition while N stock increased, and exchangeable Ca2+ and Mg2+ decreased at the balsam fir site under HN treatment. At both sites, LN treatment had nearly no impact on foliage and soil chemistry but foliar N and N:P significantly increased under HN treatment, potentially leading to foliar nutrient imbalance. Overall, our work indicates that, in the eastern Canadian boreal forest, soil and foliar nutrient concentrations and stocks are resilient to increasing N deposition potentially because, in the context of N limitation, extra N would be rapidly immobilized by soil micro-organisms and vegetation. These findings could improve modeling future boreal forest soil C stocks and biomass growth and could help in planning forest management strategies in eastern Canada.

Widespread cooling of topsoil under nitrogen enrichment and implication for soil carbon flux

Increasing reactive nitrogen (N) to terrestrial ecosystems is considered to enhance ecosystem carbon sink, which plays a critical role in ameliorating global warming. Besides this indirect buffering of temperature rise, the N-induced enhancement of vegetation growth may exert a biophysical cooling effect on soils. However, the magnitude and drivers of this cooling effect have rarely been evaluated. Here, using a global meta-analysis with 321 paired measurements, we demonstrated a widespread topsoil cooling (-0.30 °C in average) under anthropogenic N enrichment, which was primarily associated with the increase in aboveground biomass. This biophysical cooling could also buffer topsoil temperature rise by an average of 0.39 °C under experimental warming. Further, the reduced soil temperature was found to contribute to a reduction of soil respiration rate as temperature declines gradually. Overall, our results underpin a previously overlooked function of global N enrichment-the lowering of topsoil temperature, which suggests that the warming of topsoil may not be as fast as previously predicted under future global change scenarios. This biophysical cooling effect will also slow down soil carbon emissions and further mitigate climate warming.

Photosynthetic and biochemical responses of four subtropical tree seedlings to reduced dry season and increased wet season precipitation and variable N deposition

Interspecific variations in phenotypic plasticity of trees that are affected by climate change may alter the ecosystem function of forests. Seedlings of four common tree species (Castanopsis fissa, Michelia macclurei, Dalbergia odorifera and Ormosia pinnata) in subtropical plantations of southern China were grown in the field under rainout shelters and subjected to changing precipitation (48 L of water every 4 days in the dry season, 83 L of water every 1 day in the wet season; 4 g m-2 year-1 of nitrogen (N)), low N deposition (48 L of water every 2 days in the dry season, 71 L of water every 1 day in the wet season; 8 g m-2 year-1 N), high N deposition (48 L of water every 2 days in the dry season, 71 L of water every 1 day in the wet season; 10 g m-2 year-1 N) and their interactive effects. We found that the changes in seasonal precipitation reduced the light-saturated photosynthetic rate (Asat) for C. fissa due to declining area-based foliar N concentrations (Na). However, we also found that the interactive effects of changing precipitation and N deposition enhanced Asat for C. fissa by increasing foliar Na concentrations, suggesting that N deposition could alleviate N limitations associated with changing precipitation. Altered precipitation and high N deposition reduced Asat for D. odorifera by decreasing the maximum electron transport rate for RuBP regeneration (Jmax) and maximum rate of carboxylation of Rubisco (Vcmax). Ormosia pinnata under high N deposition exhibited increasing Asat due to higher stomatal conductance and Vcmax. The growth of D. odorifera might be inhibited by changes in seasonal precipitation and N deposition, while O. pinnata may benefit from increasing N deposition in future climates. Our study provides an important insight into the selection of tree species with high capacity to tolerate changing precipitation and N deposition in subtropical plantations.

Global patterns of soil available N production by mineralization-immobilization turnover in the tropical forest ecosystems

Available N (Navail) is important to nurish plant-microbial system and sequestrate carbon (C) in terrestrial ecosystems. For forest ecosystem, Navail is usually calculated as the sum of N2 fixation (NN2-fixed), N deposition (Ndeposition) and soil available N production (Navail-soil), in which Navail-soil determined the Navail production and its temporal changes. While, there is still a lack of Navail-soil estimation at the global and regional level due to the temporal and spatial variability of influencing factors, such as climate and soil physicochemical properties. By assembling a dataset of gross rates of soil N mineralization (GRmin), immobilization of ammonium (NH4+) (GRac) and nitrate (NO3-) (GRnc), as well as their corresponding geographic information, climate and main soil physicochemical properties, the Navail-soil produced from organic N (Norg) mineralization and inorganic N (Ninorg) immobilization turnover (MIT) was calculated via building a random forest (RF) model in global tropical forests. The results revealed a good fit between the observed and predicted GRmin (R2 = 0.76), GRac (R2 = 0.77) and GRnc (R2 = 0.67). We further estimated that the total mineralized N, immobilized NH4+ and NO3- was 23.97 (10.48-37.46), 17.98 (5.81-30.15) and 4.86 (1.46-8.26) Pg N year-1, respectively, leading to the total Navail-soil of 1.13 (-0.95-3.21) Pg N year-1. Referring to the reported average density of NN2-fixed and Ndeposition, the total NN2-fixed and Ndeposition was 0.03-0.05 and 0.01 Pg N year-1, respectively, by producting density and square meter of global tropic forest. Then the total Navail of global tropic forest ecosystem was 1.18 (-0.91-3.27) Pg N year-1 (Navail-soil + NN2-fixed + Ndeposition). According to the tight stoichiometric relationship between C and N in the production of gross primary productivity (GPP) and soil respiration (Rs), C:N ratio of 31.8-41.9 and 22.7-48.2 was calculated, respectively, which all fall into the C:N ratio range of plants and litter (13.9-75.9) in tropical forest ecosystem. These results confirmed the prediction of Navail-soil production from MIT was in line with theoretic estimates by applying RF machine learning. To our knowledge, this is the first estimation of Navail-soil and the results provide the theoretical basis to evaluate soil C sequestration potential in tropical (e.g. southern America, southeast Asia and Africa) forest ecosystem.

