Search

The global urban area is expanding continuously, resulting in unprecedented emissions and deposition of reactive nitrogen (N) in urban environments. However, large knowledge gaps remain in the ecological effects of N deposition on urban forests that provide key ecosystem services for an increasing majority of city dwellers. The current understanding of the spatial patterns and ecological effects of N deposition in urban forests was synthesized based on a literature review of observational and experimental studies. Nitrogen deposition generally increases closer to cities, resulting in an urban hotspot phenomenon. Chemical components of N deposition also shift across urban-suburban-rural gradients, showing higher ratios of ammonium to nitrate in and around urban areas. The ecological effects of N deposition on urban forest ecosystems are overviewed with a special focus on ecosystem N cycling, soil acidification, nutrient imbalances, soil greenhouse gas emissions, tree growth and forest productivity, and plant and soil microbial diversity. The distinct effects of unprecedented N deposition on urban forests are discussed in comparison with the common effects in natural forests. Despite the existing research efforts, several key research needs are highlighted to fill the knowledge gaps in the ecological effects of N deposition on urban forests. [ABSTRACT FROM AUTHOR]

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 N2O 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 N2O emissions (in terms of 100‐year global warming potential), and contributes ~1% to the gross forest C sink. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. Image 1 • No single factor improved forest soil N availability, alkalinity, and biodiversity. • Only carbon addition reduced N availability, only liming increased alkalinity. • Prescribed burning and thinning had weak or undesirable effects on responses. • Results are not consistently reported in the literature to support meta-analysis. • Several treatments may be needed to promote forest recovery from N deposition. Combinations of interventions in forests may be necessary to remediate soil N status, soil acidity, and plant community diversity from the effects of long-term N deposition. [ABSTRACT FROM AUTHOR]

Abstract Average nitrogen (N) deposition across Europe has declined since the 1990s. This resulted in decreased N inputs to forest ecosystems especially in Central and Western Europe where deposition levels are highest. While the impact of atmospheric N deposition on forests has been receiving much attention for decades, ecosystem responses to the decline in N inputs received less attention. Here, we review observational studies reporting on trends in a number of indicators: soil acidification and eutrophication, understory vegetation, tree nutrition (foliar element concentrations) as well as tree vitality and growth in response to decreasing N deposition across Europe. Ecosystem responses varied with limited decrease in soil solution nitrate concentrations and potentially also foliar N concentrations. There was no large-scale response in understory vegetation, tree growth, or vitality. Experimental studies support the observation of a more distinct reaction of soil solution and foliar element concentrations to changes in N supply compared to the three other parameters. According to the most likely scenarios, further decrease of N deposition will be limited. We hypothesize that this expected decline will not cause major responses of the parameters analysed in this study. Instead, future changes might be more strongly controlled by the development of N pools accumulated within forest soils, affected by climate change and forest management. Graphical abstract Image Highlights • Europe's forests show limited response to decreasing N deposition. • Potential responses have been reported for soil solution and foliage concentrations. • Delayed or marginal responses are expected for other forest ecosystem components. • Future decrease of N deposition to forests in Europe will likely be small. We find limited indication for response of Europe's forests to declining N deposition. Reactions have been reported for soil solution NO 3 − and potentially foliar N concentrations but not for other indicators. [ABSTRACT FROM AUTHOR]

Abstract 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. Graphical abstract Image 1 Highlights • Nitrogen-induced new NPP and C sequestration is estimated in global forests. • N deposition and BNF contributed 3.48 Pg C yr−1 to NPP of global forests. • N deposition and BNF contributed 1.83 Pg C yr−1 to the C sink of global forests. • BNF dominates nitrogen-induced new NPP and C sinks in global forests. • New N inputs 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. [ABSTRACT FROM AUTHOR]

Abstract: Elevated nitrogen (N) deposition may increase net primary productivity in N‐limited terrestrial ecosystems and thus enhance the terrestrial carbon (C) sink. To assess the magnitude of this N‐induced C sink, we performed a meta‐analysis on data from forest fertilization experiments to estimate N‐induced C sequestration in aboveground tree woody biomass, a stable C pool with long turnover times. Our results show that boreal and temperate forests responded strongly to N addition and sequestered on average an additional 14 and 13 kg C per kg N in aboveground woody biomass, respectively. Tropical forests, however, did not respond significantly to N addition. The common hypothesis that tropical forests do not respond to N because they are phosphorus‐limited could not be confirmed, as we found no significant response to phosphorus addition in tropical forests. Across climate zones, we found that young forests responded more strongly to N addition, which is important as many previous meta‐analyses of N addition experiments rely heavily on data from experiments on seedlings and young trees. Furthermore, the C–N response (defined as additional mass unit of C sequestered per additional mass unit of N addition) was affected by forest productivity, experimental N addition rate, and rate of ambient N deposition. The estimated C–N responses from our meta‐analysis were generally lower that those derived with stoichiometric scaling, dynamic global vegetation models, and forest growth inventories along N deposition gradients. We estimated N‐induced global C sequestration in tree aboveground woody biomass by multiplying the C–N responses obtained from the meta‐analysis with N deposition estimates per biome. We thus derived an N‐induced global C sink of about 177 (112–243) Tg C/year in aboveground and belowground woody biomass, which would account for about 12% of the forest biomass C sink (1,400 Tg C/year). [ABSTRACT FROM AUTHOR]

