Long-term nitrogen deposition increases heathland carbon sequestration

Different responses of individuals, functional groups and plant communities in CSR strategies to nitrogen deposition in high-altitude grasslands

The Competitor, Stress Tolerator, and Ruderal (CSR) theory delineates the ecological strategies of plant species. Nevertheless, how these ecological strategies shift at the levels of individuals, functional groups and plant communities to cope with increasing nitrogen deposition remains unclear. In this study, simulated nitrogen deposition experiments were performed in high-altitude grasslands of alpine meadows and alpine steppe on the Qinghai-Tibetan Plateau (QTP) by employing the strategy and functional type framework (StrateFy) methodology to evaluate plant CSR strategies. Our results indicated that the dominant ecological strategy of the high-altitude grassland on the QTP were predominantly aligned with the R-strategy. In both alpine meadow and alpine steppe grasslands, the community-weighted mean (CWM) of C scores were increased with nitrogen addition, while CWM of R and S scores were not significantly correlated with nitrogen addition. Remarkably, the increase in C scores due to nitrogen enrichment was observed solely in non-legumes, suggesting an enhanced competitive capability of non-legumes in anticipation of future nitrogen deposition. Leymus secalinus was dominated in both alpine meadow and alpine steppe grasslands across all levels of nitrogen deposition, with increasing C scores along the nitrogen gradients. Furthermore, the sensitivity of C scores of individual plant, functional group and plant community to nitrogen deposition rates was more pronounced in alpine steppe grassland than in alpine meadow grassland. These findings furnish novel insights into the alterations of ecological strategies in high-altitude alpine grasslands on the QTP and similar regions worldwide in cope with escalating nitrogen deposition.

Estimating global annual gross primary production based on satellite-derived phenology and maximal carbon uptake capacity

The high uncertainty regarding global gross primary production (GPP) remains unresolved. This study explored the relationships between phenology, physiology, and annual GPP to provide viable alternatives for accurate estimation. A statistical model of integrated phenology and physiology (SMIPP) was developed using GPP data from 145 FLUXNET sites to estimate the annual GPP for various vegetation types. By employing the SMIPP model driven by satellite-derived datasets of the global carbon uptake period (CUP) and maximal carbon uptake capacity (GPPmax), the global annual GPP was estimated for the period from 2001 to 2018. The results demonstrated that the SMIPP model accurately predicted annual GPP, with relative root mean square error values ranging from 11.20 to 19.29% for forest types and 20.49-35.71% for non-forest types. However, wetlands, shrublands, and evergreen forests exhibited relatively low accuracies. The average, trend, and interannual variation of global GPP during 2001-2018 were 132.6 Pg C yr-1, 0.25 Pg C yr-2, and 1.57 Pg C yr-1, respectively. They were within the ranges estimated in other global GPP products. Sensitivity analysis revealed that GPPmax had comparable effects to CUP in high-latitude regions but significantly greater impacts at the global scale, with sensitivity coefficients of 0.85 ± 0.23 for GPPmax and 0.46 ± 0.28 for CUP. This study provides a simple and practical method for estimating global annual GPP and highlights the influence of GPPmax and CUP on global-scale annual GPP.

Characterization of controlling factors for soil organic carbon stocks in one Karst region of Southwest China

Soil organic carbon (SOC) contributes the most significant portion of carbon storage in the terrestrial ecosystem. The potential for variability in carbon losses from soil can lead to severe consequences such as climate change. While extensive studies have been conducted to characterize how land cover type, soil texture, and topography impact the distribution of SOC stocks across different ecosystems, little is known about in Karst Region. Here, we characterized SOC stocks with intensive sampling at the local scale (495 representative samples) via Random Forest Regression (RF) and Principal Component Analysis (PCA). Our findings revealed significant differences in SOC stock among land cover types, with croplands exhibiting the lowest SOC stocks, indicating that management practices could play a crucial role in SOC stocks. Conversely, there was little correlation between SOC stock and clay percentage, suggesting that soil texture was not a primary factor influencing SOC at a local scale. Further, Annual Precipitation was identified as the key driving factor for the dynamics of SOC stocks with the help of RF and PCA. A substantial SOC deficit was observed in most soils in this study, as evaluated by a SOC/clay ratio, indicating a significant potential in SOC sequestration with practical measures in the karst region. As such, future research focused on simulating SOC dynamics in the context of climate change should consider the controlling factors at a local scale and summarize them carefully during the up-scaling process.