Nitrogen deposition raises temperature sensitivity of soil organic matter decomposition in subtropical forest

Subtropical ecosystems are strongly affected by nitrogen (N) deposition, impacting soil organic matter (SOM) availability and stocks. Here we aimed to reveal the effects of N deposition on i) the structure and functioning of microbial communities and ii) the temperature sensitivity (Q10) of SOM decomposition. Phosphorus (P) limited evergreen forest in Guangdong Province, southeastern China, was selected, and N deposition (factor level: N (100 kg N ha-1 y-1 (NH4NO3)) and control (water), arranged into randomized complete block design (n = 3)) was performed during 2.5 y. After that soils from 0 to 20 cm were collected, analyzed for the set of parameters and incubated at 15, and 25, and 35 °C for 112 days. N deposition increased the microbial biomass N and the content of fungal and Gram-positive bacterial biomarkers; activities of beta-glucosidase (BG) and acid phosphatase (ACP) also increased showing the intensification of SOM decomposition. The Q10 of SOM decomposition under N deposition was 1.66 and increased by 1.4 times than under control. Xylosidase (BX), BG, and ACP activities increased with temperature under N but decreased with the incubation duration, indicating either low production and/or decomposition of enzymes. Activities of polyphenol-(PPO) and peroxidases (POD) were higher under N than in the control soil and were constant during the incubation showing the intensification of recalcitrant SOM decomposition. At the early incubation stage (10 days), the increase of Q10 of CO2 efflux was explained by the activities of BX, BQ, ACP, and POD and the quality of the available dissolved organic matter pool. At the later incubation stages (112 days), the drop of Q10 of CO2 efflux was due to the depletion of the labile organic substances and the shift of microbial community structure to K-strategists. Thus, N deposition decoupled the effects of extracellular enzyme activities from microbial community structure on Q10 of SOM decomposition in the subtropical forest soil.

Intraspecific variations of leaf hydraulic, economic, and anatomical traits in Cinnamomum camphora along an urban-rural gradient

Urbanization brings numerous benefits to residents, but it also introduces complex, variable, and heterogeneous habitat conditions to urban plants, resulting in an arid and hot urban environment that decreases tree growth and the ecological service capacity of trees. In this study, we evaluated leaf hydraulic, economic, and anatomical traits and their covariations of Cinnamomum camphora along an urban-rural gradient in Hefei, Eastern China. We found that Cinnamomum camphora in urban adopted a conservative hydraulic strategy with low leaf turgor loss point (Tlp), leaf hydraulic conductance (Kleaf), and leaf water potential resulting in 50 % loss of hydraulic conductance (P50), as well as a quick investment-return economic strategy with low unit leaf dry matter content (LMA) and high leaf nitrogen content (Leaf N). P50, Kleaf and LMA were significantly positively correlated with the urban-rural gradient (PC1urban-rural gradient), while Leaf N exhibited a negative correlation with it. The results showed a trade-off between intraspecific safety and efficiency in leaf hydraulic traits along the urban-rural gradient and an intraspecific coordinated variation in leaf hydraulic and economic traits. In addition, based on the analysis of a trait coordination network, it was revealed that leaf mesophyll and stomata were key structures for trait adjustment and coordination. Furthermore, our findings offer a significant theoretical underpinning for the effective management of landscape trees and the strategic planning of urban tree species.

Nitrogen addition enhances tree radial growth but weakens its recovery from drought impact in a temperate forest in northern China

Forest growth in the majority of northern China is currently limited by drought and low nitrogen (N) availability. Drought events with increasing intensity have threatened multiple ecosystem services provided by forests. Whether N addition will have a detrimental or beneficial moderation effect on forest resistance and recovery to drought events was unclear. Here, our study focuses on Pinus tabulaeformis, which is the main plantation forest species in northern China. We investigated the role of climate change and N addition in driving multi-year tree growth with an 8-year soil nitrogen fertilization experiment and analyzing 184 tree ring series. A moderate drought event occurred during the experiment, providing an opportunity for us to explore the effects of drought and N addition on tree resistance and recovery. We found that N addition was beneficial for increasing the resistance of middle-aged trees, but had no effect on mature trees. The recovery of trees weakened significantly with increasing N addition, and the reduction in fine root biomass caused by multiyear N addition was a key influencing factor limiting recovery after moderate drought. Our study implies that the combined effect of increasing drought and N deposition might increase the risk of pine forest mortality in northern China.

Stability in the leaf functional traits of understory herbaceous species after 12-yr of nitrogen addition in temperate larch plantations

Leaf functional traits play critical roles in plant functioning. Although the functional traits of overstory trees have been extensively studied, minimal research has been conducted regarding understory species, despite the understory layer is an important component of temperate forests. Such insufficiency limit the broader understanding of processes and functions in forest ecosystems, particularly when under the increasing atmospheric nitrogen (N) deposition. Here, we investigated the responses of 18 leaf functional traits in six understory herbaceous species within young and mature stands (three species per stand) in larch (Larix principis-rupprechtii) plantations that subjected to 12 years of anthropogenic N addition. We found that N addition did not significantly impact the photosynthetic traits of understory herbaceous species in either stand; it only led to increased chlorophyll content in Geum aleppicum Jacq. Similarly, with the exception of decreases in the predawn leaf water potential of Sanguisorba officinalis L., N addition did not significantly affect leaf hydraulic traits. With the exception of changes to adaxial epidermis thickness in Potentilla chinensis Ser. (decreased) and G. aleppicum (increased), N addition had negligible effects on leaf anatomical traits and specific leaf area, however, interspecific variations in the plasticity of leaf anatomical traits were observed. Stable responses to N addition were also observed for nonstructural carbohydrates (NSC) and their components (soluble sugars and starch), with the exception of Polygonum divaricatum L., which exhibited increases in NSC. Overall, our results suggest that the functional traits of understory herbaceous species exhibit stability under conditions of long-term N enrichment in temperate plantations.