We modelled the effects of past and expected future changes in climate (temperature, precipitation), CO 2 concentration, nitrogen deposition (N) and ozone (O 3 ) exposure (phytotoxic ozone dose, POD) on carbon (C) sequestration by European forest ecosystems for the period 1900–2050. Tree C sequestration was assessed by using empirical response functions, while soil C sequestration was simulated with the process-based model VSD, combined with the RothC model. We evaluated two empirical growth responses to N deposition (linear and non-linear) and two O 3 exposure relationships (linear function with total biomass or net annual increment). We further investigated an ‘interactive model’ with interactions between drivers and a ‘multiplicative model’, in which the combined effect is the product of individual drivers. A single deposition and climate scenario was used for the period 1900–2050. Contrary to expectations, growth observations at European level for the period 1950–2010 compared better with predictions by the multiplicative model than with the interactive model. This coincides with the fact that carbon responses in kg C ha − 1 yr − 1 per unit change in drivers, i.e. per °C, ppm CO 2 , kg N ha − 1 yr − 1 and mmol m − 2 yr − 1 POD, are more in line with literature data when using the multiplicative model. Compared to 1900, the estimated European average total C sequestration in both forests and forest soils between 1950 and 2000 increased by 21% in the interactive model and by 41% in the multiplicative model, but observed changes were even higher. This growth increase is expected to decline between 2000 and 2050. The simulated changes between 1950 and 2000 were mainly due to the increase in both N deposition and CO 2 , while the predicted increases between 2000 and 2050 were mainly caused by the increase in CO 2 and temperature, and to lesser extent a decrease in POD, counteracted by reduced N deposition. [ABSTRACT FROM AUTHOR]

We modelled the combined effects of past and expected future changes in climate and nitrogen deposition on tree carbon sequestration by European forests for the period 1900–2050. Two scenarios for deposition (current legislation and maximum technically feasible reductions) and two climate scenarios (no change and SRES A1 scenario) were used. Furthermore, the possible limitation of forest growth by calcium, magnesium, potassium and phosphorus is investigated. The area and age structure of the forests was assumed to stay constant to observations during the period 1970–1990. Under these assumptions, the simulations show that the change in forest growth and carbon sequestration in the past is dominated by changes in nitrogen deposition, while climate change is the major driver for future carbon sequestration. However, its impact is reduced by nitrogen availability. Furthermore, limitations in base cations, especially magnesium, and in phosphorus may significantly affect predicted growth in the future. [Copyright &y& Elsevier]

In a recent study, Magnani et al. report how atmospheric nitrogen deposition drives stand-lifetime net ecosystem productivity (NEPav) for midlatitude forests, with an extremely high C to N response (725 kg C kg−1 wet-deposited N for their European sites). We present here a re-analysis of these data, which suggests a much smaller C : N response for total N inputs. Accounting for dry, as well as wet N deposition reduces the C : N response to 177 : 1. However, if covariance with intersite climatological differences is accounted for, the actual C : N response in this dataset may be <70 : 1. We then use a model analysis of 22 European forest stands to simulate the findings of Magnani et al. Multisite regression of simulated NEPav vs. total N deposition reproduces a high C : N response (149 : 1). However, once the effects of intersite climatological differences are accounted for, the value is again found to be much smaller, pointing to a real C : N response of about 50–75 : 1. [ABSTRACT FROM AUTHOR]

An estimate of net carbon (C) pool changes and long-term C sequestration in trees and soils was made at more than 100 intensively monitored forest plots (level II plots) and scaled up to Europe based on data for more than 6000 forested plots in a systematic 16 km × 16 km grid (level I plots). C pool changes in trees at the level II plots were based on repeated forest growth surveys At the level I plots, an estimate of the mean annual C pool changes was derived from stand age and available site quality characteristics. C sequestration, being equal to the long-term C pool changes accounting for CO2 emissions because of harvest and forest fires, was assumed 33% of the overall C pool changes by growth. C sequestration in the soil were based on calculated nitrogen (N) retention (N deposition minus net N uptake minus N leaching) rates in soils, multiplied by the C/N ratio of the forest soils, using measured data only (level II plots) or a combination of measurements and model calculations (level I plots). Net C sequestration by forests in Europe (both trees and soil) was estimated at 0.117 Gton yr−1, with the C sequestration in stem wood being approximately four times as high (0.094 Gton yr−1) as the C sequestration in the soil (0.023 Gton yr−1). The European average impact of an additional N input on the net C sequestration was estimated at approximately 25 kg C kg−1 N for both tree wood and soil. The contribution of an average additional N deposition on European forests of 2.8 kg ha−1 yr−1 in the period 1960–2000 was estimated at 0.0118 Gton yr−1, being equal to 10% of the net C sequestration in both trees and soil in that period (0.117 Gton yr−1). The C sequestration in trees increased from Northern to Central Europe, whereas the C sequestration in soil was high in Central Europe and low in Northern and Southern Europe. The result of this study implies that the impact of forest management on tree growth is most important in explaining the C pool changes in European forests. [ABSTRACT FROM AUTHOR]

