Atmospheric Nitrogen Deposition Research Articles

Climate change could negate U.S. forest ecosystem service benefits gained through reductions in nitrogen and sulfur deposition

Climate change and atmospheric deposition of nitrogen (N) and sulfur (S) impact the health and productivity of forests. Here, we explored the potential impacts of these environmental stressors on ecosystem services provided by future forests in the contiguous U.S. We found that all stand-level services benefitted (+ 2.6 to 8.1%) from reductions in N+S deposition, largely attributable to positive responses to reduced S that offset the net negative effects of lower N levels. Sawtimber responded positively (+ 0.5 to 0.6%) to some climate change, but negatively (− 2.4 to − 3.8%) to the most extreme scenarios. Aboveground carbon (C) sequestration and forest diversity were negatively impacted by all modelled changes in climate. Notably, the most extreme climate scenario eliminated gains in all three services achieved through reduced deposition. As individual tree species responded differently to climate change and atmospheric deposition, associated services unique to each species increased or decreased under future scenarios. Our results suggest that climate change should be considered when evaluating the benefits of N and S air pollution policies on the services provided by U.S. forests.

A Decadal Change in Atmospheric Nitrogen Deposition at a Rural Site in Southern China

A Decadal Change in Atmospheric Nitrogen Deposition at a Rural Site in Southern China

Response patterns of moss to atmospheric nitrogen deposition and nitrogen saturation in an urban–agro–forest transition

Abstract. Increasing trends of atmospheric nitrogen (N) deposition resulting from a large number of anthropogenic emissions of reactive N are dramatically altering the global biogeochemical cycle of N. Nitrogen uptake by mosses occurs mainly from the atmosphere, making it a competent bio-indicator of N deposition. However, high uncertainties exist when using mosses to indicate N deposition, especially in choosing sampling periods and sampling frequencies. In this study, atmospheric N deposition and moss N content in the urban–agro–forest transition, a region with a high N deposition level of 27.46–43.70 kg N hm−2 yr−1, were monitored, and the method for monitoring atmospheric N deposition by mosses was optimized. We found that the optimal sampling frequency is within 6 months per time, and the optimal sampling times are winter (January and February), autumn (October and November), and summer (July and August), which provides us with a more accurate estimation of atmospheric N deposition than other scenarios. In addition, the moss N content serves as a more reliable N species. This study eventually allowed mosses to be used more effectively and sensibly as an indicator of atmospheric N deposition and helped to improve the accuracy of the model for quantifying N deposition.

Driving mechanisms of soil bacterial α and β diversity under long-term nitrogen addition: Subtractive heterogenization based on the environment selection

Soil bacterial α and β diversity patterns under nitrogen (N) addition have been intensively examined, but γ diversity patterns remain largely unknown, especially, the mechanisms that concurrently control changes in α, β and γ diversity remain elusive. Therefore, we formulated a conceptual framework that simultaneously considers candidate drivers including ubiquitous species, rare species, and community assembly processes to elucidate the driving mechanisms of α, β and γ diversity under N addition. The conceptual framework was tested by compiling the sequence data of seven studies published from January 1997 to May 2022 and following the same analysis as our own two long-term multilevel N addition experiments. We demonstrate that subtractive heterogenization based on environment selection simultaneously predicts the changes in α, β and γ diversity under long-term N addition. That is, long-term N addition led to the decline of ubiquitous species (subtractive processes) through low soil pH, and promoted the strength of heterogeneous selection (heterogeneous processes) via enhancing environmental heterogeneity, subsequently causing lower α diversity and γ diversity but higher β diversity. These results mean that N addition may lead to a significant loss of soil bacterial diversity around the world. Together, these findings offer a way to simultaneously predict soil bacterial α, β and γ diversity responses to the ongoing atmospheric nitrogen deposition.

Short-versus long-term effects of nitrogen addition and warming on soil nitrogen mineralization and leaching in a grass-dominated old field.

Increased atmospheric nitrogen (N) deposition and climate warming are both anticipated to influence the N dynamics of northern temperate ecosystems substantially over the next century. In field experiments with N addition and warming treatments, cumulative treatment effects can be important for explaining variation in treatment effects on N dynamics over time; however, comparisons between data collected in the early vs. later years potentially can be confounded with interactions between treatment effects and inter-annual variation in environmental conditions or other factors. We compared the short-term versus long-term effects of N addition and warming on net N mineralization and N leaching in a grass-dominated old field using in situ soil cores. We added new N addition and warming plots (3 years old) to an existing field experiment (16 years old), which enabled comparison of the treatment effects at both time scales while controlling for potential inter-annual variation in other factors. For net N mineralization, there was a significant interaction between plot age and N addition over the growing season, and for extractable inorganic N there was a significant interaction between plot age and warming over winter. In both cases, the directions of the treatment effects differed among old and new plots. Moreover, the responses in the new plots differed from the responses observed previously when the 16-year-old plots had been new. These results demonstrate how inter-annual variation in responses, independent from cumulative treatment effects, can play an important role in interpreting long-term effects on soil N cycling in global change field experiments.

