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Mitigation of temperate broadleaved trees into southern boreal forest has occurred in response to rapid climatic warming, consequently resulting in profound changes in species composition and ecosystem functions of southern boreal forest. However, the biogeochemical effect of migrating temperate trees on boreal forest trees remains poorly understood. Here we performed a 52-sites survey along the temperate-boreal forest ecotones in Northeastern China to uncover that the encroaching Mongolian oak, dominant trees in temperate forest, has affected N nutrition of Dahurian larch, the dominant trees of regional boreal forest. Specifically, we tested following hypotheses: (i) encroaching Mongolian oak affects N availability for Dahurian larch via modifying soil N availability; (ii) Mongolian oak directly affects N dynamic of Dahurian larch via competing for available N against Dahurian larch. Our results show that the foliar 15N is significantly lower in Mongolian oak than in co-occurring Dahurian larch. Soil 15N is negatively correlated with soil C:N ratio and stand slope but is not affected by the encroachment of Mongolian oak. Both foliar 15N abundance and difference (δ15Nfoliage-δ15Nsoil) of Dahurian larch are significantly affected by the dominance of Mongolian oak, suggesting that encroaching Mongolian oak aggravates N limitation of boreal Dahurian larch. Our findings highlight an unexpected biogeochemical effect of migrating temperate trees on boreal forest.

Purpose: Rapid urbanization has altered regional nutrient cycles and consequently affected soil nutrient availability for plant growth. This study aims to explore the way in which urbanization shapes the spatial patterns of soil phosphorus (P) concentrations as well as the imprint on tree leaf P concentrations across gradients of urban-rural forests. Methods: We collected soil samples of the surface (0-10 cm) and subsurface (10-20 cm) layers and leaf samples of three common tree species (Chinese pine, Pinus tabuliformis; Chinese scholar tree, Sophora japonica; golden rain tree, Koelreuteria paniculata) from forest patches in twenty parks across urban-rural transects in the Beijing metropolitan region, China. By measuring soil total P, soil available P, and leaf P concentrations, we tested the urban soil P hotspot hypothesis and assessed the consequent imprint on tree leaf P concentrations. Results: Soil total P and available P concentrations in the surface and subsurface layers both increased significantly with closer distance to the urban core. Spatially, leaf P concentrations showed an increase with soil available P concentrations only for the legume Chinese scholar tree, while leaf P concentrations of Chinese pine and broadleaf golden rain tree both increased significantly with the diameter at breast height. Conclusion: Our results confirmed the occurrence of an urban soil P hotspot and identified its significant imprint on leaf P nutrition of legume trees across transects of urban-rural forests. The findings improve the understanding of the way that urbanization alters regional P cycles and consequent P availability for plant growth.

11 páginas.- 7 figuras.- 1 tabla.- 41 referencias.- Additional supporting information can be found online in the Supporting Information section at the end of this article..- Read the free Plain Language Summary for this article on the Journal blog., Nitrogen (N) and phosphorus (P) are essential elements limiting plant–microbial growth in forest ecosystems. However, whether the pattern of plant–microbe nutrient limitation is consistent across forest biomes and the associated potential mechanisms remain largely unclear, limiting us to better understand the biogeochemical processes under future climate change. Here, we investigated patterns of plant–microbial N/P limitation in forests across a wide environmental gradient and biomes in China to explore the divergence of plant–microbial N/P limitation and the driving mechanisms. We revealed that 42.6% of the N/P limitation between plant–microbial communities was disconnected. Patterns in plant–microbial N/P limitations were consistent only for 17.7% of N and 39.7% of P. Geospatially, the inconsistency was more evident at mid-latitudes, where plants were mainly N limited and microbes were mainly P limited. Furthermore, our findings were consistent with the ecological stoichiometry of plants and microbes themselves and their requirements. Whereas plant N and P limitation was more strongly responsive to meteorological conditions and atmospheric deposition, that of microbes was more strongly responsive to soil chemistry, which exacerbated the plant–microbe N and P limitation divergence. Our work identified an important disconnection between plant and microbial N/P limitation, which should be incorporated into future Earth System Model to better predict forest biomes–climate change feedback. Read the free Plain Language Summary for this article on the Journal blog. © 2023 The Authors. Functional Ecology © 2023 British Ecological Society, National Natural Science Foundation of China, Grant/Award Number: 42207107; Catalan Government Grant, Grant/Award Number: SGR2017-1005; Fundación Ramón Areces grant, Grant/Award Number: CIVP20A6621; National Key Research and Development Program of China, Grant/Award Number: 2021YFD1901205; Open Fund of Key Laboratory of Agro-Ecological Processes in Subtropical Region, Chinese Academy of Sciences, Grant/Award Number: ISA2021101; Spanish Government, Grant/Award Number: PID2019-110521GB-I00 and PID2020-115770RB-I00; Strategic Priority Research Program of Chinese Academy of Sciences, Grant/Award Number: XDB40020202

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.

