Spatial boundary of urban 'acid islands' in southern China

Review of health effects driven by aerosol acidity: Occurrence and implications for air pollution control

Acidity, generally expressed as pH, plays a crucial role in atmospheric processes and ecosystem evolution. Atmospheric acidic aerosol, triggering severe air pollution in the industrialization process (e.g., London Great Smoke in 1952), has detrimental effects on human health. Despite global endeavors to mitigate air pollution, the variation of aerosol acidity remains unclear and further restricts the knowledge of the acidity-driven toxicity of fine particles (PM2.5) in the atmosphere. Here, we summarize the toxicological effects and mechanisms of inhalable acidic aerosol and its response to air pollution control. The acidity could adjust toxic components (e.g., metals, quinones, and organic peroxides) bonded in aerosol and synergize with oxidant gaseous pollutants (e.g., O3 and NO2) in epithelial lining fluid to induce oxidative stress and inflammation. The inhaled aerosol from the ambient air with higher acidity might elevate airway responsiveness and cause worse pulmonary dysfunction. Furthermore, historical observation data and model simulation indicate that PM2.5 can retain its acidic property despite considerable reductions in acidifying gaseous pollutants (e.g., SO2 and NOx) from anthropogenic emissions, suggesting its continuing adverse impacts on human health. The study highlights that aerosol acidity could partially offset the health benefits of emission reduction, indicating that acidity-related health effects should be considered for future air pollution control policies.

Distinct urban-rural gradients of air NO2 and SO2 concentrations in response to emission reductions during 2015-2022 in Beijing, China

Nitrogen dioxide (NO2) and sulfur dioxide (SO2) are two major air pollutants in urban environment. Emission reduction policies have thus been implemented to improve urban air quality, especially in the metropolises. However, it remains unclear whether the air concentrations of NO2 and SO2 in and around large cities follow a same spatial pattern and how their characteristics change over time in response to the emission reductions. Using ground-based monitoring datasets of air NO2 and SO2 concentrations in Beijing, China, we tested the hypothesis of urban air pollutant islands and evaluated their seasonal and inter-annual variations during 2015-2022. The results showed that air NO2 concentrations increased significantly towards the urban core, being in line with the hypothesis of urban air pollutant island, while air SO2 concentrations showed no such spatial patterns. The urban air NO2 island varied seasonally, with larger radius and higher air NO2 concentrations in spring and winter. In response to the emission reduction, the annual mean radius of the urban air NO2 island showed a rapid decrease from 45.8 km to zero km during the study period. The annual mean air NO2 concentration at the urban core showed a linear decrease at a rate of 4.5 μg m-3 yr-1. In contrast, air SO2 concentration decreased nonlinearly over time and showed a legacy in comparison to the emission reduction. Our findings suggest different urban-rural gradients of air NO2 and SO2 concentrations and highlight their distinct responses to the regional reductions of anthropogenic emissions.

Wet nitrogen (N) deposition to urban Latin America: filling in the gaps with GEOS-Chem

In Latin America, atmospheric deposition is a major vector of nitrogen (N) input to urban systems. Yet, measurements of N deposition are sparse, precluding analysis of spatial patterns, temporal trends, and ecosystem impacts. Chemical transport models can be used to fill these gaps in the absence of dense measurements. Here, we evaluate the performance of a global 3-D chemical transport model in simulating spatial and interannual variation in wet inorganic N (NH₄-N + NO₃-N) deposition across urban areas in Latin America. Monthly wet and dry inorganic N deposition to Latin America were simulated for the period 2006-2010 using the GEOS-Chem Chemical Transport Model. Published estimates of observed wet or bulk inorganic N deposition measured between 2006-2010 were compiled for 16 urban areas and then compared with model output from GEOS-Chem. Observed mean annual inorganic N deposition to the urban study sites ranged from 5.7-14.2 kg ha^-1 yr^-1, with NH₄-N comprising 48-90% of the total. Results show that simulated N deposition was highly correlated with observed N deposition across sites (R² = 0.83, NMB = -50%). However, GEOS-Chem generally underestimated N deposition to urban areas in Latin America compared to observations. Underestimation due to bulk sampler dry deposition artifacts was considered and improved bias without improving correlation. In contrast to spatial variation, the model did not capture year-to-year variation well. Discrepancies between modeled and observed values exist, in part, because of uncertainties in Latin American N emissions inventories. Our findings indicate that even at coarse spatial resolution, GEOS-Chem can be used to simulate N deposition to urban Latin America, improving understanding of regional deposition patterns and potential ecological effects.