Water status dynamics and drought tolerance of juvenile European beech, Douglas fir and Norway spruce trees as dependent on neighborhood and nitrogen supply.

To increase the resilience of forests to drought and other hazards, foresters are increasingly planting mixed stands. This requires knowledge about the drought response of tree species in pure- and mixed-culture neighborhoods. In addition, drought frequently interacts with continued atmospheric nitrogen (N) deposition. To disentangle these factors for European beech, Norway spruce and Douglas fir, we conducted a replicated three-factorial sapling growth experiment with three moisture levels, (high, medium and low), two N levels (high and ambient) and pure and mixed-culture neighborhoods. We measured biomass, stomatal conductance (GS), shoot water potential (at predawn: ΨPD, midday, and turgor loss point: ΨTLP), branch xylem embolism resistance (Ψ50), and minimum epidermal conductance (Gmin). The three species differed most with respect to Gmin (10-fold higher in beech than in the conifers), hydroscape area (larger in beech), and the time elapsed to reach stomatal closure (TΨGS90) and ΨTLP (TTLP; shorter in beech), while Ψ50 and ΨTLP were remarkably similar. Neighborhood (pure vs. mixed-culture) influenced biomass production, water status and hydraulic traits, notably GS (higher in Douglas fir, but lower in spruce and beech, in mixtures than pure culture), hydraulic safety margin (smaller for beech in mixtures), and TΨGS90 and TTLP (shorter for spruce in mixture). High N generally increased GS, but no consistent N effects on leaf water status and hydraulic traits were detected, suggesting that neighbor identity had a larger effect on plant water relations than N availability. We conclude that both tree neighborhood and N availability modulate the drought response of beech, spruce and Douglas fir. Species mixing can alleviate the drought stress of some species, but often by disadvantaging other species. Thus, our study suggests stabilizing and building resilience of production forests against a drier and warmer climate may depend primarily on the right species choice; species mixing can support the agenda.

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

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

Losses of native mineral-associated organic nitrogen through microbial mineralization and gaseous emissions induced by ammonium and nitrate addition

Atmospheric nitrogen (N) deposition has a significant impact on terrestrial N cycling. However, it has not been well understood how external inorganic N addition affects the dynamics of soil organic N (SON) that supports long-term plant productivity and carbon (C) sequestration. Here, we added six levels of ammonium (NH4+-N) or nitrate (NO3−-N) to a temperate grassland soil to reveal the mechanisms of SON responses to inorganic N addition. Results showed that the SON content decreased by 7% ± 2%–15% ± 1%, which corresponded with the increase in the abundance of functional gene related to SON mineralization (i.e., chiA) by both NH4+-N and NO3−-N. The inorganic N addition stimulated gaseous losses from the native soil N, leading to at least 6% ± 2%–12% ± 1% of the initial SON lost at different N addition levels (i.e., N-priming). However, the responses of nitrification- and denitrification-related functional genes to NH4+-N and NO3−-N treatments varied remarkably. The positive responses of nitrification- and denitrification-related functional genes (i.e., hao, nirS, and narG) to NO3−-N addition contributed to gaseous N losses. The negative responses of some denitrification-related functional genes (i.e., nirK, narG, and napA) to NH4+-N addition resulted in less gaseous N losses by NH4+-N compared with that by NO3−-N addition. Additionally, the release of H+ by NH4+-N addition may liberate organic matter from associated minerals, showing a decrease in mineral-associated organic matter (MAOM) but an increase in extractable Fe. These findings provide direct evidence that the addition of inorganic N results in a net native SON loss through microbe-mediated mineralization and gaseous emissions. The study also emphasizes the different mechanisms of reduced and oxidized N additions in affecting SON mineralization and gaseous N losses.

Increase in Agricultural-Derived NHx and Decrease in Coal Combustion-Derived NOx Result in Atmospheric Particulate N-NH4+ Surpassing N-NO3- in the South China Sea.