Long-term temporal patterns in ecosystem carbon flux components and overall balance in a heathland ecosystem

Terrestrial ecosystems have strong feedback to atmospheric CO₂ concentration and climate change. However, the long-term whole life cycle dynamics of ecosystem carbon (C) fluxes and overall balance in some ecosystem types, such as heathland ecosystems, have not been thoroughly explored. We studied the changes in ecosystem CO₂ flux components and overall C balance over a full ecosystem lifecycle in stands of Calluna vulgaris (L.) Hull by using a chronosequence of 0, 12, 19 and 28 years after vegetation cutting. Overall, the ecosystem C balance was highly nonlinear over time and exhibited a sinusoidal-like curvature of C sink/source change over the three-decade timescale. After cutting, plant-related C flux components of gross photosynthesis (P_G)_, aboveground autotrophic respiration (R_aa) and belowground autotrophic respiration (R_ba) were higher at the young age (12 years) than at middle (19 years) and old (28 years) ages. The young ecosystem was a C sink (12 years: -0.374 kg C m^-2 year^-1) while it became a C source with aging (19 years: 0.218 kg C m^-2 year^-1) and when dying (28 years: 0.089 kg C m^-2 year^-1). The post-cutting C compensation point was observed after four years, while the cumulative C loss in the period after cutting had been compensated by an equal amount of C uptake after seven years. Annual ecosystem C payback from the ecosystem to the atmosphere started after 16 years. This information may be used directly for optimizing vegetation management practices for maximal ecosystem C uptake capacity. Our study highlights that whole life cycle observational data of changes in C fluxes and balance in ecosystems are important and the ecosystem model needs to take the successional stage and vegetation age into account when projecting component C fluxes, ecosystem C balance, and overall feedback to climate change.

Influence of soil properties, topography, and land cover on soil organic carbon and total nitrogen concentration: A case study in Qinghai-Tibet plateau based on random forest regression and structural equation modeling

The effects of topography, land cover type, and soil physicochemical properties on the distribution of soil organic carbon (SOC) and total nitrogen (TN) have drawn growing attention recently, but little is known about how these features-associated interactions impact SOC and TN. To elucidate how these interactions affect the preservation of carbon and nitrogen in soils, we used data-driven models (random forest regression and structural equation modeling) to identify the dominant environmental factors affecting the distribution of SOC and TN in two different soil layers (0-20 and 20-40 cm) of the Qinghai-Tibet plateau. In addition, an algorithm based on random forest ("Boruta") was chosen to identify the relevant influencing factors and partial dependence was used to depict the two most important factors. We found that rather than land cover type, environmental properties, such as soil physicochemical characteristics and altitude had the most significant effects on the distribution of SOC and TN. Our findings indicate that elevation and TN are the two most important factors influencing SOC in the surface and subsurface soil layers. Moreover, total potassium (TK) impacts TN content in the surface soil layer, but only in a specific range of concentrations, which could be attributed to anthropogenic activities such as applying nitrogen and potassium fertilizers to increase the yields of local food crop, Tibetan hulless barley. These findings provide a scientific perspective on soil nutrient preservation.