Rapid urbanization has occurred globally and resulted in increasing CO

Increased nitrogen (N) inputs are widely recognised to reduce soil respiration (Rs), but how N deposition affects the temporal dynamics of Rs remains unclear. Using a decade-long fertilisation experiment in a boreal larch forest (Larix gmelini) in northeast China, we found that the effects of N additions on Rs showed a temporal shift from a positive effect in the short-term (increased by 8% on average in the first year) to a negative effect over the longer term (decreased by 21% on average in the 11th year). The rates of decrease in Rs for the higher N levels were almost twice as high as those of the low N level. Our results suggest that the reduction in Rs in response to increased N input is accelerated by high-level N additions, and experimental high N applications are likely to overestimate the contribution of N deposition to soil carbon sequestration in a boreal forest.

Global urbanization has profoundly altered regional biogeochemical cycles across urban-rural-natural continuums. A better insight in the changes in soil nitrogen (N) properties across urban-rural-natural forests sheds lights on the impacts of urbanization on regional N cycles and ecological consequences. Based on a systematic survey in forest patches of twenty parks across the urban-rural-natural gradients in the Beijing metropolitan region, China, we analyzed the spatial variations of three soil N properties (i.e., total N, alkali-hydrolyzable N, and δ15N) and their potential drivers that are indicative for anthropogenic N sources (trunk road density for traffic N emissions; cropland share for agricultural N emissions), climate conditions (mean annual temperature, MAT; mean annual precipitation, MAP), forest conditions (forest coverage and the number of common tree species) and time for soil N accumulation since park establishment (park age). Soil total N concentration, alkali-hydrolyzable N concentration and δ15N values in the surface (0–10 cm) and subsurface layers (10–20 cm) all showed a decrease up to 40 km (i.e., the urban fertile island phenomenon) from the urban core and then an increase to values in peri-urban natural forests as high as those in urban forests. The spatial variations of the surface and subsurface soil total N concentrations and alkali-hydrolyzable N concentrations were, respectively, mainly and exclusively explained by park age, while trunk road density, MAP and MAT played a less important role in regulating the soil N variation in the surface layer. The spatial variations of the surface and subsurface soil δ15N values were respectively, mainly, and exclusively controlled by cropland share, while park age and MAP played a limited role in regulating the soil δ15N variation in the surface layer but not in the subsurface layer. Our findings reveal the way in which anthropogenic and climatic drivers shape soil N properties across the urban-rural-natural forests.

Although the above and belowground sizes and shapes of plants strongly influence plant competition, community structure, and plant-environment interactions, plant sizes and shapes remain poorly characterized across climate regimes. We investigated relationships among shoot and root system size and climate. We assembled and analyzed, to our knowledge, the largest global database describing the maximum rooting depth, lateral spread, and shoot size of terrestrial plants - more than doubling the Root Systems of Individual Plants database to 5647 observations. Water availability and growth form greatly influence shoot size, and rooting depth is primarily influenced by temperature seasonality. Shoot size is the strongest predictor of lateral spread, with root system diameter being two times wider than shoot width on average for woody plants. Shoot size covaries strongly with rooting system size; however, the geometries of plants differ considerably across climates, with woody plants in more arid climates having shorter shoots, but deeper, narrower root systems. Additionally, estimates of the depth and lateral spread of plant root systems are likely underestimated at the global scale.