The atmospheric deposition of anthropogenic active nitrogen significantly influences marine primary productivity and contributes to eutrophication. The form of nitrogen deposition has been evolving annually, alongside changes in human activities. A disparity arises between observation results and simulation conclusions due to the limited field observation and research in the ocean. To address this gap, our study undertook three field cruises in the South China Sea in 2021, the largest marginal sea of China. The objective was to investigate the latest atmospheric particulate inorganic nitrogen deposition pattern and changes in nitrogen sources, employing nitrogen-stable isotopes of nitrate (δ15N-NO3-) and ammonia (δ15N-NH4+) linked to a mixing model. The findings reveal that the N-NH4+ deposition generally surpasses N-NO3- deposition, attributed to a decline in the level of NOx emission from coal combustion and an upswing in the level of NHx emission from agricultural sources. The disparity in deposition between N-NH4+ and N-NO3- intensifies from the coast to the offshore, establishing N-NH4+ as the primary contributor to oceanic nitrogen deposition, particularly in ocean background regions. Fertilizer (33 ± 21%) and livestock (20 ± 6%) emerge as the primary sources of N-NH4+. While coal combustion continues to be a significant contributor to marine atmospheric N-NO3-, its proportion has diminished to 22 (Northern Coast)-35% (background area) due to effective NOx emission controls by the countries surrounding the South China Sea, especially the Chinese Government. As coal combustion's contribution dwindles, the significance of vessel and marine biogenic emissions grows. The daytime higher atmospheric N-NO3- concentration and lower δ15N-NO3- compared with nighttime further underscore the substantial role of marine biogenic emissions.

Foliar nitrogen uptake in broadleaf evergreen Mediterranean forests: Fertilisation experiment with labelled nitrogen

Atmospheric nitrogen (N) deposition in Mediterranean sclerophyllous forests of Holm oak (Quercus rotundifolia, Q. ilex) in Spain often exceeds empirical critical loads established for ecosystem conservation. There are still uncertainties on the capacity of canopy retention and uptake of the atmospheric N deposited of these forests. Studying and analysing all the forest nitrogen-cycle processes is essential to understand the potential effect of N deposition in these ecosystems. This study conducted a year-long short-term fertilisation experiment with labelled ammonium (15N-NH4) and nitrate (15N-NO3) to estimate foliar N absorption rates and assess the influence of leaf phenology and meteorological seasonal variations. Fertilising solutions were prepared to simulate low and high wet N deposition concentration, based on data reported from previous studies. Additionally, ecophysiological and meteorological measurements were collected to explore potential relationships between absorption rates, plant activity, and weather conditions. The results showed that Holm oak leaves were able to absorb both oxidised and reduced N compounds, with higher rates of NH4+ absorption. N recovery of both NH4+ and NO3− was higher in the low concentration treatments, suggesting reduced effectiveness of absorption as concentration increases. Foliar absorption rates were leaf-age dependent, with the highest values observed in young developing leaves. Foliar uptake showed seasonal changes with a clear reduction during the summer, linked to drought and dry weather conditions, and showing also smaller leaf net assimilation and stomatal conductance. During the rest of the year, foliar N absorption was not clearly associated to plant physiological activity but with environmental conditions. Our findings suggest that Holm oak canopies could absorb an important part of the incoming N deposition, but this process is compound, season and leaf phenology dependent. Further research is therefore needed to better understand and model this part of the N cycle.

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

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

Drought effects on growth and density of temperate tree regeneration under different levels of nitrogen deposition

Temperate forests in Central Europe suffer from climate change-induced productivity and vitality reductions and increased tree mortality. Most field work assessing climate change effects in forests refers to mature trees and does not cover interaction between climate change and nitrogen deposition. Here we show in a study of 54 forest sites representing different combinations of climatic conditions and atmospheric nitrogen deposition across Germany that nitrogen significantly affects the drought tolerance of tree regeneration in the field. We compared shoot length increment and regeneration density of Central Europe’s naturally most dominant tree species, European beech (Fagus sylvatica) with three species, which are discussed as potentially more drought-tolerant replacement tree species for climate change adaptation of forestry. Growth of beech was reduced with increasing nitrogen deposition identifying high reactive nitrogen loads as an additional threat for this species, but between-site variation of beech growth was not dependent on climatic parameters despite growth reductions after the drought summer 2018 across all study sites. Remarkably, the only species in the between-site comparisons where low growth was attributable to dry climate was Douglas fir (Pseudotsuga menziesii), as shoot length increment was reduced at sites with dry spring climate. Silver fir (Abies alba) showed increasing growth with increasing nitrogen deposition in combination with increasing drought, probably due to reduced ammonium uptake. Shoot length increment in sessile oak (Quercus petraea) was not affected by the between-site variation of either drought or nitrogen. Our results (combined with results of regeneration density) indicate that high atmospheric nitrogen deposition, which is primarily found in regions with intensive livestock farming in Central Europe, modifies the climate change response of temperate tree species. Interaction of drought and nitrogen deposition should thus be addressed in the debate on climate change adaptation of forestry.