Nitrogen (N) deposition has been conventionally thought to decrease forest soil methane (CH 4) 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 CH 4 uptake fluxes across global forest biomes. When combining all N addition levels, soil CH 4 uptake was insignificantly decreased by 7% in boreal forests, while N addition significantly decreased soil CH 4 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 CH 4 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 CH 4 uptake in both forest biomes. Our results provide new insights on the biome specific and dose dependent effect of N addition on soil CH 4 sink in global forests and suggest that the current understanding that N deposition decreases forest soil CH 4 uptake is flawed by high levels of experimental N addition. Image 1 • The effects of N addition on soil CH 4 flux were evaluated in global forest biomes. • The effect of N addition was biome specific and dose dependent. • A shift from a positive to negative effect occurred with increasing N additions in boreal and temperate forests. • No shift of effect occurred with increasing N additions in subtropical and tropical forests. • Previous meta-analyses are likely flawed by high-level N additions. A meta-analysis indicates a biome specific and dose dependent effect of N addition on soil CH 4 sink in global forests and suggests that the current understanding that N deposition reduces forest soil CH 4 uptake is flawed by high levels of experimental N addition. [ABSTRACT FROM AUTHOR]

Hypotheses about the impacts of elevated atmospheric deposition of nitrogen on the forest ecosystem include an increased sensitivity to natural stress, impacts on roots, reduced species diversity of the ground vegetation, reduced growth, and unbalanced nutritional status dueto eutrophication and acidification. The impact of N deposition has gained in ecological importance during recent decades, in part due tothe steady decline in S emissions. Results of throughfall and deposition measurements at 163 plots in Europe show that total deposition of S and N compounds ranged from 100 to 3000 mol ha-1 yr-1 in approximately 90% of the plots, but values up to 4000-8000 mol ha-1 yr-1 were also observed. Approximately 50% of the plots received N inputs, dominated by NH4,above 1000 mol ha-1 yr-1, which is a deposition level at which species diversity of the ground vegetation may be atrisk. Results of input-output budgets for plots concentrated in Northern and Western Europe indicate that nitrate leaching starts to occur at throughfall inputs above 10 kg ha-1 yr-1, specifically in soils with C/N ratios in the humus layer below 25. Examples are given of field evidence for impacts of elevated N deposition, including elevated N contents in foliage and soil, Al release in soil response to increased nitrate concentrations, reduced shoot/rootratios, and a reduction in species diversity. Although knowledge about the response of forest ecosystems to N inputs has increased over the last decade, there is still a lack of information on the dynamics of N accumulation and related critical N loads in a range of environmental conditions. Furthermore, a European-wide perspective of N saturation in forest ecosystems is still lacking. [ABSTRACT FROM AUTHOR]

Nitrogen (N) deposition in China has been dramatically enhanced by anthropogenic emissions and has aroused great concerns of its impacts on forest ecosystems. This study synthesized data on ammonium (NH 4 + ) and nitrate (NO 3 − ) contents in bulk precipitation and throughfall from 38 forest stands in published literature to assess the status and characteristics of N deposition to typical forests in China between 1995 and 2010. Our results showed that ammonium dominated N deposition in this period, with a mean NH 4 + –N:NO 3 − –N ratio of ∼2.5 in bulk deposition and throughfall. Mean throughfall N deposition in China's forests was as high as 14.0 kg N ha −1 yr −1 for ammonium, 5.5 kg N ha −1 yr −1 for nitrate and 21.5 kg N ha −1 yr −1 for total inorganic N (TIN), respectively. Mean bulk deposition was 9.4 kg N ha −1 yr −1 for ammonium, 3.9 kg N ha −1 yr −1 for nitrate and 14.0 kg N ha −1 yr −1 for TIN, respectively. Canopy captured dry deposition, calculated as the difference between throughfall and bulk deposition, was thus approximately half of the bulk deposition. Spatial patterns of N deposition were in accordance with our urban hotspot hypothesis, showing a strong power-law reduction of ammonium with increasing distance to large cities but only slightly lower nitrate deposition. Our results suggest that high N deposition, especially of ammonium, exceeds critical N loads for large areas of China's forests. [ABSTRACT FROM AUTHOR]