Nitrogen (N) deposition and biological N fixation (BNF) are main external N inputs into terrestrial ecosystems. However, few studies have simultaneously quantified the contribution of these two external N inputs to global NPP and consequent C sequestration. Based on literature analysis, we estimated new net primary production (NPP) due to external N inputs from BNF and N deposition and the consequent C sinks in global boreal, temperate and tropical forest biomes via a stoichiometric scaling approach. Nitrogen-induced new NPP is estimated to be 3.48 Pg C yr−1 in global established forests and contributes to a C sink of 1.83 Pg C yr−1. More specifically, the aboveground and belowground new NPP are estimated to be 2.36 and 1.12 Pg C yr−1, while the external N-induced C sinks in wood and soil are estimated to be 1.51 and 0.32 Pg C yr−1, respectively. BNF contributes to a major proportion of N-induced new NPP (3.07 Pg C yr−1) in global forest, and accounts for a C sink of 1.58 Pg C yr−1. Compared with BNF, N deposition only makes a minor contribution to new NPP (0.41 Pg C yr−1) and C sinks (0.25 Pg C yr−1) in global forests. At the biome scale, rates of N-induced new NPP and C sink show an increase from boreal forest towards tropical forest, as mainly driven by an increase of BNF. In contrast, N deposition leads to a larger C sink in temperate forest (0.11 Pg C yr−1) than boreal (0.06 Pg C yr−1) and tropical forest (0.08 Pg C yr−1). Our estimate of total C sink due to N-induced new NPP approximately matches an independent assessment of total C sink in global established forests, suggesting that external N inputs by BNF and atmospheric deposition are key drivers of C sinks in global forests. Based on a stoichiometric scaling approach, we estimated that nitrogen-induced new net primary production is a key driver of C sinks in global forests.

Combined effects of cumulative nutrient inputs and biogeochemical processes that occur in freshwater under anthropogenic eutrophication could lead to myriad shifts in nitrogen (N):phosphorus (P) stoichiometry in global freshwater ecosystems, but this is not yet well-assessed. Here we evaluated the characteristics of N and P stoichiometries in bodies of freshwater and their herbaceous macrophytes across human-impact levels, regions and periods. Freshwater and its macrophytes had higher N and P concentrations and lower N : P ratios in heavily than lightly human-impacted environments, further evidenced by spatiotemporal comparisons across eutrophication gradients. N and P concentrations in freshwater ecosystems were positively correlated and N : P was negatively correlated with population density in China. These results indicate a faster accumulation of P than N in human-impacted freshwater ecosystems, which could have large effects on the trophic webs and biogeochemical cycles of estuaries and coastal areas by freshwater loadings, and reinforce the importance of rehabilitating these ecosystems.

Nitrogen (N) and phosphorus (P) are essential nutrients that widely limit plant growth in global terrestrial ecosystems. Rising atmospheric CO2 concentration generally stimulates terrestrial net primary productivity and consequently may cause or aggravate N and P limitation due to a dilution effect, but the spatial variation of temporal trends in N versus P limitation and its key regulating factors is poorly understood. Using the leaf N:P ratio of 15 dominant tree species as an indicator, we analysed the spatial variation of plot-level shift towards N or P limitation across 163 European forest plots during 1995–2017. Phosphorus limitation increased from 25% to 33% of the studied plots between 1995–1997 and 2015–2017, while N limitation occurred in a negligible number of plots. A major proportion (56%) of the plots showed no significant trend in leaf N:P ratio, implying no shifts in N versus P limitation status. In the remaining plots, 38% of the plots showed a significant increase of leaf N:P ratio and only 6% of the plots showed a significant decrease of leaf N:P ratio. The spatial variation in the rate of decrease in leaf N:P ratio was associated with a significant decrease in leaf N concentration and mainly explained by the rate of decrease in N deposition. In contrast, the spatial variation in the rate of increase in leaf N:P ratio was associated with a significant decrease in leaf P concentration and mainly explained by forest category (broadleaf vs. conifer), mean annual temperature and soil C:N ratio. Our findings highlight a remarkable spatial divergence in temporal trends of nutrient limitation status across European forests over the past two decades, but overall, P is becoming more limiting versus N, especially in broadleaved forests.