Nitrogen addition increased resistance of resident plant community to Solidago canadensis invasion by altering allelopathic effect

Abstract Allelopathy plays an important role in the interaction between invasive and resident plants. Atmospheric nitrogen (N) deposition has become a global problem, but it is unclear whether N enrichment affects the interaction between invasive and resident plants by affecting their allelopathy. Thus, we performed a greenhouse experiment in which the resident plant community was grown under two levels of invasion by S. canadensis (invasion vs. no invasion) and fully crossed with two levels of allelopathy (with or without adding activated carbon) and two levels of N addition (with or without). The resident plant communities were constructed with eight herbaceous species that often co-occur with S. canadensis. The results showed that both allelopathy of S. canadensis and the resident plants had obvious positive effects on their own growth. Nitrogen addition had more obvious positive effects on the resident plants under invasion than those that were not invaded. Moreover, N addition also altered the allelopathy of resident plants. Specifically, N addition improved the allelopathy of resident plants when they were invaded but decreased the allelopathy of resident plants when they grew alone. Although nitrogen addition had no obvious effect on S. canadensis, it reduced the allelopathy of S. canadensis. These results suggest that N addition could improve the resistance of resident plants to invasion by improving the allelopathy of resident plants and reducing the allelopathy of S. canadensis. These findings provide a scientific basis to manage and control the S. canadensis invasion.

Climate warming impacts chewing Spodoptera litura negatively but sucking Corythucha marmorata positively on native Solidago canadensis

Insect-plant interactions are among importantly ecological processes, and rapid environmental changes such as temperature and resource fluctuations can disrupt long-standing insect-plant interactions. While individual impacts of climate warming, atmospheric nitrogen (N) deposition, and plant provenance on insect-plant interactions are well studied, their joint effects on insect-plant interactions are less explored in ecologically realistic settings. To this end, we performed five experiments with native and invasive Solidago canadensis populations from home and introduced ranges and two insect herbivores (leaf-chewing Spodoptera litura and sap-sucking Corythucha marmorata) in the context of climate warming and N deposition. We determined leaf defensive traits, feeding preference, and insect growth and development, and quantified the possible associations among climate change, host-plant traits, and insect performance with structural equation modeling. First, native S. canadensis populations experienced higher damage by S. litura but lower damage by C. marmorata than invasive S. canadensis populations in the ambient environment. Second, warming decreased the leaf consumption, growth, and survival of S. litura on native S. canadensis populations, but did not affect these traits on invasive S. canadensis populations; warming increased the number of C. marmorata on native S. canadensis populations via direct facilitation, but decreased that on invasive S. canadensis populations via indirect suppression. Third, N addition enhanced the survival of S. litura on native S. canadensis populations, and its feeding preference and leaf consumption on invasive S. canadensis populations. Finally, warming plus N addition exhibited non-additive effects on insect-plant interactions. Based on these results, we tentatively conclude that climate warming could have contrasting effects on insect-plant interactions depending on host-plant provenance and that the effects of atmospheric N deposition on insects might be relatively weak compared to climate warming. Future studies should focus on the molecular mechanisms underlying these different patterns.

Shifts of arbuscular mycorrhizal fungal functioning along a simulated nitrogen deposition gradient

AbstractThe deposition of atmospheric nitrogen can significantly boost the amount of nitrogen available in various ecosystems, potentially altering the mutualistic association between arbuscular mycorrhizal fungi (AMF) and their host plants. Nevertheless, the precise mechanisms and the degree to which externally induced nitrogen‐related changes in AMF functionality might impact Sorghum bicolor (L.) Moench, a plant known for its high mycorrhizal colonization, remains unclear. In this study, the mycorrhizal response affected by environmental N enrichment was addressed by conducting a glasshouse experiment, and four fertilization treatments (N1, N2, N3, and N4, 0, 15, 30, and 60 kg N hm−1 a−1, respectively) were used to simulate N deposition differences over the mycorrhizal response. The changes in mycorrhizal colonization and plant variables during different AMF and N fertilizer applications were investigated. When the gradient's nitrogen levels increased, the mycorrhizal growth response and mycorrhizal nitrogen response showed a pattern of first dropping and then increasing. N‐induced changes in the mycorrhizal response were associated with vesicular colonization, arbuscular colonization, and root‐length colonization. The variation in the mycorrhizal response over the N concentration gradient highlights the critical role of AMF in agroecosystems.