Effects of 5-Year Nitrogen Addition on Species Composition and Diversity of an Alpine Steppe Plant Community on Qinghai-Tibetan Plateau

The N deposition rate is notably increased in China, especially in the Qinghai-Tibetan Plateau (QTP). How plants respond to the projected N deposition on the alpine steppe is still in debate. In this study, to investigate the effects of N deposition on the plant community of the alpine steppe, we simulated N deposition at six different N addition rate levels (0, 8, 24, 40, 56, 72 kg N ha⁻¹ y⁻¹) from 2015 to 2019. Species composition and diversity were investigated as the assessment indices. The results showed that the importance value of grasses significantly increased with the increase of the N addition rate, while that of forbs significantly decreased. A high N addition rate (72 kg N ha⁻¹ y⁻¹) induced species composition change, making Leymus secalinus become the most dominant species within the entire plant community. Compared with the control (without N addition), species richness, Shannon–Weiner diversity, Simpson dominance and Pielou Evenness were significantly reduced under a high N addition rate. The changes of plant diversity in the alpine steppe were closely correlated with dynamics of soil nutrients, especially total carbon (TC), total phosphorus (TP) and ammonia nitrogen (NH₄-N). Our findings suggested that a high N deposition rate (72 kg N ha⁻¹ y⁻¹) could significantly change plant composition and reduce the diversity of the alpine steppe, though they were less affected by low N deposition rates at present. With the increase of the N deposition rate, plant composition and diversity of the alpine steppe may be negatively affected in the future. In addition, Leymus secalinus is more competitive than other species with an N deposition rate increase. Soil C, soil P and soil NH⁴-N variation induced by N deposition might play a key role in regulating changes in plant composition and diversity in the alpine steppe. In addition, longer term field investigation needs to be carried out to testify to this phenomenon with the increase of N deposition in the future.

Long-term effects of atmospheric deposition on British plant species richness

The effects of atmospheric pollution on plant species richness (n_sp) are of widespread concern. We carried out a modelling exercise to estimate how n_sp in British semi-natural ecosystems responded to atmospheric deposition of nitrogen (N_dep) and sulphur (S_dep) between 1800 and 2010. We derived a simple four-parameter equation relating n_sp to measured soil pH, and to net primary productivity (NPP), calculated with the N14CP ecosystem model. Parameters were estimated from a large data set (n = 1156) of species richness in four vegetation classes, unimproved grassland, dwarf shrub heath, peatland, and broadleaved woodland, obtained in 2007. The equation performed reasonably well in comparisons with independent observations of n_sp. We used the equation, in combination with modelled estimates of NPP (from N14CP) and soil pH (from the CHUM-AM hydrochemical model), to calculate changes in average n_sp over time at seven sites across Britain, assuming that variations in n_sp were due only to variations in atmospheric deposition. At two of the sites, two vegetation classes were present, making a total of nine site/vegetation combinations. In four cases, n_sp was affected about equally by pH and NPP, while in another four the effect of pH was dominant. The ninth site, a chalk grassland, was affected only by NPP, since soil pH was assumed constant. Our analysis suggests that the combination of increased NPP, due to fertilization by N_dep, and decreased soil pH, primarily due to S_dep, caused an average species loss of 39% (range 23-100%) between 1800 and the late 20th Century. The modelling suggests that in recent years n_sp has begun to increase, almost entirely due to reductions in S_dep and consequent increases in soil pH, but there are also indications of recent slight recovery from the eutrophying effects of N_dep.

Legacy effects of nitrogen and phosphorus additions on vegetation and carbon stocks of upland heaths

Soil carbon (C) pools and plant community composition are regulated by nitrogen (N) and phosphorus (P) availability. Atmospheric N deposition impacts ecosystem C storage, but the direction of response varies between systems. Phosphorus limitation may constrain C storage response to N, hence P application to increase plant productivity and thus C sequestration has been suggested. We revisited a 23-yr-old field experiment where N and P had been applied to upland heath, a widespread habitat supporting large soil C stocks. At 10 yr after the last nutrient application we quantified long-term changes in vegetation composition and in soil and vegetation C and P stocks. Nitrogen addition, particularly when combined with P, strongly influenced vegetation composition, favouring grasses over Calluna vulgaris, and led to a reduction in vegetation C stocks. However, soil C stocks did not respond to nutrient treatments. We found 40% of the added P had accumulated in the soil. This study showed persistent effects of N and N + P on vegetation composition, whereas effects of P alone were small and showed recovery. We found no indication that P application could mitigate the effects of N on vegetation or increase C sequestration in this system.