Microbial controls over soil priming effects under chronic nitrogen and phosphorus additions in subtropical forests

The soil priming effect (PE), defined as the modification of soil organic matter decomposition by labile carbon (C) inputs, is known to influence C storage in terrestrial ecosystems. However, how chronic nutrient addition, particularly in leguminous and non-leguminous forests, will affect PE through interaction with nutrient (e.g., nitrogen and phosphorus) availability is still unclear. Therefore, we collected soils from leguminous and non-leguminous subtropical plantations across a suite of historical nutrient addition regimes. We added 13C-labeled glucose to investigate how background soil nutrient conditions and microbial communities affect priming and its potential microbial mechanisms. Glucose addition increased soil organic matter decomposition and prompted positive priming in all soils, regardless of dominant overstory tree species or fertilizer treatment. In non-leguminous soil, only combined nitrogen and phosphorus addition led to a higher positive priming than the control. Conversely, soils beneath N-fixing leguminous plants responded positively to P addition alone, as well as to joint NP addition compared to control. Using DNA stable-isotope probing, high-throughput quantitative PCR, enzyme assays and microbial C substrate utilization, we found that positive PE was associated with increased microbial C utilization, accompanied by an increase in microbial community activity, nutrient-related gene abundance, and enzyme activities. Our findings suggest that the balance between soil available N and P effects on the PE,  was dependent on rhizosphere microbial community composition. Furthermore, these findings highlight the roles of the interaction between plants and their symbiotic microbial communities in affecting soil priming and improve our understanding of the potential microbial pathways underlying soil PEs.

Patterns of δ15N in forest soils and tree foliage and rings between climate zones in relation to atmospheric nitrogen deposition: A review

The stable nitrogen (N) isotope ratio (δ15N) of forest samples (soils, tree foliage, and tree rings) has been used as a powerful indicator to explore the responses of forest N cycling to atmospheric N deposition. This review investigated the patterns of δ15N in forest samples between climate zones in relation to N deposition. Forest samples exhibited distinctive δ15N patterns between climate zones due to differences in site conditions (i.e., N availability and retention capacity) and the atmospheric N deposition characteristics (i.e., N deposition rate, N species, and δ15N of deposited N). For example, the δ15N of soil and foliage was higher for tropical forests than for other forests by >1.2 ‰ and 4 ‰, respectively due to the site conditions favoring N losses coupled with relatively low N deposition for tropical forests. This was further supported by the unchanged or increased δ15N of tree rings in tropical forests, which contrasts with other climate zones that exhibited a decreased wood δ15N since the 1920s. Subtropical forests under a high deposition of reduced N (NHy) had a lower δ15N by 2-5 ‰ in the organic layer compared with the other forests, reflecting high retention of 15N-depleted NHy deposition. At severely polluted sites in East Asia, the decreased δ15N in wood also reflected the consistent deposition of 15N-depleted NHy. Though our data analysis represents only a subset of global forest sites where atmospheric N deposition is of interest, the results suggest that the direction and magnitude of the changes in the δ15N of forest samples are related to both atmospheric N and site conditions particularly for tropical vs. subtropical forests. Site-specific information on the atmospheric N deposition characteristics would allow more accurate assessment of the variations in the δ15N of forest samples in relation to N deposition.

Enhanced CO2 uptake is marginally offset by altered fluxes of non-CO2 greenhouse gases in global forests and grasslands under N deposition

Despite the increasing impact of atmospheric nitrogen (N) deposition on terrestrial greenhouse gas (GHG) budget, through driving both the net atmospheric CO2 exchange and the emission or uptake of non-CO2 GHGs (CH4 and N2 O), few studies have assessed the climatic impact of forests and grasslands under N deposition globally based on different bottom-up approaches. Here, we quantify the effects of N deposition on biomass C increment, soil organic C (SOC), CH4 and N2 O fluxes and, ultimately, the net ecosystem GHG balance of forests and grasslands using a global comprehensive dataset. We showed that N addition significantly increased plant C uptake (net primary production) in forests and grasslands, to a larger extent for the aboveground C (aboveground net primary production), whereas it only caused a small or insignificant enhancement of SOC pool in both upland systems. Nitrogen addition had no significant effect on soil heterotrophic respiration (RH ) in both forests and grasslands, while a significant N-induced increase in soil CO2 fluxes (RS , soil respiration) was observed in grasslands. Nitrogen addition significantly stimulated soil N2 O fluxes in forests (76%), to a larger extent in grasslands (87%), but showed a consistent trend to decrease soil uptake of CH4 , suggesting a declined sink capacity of forests and grasslands for atmospheric CH4 under N enrichment. Overall, the net GHG balance estimated by the net ecosystem production-based method (forest, 1.28 Pg CO2 -eq year-1 vs. grassland, 0.58 Pg CO2 -eq year-1 ) was greater than those estimated using the SOC-based method (forest, 0.32 Pg CO2 -eq year-1 vs. grassland, 0.18 Pg CO2 -eq year-1 ) caused by N addition. Our findings revealed that the enhanced soil C sequestration by N addition in global forests and grasslands could be only marginally offset (1.5%-4.8%) by the combined effects of its stimulation of N2 O emissions together with the reduced soil uptake of CH4 .

Nitrogen availability mediates soil carbon cycling response to climate warming: A meta-analysis

Global climate warming may induce a positive feedback through increasing soil carbon (C) release to the atmosphere. Although warming can affect both C input to and output from soil, direct and convincing evidence illustrating that warming induces a net change in soil C is still lacking. We synthesized the results from field warming experiments at 165 sites across the globe and found that climate warming had no significant effect on soil C stock. On average, warming significantly increased root biomass and soil respiration, but warming effects on root biomass and soil respiration strongly depended on soil nitrogen (N) availability. Under high N availability (soil C:N ratio < 15), warming had no significant effect on root biomass, but promoted the coupling between effect sizes of root biomass and soil C stock. Under relative N limitation (soil C:N ratio > 15), warming significantly enhanced root biomass. However, the enhancement of root biomass did not induce a corresponding C accumulation in soil, possibly because warming promoted microbial CO₂ release that offset the increased root C input. Also, reactive N input alleviated warming-induced C loss from soil, but elevated atmospheric CO₂ or precipitation increase/reduction did not. Together, our findings indicate that the relative availability of soil C to N (i.e., soil C:N ratio) critically mediates warming effects on soil C dynamics, suggesting that its incorporation into C-climate models may improve the prediction of soil C cycling under future global warming scenarios.