Morphological, Physiological and Photophysiological Responses of Critically Endangered Acer catalpifolium to Acid Stress

Acid rain deposition (AR) has long-lasting implications for the community stability and biodiversity conservation in southwest China. Acer catalpifolium is a critically endangered species in the rain zone of Western China where AR occurs frequently. To understand the effects of AR on the morphology and physiology of A. catalpifolium, we conducted an acid stress simulation experiment for 1.5 years. The morphological, physiological, and photosynthetic responses of A. catalpifolium to the acidity, composition, and deposition pattern of acid stress was observed. The results showed that simulated acid stress can promote the growth of A. catalpifolium via the soil application mode. The growth improvement of A. catalpifolium under nitric-balanced acid rain via the soil application mode was greater than that of sulfuric-dominated acid rain via the soil application mode. On the contrary, the growth of A. catalpifolium was significantly inhibited by acid stress and the inhibition increased with the acidity of acid stress applied via leaf spraying. The inhibitory impacts of nitric-balanced acid rain via the leaf spraying of A. catalpifolium were greater than that of sulfur-dominant acid rain via leaf spraying. The observations presented in this work can be utilized for considering potential population restoration plans for A. catalpifolium, as well as the forests in southwest China.

Ambient urban N deposition drives increased biomass and total plant N in two native prairie grass species in the U.S. Southern Great Plains

Remnants of native tallgrass prairie experience elevated atmospheric nitrogen (N) deposition in urban areas, with potential effects on species traits that are important for N cycling and species composition. We quantified bulk (primarily wet) inorganic N (NH₄⁺-N + NO₃^--N) deposition at six sites along an urban development gradient (6–64% urban) in the Dallas-Fort Worth metropolitan area from April 2014 to October 2015. In addition, we conducted a phytometer experiment with two common native prairie bunchgrass species––one well studied (Schizachyrium scoparium) and one little studied (Nasella leucotricha)––to investigate ambient N deposition effects on plant biomass and tissue quality. Bulk inorganic N deposition ranged from 6.1–9.9 kg ha^-1 yr^-1, peaked in spring, and did not vary consistently with proportion of urban land within 10 km of the sites. Total (wet + dry) inorganic N deposition estimated using bulk deposition measured in this study and modeled dry deposition was 12.9–18.2 kg ha^-1 yr^-1. Although the two plant species studied differ in photosynthetic pathway, biomass, and tissue N, they exhibited a maximum 2-3-fold and 2-4-fold increase in total biomass and total plant N, respectively, with 1.6-fold higher bulk N deposition. In addition, our findings indicate that while native prairie grasses may exhibit a positive biomass response to increased N deposition up to ~18 kg ha^-1 yr^-1, total inorganic N deposition is well above the estimated critical load for herbaceous plant species richness in the tallgrass prairie of the Great Plains ecoregion and thus may negatively affect these plant communities.