Atmospheric nitrogen (N) deposition has a significant influence on soil organic carbon (SOC) accumulation in forest ecosystems. Microbial residues, as by-products of microbial anabolism, account for a significant fraction of soil C pools. However, how N deposition affects the accumulation of soil microbial residues in different forest biomes remains unclear. Here, we investigated the effects of six/seven-year N additions on microbial residues (amino sugar biomarkers) in eight forests from tropical to boreal zone in eastern China. Our results showed a minor change in the soil microbial residue concentrations but a significant change in the contribution of microbial residue-C to SOC after N addition. The contribution of fungal residue-C to SOC decreased under low N addition (50 kg N ha−1 yr−1) in the tropical secondary forest (−19%), but increased under high N addition (100 kg N ha−1 yr−1) in the temperate Korean pine mixed forest (+21%). The contribution of bacterial residue-C to SOC increased under the high N addition in the subtropical Castanopsis carlesii forest (+26%) and under the low N addition in the temperate birch forest (+38%), respectively. The responses of microbial residue-C in SOC to N addition depended on the changes in soil total N concentration and fungi to bacteria ratio under N addition and climate. Taken together, these findings provide the experimental evidence that N addition diversely regulates the formation and composition of microbial-derived C in SOC in forest ecosystems.

Nitrogen (N) and phosphorus (P) limitation constrains the magnitude of terrestrial carbon uptake in response to elevated carbon dioxide and climate change. However, global maps of nutrient limitation are still lacking. Here we examined global N and P limitation using the ratio of site-averaged leaf N and P resorption efficiencies of the dominant species across 171 sites. We evaluated our predictions using a global database of N- and P-limitation experiments based on nutrient additions at 106 and 53 sites, respectively. Globally, we found a shift from relative P to N limitation for both higher latitudes and precipitation seasonality and lower mean annual temperature, temperature seasonality, mean annual precipitation and soil clay fraction. Excluding cropland, urban and glacial areas, we estimate that 18% of the natural terrestrial land area is significantly limited by N, whereas 43% is relatively P limited. The remaining 39% of the natural terrestrial land area could be co-limited by N and P or weakly limited by either nutrient alone. This work provides both a new framework for testing nutrient limitation and a benchmark of N and P limitation for models to constrain predictions of the terrestrial carbon sink. Spatial patterns in the phosphorus and nitrogen limitation in natural terrestrial ecosystems are reported from analysis of a global database of the resorption efficiency of nutrients by leaves.

Atmospheric reactive nitrogen (N r ) has been a cause of serious environmental pollution in China. Historically, China used too little N r in its agriculture to feed its population. However, with the rapid increase in N fertilizer use for food production and fossil fuel consumption for energy supply over the last four decades, increasing gaseous N r species (e.g. NH 3 and NO x ) have been emitted to the atmosphere and then deposited as wet and dry deposition, with adverse impacts on air, water and soil quality as well as plant biodiversity and human health. This paper reviews the issues associated with this in a holistic way. The emissions, deposition, impacts, actions and regulations for the mitigation of atmospheric N r are discussed systematically. Both NH 3 and NO x make major contributions to environmental pollution but especially to the formation of secondary fine particulate matter (PM 2.5 ), which impacts human health and light scattering (haze). In addition, atmospheric deposition of NH 3 and NO x causes adverse impacts on terrestrial and aquatic ecosystems due to acidification and eutrophication. Regulations and practices introduced by China that meet the urgent need to reduce N r emissions are explained and resulting effects on emissions are discussed. Recommendations for improving future N management for achieving ‘win-win’ outcomes for Chinese agricultural production and food supply, and human and environmental health, are described. This article is part of a discussion meeting issue ‘Air quality, past present and future’.

Nitrogen (N) deposition has been conventionally thought to decrease forest soil methane (CH4) uptake, while the biome specific and dose dependent effect is poorly understood. Based on a meta-analysis of 63 N addition trials from 7 boreal forests, 8 temperate forests, 13 subtropical and 4 tropical forests, we evaluated the effects of N addition on soil CH4 uptake fluxes across global forest biomes. When combining all N addition levels, soil CH4 uptake was insignificantly decreased by 7% in boreal forests, while N addition significantly decreased soil CH4 uptake by 39% in temperate forests and by 21% in subtropical and tropical forests, respectively. Meta-regression analyses, however, indicated a shift from a positive to a negative effect on soil CH4 uptake with increasing N additions both in boreal forests (threshold = 48 kg N ha−1 yr−1) and temperate forests (threshold = 27 kg N ha−1 yr−1), while no such shift was found in subtropical and tropical forests. Considering that current N deposition to most boreal and temperate forests is below the abovementioned thresholds, N deposition likely exerts a positive to neutral effect on soil CH4 uptake in both forest biomes. Our results provide new insights on the biome specific and dose dependent effect of N addition on soil CH4 sink in global forests and suggest that the current understanding that N deposition decreases forest soil CH4 uptake is flawed by high levels of experimental N addition.