Nitrogen deposition-induced stimulation of soil heterotrophic respiration is counteracted by biochar in a subtropical forest

Both atmospheric nitrogen (N) deposition and biochar application can markedly impact soil heterotrophic respiration (RH), an important component of the global carbon cycle. However, the interactive effects of N deposition and biochar application on soil RH in subtropical forest ecosystems remain unclear. Here, we conducted a three-year (2019–2022) field trial within a bamboo forest in subtropical China to examine the responses of soil physicochemical and microbial properties to N deposition and biochar application, and to elucidate how biochar regulates N deposition-induced change in soil RH. Nitrogen deposition stimulated soil RH by 8.1–9.8 % annually over three years compared to the control, and this stimulation was mitigated (by 8.1–8.9 % annually) with biochar addition. In the context of N deposition, the decrease of soil RH by biochar application was not through changing soil temperature, moisture or labile organic carbon content. Biochar treatment reduced the abundances of bacterial glycoside hydrolase family 48 gene (GH48) and fungal glycoside hydrolase family 7 cellobiohydrolase I gene (cbhI) and the activities of β-glucosidase and cellobiohydrolase (CBH), but increased the abundance of cbbL gene and activity of RubisCO enzyme. Furthermore, the RH was correlated positively (P < 0.01) with β-glucosidase and CBH activities and negatively (P < 0.01) with RubisCO enzyme activity. Structural equation modeling revealed that the biochar-induced reduction of soil RH under N deposition was associated with decreases in the abundance of cbhI gene and the activity of CBH in soils. We highlight that management practices can mitigate soil carbon loss in forests through modulating soil microbial functions under atmospheric N deposition, and that biochar application in Moso bamboo forests has the potential to reduce RH by approximately 7.6 × 106 t CO2 yr−1 under atmospheric N deposition.

Nitrogen addition does not alter symmetric responses of soil respiration to changing precipitation in a semi-arid grassland

Concurrent changing precipitation regimes and atmospheric nitrogen (N) deposition can have profound influences on soil carbon (C) cycling. However, how N enrichment regulates the responses of soil C fluxes to increasing variability of precipitation remains elusive. As part of a field precipitation gradient experiment with nine levels of precipitation amounts (−60 %, −45 %, −30 %, −15 %, ambient precipitation, +15 %, +30 %, +45 %, and +60 %) and two levels of N addition (0 and 10 g N m−2 yr−1) in a semi-arid temperate steppe on the Mongolian Plateau, this work was conducted to investigate the responses of soil respiration to decreased and increased precipitation (DP and IP), N addition, and their possible interactions. Averaged over the three years from 2019 to 2021, DP suppressed soil respiration by 16.1 %, whereas IP stimulated it by 27.4 %. Nitrogen addition decreased soil respiration by 7.1 % primarily via reducing microbial biomass C. Soil respiration showed symmetric responses to DP and IP within all the four precipitation variabilities (i.e., 15 %, 30 %, 45 %, and 60 %) under ambient N. Nevertheless, N addition did not alter the symmetric responses of soil respiration to changing precipitation due to the comparable sensitivities of microbial biomass and root growth to DP and IP under the N addition treatment. These findings indicate that intensified precipitation variability does not change but N addition could alleviate soil C releases. The unchanged symmetric responses of soil respiration to precipitation variability under N addition imply that N deposition may not change the response pattern of soil C releases to predicted increases in precipitation variability in grasslands, facilitating the robust projections of ecosystem C cycling under future global change scenarios.