Higher soil acidification risk in southeastern Tibetan Plateau

Stable soil pH is a key property in maintaining an ecosystem's structure, function, and sustainability. Increasing atmospheric deposition and grassland use on the Tibetan Plateau (TP) may increase the soil acidification risk, but we lack such information to date. Here, we evaluated the soil acidification risk in the TP, by comparing it with that in the Mongolia Plateau (MP) and applying the acid-base balance principles on atmospheric inputs, soils, and plants from 1980 to 2019. Cumulative acid input was lower in the TP than in the MP. Sulfur contributed more to acidity than nitrogen and atmospheric deposition contributed more to acidity than grassland use. Acid input was mainly influenced by local industry, animal husbandry and transportation in the MP, while in the TP it was also affected by the long-distance transportation of pollutants from South Asia and southern China. Overall, the TP was less acid-sensitive than the MP because of higher inorganic carbon content. However, soils in the southeastern TP, covering 21% of the total area, were acid-sensitive due to low levels of soil exchangeable base cation (EBCs) and lack of calcium carbonate. Coincidentally, the southeastern region has the highest concentration of acid input in the TP due to more rapid development and stronger influence of adjacent high acid deposition regions than others. Therefore, the acidification risk to the southeastern region is much higher than to other regions of the TP and the MP; in this region, the EBCs are likely to be depleted approximately 95 years earlier than in the MP. The findings of this study provide insights into the response of the TP to global change. For the ecosystem sustainability of southeastern TP, control of atmospheric acid deposition, especially sulfur deposition, in both local and adjacent regions and nations is required.

The Additions of Nitrogen and Sulfur Synergistically Decrease the Release of Carbon and Nitrogen from Litter in a Subtropical Forest

Atmospheric nitrogen (N) and sulfur (S) deposition in subtropical forests has increased rapidly and the current level is very high, thus seriously affecting nutrient (e.g., N and phosphorus (P)) release from litter. However, the specific effects of S addition and its interaction with N on the release of carbon (C), N, and P from litter in subtropical evergreen broadleaved forests are unclear. Therefore, a two-year field experiment was performed using a litterbag method in a subtropical evergreen broadleaved forest in western China to examine the responses of litter decomposition and nutrient release to the control (CK), added N (+N), added S (+S), and added N and S (+NS) treatments. The results showed that the remaining litter mass, lignin, cellulose, C, N, P, and litter N/P ratio were higher, whereas the litter C/N ratio and soil pH were lower in the fertilization treatments than in CK. The annual decomposition coefficients (k-values) in the +N, +S, and +NS treatments were 0.384 ± 0.002, 0.378 ± 0.002, and 0.374 ± 0.001 year−1, respectively, which were significantly lower than the k-values in CK (0.452 ± 0.005 year−1, p < 0.05). The remaining mass, lignin, cellulose, C, and litter N/P ratio were higher, whereas the soil pH was lower in the +NS treatment than in the +N and +S. The interactive effects of N addition and S addition on the remaining litter lignin, cellulose, C, N, and P; the litter C/N, C/P, and N/P ratios; and the soil pH were significant (p < 0.05). In conclusion, the addition of N and S synergistically decreased the degradation of lignin and cellulose and the release of C and N and increased the litter N/P ratio, suggesting that external N and S inputs synergistically slowed the release of C and N from litter and exacerbated litter P limitation during decomposition in this forest.

Municipal solid waste incineration fly ash exposed to carbonation and acid rain corrosion scenarios: Release behavior, environmental risk, and dissolution mechanism of toxic metals