Impacts of nitrogen deposition on carbon and nitrogen cycling in alpine Racomitrium heath in the UK and prospects for recovery

Deposition of reactive nitrogen (N) is a major threat to terrestrial ecosystems associated with impacts on ecosystem properties and functions including carbon (C) and nutrient stocks, soil water quality and nutrient retention. In the oceanic-alpine Racomitrium heath habitat, N deposition is associated with moss mat degradation and a shift from bryophyte to graminoid dominance. To investigate the effects of moss mat decline on C and N stocks and fluxes, we collected Racomitrium heath vegetation/soil cores from sites along a gradient of N deposition in the UK. Cores were maintained under controlled conditions and exposed to scenarios of current (8-40 kg N ha^-1 y^-1), reduced (8 kg N ha^-1 y^-1) and elevated (50 kg N ha^-1 y^-1) N deposition. Cores from high N deposition sites had smaller aboveground C and N stocks and, under current conditions, leached large amounts of inorganic N and had low soil water pH compared with low N deposition sites. With reduced N deposition there was evidence for rapid recovery of soil water quality in terms of reduced N leaching and small increases in pH. Under high N deposition, cores from low N deposition sites retained much of the applied N while those with a history of high N deposition leached large amounts of inorganic N. Carbon fluxes in soil water and net CO₂ fluxes varied according to core source site but were not affected by the N deposition scenarios. We conclude that C and N stocks and cycling in Racomitrium heath are strongly affected by long-term exposure to N deposition but that soil water quality may improve rapidly, if N deposition rates are reduced. The legacy of N deposition impacts on moss mat cover and vegetation composition however, mean that the ecosystem remains sensitive to future pulses in N input.

Effects of simulated N deposition on photosynthesis and productivity of key plants from different functional groups of alpine meadow on Qinghai-Tibetan plateau

Nitrogen (N) deposition may alter physiological process of plants in grassland ecosystem. However, little is known about the response mechanism of individual plants in alpine regions to N deposition. We conducted a field experiment, and three treatments including 0 kg Nha^-1year^-1 (CK), 8 kgNha^-1year^-1 (Low N), and 72 kg N ha^-1 year^-1 (High N) were established to simulate N deposition in alpine meadow of Qinghai-Tibetan plateau. Our objectives were to determine the influence of N deposition on photosynthesis of different functional types of herbage species in alpine meadow, and finally characterize the links of plant productivity and photosynthesis with soil nutrients. The results showed that responses of alpine plants were species-specific under N deposition. Compared with grass species Agropyron cristatum and forb species Thalictrum aquilegifolium, the sedge species Carex melanantha was much more sensitive to N deposition; a lower N load (8 kgNha^-1year^-1) can cause a negative effect on its photosynthesis and productivity. Additionally, N deposition can promote plant N uptake and significantly decreased the C (carbon)/N (nitrogen) ratio. Compared with CK and low N deposition, high N deposition inhibited the photosynthesis and growth of the forb species Thalictrum aquilegifolium and sedge species Carex melanantha. In all three functional types of herbage species, the grass species A. cristatum tended to show a much higher photosynthetic capacity and better growth potential; thus, suggesting that grass species A. cristatum will be a more adaptative alpine plants under N deposition. Our findings suggested that plant photosynthetic responses to N deposition were species-specific, low N deposition was not beneficial for all the herbage species, and N deposition may change plant composition by the differential photosynthetic responses among species in alpine grassland. Plant composition shift to grass-dorminant in alpine regions might be attributed to a much higher photosynthetic potential and N use efficiency of grass species.