Spatial predictors and temporal forecast of total organic carbon levels in boreal lakes

Browning of Fennoscandian boreal lakes is raising concerns for negative ecosystem impacts as well as reduced drinking water quality. Declined sulfur deposition and warmer climate, along with afforestation, other climate impacts and less outfield grazing, have resulted in increased fluxes of Total Organic Carbon (TOC) from catchments to freshwater, and subsequently to coastal waters. This study assesses the major governing factors for increased TOC levels among several catchment characteristics in almost 5000 Fennoscandian lakes and catchments. Normalized Difference Vegetation Index (NDVI), a proxy for plant biomass, and the proportions of peatland in the catchment, along with surface runoff intensity and nitrogen deposition loading, were identified as the main spatial predictors for lake TOC concentrations. A multiple linear model, based on these explanatory variables, was used to simulate future TOC concentration in surface runoff from coastal drainage basins in 2050 and 2100, using the forecasts of climatic variables in two of the Shared Socio-economic Pathways (SSP): 1-2.6 (+2 °C) and 3-7.0 (+4,5 °C). These scenarios yield contrasting effects. SSP 1-2.6 predicts an overall decrease of TOC export to coastal waters, while SSP 3-7.0 in contrast leads to an increase in TOC export.

No impact of nitrogen fertilization on carbon sequestration in a temperate Pinus densiflora forest

Carbon (C) sequestration capacity in forest ecosystems is generally constrained by soil nitrogen (N) availability. Consequently, N fertilization is seen as a promising tool for enhancing ecosystem-level C sequestration in N-limited forests. We examined the responses of ecosystem C (vegetation and soil) and soil N dynamics to 3 years of annual nitrogen-phosphorus-potassium (N₃P₄K₁ = 11.3 g N, 15.0 g P, 3.7 g K m^-2 year^-1) or PK fertilization (P₄K₁), observed over 4 years in a 40-year-old Pinus densiflora forest with poor N nutrition in South Korea. PK fertilization without N was performed to test for PK limitation other than N. Neither tree growth nor soil C fluxes responded to annual NPK or PK fertilization despite an increase in soil mineral N fluxes following NPK fertilization. NPK fertilization increased the rate of N immobilization and 80% of the added N was recovered from mineral soil in the 0-5 cm layer, suggesting that relatively little of the added N was available to trees. These results indicate that N fertilization does not always enhance C sequestration even in forests with poor N nutrition and should therefore be applied with caution.

Contrasting effects of nitrogen and phosphorus additions on nitrogen competition between coniferous and broadleaf seedlings

Nitrogen (N) is a major element limiting plant growth and metabolism. Nitrogen addition can influence plant growth, N uptake, and species interactions, while phosphorus (P) addition may affect N acquisition. However, knowledge of how nutrient availability influences N uptake and species interactions remains limited and controversial. Here, pot experiments were conducted for 14 months, in which conifers (Pinus massoniana and Pinus elliottii) and broadleaved trees (Michelia maudiae and Schima superba) were planted in monoculture or mixture, and provided additional N and P in a full-factorial design. Nitrogen addition increased the biomass, but P addition did not significantly affect the biomass of the four subtropical species. Combined N and P (NP) addition had no additive effect on plant biomass over N addition. Total plant biomass was significantly positively correlated to root traits (branching intensity and root tissue density) and leaf traits (net photosynthetic rate, stomatal conductance, and transpiration rate), but negatively correlated to root diameter in response to nutrient addition. Plant uptake rates of NH₄⁺ or NO₃^- were not altered by N addition, but P or NP additions decreased NH₄⁺ uptake rates and increased NO₃^- uptake rates. Neighboring conifers significantly inhibited NH₄⁺ and NO₃^- uptake rates of the two broadleaf species, but neighboring broadleaves had no effects on the N uptake rates of pine species. The effects of nutrient additions on interspecific interactions differed among species. Nitrogen addition altered the interaction of P. elliottii and M. maudiae from neutral to competition, while P addition altered the interaction of P. massoniana and M. maudiae from neutral to favorable effects. Increasing nutrient availability switched the direction of interspecific interaction in favor of pines. This study provides insights into forest management for productivity improvement and optimizing the selection of broadleaf species regarding differences in soil fertility of subtropical plantations.

Above-ground tree carbon storage in response to nitrogen deposition in the U.S. is heterogeneous and may have weakened

Changes in nitrogen (N) availability affect the ability for forest ecosystems to store carbon (C). Here we extend an analysis of the growth and survival of 94 tree species and 1.2 million trees, to estimate the incremental effects of N deposition on changes in aboveground C (dC/dN) across the contiguous U.S. (CONUS). We find that although the average effect of N deposition on aboveground C is positive for the CONUS (dC/dN=+9 kg C per kg N), there is wide variation among species and regions. Furthermore, in the Northeastern U.S. where we may compare responses from 2000-2016 with those from the 1980s-90s, we find the recent estimate of dC/dN is weaker than from the 1980s-90s due to species-level changes in responses to N deposition. This suggests that the U.S. forest C-sink varies widely across forests and may be weakening overall, possibly necessitating more aggressive climate policies than originally thought.

Nitrate as an alternative electron acceptor destabilizes the mineral associated organic carbon in moisturized deep soil depths

Numerous studies have investigated the effects of nitrogen (N) addition on soil organic carbon (SOC) decomposition. However, most studies have focused on the shallow top soils <0.2 m (surface soil), with a few studies also examining the deeper soil depths of 0.5-1.0 m (subsoil). Studies investigating the effects of N addition on SOC decomposition in soil >1.0 m deep (deep soil) are rare. Here, we investigated the effects and the underlying mechanisms of nitrate addition on SOC stability in soil depths deeper than 1.0 m. The results showed that nitrate addition promoted deep soil respiration if the stoichiometric mole ratio of nitrate to O₂ exceeded the threshold of 6:1, at which nitrate can be used as an alternative acceptor to O₂ for microbial respiration. In addition, the mole ratio of the produced CO₂ to N₂O was 2.57:1, which is close to the theoretical ratio of 2:1 expected when nitrate is used as an electron acceptor for microbial respiration. These results demonstrated that nitrate, as an alternative acceptor to O₂, promoted microbial carbon decomposition in deep soil. Furthermore, our results showed that nitrate addition increased the abundance of SOC decomposers and the expressions of their functional genes, and concurrently decreased MAOC, and the ratio of MAOC/SOC decreased from 20% before incubation to 4% at the end of incubation. Thus, nitrate can destabilize the MAOC in deep soils by stimulating microbial utilization of MAOC. Our results imply a new mechanism on how above-ground anthropogenic N inputs affect MAOC stability in deep soil. Mitigation of nitrate leaching is expected to benefit the conservation of MAOC in deep soil depths.