This study investigated the leaching behavior, environmental risk, and dissolution mechanism of toxic metals (TMs) in solidified/stabilized municipal solid waste incineration fly ash (MSWI FA) exposed to alternative "carbonation + acid rain corrosion" disposal scenarios. The content of TMs (mg/kg) showed a trend of Zn (12,187.10 ± 168.60) > Pb (3374.43 ± 66.12) > Cu (1055.14 ± 32.52) > Cr (127.95 ± 8.12) > Cd (119.05 ± 6.26) > Ni (49.50 ± 3.20). Initial leaching of CO₂-saturated water (CSW) and replacement of simulated acid rain (SAR) increased the environmental risk of leached TMs. The results of "average release rate" (mg/(kg·d)) of TMs indicated that Zn (0.8307)/Cu (0.0278)/Cd (0.0109) and Cu (0.0581)/Cr (0.001176)/Ni (0.004339) in phosphoric acid stabilized FA and Pb (0.0753)/Cr (0.001921)/Ni (0.00111) and Pb (0.0656)/Zn (1.0560)/Cd (0.0050) in Portland cement solidified FA were the key "problem TMs" during carbonation and acid rain corrosion, respectively. CSW leaching increased the independent environmental risk of most TMs in residual FA (especially Zn/Cd) due to the increased carbonate-bound fraction. Compared with independent carbonation, alternative "carbonation + acid rain corrosion" contributed to a higher comprehensive environmental risk for TMs in residual FA. CSW leaching system was an indirect carbonation based on CO₂-water and FA matrix, in which "nucleation and dissolution" of carbonates and "immobilization and dissolution" of TMs coexisted. The dissolution mechanism of TMs was mainly controlled by reaction equilibrium of nucleation and dissolution of carbonates containing TMs. Dissolution and nucleation were the dominant mechanism in the early and later periods of CSW leaching, respectively. Carbonate layer dissolution, H⁺ corrosion/displacement, and counter-ion effect (SO₄^2- > NO₃^- > Cl^-) were the main mechanisms affecting TM dissolution during SAR leaching.

Reduced Lignin Decomposition and Enhanced Soil Organic Carbon Stability by Acid Rain: Evidence from 13C Isotope and 13C NMR Analyses

Due to the emissions of air pollutants, acid rain in southern China poses a great threat to terrestrial ecosystems. However, its influences on ecological processes such as litter decomposition and soil organic carbon (SOC) accumulation are still not clear. The aim of this study was to understand the potential mechanisms of carbon sequestration change in response to long-term acid rain in a subtropical forest. A field experiment with simulated acid rain (SAR) treatment was conducted in a monsoon evergreen broadleaf forest in southern China. Four levels of SAR treatment were implemented by irrigating the plots with water of different pH values (4.5 as a control, 4.0, 3.5, and 3.0). The results showed that the concentration of SOC and recalcitrant index for the SAR pH = 3.0 treatment were significantly higher compared to the control. Lignin fractions in litter residue layers were significantly increased, while soil microbial biomass carbon and soil ligninolytic enzyme activities were reduced under the SAR treatment. The concentration of SOC and recalcitrant index had positive relationships with the litter residue lignin fraction, but negative relationships with soil ligninolytic enzyme activity. These findings indicate that soil carbon accumulation could be enhanced with more stable lignin input under prolonged acid rain in forest ecosystems in southern China.

Leaching behavior and potential ecological risk of heavy metals in Southwestern China soils applied with sewage sludge compost under acid precipitation based on lysimeter trials

The ecological risk of heavy metals (HM) resulting from the use of sewage sludge compost (SSC) as an amendment to flower garden soil (FGS) and to abandoned phosphate mine soil (APMS) influenced by acid rain were simulated in lysimeter trials and the potential ecological risk index (PERI) was evaluated with minor modifications. The use of SSC indeed increased the mobility and release of HMs in FGS and APMS under conditions of acid rain. The leaching dynamics of HMs was found to be influenced by Fe/Al oxides and organic matter (OM) in the soil. The application of SSC as a fertilizer to barren APMS dramatically decreased the mobility of Cr, Cu and Pb by 51-56% due to their retention by particulate organic matter, while the leaching of As, Cd and Ni was increased as the result of competition with OM for available Fe/Al oxides (As) and proton-metal exchange reactions that occurred in HM-OM complexes (Cd and Ni). The ecological risk of FGS and APMS resulting from HM migration was actually low (PERI = 0.07-0.12), but the increased potential ecological risk resulting from the use of SSC were estimated to be moderate (a 16.0-33.5% increase in PERI for SSC-amended FGS) or high (a 140% increase in PERI for SSC-amended APMS). Ni, Cd and Cu were identified as the three main HMs responsible for increasing the ecological risk in soil which was mainly composed of fine-grained particles, whereas Cd and As were key ecological risks HMs in soil that was mainly composed of coarse-grained particles.