Plant and vegetation functional responses to cumulative high nitrogen deposition in rear-edge heathlands

Elevated atmospheric nitrogen (N) deposition is a major driver of change, altering the structure/functioning of nutrient-poor Calluna vulgaris-heathlands over Europe. These effects amply proven for north-western/central heathlands may, however, vary across the ecosystem's distribution, especially at the range limits, as heathlands are highly vulnerable to land-use changes combined with present climate change. This is an often overlooked and greatly understudied aspect of the ecology of heathlands facing global change. We investigated the effects of five N-fertilisation treatments simulating a range of N deposition rates (0, 10, 20, and 50 kg N ha^-1 yr^-1 for 1 year; and 56 kg N ha^-1 yr^-1 for 9 years) on the Calluna-plants, the plant functional groups, species composition and richness of two life-cycle stages (building/young- and mature-phase) of Calluna-heathlands at their rear-edge limit. Our findings revealed a dose-related response of the shoot length and number of flowers of young and mature Calluna-plants to the addition of N, adhering to the findings from other heathland locations. However, cumulative high-N loading reduced the annual growth and flowering of young plants, showing early signs of N saturation. The different plant functional groups showed contrasting responses to the cumulative addition of N: annual/perennial forbs and annual graminoids increased with quite low values; perennial graminoids were rather abundant in young heathlands but only slightly augmented in mature ones; while bryophytes and lichens strongly declined at the two heathland life-cycle stages. Meanwhile there were no significant N-driven changes in plant species composition and richness. Our results demonstrated that Calluna-heathlands at their low-latitude distribution limit are moderately resistant to cumulative high-N loading. As north-western/central European heathlands under high-N inputs broadly experienced the loss of plant diversity and pronounced changes in plant species dominance, rear-edge locations may be of critical importance to unravel the mechanisms of heathland resilience to future global change.

Influence of enhanced ultraviolet-B radiation during rice plant growth on rice straw decomposition with nitrogen deposition

Although straw decomposition is important for ecosystem fertility and carbon balance, influence of ultraviolet-B (UV-B) radiation and nitrogen (N) deposition on this process is unclear. In this study, UV-B-exposed rice straw was decomposed under different N addition treatments for 15 months to investigate the indirect effects of UV-B radiation on straw chemistry and direct effects of N deposition on decomposition. UV-B exposure during rice plant growth changed the rice straw chemical composition, increasing the concentrations of acid-insoluble fraction (AIF), acid-soluble fraction, and UV-B-absorbing compounds. High N content had a negative effect on decomposition of rice straw exposed to enhanced and ambient UV-B radiation. Both AIF concentration and FTIR peak intensities indicated that lignin in rice straw was selectively preserved following N addition and UV-B radiation, reducing straw decomposition rate, which corresponded to lower activities of lignin-degrading enzymes in the later stage of decomposition. Thus, enhanced UV-B radiation during rice plant growth produced more recalcitrant substrates (lignin) and N reacted with lignin to produce more resistant compounds, further decreasing straw decomposition rate. UV-B radiation during plant growth and N deposition inhibit litter decomposition in agroecosystem, and their effects should be considered when establishing biogeochemical models in response to global changes.

Comparison of the impacts of acid and nitrogen additions on carbon fluxes in European conifer and broadleaf forests