Forests are an important biome that covers about one third of the global land surface and provides important ecosystem services. Since atmospheric deposition of nitrogen (N) can have both beneficial and deleterious effects, it is important to quantify the amount of N deposition to forest ecosystems. Measurements of N deposition to the numerous forest biomes across the globe are scarce, so chemical transport models are often used to provide estimates of atmospheric N inputs to these ecosystems. We provide an overview of approaches used to calculate N deposition in commonly used chemical transport models. The Task Force on Hemispheric Transport of Air Pollution (HTAP2) study intercompared N deposition values from a number of global chemical transport models. Using a multi-model mean calculated from the HTAP2 deposition values, we map N deposition to global forests to examine spatial variations in total, dry and wet deposition. Highest total N deposition occurs in eastern and southern China, Japan, Eastern U.S. and Europe while the highest dry deposition occurs in tropical forests. The European Monitoring and Evaluation Program (EMEP) model predicts grid-average deposition, but also produces deposition by land use type allowing us to compare deposition specifically to forests with the grid-average value. We found that, for this study, differences between the grid-average and forest specific could be as much as a factor of two and up to more than a factor of five in extreme cases. This suggests that consideration should be given to using forest-specific deposition for input to ecosystem assessments such as critical loads determinations. Estimates of nitrogen deposition to global forests by global models may be a factor of 2 or more higher if the forest-specific deposition is used, compared to the grid cell average value and is on average 12% higher for all global forests.

China is experiencing severe tropospheric ozone pollution, especially during the summer period in cities. Previous studies have assessed the role of meteorological conditions and anthropogenic precursors in shaping the diurnal variation of ozone concentration in some Chinese cities or the spatial patterns of daytime ozone concentration, but less is known about the spatial variation and main regulators of the diurnal cycle of summer ozone concentrations in Chinese cities. Using monitoring data from 367 cities, we analyzed the spatial patterns and main regulators of daytime maximum, nighttime minimum and diurnal difference of summer (June–August) ozone concentration during 2015–2019. National mean values and standard deviations of daytime maximum and nighttime minimum of summer surface ozone concentration were 124.1 ± 27.5 and 33.4 ± 13.0 μg m−3, resulting in a diurnal difference of 90.7 ± 25.2 μg m−3. High values of daytime maximum, nighttime minimum, and diurnal difference of summer ozone concentration occurred in cities in northern China, especially in the North China Plain, and several city agglomerations in southern China. Daytime maximum ozone concentration was higher in cities with higher daytime PM2.5 and NO2 concentrations, lower daytime precipitation and lower elevation. Nighttime minimum ozone concentration increased with lower nighttime precipitation, lower NO2 concentration and CO concentration, higher nighttime maximum PM2.5 concentration and higher elevation. Diurnal difference of ozone concentration increased with lower elevation, lower daytime precipitation, and higher diurnal difference of CO and NO2 concentrations. Our findings highlight different regulators for daytime and nighttime ozone and imply the need of joint regulation of PM2.5 and NO2 emissions to control ozone pollution.

There is increasing interest in understanding atmospheric phosphorus (P) deposition and its impacts on plant productivity and carbon sinks in ecosystems. However, the global pattern of P deposition remains poorly understood, primarily due to the sparseness of data in Asia. In this study, the authors compiled 396 published observations of atmospheric P deposition from 1959 to 2020 on the global scale. The results gave a geometric mean bulk P deposition value of 0.32 kg ha−1 yr−1, or a global P budget of 4.4 Tg yr−1. Compared with the period 1959–2000, the authors found an elevated P deposition in Europe and Asia during 2001–2020, likely due to the increasing agricultural emissions and fossil fuel combustion–related sources in addition to dust emissions. The findings highlight the need to quantify the impacts of elevated P deposition from anthropogenic emissions on long-term ecosystem development in the context of carbon neutrality and clean-air actions. 摘要 磷是植物生长必需的元素, 磷缺乏将限制植被生产力及碳汇功能. 为厘清全球大气磷沉降的时空格局, 本文收集了1959–2020年发表的396条观测资料. 结果发现, 全球大气磷沉降的几何均值为0.32 kg ha−1 yr−1, 全球大气磷库约4.4 Tg yr−1. 与1959–2000年相比, 近20年亚洲和欧洲大气磷沉降呈现上升趋势, 主要来源是农业活动,沙尘传输和燃烧源排放. 碳中和背景下, 清洁空气行动是否会改变磷沉降的途径和形态, 进而影响到生态系统的结构和功能, 是需要回答的科学问题.