Sulfur deposition still contributes to forest soil acidification in the Pearl River Delta, South China, despite the control of sulfur dioxide emission since 2001

Sulfur dioxide emissions have been regulated at a global scale; sulfur (S) deposition no longer contributes to soil acidification instead of an alleviation effect in temperate regions; however, it remains unclear whether S deposition still contributes to soil acidification in the tropics. The Pearl River Delta (PRD), South China, has been suffering serious soil acidification, but the contribution of S deposition was ignored because of the regulation of S emission since 2001. Here, we chose the evergreen broadleaf forests, which are the typical forest type at the regional scale in PRD to examine the contribution of S deposition and its characteristics in this acidification, based on an established urban-rural gradient in the range of 260 km. A substantial acidification was evidenced by the significant decline of soil pH from rural to urban sites, with mean pH values decreased by more than 0.60 U through the whole 40-cm depths. However, there was no significant difference in soil pH from 0-10 cm, 10-20 cm, and to 20-40 cm at each site (P > 0.05). Acid-neutralizing capacity (ANC) showed a similar trend to soil pH, with a significant decline along the urbanization gradient and no significant effect of soil depths. Soil sulfate (SO₄^2-), as the most abundant species in ANC, contributed greatly to soil acidification for the whole 40-cm depth, as shown by the significant positive relationships between it with soil pH and base cations. Soils also exhibited the depletion of base cations with low base saturation (< 20%) and the release of Al and Fe. Our research demonstrated that the severe soil acidification in the PRD region has extended to the subsoil level (40-cm depth), and S deposition is still an important driver to this acidification. Therefore, both recovering the acidified soils and controlling the acidifying pollutants, especially S, are particularly difficult in southern China.

Responses of forest ecosystems to increasing N deposition in China: A critical review

China has been experiencing a rapid increase in nitrogen (N) deposition due to intensified anthropogenic N emissions since the late 1970s. By synthesizing experimental and observational data taken from literature, we reviewed the responses of China's forests to increasing N deposition over time, with a focus on soil biogeochemical properties and acidification, plant nutrient stoichiometry, understory biodiversity, forest growth, and carbon (C) sequestration. Nitrogen deposition generally increased soil N availability and soil N leaching and decreased soil pH in China's forests. Consequently, microbial biomass C and microbial biomass N were both decreased, especially in subtropical forests. Nitrogen deposition increased the leaf N concentration and phosphorus resorption efficiency, which might induce nutrient imbalances in the forest ecosystems. Although experimental N addition might not affect plant species richness in the overstorey, it did significantly alter species composition of understory plants. Increased N stimulated tree growth in temperate forests, but this effect was weak in subtropical and tropical forests. Soil respiration in temperate forests was non-linearly responsive to N additions, with an increase at dosages of <60 kg N ha^-1 yr^-1 and a decrease at dosages of >60 kg N ha^-1 yr^-1. However, it was consistently decreased by increased N inputs in subtropical and tropical forests. In light of future trends in the composition (e.g., reduced N vs. oxidized N) and the loads of N deposition in China, further research on the effects of N deposition on forest ecosystems will have critical implications for the management strategies of China's forests.

A database of annual atmospheric acid and nutrient deposition to China's forests

Anthropogenic emissions have substantially altered atmospheric acid and nutrient deposition in China. Understanding the status and characteristics of acid and nutrient deposition to China’s forests is crucial to assess the consequent impacts, and to better guide forest management options. Based on a comprehensive literature review, here I present an updated database for annual acid and nutrient deposition during the period 1991-2015 in China’s forests (CFAND 2.0). The database includes information from 56 forested sites on the water fluxes of bulk precipitation and throughfall, the concentrations of H⁺ (pH), dissolved inorganic nitrogen (N) (NH₄⁺ and NO₃⁻), sulfur (S), dissolved phosphorus (P), potassium (K⁺), calcium (Ca²⁺) and magnesium (Mg²⁺) in bulk precipitation and throughfall, and the fluxes of dissolved inorganic N, S, dissolved P, K⁺, Ca²⁺ and Mg²⁺ in bulk deposition and throughfall. This database will help to understand the spatial patterns of acid and nutrient deposition, validate modelling results of acid and nutrient deposition and assess the ecological effects of acid and nutrient deposition in China’s forests.