Patterns and controlling factors of soil carbon sequestration in nitrogen-limited and -rich forests in China-a meta-analysis

Soil organic carbon (SOC) management has the potential to contribute to climate change mitigation by reducing atmospheric carbon dioxide (CO2). Understanding the changes in forest nitrogen (N) deposition rates has important implications for C sequestration. We explored the effects of N enrichment on soil carbon sequestration in nitrogen-limited and nitrogen-rich Chinese forests and their controlling factors. Our findings reveal that N inputs enhanced net soil C sequestration by 5.52-18.46 kg C kg-1 N, with greater impacts in temperate forests (8.37-13.68 kg C kg-1 N), the use of NH4NO3 fertilizer (7.78 kg Ckg-1 N) at low N levels (<30 kg Ckg-1 N; 9.14 kg Ckg-1 N), and in a short period (<3 years; 12.95 kg C kg-1 N). The nitrogen use efficiency (NUE) varied between 0.24 and 13.3 (kg C kg-1 N) depending on the forest type and was significantly controlled by rainfall, fertilizer, and carbon-nitrogen ratio rates. Besides, N enrichment increased SOC concentration by an average of 7% and 2% for tropical and subtropical forests, respectively. Although soil carbon sequestration was higher in the topsoil compared to the subsoil, the relative influence indicated that nitrogen availability strongly impacts the SOC, followed by dissolved organic carbon concentration and mean annual precipitation. This study highlights the critical role of soil NUE processes in promoting soil C accumulation in a forest ecosystem.

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.

Differential responses of fungal and bacterial necromass accumulation in soil to nitrogen deposition in relation to deposition rate

Influenced by nitrogen (N) deposition, changes in soil organic carbon (SOC) sequestration in terrestrial ecosystems could provide strong feedback to climate change. Mounting evidence showed that microbial necromass contributes substantially to SOC sequestration; however, how N deposition influences microbial necromass accumulation in soils remains elusive. We investigated the impacts of N deposition on soil microbial necromass, assessed by amino sugars, at seven forest sites along a north-south transect in eastern China. We found that the responses of fungal and bacterial necromass accumulation to N deposition depended on the deposition rate, with high N deposition (>50 kg N ha^-1 yr^-1) stimulating fungal necromass accumulation from 29.1 % to 35.2 %, while low N deposition damaging the accumulation of bacterial necromass in soil by 12.1 %. On the whole, N deposition benefitted the dominance of fungal over bacterial necromass, with their ratio being significantly greater at high-N level. The accumulation of microbial necromass was primarily governed by soil properties, including nutrients stoichiometry, clay content and pH, while the composition of microbial necromass was conjointly affected by soil properties and microbial community structure. The latitudinal distribution of microbial necromass contributions to SOC pool was not altered by N deposition, and was firmly controlled by the climatic and edaphic factors. Collectively, our results reveal the impacts of N deposition on microbial necromass accumulation in soil and the geographical pattern across forest ecosystems in eastern China, providing implications for our accurate predictions of global change impacts on SOC sequestration.

Mowing enhances the positive effects of nitrogen addition on ecosystem carbon fluxes and water use efficiency in a semi-arid meadow steppe

Grasslands are now facing a continuously increasing supply of nitrogen (N) fertilizers, resulting in alterations in ecosystem functioning, including changes in carbon (C) and water cycling. Mowing, one of the most widely used grassland management techniques, has been shown to mitigate the negative impacts of increased N availability on species richness. However, knowledge of how N addition and mowing, alone and/or in combination, affect ecosystem-level C fluxes and water use efficiency (W_N) is still limited. We experimentally manipulated N fertilization (0 and 10 g N m^-2 yr^-1) and mowing (once per year at the end of the growing season) following a randomized block design in a meadow steppe characterized by salinization and alkalinization in northeastern China. We found that, compared to the control plots, N addition, mowing, and their interaction increased net ecosystem CO₂ exchange by 65.1%, 14.7%, and 133%, and W_N by 40.7%, 18.5%, and 96.1%, respectively. Nitrogen enrichment also decreased soil pH, which resulted in greater aboveground biomass (AGB). Moreover, N addition indirectly increased AGB by inducing changes in species richness. Our results indicate that mowing enhances the positive effects of N addition on ecosystem C fluxes and W_N. Therefore, appropriate grassland management practices are essential to improve ecosystem C sequestration, W_N, and mitigate future species diversity declines due to ecosystem eutrophication.

Quantifying climate change effects on future forest biomass availability using yield tables improved by mechanistic scaling

Forests and wood products play a major role in climate change mitigation strategies and the transition from a fossil-based economy to a circular bioeconomy. Accurate estimates of future forest productivity are crucial to predict the carbon sequestration and wood provision potential of forests. Since long, forest managers have used empirical yield tables as a cost-effective and reliable way to predict forest growth. However, recent climate change-induced growth shifts raised doubts about the long-term validity of these yield tables. In this study, we propose a methodology to improve available yield tables of 11 tree species in the Netherlands and Flanders, Belgium. The methodology uses scaling functions derived from climate-sensitive process-based modelling (PBM) that reflect state-of-the-art projections of future growth trends. Combining PBM and stand information from the empirical yield tables for the region of Flanders, we found that for the period 1987-2016 stand productivity has on average increased by 13% compared to 1961-1990. Furthermore, simulations indicate that this positive growth trend is most likely to persist in the coming decades, for all considered species, climate or site conditions. Nonetheless, results showed that local site variability is equally important to consider as the in- or exclusion of the CO₂ fertilization effect or different climate projections, when assessing the magnitude of forests' response to climate change. Our projections suggest that incorporating these climate change-related productivity changes lead to a 7% increase in standing stock and a 22% increase in sustainably potentially harvestable woody biomass by 2050. The proposed methodology and resulting estimates of climate-sensitive projections of future woody biomass stocks will facilitate the further incorporation of forests and their products in global and regional strategies for the transition to a climate-smart circular bioeconomy.