Increased reactive nitrogen (N) loadings to terrestrial ecosystems are believed to have positive effects on ecosystem carbon (C) sequestration. Global "hot spots" of N deposition are often associated with currently or formerly high deposition of sulphur (S); C fluxes in these regions might therefore not be responding solely to N loading, and could be undergoing transient change as S inputs change. In a four-year, two-forest stand (mature Norway spruce and European beech) replicated field experiment involving acidity manipulation (sulphuric acid addition), N addition (NH₄NO₃) and combined treatments, we tested the extent to which altered soil solution acidity or/and soil N availability affected the concentration of soil dissolved organic carbon (DOC), soil respiration (Rs), microbial community characteristics (respiration, biomass, fungi and bacteria abundances) and enzyme activity. We demonstrated a large and consistent suppression of soil water DOC concentration driven by chemical changes associated with increased hydrogen ion concentrations under acid treatments, independent of forest type. Soil respiration was suppressed by sulphuric acid addition in the spruce forest, accompanied by reduced microbial biomass, increased fungal:bacterial ratios and increased C to N enzyme ratios. We did not observe equivalent effects of sulphuric acid treatments on Rs in the beech forest, where microbial activity appeared to be more tightly linked to N acquisition. The only changes in C cycling following N addition were increased C to N enzyme ratios, with no impact on C fluxes (either Rs or DOC). We conclude that C accumulation previously attributed solely to N deposition could be partly attributable to their simultaneous acidification.

Effects of revegetation and precipitation gradient on soil carbon and nitrogen variations in deep profiles on the Loess Plateau of China

Precipitation is one of the most important factors affecting the variations in soil carbon (C) and nitrogen (N) following revegetation. However, the effects of revegetation and precipitation gradients on soil organic carbon (SOC), total nitrogen (TN) and C-N interactions in deep profiles over large scales are poorly understood. This study measured the SOC and TN stocks to depth of 300 cm in three revegetation types (grassland, shrubland and forestland) and paired cropland stands at seven sites along a precipitation gradient with mean annual precipitation (MAP) from 280 to 540 mm yr^-1 in the Loess Plateau of China. The results showed that the SOC and TN stocks in the 0-300 cm profile increased along the precipitation gradient. Revegetation did not always result in accumulation of SOC and TN stocks, which depended on the precipitation condition and varied among different vegetation types. Grassland restoration resulted in more SOC and TN accumulation than shrubland and forestland in areas with MAP < 510 mm, whereas there were losses in SOC and TN following grass plantation in sites with MAP > 510 mm. The changes in SOC and TN stocks following revegetation (∆SOC and ∆TN) were significantly correlated with MAP in only the 0-20 cm layer, whereas the changes in the C/N ratio of each depth were significantly and negatively correlated with MAP. The correlations between ∆SOC and ∆TN were stronger in the 0-60 cm layer than that in the 60-300 cm layer, and an accumulation of 1 g TN was associated with approximately 7.9 g increase of SOC in the 0-300 cm profile following revegetation. This study indicated that the changes in soil C and N stocks following revegetation had different patterns along precipitation gradient and among depths, and grassland restoration and N fertilizer input benefitted soil C and N sequestration in drier areas.

Nitrogen deposition drives loss of moss cover in alpine moss-sedge heath via lowered C : N ratio and accelerated decomposition

In alpine ecosystems, nitrogen (N) deposition has been linked to plant community composition change, including loss of bryophytes and increase of graminoids. Since bryophyte growth is stimulated by increased N availability, it has been hypothesized that loss of bryophyte cover is driven by enhanced decomposition. As bryophyte mats are a significant carbon (C) store, their loss may impact C storage in these ecosystems. We used an N deposition gradient across 15 sites in the UK to examine effects of N deposition on bryophyte litter quality, decomposition and C and N stocks in Racomitrium moss-sedge heath. Increasing N deposition reduced C : N in bryophyte litter, which in turn enhanced decomposition. Soil N stocks increased significantly in response to increased N deposition, and soil C : N declined. However, depletion of the bryophyte mat and its replacement by graminoids under high N deposition was not associated with a change in total ecosystem C stocks. We conclude that decomposition processes in Racomitrium heath are very sensitive to N deposition and provide a mechanism by which N deposition drives depletion of the bryophyte mat. Nitrogen deposition did not measurably alter C stocks, but changes in soil N stocks and C : N suggest the ecosystem is becoming N saturated.