Effects of elevated ozone concentration and nitrogen addition on ammonia stomatal compensation point in a poplar clone

The stomatal compensation point of ammonia (χ_s) is a key factor controlling plant-atmosphere NH₃ exchange, which is dependent on the nitrogen (N) supply and varies among plant species. However, knowledge gaps remain concerning the effects of elevated atmospheric N deposition and ozone (O₃) on χ_s for forest species, resulting in large uncertainties in the parameterizations of NH₃ incorporated into atmospheric chemistry and transport models (CTMs). Here, we present leaf-scale measurements of χ_s for hybrid poplar clone '546' (Populusdeltoides cv. 55/56 x P. deltoides cv. Imperial) growing in two N treatments (N0, no N added; N50, 50 kg N ha^-1 yr^-1 urea fertilizer added) and two O₃ treatments (CF, charcoal-filtered air; E-O₃, non-filtered air plus 40 ppb) for 105 days. Our results showed that χ_s was significantly reduced by E-O₃ (41%) and elevated N (19%). The interaction of N and O₃ was significant, and N can mitigate the negative effects of O₃ on χ_s. Elevated O₃ significantly reduced the light-saturated photosynthetic rate (A_sat) and chlorophyll (Chl) content and significantly increased intercellular CO₂ concentrations (Ci), but had no significant effect on stomatal conductance (g_s). By contrast, elevated N did not significantly affect all measured photosynthetic parameters. Overall, χ_s was significantly and positively correlated with A_sat, g_s and Chl, whereas a significant and negative relationship was observed between χ_s and Ci. Our results suggest that O₃-induced changes in A_sat, Ci and Chl may affect χ_s. Our findings provide a scientific basis for optimizing parameterizations of χ_s in CTMs in response to environmental change factors (i.e., elevated N deposition and/or O₃) in the future.

Direct effect of acid rain on leaf chlorophyll content of terrestrial plants in China

Anthropogenic emissions of acid precursors in China have resulted in widespread acid rain since the 1980s. Although efforts have been made to assess the indirect, soil mediated ecological effects of acid rain, a systematic assessment of the direct foliage injury by acid rain across terrestrial plants is lacking. Leaf chlorophyll content is an important indicator of direct foliage damage and strongly related to plant productivity. We synthesized data from published literature on experiments of simulated acid rain, by directly exposing plants to acid solutions with varying pH levels, to assess the direct effect of acid rain on leaf chlorophyll content across 67 terrestrial plants in China. Our results indicate that acid rain substantially reduces leaf chlorophyll content by 6.71% per pH unit across the recorded plant species. The direct reduction of leaf chlorophyll content due to acid rain exposure showed no significant difference across calcicole, ubiquist or calcifuge species, implying that soil acidity preference does not influence the sensitivity to leaf injury by acid rain. On average, the direct effects of acid rain on leaf chlorophyll on trees, shrubs and herbs were comparable. The effects, however varied across functional groups and economic use types. Specifically, leaf chlorophyll content of deciduous species was more sensitive to acid rain in comparison to evergreen species. Moreover, vegetables and fruit trees were more sensitive to acid rain than other economically used plants. Our findings imply a potential production reduction and economic loss due to the direct foliage damage by acid rain.