Spatiotemporal dynamics of forest ecosystem carbon budget in Guizhou: customisation and application of the CBM-CFS3 model for China

BACKGROUND

Countries seeking to mitigate climate change through forests require suitable modelling approaches to predict carbon (C) budget dynamics in forests and their responses to disturbance and management. The Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) is a feasible and comprehensive tool for simulating forest C stock dynamics across broad levels, but discrepancies remain to be addressed in China. Taking Guizhou as the case study, we customised the CBM-CFS3 model according to China's context, including the modification of aboveground biomass C stock algorithm, addition of C budget accounting for bamboo forests, economic forests, and shrub forests, improvement of non-forest land belowground slow dead organic matter (DOM) pool initialisation, and other model settings.

RESULTS

The adequate linear relationship between the estimated and measured C densities (R² = 0.967, P < 0.0001, slope = 0.904) in the model validation demonstrated the high accuracy and reliability of our customised model. We further simulated the spatiotemporal dynamics of forest C stocks and disturbance impacts in Guizhou for the period 1990-2016 using our customised model. Results shows that the total ecosystem C stock and C density, and C stocks in biomass, litter, dead wood, and soil in Guizhou increased continuously and significantly, while the soil C density decreased over the whole period, which could be attributed to deforestation history and climate change. The total ecosystem C stock increased from 1220 Tg C in 1990 to 1684 Tg C in 2016 at a rate of 18 Tg C yr^-1, with significant enhancement in most areas, especially in the south and northwest. The total decrease in ecosystem C stock and C expenditure caused by disturbances reached 97.6 Tg C and 120.9 Tg C, respectively, but both represented significant decreasing trends owing to the decline of disturbed forest area during 1990-2016. Regeneration logging, deforestation for agriculture, and harvest logging caused the largest C stock decrease and C expenditure, while afforestation and natural expansion of forest contributed the largest increases in C stock.

CONCLUSIONS

The forests in Guizhou were a net carbon sink under large-scale afforestation throughout the study period; Our customised CBM-CFS3 model can serve as a more effective and accurate method for estimating forest C stock and disturbance impacts in China and further enlightens model customisation to other areas.

Thawing Permafrost as a Nitrogen Fertiliser: Implications for Climate Feedbacks

Studies for the northern high latitudes suggest that, in the near term, increased vegetation uptake may offset permafrost carbon losses, but over longer time periods, permafrost carbon decomposition causes a net loss of carbon. Here, we assess the impact of a coupled carbon and nitrogen cycle on the simulations of these carbon fluxes. We present results from JULES-IMOGEN—a global land surface model coupled to an intermediate complexity climate model with vertically resolved soil biogeochemistry. We quantify the impact of nitrogen fertilisation from thawing permafrost on the carbon cycle and compare it with the loss of permafrost carbon. Projections show that the additional fertilisation reduces the high latitude vegetation nitrogen limitation and causes an overall increase in vegetation carbon uptake. This is a few Petagrams of carbon (Pg C) by year 2100, increasing to up to 40 Pg C by year 2300 for the RCP8.5 concentration scenario and adds around 50% to the projected overall increase in vegetation carbon in that region. This nitrogen fertilisation results in a negative (stabilising) feedback on the global mean temperature, which could be equivalent in magnitude to the positive (destabilising) temperature feedback from the loss of permafrost carbon. This balance depends on the future scenario and initial permafrost carbon. JULES-IMOGEN describes one representation of the changes in Arctic carbon and nitrogen cycling in response to climate change. However there are uncertainties in the modelling framework, model parameterisation and missing processes which, when assessed, will provide a more complete picture of the balance between stabilising and destabilising feedbacks.

High Level of Ammonium Nitrogen Increases Net Ecosystem Productivity in a Quercus liaotungensis Forest in Northern China

Forest ecosystems are vital to the terrestrial ecosystem’s carbon (C) cycle. The effects of nitrogen (N) addition on C sequestration in forest ecosystems are critical for better understanding C dynamics when facing an increase in N availability. We conducted a six-year field experiment to examine the effects of N addition on C sequestration and net ecosystem productivity (NEP) in a Quercus liaotungensis forest in northern China. N addition resulted in a significant increase in biomass C storage (17.54–48.62%) and changed the distribution patterns of above and belowground biomass C storage, resulting in a 9.64 to 23.23% reduction in the proportion of belowground biomass C compared with the control. The annual average heterotrophic respiration was significantly increased by the additional N (by 0.06–0.94 Mg C ha−1 yr1). In comparison with the control, the C sequestration efficiency driven by N addition ranged from 7.12 to 33.50 kg C/kg N. High-level N addition exerted stronger effects on ecosystem C sequestration than low-level N addition. NH4+-N, rather than NO3−-N, dominated the increase in ecosystem C sequestration. We found that Q. liaotungensis forest acted as a C sink. The increase in NEP in the study forest in northern China was mainly due to an increase in net primary productivity (NPP) caused by N addition. Atmospheric N deposition increased the C sequestration efficiency depending on the rate and form of N deposition.

Inorganic Nitrogen Enhances the Drought Tolerance of Evergreen Broad-Leaved Tree Species in the Short-Term, but May Aggravate Their Water Shortage in the Mid-Term

With global climate change, atmospheric nitrogen (N) deposition and drought have been well documented to cause substantial challenges for tropical and subtropical evergreen broad-leaved forests. Here, we conducted an experiment that measured the physiological responses of the seedlings of three dominant tree species (Tabebuia chrysantha, Elaeocarpus sylvestris, and Bischofia javanica) of the evergreen broad-leaved forests in South China under control (CT), drought stress (D), N addition (N), and drought stress plus N addition (N+D). We found that N addition significantly decreased malondialdehyde (MDA) content, abscisic acid (ABA) content, total antioxidant capacity (T-AOC), but significantly increased the content of proline (PRO), and the activities of ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco), nitrate reductase (NR), nitrite reductase (NiR), and glutamine synthetase (GS) in the three species under D. Meanwhile, we also found that under drought conditions, N addition promoted the leaf transpiration rate (E), stomatal conductance (g _s ), and light-saturated net photosynthetic rate (A _max ) of the three species. These results indicate that N addition can enhance the drought tolerance of the three species by osmotic adjustment and protecting the photosystem. However, the enhancement in A _max and E will cause plants to face more severe drought conditions, especially B. javanica (large tree species). This study helps to explain why the evergreen broad-leaved forests in South China are gradually degrading to shrublands in recent decades.