Characteristics of labile organic carbon fractions in reclaimed mine soils: Evidence from three reclaimed forests in the Pingshuo opencast coal mine, China

The reclamation of discarded spoils has the potential to stimulate carbon (C) sequestration in reclaimed mine soils (RMSs). Nevertheless, to date the temporal dynamics of labile organic C fractions have not been sufficiently elucidated in RMSs. In this study, soil organic carbon (SOC) and labile organic C fractions, including microbial biomass organic C (MBC), easily oxidizable organic C (EOC) and dissolved organic C (DOC), were determined in Robinia pseudoacacia monoculture forests (reclamation periods of 0, 8, 10, 13, 15, 18 and 30years), Pinus tabuliformis forests (reclamation periods of 0, 10, 19, 23 and 25years) and Ulmus pumila forests (reclamation periods of 0, 18, 20 and 22years) situated on RMSs in the Pingshuo opencast coal mine, China. Changes in labile organic C fractions within the soil profiles (0-100cm) were also identified at the 18- or 19-year plots under the three monoculture forests. Our results showed that, SOC and labile organic C fractions, together with soil microbial quotient (SMQ) and C management index (CMI), increased with time since reclamation, indicating that the quality of RMSs improved over time after initial reclamation under the three forest types. R. pseudoacacia significantly increased the accretion of SOC and EOC in the early stage of reclamation while P. tabuliformis accelerated the accumulation of the MBC fraction. Results for U. pumila indicated that this species had a better ability to store C in RMSs 10years or more after reclamation. SOC and labile organic C fractions both had S-shaped distributions within the soil profiles (0-100cm), with the 0-20cm layer recording the highest values (P<0.05). Labile organic C fractions were closely associated and correlated with soil physicochemical properties; our results also showed that nitrogen played an important role in the development of labile organic C fractions. Overall, reclamation accelerated the accretion of both SOC and labile organic C fractions, results of which varied among the reclaimed forests.

Effects of three years of simulated nitrogen deposition on soil nitrogen dynamics and greenhouse gas emissions in a Korean pine plantation of northeast China

Continuously enhanced nitrogen (N) deposition alters the pattern of N and carbon (C) transformations, and thus influences greenhouse gas emissions. It is necessary to clarify the effect of N deposition on greenhouse gas emissions and soil N dynamics for an accurate assessment of C and N budgets under increasing N deposition. In this study, four simulated N deposition treatments (control [CK: no N addition], low-N [L: 20kgNha^-1yr^-1], medium-N [M: 40kgNha^-1yr^-1], and high-N [H: 80kgNha^-1yr^-1]) were operated from 2014. Carbon dioxide, methane and nitrous oxide fluxes were monitored semimonthly, as were soil variables such as temperature, moisture and the concentrations of total dissolved N (TDN), NO₃^-, NO₂^-, NH₄⁺, and dissolved organic N (DON) in soil solutions. The simulated N deposition resulted in a significant increase in TDN, NO₃^- and DON concentrations in soil solutions. The average CO₂ emission rate ranged from 222.6mgCO₂m^-2h^-1 in CK to 233.7mgCO₂m^-2h^-1 in the high-N treatment. Three years of simulated N deposition had no effect on soil CO₂ emission, which was mainly controlled by soil temperature. The mean N₂O emission rate during the whole 3years was 0.02mgN₂Om^-2h^-1 for CK, which increased significantly to 0.05mgN₂Om^-2h^-1 in the high-N treatment. The N₂O emission rate positively correlated with NH₄⁺ concentrations, and negatively correlated with soil moisture. The average CH₄ flux during the whole 3years was -0.74μgCH₄m^-2h^-1 in CK, which increased to 1.41μgCH₄m^-2h^-1 in the low-N treatment. CH₄ flux positively correlated with NO₃^- concentrations. These results indicate that short-term N deposition did not affect soil CO₂ emissions, while CH₄ and N₂O emissions were sensitive to N deposition.