Development of atmospheric acid deposition in China from the 1990s to the 2010s

Atmospheric acid deposition is a global environmental issue. China has been experiencing serious acid deposition, which is anticipated to become more severe with the country's economic development and increasing consumption of fossil fuels in recent decades. We explored the spatiotemporal variations of acid deposition (wet acid deposition) and its influencing factors by collecting nationwide data on pH and concentrations of sulfate (SO₄^2-) and nitrate (NO₃^-) in precipitation between 1980 and 2014 in China. Our results showed that average precipitation pH values were 4.59 and 4.70 in the 1990s and 2010s, respectively, suggesting that precipitation acid deposition in China has not seriously worsened. Average SO₄^2- deposition declined from 40.54 to 34.87 kg S ha^-1 yr^-1 but average NO₃^- deposition increased from 4.44 to 7.73 kg N ha^-1 yr^-1. Specifically, the area of severe precipitation acid deposition in southern China has shrunk to some extent as a result of controlling the pollutant emissions; but the area of moderate precipitation acid deposition has expanded in northern China, associated with rapid industrial and transportation development. Furthermore, we found significant positive correlations between precipitation acid deposition, energy consumption, and rainfall. Our findings provide a relatively comprehensive evaluation of the spatiotemporal dynamics of precipitation acid deposition in China over past three decades, and confirm the idea that strategies implemented to save energy and control pollutant emissions in China have been effective in alleviating precipitation acid deposition. These findings might be used to demonstrate how developing countries could achieve economic development and environmental protection through the implementation of advanced technologies to reduce pollutant emissions.

Non-linear direct effects of acid rain on leaf photosynthetic rate of terrestrial plants

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

Long-term increases in soil carbon due to ecosystem fertilization by atmospheric nitrogen deposition demonstrated by regional-scale modelling and observations

Fertilization of nitrogen (N)-limited ecosystems by anthropogenic atmospheric nitrogen deposition (Ndep) may promote CO2 removal from the atmosphere, thereby buffering human effects on global radiative forcing. We used the biogeochemical ecosystem model N14CP, which considers interactions among C (carbon), N and P (phosphorus), driven by a new reconstruction of historical Ndep, to assess the responses of soil organic carbon (SOC) stocks in British semi-natural landscapes to anthropogenic change. We calculate that increased net primary production due to Ndep has enhanced detrital inputs of C to soils, causing an average increase of 1.2 kgCm-2 (c. 10%) in soil SOC over the period 1750-2010. The simulation results are consistent with observed changes in topsoil SOC concentration in the late 20th Century, derived from sample-resample measurements at nearly 2000 field sites. More than half (57%) of the additional topsoil SOC is predicted to have a short turnover time (c. 20 years), and will therefore be sensitive to future changes in Ndep. The results are the first to validate model predictions of Ndep effects against observations of SOC at a regional field scale. They demonstrate the importance of long-term macronutrient interactions and the transitory nature of soil responses in the terrestrial C cycle.

Prolonged acid rain facilitates soil organic carbon accumulation in a mature forest in Southern China

With the continuing increase in anthropogenic activities, acid rain remains a serious environmental threat, especially in the fast developing areas such as southern China. To detect how prolonged deposition of acid rain would influence soil organic carbon accumulation in mature subtropical forests, we conducted a field experiment with simulated acid rain (SAR) treatments in a monsoon evergreen broadleaf forest at Dinghushan National Nature Reserve in southern China. Four levels of SAR treatments were set by irrigating plants with water of different pH values: CK (the control, local lake water, pH ≈ 4.5), T1 (water pH=4.0), T2 (water pH=3.5), and T3 (water pH=3.0). Results showed reduced pH measurements in the topsoil exposed to simulated acid rains due to soil acidification. Soil respiration, soil microbial biomass and litter decomposition rates were significantly decreased by the SAR treatments. As a result, T3 treatment significantly increased the total organic carbon by 24.5% in the topsoil compared to the control. Furthermore, surface soil became more stable as more recalcitrant organic matter was generated under the SAR treatments. Our results suggest that prolonged acid rain exposure may have the potential to facilitate soil organic carbon accumulation in the subtropical forest in southern China.