Osmotic Adjustment and Antioxidant System Regulated by Nitrogen Deposition Improve Photosynthetic and Growth Performance and Alleviate Oxidative Damage in Dwarf Bamboo Under Drought Stress

Dwarf bamboo (Fargesia denudata) is a staple food for the endangered giant pandas and plays a critical role in the sub-alpine ecosystem. Characterized by shallow roots and expeditious growth, it is exceedingly susceptible to drought stress and nitrogen (N) deposition in the context of a changing global environment. However, a comprehensive picture about the interactive response mechanism of dwarf bamboo to the two factors, water regime and N deposition, is far from being given. Therefore, a completely randomized design with two factors of water regimes (well-watered and water-stressed) and N deposition levels (with and without N addition) of F. denudata was conducted. In view of the obtained results, drought stress had an adverse impact on F. denudata, showing that it destroyed ultrastructure integrity and induced oxidative damage and restricted water status in leaves and roots, as well as declined photosynthetic efficiency in leaves, especially in N non-deposition plants. Nevertheless, F. denudata significantly increased heat dissipation in leaves, regulated antioxidant enzymes activities, antioxidants contents, and osmoregulation substances concentrations in leaves and roots, as well as shifted biomass partitioning in response to drought stress. However, regardless of water availability, N deposition maintained better ultrastructure in leaves and roots, resulting in superior photosynthesis and growth of F. denudata. Additionally, although N deposition did not cause oxidative damage in well-watered plants, ameliorated the effects of drought stress on F. denudata through co-deploying heat dissipation in leaves, the antioxidant system in roots as well as osmotic adjustment in leaves and roots. Noticeably, the leaves and roots of F. denudata expressed quite distinct acclimation responses to drought resistance under N deposition.

Effects of nitrogen addition and increased precipitation on xylem growth of Quercus acutissima Caruth. in central China

Atmospheric nitrogen (N) deposition and increasing precipitation affect carbon sequestration in terrestrial ecosystems, but how these two concurrent global change variables affect xylem growth in trees (i.e., independently or interactively) remains unclear. We conducted novel experiments in central China to monitor the xylem growth in a dominant species (Quercus acutissima Caruth.) in response to N addition (CN), supplemental precipitation (CW) or both treatments (CNW), compared with untreated controls (C). Measurements were made at weekly intervals during 2014-15. We found that supplemental precipitation significantly enhanced xylem growth in the dry spring of 2015, indicating a time-varying effect of increased precipitation on intra-annual xylem growth. Elevated N had no significant effect on xylem increment, xylem growth rate, and lumen diameters and potential hydraulic conductivity (Ks) of earlywood vessels, but Ks with elevated N was significantly negatively related to xylem increment. The combination of additional N and supplemental precipitation suppressed the positive effect of supplemental precipitation on xylem increment in the dry spring of 2015. These findings indicated that xylem width was more responsive to supplemental precipitation than to increasing N in a dry early growing season; the positive effect of supplemental precipitation on xylem growth could be offset by elevated N resources. The negative interactive effect of N addition and supplemental precipitation also suggested that increasing N deposition and precipitation in the future might potentially affect carbon sequestration of Q. acutissima during the early growing season in central China. The effects of N addition and supplemental precipitation on tree growth are complex and might vary depending on the growth period and local climatic conditions. Therefore, future models of tree growth need to consider multiple-time scales and local climatic conditions when simulating and projecting global change.

Retention of deposited ammonium and nitrate and its impact on the global forest carbon sink

The impacts of enhanced nitrogen (N) deposition on the global forest carbon (C) sink and other ecosystem services may depend on whether N is deposited in reduced (mainly as ammonium) or oxidized forms (mainly as nitrate) and the subsequent fate of each. However, the fates of the two key reactive N forms and their contributions to forest C sinks are unclear. Here, we analyze results from 13 ecosystem-scale paired ¹⁵N-labelling experiments in temperate, subtropical, and tropical forests. Results show that total ecosystem N retention is similar for ammonium and nitrate, but plants take up more labelled nitrate ([Formula: see text]%) ([Formula: see text]) than ammonium ([Formula: see text]%) while soils retain more ammonium ([Formula: see text]%) than nitrate ([Formula: see text]%). We estimate that the N deposition-induced C sink in forests in the 2010s  is [Formula: see text] Pg C yr^-1, higher than previous estimates because of a larger role for oxidized N and greater rates of global N deposition.

Experimental evidence shows minor contribution of nitrogen deposition to global forest carbon sequestration

Human activities have drastically increased nitrogen (N) deposition onto forests globally. This may have alleviated N limitation and thus stimulated productivity and carbon (C) sequestration in aboveground woody biomass (AGWB), a stable C pool with long turnover times. This 'carbon bonus' of human N use partly offsets the climate impact of human-induced N₂ O emissions, but its magnitude and spatial variation are uncertain. Here we used a meta-regression approach to identify sources of heterogeneity in tree biomass C-N response (additional C stored per unit of N) based on data from fertilization experiments in global forests. We identified important drivers of spatial variation in forest biomass C-N response related to climate (potential evapotranspiration), soil fertility (N content) and tree characteristics (stand age), and used these relationships to quantify global spatial variation in N-induced forest biomass C sequestration. Results show that N deposition enhances biomass C sequestration in only one-third of global forests, mainly in the boreal region, while N reduces C sequestration in 5% of forests, mainly in the tropics. In the remaining 59% of global forests, N addition has no impact on biomass C sequestration. Average C-N responses were 11 (4-21) kg C per kg N for boreal forests, 4 (0-8) kg C per kg N for temperate forests and 0 (-4 to 5) kg C per kg N for tropical forests. Our global estimate of the N-induced forest biomass C sink of 41 (-53 to 159) Tg C yr^-1 is substantially lower than previous estimates, mainly due to the absence of any response in most tropical forests (accounting for 58% of the global forest area). Overall, the N-induced C sink in AGWB only offsets ~5% of the climate impact of N₂ O emissions (in terms of 100-year global warming potential), and contributes ~1% to the gross forest C sink.