Atmospheric reactive nitrogen (N) has induced large impacts on air pollution and ecosystem health worldwide. Atmospheric reactive N emission and deposition have largely increased in China since 1980 due to rapid agricultural, industrial, and urban development. But scientific gaps still remain in the regional and temporal variability in atmospheric N emissions and deposition. Meanwhile, the environmental impacts of N pollution and deposition are of great concern in China. This paper overviews the status of anthropogenic N emissions and deposition and their linkages to air pollution in China. The major findings include two aspects: (1) anthropogenic reactive N (e.g., NH3 and NOx) emissions contribute greatly to secondary inorganic aerosol formation and haze pollution and (2) dry N deposition is comparable in importance to wet N deposition, suggesting that both dry and wet deposition should be quantified simultaneously. Future research challenges on atmospheric N emission and deposition are discussed as well. China needs to (1) reduce the uncertainties of national emission inventory of various N species, especially organic N compounds; (2) establish national networks for atmospheric N concentration and deposition monitoring; and (3) evaluate ecological and environmental impacts of N pollution and deposition in typical ecosystems. Last but not least, N deposition modeling tools should be improved based on localized parameters and further used in future N regulation.

Anthropogenic emissions of acid precursors have enhanced global occurrence of acid rain, especially in East Asia. Acid rain directly suppresses leaf function by eroding surface waxes and cuticle and leaching base cations from mesophyll cells, while the simultaneous foliar uptake of nitrates in rainwater may directly benefit leaf photosynthesis and plant growth, suggesting a non-linear direct effect of acid rain. By synthesizing data from literature on acid rain exposure experiments, we assessed the direct effects of acid rain on leaf photosynthesis across 49 terrestrial plants in China. Our results show a non-linear direct effect of acid rain on leaf photosynthetic rate, including a neutral to positive effect above pH 5.0 and a negative effect below that pH level. The acid rain sensitivity of leaf photosynthesis showed no significant difference between herbs and woody species below pH 5.0, but the impacts above that pH level were strongly different, resulting in a significant increase in leaf photosynthetic rate of woody species and an insignificant effect on herbs. Our analysis also indicates a positive effect of the molar ratio of nitric versus sulfuric acid in the acid solution on leaf photosynthetic rate. These findings imply that rainwater acidity and the composition of acids both affect the response of leaf photosynthesis and therefore result in a non-linear direct effect.

Anthropogenic nitrogen (N) deposition, mainly driven by reactive N emissions from agricultural and industrial activities, has been enhanced dramatically in China. The enhancement of N deposition has aroused increasing concerns about its effects on ecosystem health and function. Forest covers more than one fifth of the national land area in China and provides fundamental ecosystem services. There have been emerging experimental, observational and modelling efforts to assess the impacts of N deposition on forest ecosystems since the 2000s. Here we summarized these research progresses with a focus on the effects of N deposition on soil chemistry and N transformation, soil microorganisms and enzymes, plant physiology, biodiversity and ecosystem carbon (C) balance. Experimental results and modelling estimates generally indicate a fertilization effect of N deposition on forest growth and consequent C sequestration. However, high-level N deposition has been increasingly evidenced to cause N leaching, soil acidification, nutrient imbalance, increased N2O emissions and decreased soil CH4 uptake, which likely offset the positive effect on ecosystem C storage over time. Meanwhile, N deposition has changed both species composition and richness of plant and soil microbial communities, consequently altering C and nutrient cycling. Moreover, the effects of N deposition are likely non-linear with N doses, and thus the assessment on the threshold values for N deposition would have important implications for N emission regulation and forest management options. We conclude that elevated N deposition is altering ecosystem structure and function of China’s forests, especially in the eastern and southern regions. In addition, we highlight research efforts to jointly consider multiple abiotic factors (e.g. climate warming, CO2 enrichment, drought and increase in surface ozone) and biotic factors (e.g. insect outbreaks and invasive species), when evaluating ecological impacts of N deposition in the future.