Geographic variation in projected US forest aboveground carbon responses to climate change and atmospheric deposition

Forest composition and ecosystem services are sensitive to anthropogenic pressures like climate change and atmospheric deposition of nitrogen (N) and sulfur (S). Here we extend recent forest projections for the current cohort of trees in the contiguous US, characterizing potential changes in aboveground tree carbon at the county level in response to varying mean annual temperature, precipitation, and N and S deposition. We found that relative to a scenario with N and S deposition reduction and no climate change, greater climate change led generally to decreasing aboveground carbon (mean −7.5% under RCP4.5, −16% under RCP8.5). Keeping climate constant, reduced N deposition tended to lessen aboveground carbon (mean −7%), whereas reduced S deposition tended to increase aboveground carbon (+3%) by 2100. Through mid-century (2050), deposition was more important for predicting carbon responses except under the extreme climate scenarios (RCP8.5); but, by 2100, climate drivers generally outweighed deposition. While more than 70% of counties showed reductions in aboveground carbon relative to the reference scenario, these were not evenly distributed across the US. Counties in the Northwest and Northern Great Plains, and the northern parts of New England and the Midwest, primarily showed positive responses, while counties in the Southeast showed negative responses. Counties with greater initial biomass showed less negative responses to climate change while those which exhibited the greatest change in composition (>15%) had a 95% chance of losing carbon relative to a no-climate change scenario. This analysis highlights that declines in forest growth and survival due to increases in mean temperature and reductions in atmospheric N deposition are likely to outweigh positive impacts of reduced S deposition and potential increases in precipitation. These effects vary at the regional and county level, however, so forest managers must consider local rather than national dynamics to maximize forest carbon sinks in the future.

Biochar mediates nitrogen investment strategy involved in Chinese fir growth as revealed by molecular information on soil dissolved organic matter

Soil dissolved organic matter (DOM) is the most reactive pool of soil organic matter. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) is a powerful tool for obtaining molecular-level DOM information. The ongoing atmospheric nitrogen (N) deposition could cause some negative effects on tropical and subtropical forest ecosystems, and biochar is considered to be an effective soil ameliorator. However, few studies have evaluated the effects of biochar and N deposition, either separately or in combination, on the molecular composition of soil DOM. In this study, we conducted a pot experiment using Chinese fir seedlings amended with biochar (0, 12, and 36 t biochar ha−1, prepared from corn stover at 450 °C) and inorganic N (NH4NO3: 0, 40, and 80 kg N ha−1 y-1) in a full factorial design (four replications per treatment) and characterized soil DOM using FT-ICR-MS. A total of 6084 types of formulas were identified in this study. All 36 samples had 2109 types of formulas in common, which had much lower N/C and P/C ratios than the bulk DOM samples. Permutation-based multivariate analysis of variance revealed that both N addition and biochar application had a significant impact on DOM chemodiversity. N addition increased the relative abundance of lignins and lipids, which may be a way for Chinese fir to acclimate to an acidic environment by expanding roots and fixing N. Biochar amendment increased the relative abundance of carbohydrates and tannins in soil DOM at each N application level. Soil pH is a key soil variable controlling DOM composition, and an increase in pH after biochar amendment contributed greatly to the occurrence of DOM compounds with an O/C > 0.5, which are easily degradable. In particular, lignins associated with low N/C were more abundant at higher pH. Biochar application also improved the efficiency of roots in acquiring nutrients, reduced the relative biomass of roots, and decreased the relative content of lignins or lipids, which primarily originated from the roots. In summary, biochar amendment effectively alleviated the negative effects of N addition by investing more N into the production of easily degradable DOM compounds and stimulating the biogeochemical cycle of N.

Contrasting Effects of Nitrogen Deposition and Phosphorus Addition on Soil Organic Carbon in a Subtropical Forest: Physical Protection versus Chemical Stability

Soil organic carbon (SOC) not only contributes to maintain soil health, but is also important in regulating global climate change. How atmospheric nitrogen (N) deposition and phosphorus (P) addition affects SOC dynamics remains unclear, especially in subtropical forests. The response of SOC in three layers to N deposition and P addition in this study is estimated by analyzing the soil aggregates and C chemical stability composition fertilized with N (100 kg N hm−2 a−1) and/or P (50 kg P hm−2 a−1) over 9 years in a Chinese fir (Cunninghamia lanceolata) plantation. Treatments involving N deposition increased the SOC concentration, while P addition alone decreased the SOC concentration in soil layers above 10 cm. The addition of N significantly increased the mean diameter of topsoil aggregates, macroaggregates SOC concentration, and the contribution of N to total SOC. P addition decreased the relative abundances of aromatic and aliphatic functional groups while decreasing the chemical stability of SOC in the topsoil. A structural equation model indicated that N deposition promoted SOC concentration by mainly improving the physical protection of soil aggregates, while P addition reduced SOC sequestration by decreasing the chemical stability of SOC. Our research suggested that elevated N deposition might promote the soil C sink, while P fertilization would not be recommended under increased N deposition to protect soil C storage in subtropical forests.