The gap between atmospheric nitrogen deposition experiments and reality

Anthropogenic activities have dramatically altered the global nitrogen (N) cycle. Atmospheric N deposition, primarily from combustion of biomass and fossil fuels, has caused acidification of precipitation and freshwater, and triggered intense research into ecosystem responses to this pollutant. Experimental simulations of N deposition have been the main scientific tool to understand ecosystem responses, revealing dramatic impacts on soil microbes, plants, and higher trophic levels. However, comparison of the experimental treatments applied in the vast majority of studies with observational and modelled N deposition reveals a wide gulf between research and reality. While the majority of experimental treatments exceed 100 kg N ha^-1 y^-1, global median land surface deposition rates are around 1 kg N ha^-1 y^-1 and only exceed 10 kg N ha^-1 y^-1 in certain regions, primarily in industrialized areas of Europe and Asia and particularly in forests. Experimental N deposition treatments are in fact similar to mineral fertilizer application rates in agriculture. Some ecological guilds, such as saprotrophic fungi, are highly sensitive to N and respond differently to low and high N availability. In addition, very high levels of N application cause changes in soil chemistry, such as acidification, meaning that unrealistic experimental treatments are unlikely to reveal true ecosystem responses to N. Hence, despite decades of research, past experiments can tell us little about how the biosphere has responded to anthropogenic N deposition. A new approach is required to improve our understanding of this important phenomenon. First, characterization of N response functions using observed N deposition gradients. Second, application of experimental N addition gradients at realistic levels over long periods to detect cumulative effects. Third, application of non-linear meta-regressions to detect non-linear responses in meta-analyses of experimental studies.

Global sensitivities of forest carbon changes to environmental conditions

The responses of forest carbon dynamics to fluctuations in environmental conditions at a global scale remain elusive. Despite the understanding that favourable environmental conditions promote forest growth, these responses have been challenging to observe across different ecosystems and climate gradients. Based on a global annual time series of aboveground biomass (AGB) estimated from radar satellites between 1992 and 2018, we present forest carbon changes and provide insights on their sensitivities to environmental conditions across scales. Our findings indicate differences in forest carbon changes across AGB classes, with regions with carbon stocks of 50-125 MgC ha^-1 depict the highest forest carbon gains and losses, while regions with 125-150 MgC ha^-1  have the lowest forest carbon gains and losses in absolute terms. Net forest carbon change estimates show that the arc-of-deforestation and the Congo Basin were the main hotspots of forest carbon loss, while a substantial part of European forest gained carbon during the last three decades. Furthermore, we observe that changes in forest carbon stocks were systematically positively correlated with changes in forest cover fraction. At the same time, it was not necessarily the case with other environmental variables, such as air temperature and water availability at the bivariate level. We also used a model attribution method to demonstrate that atmospheric conditions were the dominant control of forest carbon changes (56% of the total study area) followed by water-related (29% of the total study area) and vegetation (15% of the total study area) conditions. Regionally, we find evidence that carbon gains from long-term forest growth covary with long-term carbon sinks inferred from atmospheric inversions. Our results describe the contributions from the atmosphere, water-related and vegetation conditions to forest carbon changes and provide new insights into the underlying mechanisms of the coupling between forest growth and the global carbon cycle.

The impacts of nitrogen addition on upland soil methane uptake: A global meta-analysis

Elevated nitrogen (N) addition from anthropogenic activities has great impacts on soil methane (CH₄) uptake, which could interrupt the existing global CH₄ balance and cause feedbacks to climate and biogeochemical processes. Previous studies have come to inconsistent conclusions on both the quantification of the response of CH₄ uptake to N addition and understanding of its underlying mechanisms, probably due to the lack of experimental data. Here, we conduct a broad meta-analysis of 90 papers to quantify the responses of CH₄ uptake to N addition in upland soil. The results show that N addition has a significant negative impact on soil CH₄ uptake (-19.25%), which is termed the N inhibition effect. Soil pH is identified as the dominant factor, with the other factors affecting the CH₄ uptake through the alteration of soil pH. The N inhibition effect is observed to be large and significant in forest and grassland, but small and insignificant in farmland, because of the distinct composition of their methanotrophic communities. A threshold of the N addition level is identified at about 68 kg N ha^-1 year^-1, which indicates the lowest N inhibition effect. Furthermore, the convex relationship between response ratio of CH₄ uptake (negative) and N addition duration indicates that a medium level of N addition duration has the largest N inhibition effect, and longer or shorter durations will both reduce the effect. Our analysis of the N inhibition effect implies that controlling the N addition level could effectively reduce the CH₄ concentration in the atmosphere and thus relieve global warming.

Nonlinear responses of ecosystem carbon fluxes to nitrogen deposition in an old-growth boreal forest

Nitrogen (N) deposition is known to increase carbon (C) sequestration in N-limited boreal forests. However, the long-term effects of N deposition on ecosystem carbon fluxes have been rarely investigated in old-growth boreal forests. Here we show that decade-long experimental N additions significantly stimulated net primary production (NPP) but the effect decreased with increasing N loads. The effect on soil heterotrophic respiration (Rh) shifted from a stimulation at low-level N additions to an inhibition at higher levels of N additions. Consequently, low-level N additions resulted in a neutral effect on net ecosystem productivity (NEP), due to a comparable stimulating effect on NPP and Rh, while NEP was increased by high-level N additions. Moreover, we found nonlinear temporal responses of NPP, Rh and NEP to low-level N additions. Our findings imply that actual N deposition in boreal forests likely exerts a minor contribution to their soil C storage.