Atmospheric reactive nitrogen (N), as an important component of global N cycle, has been significantly altered by anthropogenic emissions and consequently induced worldwide impacts on air pollution and ecosystem services. Due to rapid agricultural, industrial, and urban development, China has been experiencing a rapid increase in reactive N emissions and deposition since the late 1970s. Based on a literature review, this book summarizes recent research on (1) atmospheric reactive N as a global environmental issue (Chap. 1), (2) the emission, deposition, and budget of atmospheric reactive N (Chaps. 2, 3, 4 and 5), (3) the contribution of reactive N to air pollution (e.g., haze, surface O3, and acid deposition) (Chaps. 6, 7 and 8), (4) the impacts of N deposition on sensitive ecosystems (e.g., forests, grasslands, deserts, and lakes) (Chaps. 9, 10, 11 and 12), and (5) the regulatory strategies for mitigation of atmospheric reactive N pollution from agricultural and nonagricultural sectors in China (Chaps. 13 and 14).

Atmospheric nitrogen (N) deposition, one of the main pathways of loss for reactive N (Nr) species in the atmosphere, refers to the removal of Nr species as dry and wet deposition. It is an N input to terrestrial and water ecosystems and can have both substantially positive (as a nutrient source) and negative (e.g. biodiversity decline via N deposition-induced acidification and eutrophication) effects on ecosystems. Monitoring N deposition is crucial for a better understanding of its magnitude, optimizing deposition parameters in chemical transport models and evaluating the helpfulness of policies to reduce Nr emissions and deposition. This chapter reviews research progress in field and satellite monitoring of N depositions. Generally, dry and wet N deposition are of equal importance, and both vary greatly in space and time, with the former dominating mainly in northern China and the latter in southern China. Nitrogen deposition significantly increased during the period from 1980 to 2010 but has stabilized afterwards. We discuss the uncertainties in quantifying dry, wet and total N deposition. We also propose recommendations to address current challenges in developing a long-term, open-access deposition monitoring network with reasonable accuracy and representativeness in China.

Leaf nitrogen (N) and phosphorus (P) concentrations are critical for photosynthesis, growth, reproduction and other ecological processes of plants. Previous studies on large-scale biogeographic patterns of leaf N and P stoichiometric relationships were mostly conducted using data pooled across taxa, while family/genus-level analyses are rarely reported. Here, we examined global patterns of family-specific leaf N and P stoichiometry using a global data set of 12,716 paired leaf N and P records which includes 204 families, 1,305 genera, and 3,420 species. After determining the minimum size of samples (i.e., 35 records), we analyzed leaf N and P concentrations, N:P ratios and N∼P scaling relationships of plants for 62 families with 11,440 records. The numeric values of leaf N and P stoichiometry varied significantly across families and showed diverse trends along gradients of mean annual temperature (MAT) and mean annual precipitation (MAP). The leaf N and P concentrations and N:P ratios of 62 families ranged from 6.11 to 30.30 mg g−1, 0.27 to 2.17 mg g−1, and 10.20 to 35.40, respectively. Approximately 1/3–1/2 of the families (22–35 of 62) showed a decrease in leaf N and P concentrations and N:P ratios with increasing MAT or MAP, while the remainder either did not show a significant trend or presented the opposite pattern. Family-specific leaf N∼P scaling exponents did not converge to a certain empirical value, with a range of 0.307–0.991 for 54 out of 62 families which indicated a significant N∼P scaling relationship. Our results for the first time revealed large variation in the family-level leaf N and P stoichiometry of global terrestrial plants and that the stoichiometric relationships for at least one-third of the families were not consistent with the global trends reported previously. The numeric values of the family-specific leaf N and P stoichiometry documented in the current study provide critical synthetic parameters for biogeographic modeling and for further studies on the physiological and ecological mechanisms underlying the nutrient use strategies of plants from different phylogenetic taxa.

Anthropogenic alteration of global nitrogen (N) deposition has resulted in profound impacts on soil fluxes of greenhouse gases in terrestrial ecosystems. However, the response of soil methane (CH4) flux to N deposition remains poorly quantified in global forest. Based on a synthesis of experimental results from literature, we evaluated the effects of N deposition on growing-season soil CH4 flux across forest biomes. A distinction was made between low-level N addition that is comparable with the worldwide range in N deposition (−1 yr−1) and high-level N addition (> 60 kg N−1 yr−1. The results showed that growing-season soil CH4 flux was significantly affected by N additions, the value being dependent on the N addition level and forest biome. Low-level N addition significantly increased growing-season soil CH4 uptake in boreal forest, while an opposite effect occurred in temperate and subtropical forests. However, high-level N addition significantly decreased growing-season soil CH4 uptake across boreal, temperate, and subtropical forests. At biome scale, current N deposition was estimate to increase growing-season soil CH4 sink by 0.029 Tg CH4 in boreal forest, while it decreased growing-season soil CH4 sink by 0.025 Tg CH4 and 0.051 Tg CH4 in temperate and subtropical forests, respectively. This work improves our understanding of biome-specific effect of N deposition on soil CH4 uptake and identifies knowledge gaps in the effect of N deposition on soil CH4 flux in tropical forest.

This is the Preface paper for a special issue of Environmental Pollution that includes ten papers that synthesize cutting-edge studies on: 1) the characteristics of N deposition to global forests, 2) the impacts of N deposition on forest structure and function, 3) the responses of forest ecosystems to regional trends of N deposition and 4) the management options to mitigate negative impacts of N deposition in forest ecosystems. Among the ten papers were reviews on the influence of chronic N deposition on: the structure and function of forest mycorrhizal communities; understory plant community responses; trends in N deposition in key regions and how forests are responding to these temporal trends; ecosystem management strategies for forests in the face of chronic N deposition; and N-induced net primary production and C sequestration in global forests.

Total nitrogen (N) deposition has declined in many parts of the U.S. and Europe since the 1990s. Even so, it appears that decreased N deposition alone may be insufficient to induce recovery from the impacts of decades of elevated deposition, suggesting that management interventions may be necessary to promote recovery. Here we review the effectiveness of four remediation approaches (prescribed burning, thinning, liming, carbon addition) on three indicators of recovery from N deposition (decreased soil N availability, increased soil alkalinity, increased plant diversity), focusing on literature from the U.S. We reviewed papers indexed in the Web of Science since 1996 using specific key words, extracted data on the responses to treatment along with ancillary data, and conducted a meta-analysis using a three-level variance model structure. We found 69 publications (and 2158 responses) that focused on one of these remediation treatments in the context of N deposition, but only 29 publications (and 408 responses) reported results appropriate for our meta-analysis. We found that carbon addition was the only treatment that decreased N availability (effect size: −1.80 to −1.84 across metrics), while liming, thinning, and prescribed burning all tended to increase N availability (effect sizes: +0.4 to +1.2). Only liming had a significant positive effect on soil alkalinity (+10.5%–82.2% across metrics). Only prescribed burning and thinning affected plant diversity, but with opposing and often statistically marginal effects across metrics (i.e., increased richness, decreased Shannon or Simpson diversity). Thus, it appears that no single treatment is effective in promoting recovery from N deposition, and combinations of treatments should be explored. These conclusions are based on the limited published data available, underscoring the need for more studies in forested areas and more consistent reporting suitable for meta-analyses across studies.

Responses of tree growth to climate are usually spatially heterogeneous. Besides regionally varying external environments, species specificity is a crucial factor in determining said spatial heterogeneity. A better understanding of this species specificity would improve our estimations of the warming effects on forests. In this study, we selected two widely-distributed boreal conifers, Dahurian larch (Larix gmelinii) and Mongolian pine (Pinus sylvestris var. mongolica), to compare their growth-climate responses, including long-term growth-climate correlations and short-term growth resilience to drought. We sampled 160 trees and 481 tree-ring cores from the two species in two pure and two mixed forests, located in the Greater Khingan Range, northeast China. We found that Dahurian larch was generally positively correlated with spring temperature and negatively correlated with summer temperature. In contrast, Mongolian pine was more sensitive to summer moisture. Our results suggest that the main climatic limitations were low spring temperatures for Dahurian larch and summer moisture deficits for Mongolian pine. Dahurian larch represented higher growth resistance to drought, while Mongolia pine represented higher recovery. Based on this, we inferred that Dahurian larch was more vulnerable to extreme droughts, while Mongolian pine was more vulnerable to frequent droughts. We also demonstrated the effects of forest type on growth-climate responses. The negative effects of summer temperatures on Mongolian pine seemed to be more significant in mixed forests. As warming continued, Mongolian pine in this area would suffer severer moisture deficits, especially when coexisting with Dahurian larch. Our results suggest that Dahurian larch gained an advantage in the competition with Mongolian pine during high moisture stress. Driven by the warming trends, the species specificity in growth response would ultimately promote the separation of the two species in distribution. This study will help improve our estimations of the warming effects on forests and develop more species-targeted forest management practices.