Acid Rain Treatment Research Articles

Soil aggregate size distribution mediates microbial responses to prolonged acid deposition in a subtropical forest in south China

Extended exposure to acid rain has vastly limited soil microbial activity with the consequences for soil carbon (C) storage, but less is known about the microbial responses within soil aggregates that to some extent determine soil C stabilization. Here, we investigated the main microbial group compositions and the relevant potential enzyme activities within different soil aggregates sizes (microaggregates (<250 μm), small macroaggregates (250–2000 μm), and microaggregates (>2000 μm)) in a subtropical forest with decade-long simulated acid rain (SAR) treatments. Four SAR treatments were set by irrigating plots with water of different pH values (i.e., 3.0, 3.5, 4.0, and 4.5 as a control). Results showed that the SAR treatment significantly inhibited microbial activities, specifically decreasing both bacterial and fungal abundances, leading to declines in C-degrading potential enzyme activities. Conversely, potential enzyme activities related to phosphorus (P) and nitrogen (N) mineralization as well as the enzyme stoichiometry for P/N ratio significantly increased under the SAR treatment. The SAR treatment showed no significant differences in microbial abundance across the three soil aggregate sizes. However, it had a more pronounced effect on potential enzyme activities in their optimal aggregate sizes, such as hydrolytic enzymes like β-glucosidase in macroaggregates and oxidases like phenol oxidase and peroxidase in microaggregates. Overall, C-degrading potential enzyme activities were more strongly decreased in the microaggregates than in macroaggregates, and the distribution in aggregates was significantly altered, transforming from large to small sizes under the SAR treatment, which together may boost SOC stabilization and accumulation. Additionally, our findings indicate that prolonged acid rain also caused soil nutrient limitation and imbalance, particularly for P, in subtropical forests. This study highlights the importance of soil aggregate size in regulating microbial responses to acid rain, which should be integrated into ecosystem models to predict soil biogeochemical cycles under future climate conditions.

Considering the bioavailability and bioaccessibility of metal(loid)s for risk assessment of soils affected by different non-ferrous metal activities in Southwest China

Toxic metal(loid)s released into the soil by non-ferrous metal mining and smelting activities pose a serious threat to residents and the surrounding ecosystem. Considering only total metal(loid) concentrations likely overestimates routine (eco)toxicological risk assessment of soil. We hypothesize that considering metal(loid) bioavailability/accessibility will improve the accuracy of risk assessment. To test this hypothesis, four mining areas in Southwest China, including mining and surrounding sites, were studied. Bioavailability was determined considering metal(loid)s leached by a simulated strong acid rain (SSAR) treatment. In the four areas, the mining site showed higher cumulative releases of metal(loid)s under SSAR treatment than the agricultural field located in the surrounding sites. Thus, the bioavailable metal(loid)s contents were continuously being released during SSAR treatment and likely increased the environmental risk. Ecological and health risk assessment of soil, calculated using total metal(loid)s content, was corrected considering bioavailable/accessible metal(loid)s, which was determined by the heavy metal(loid)s forms and in vitro simulated intestinal stages. Although the corrected indices indicated that the risk of metal(loid)s-contaminated soil was reduced, unfavorable ecological and health risks remained in the four areas. Our study provides new perspectives to better predict the risk of bioavailable/accessible metal(loid)s in non-ferrous metal contaminated and surrounding soils.

Effects of Acid Rain Stress on the Physiological and Biochemical Characteristics of Three Plant Species

The physiological and biochemical indicators of plants reflect the plant’s adaptation to environmental changes and provide information for the planting and management of acid-resistant tree species. To analyze the responses of typical tree species to recent changes in acid rain conditions in Jinyun Mountain, Chongqing, we focused on three representative tree species in the Jinyun Mountain area of Chongqing: Pinus massoniana, Phyllostachys edulis, and Cinnamomum camphora. A mixed acid rain experiment with five gradients of natural rainfall (NR) and pH values of 7.0, 4.5, 3.5, and 2.5 was conducted in May 2021. The changes in physiological and biochemical indicators (net photosynthetic rate, stomatal conductance, intercellular CO2 concentration, transpiration rate, light saturation point, light compensation point, apparent quantum efficiency, dark respiration rate, soluble sugar, starch, soluble protein, proline, malondialdehyde, and antioxidant enzyme activity) were determined. The results show the following: 1. Compared with other treatments, NR and slightly acidic rain increased the relative chlorophyll content in plant seedlings. 2. The synthesis of soluble sugars, starches, and soluble proteins was inhibited to different degrees in the three species under acid rain stress at pH ≤ 3.5. 3. The enzyme activity of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) initially increased and then decreased with an increase in acidity. 4. Acid rain treatments with pH ≤ 4.5 reduced the net photosynthetic rate (Pn) of plants; the higher the acidity, the lower the Pn. Conclusion: A comprehensive comparison of the indicators revealed that NR and mild acid rain enhanced the plant seedlings’ physiological and biochemical characteristics. A pH of 3.5 was the threshold where acid rain had an adverse effect on Pinus massoniana, Phyllostachys edulis, and Cinnamomum camphora. The high indicator values for NR indicate that these tree species have adapted to current conditions in the Jinyun Mountain area of Chongqing. This study provides new information for selecting tree species adapted to the acid rain environment in Jinyun Mountain, Chongqing.

Effects of simulated acid rain on rhizosphere microorganisms of invasive Alternanthera philoxeroides and native Alternanthera sessilis.

Acid rain not only has serious harm to the environment, but also has the same threat to plants, but the invasive plant Alternanthera philoxeroides still grows well compared to the native plant Alternanthera sessilis under acid rain stress. However, the underlying mechanism of resistance to the acid rain environment in invasive Alternanthera philoxeroides remains unclear. In the current study, we comparatively analyzed the plant physiological characteristics, soil physicochemical properties, and rhizosphere microbial communities of invasive A. philoxeroides and native A. sessilis under different pH condition. The simulated acid rain had a significant inhibitory effect on the morphological and physiological traits of A. philoxeroides and A. sessilis and reduced the soil nutrient content. However, A. philoxeroides was more tolerant of acid rain. Compared with CK, simulated acid rain treatment at pH 2.5 significantly increased the Chao1, ACE, and Shannon indexes of A. philoxeroides microorganisms. Under simulated acid rain treatment at pH 2.5, the fungal flora Chao1, ACE and Shannon index were significantly higher than those of CK by 14.5%, 12.4%, and 30.4%, respectively. The dominant bacterial phyla of soil bacteria were Proteobacteria, Actinobacteria, Bacteroidota, Actinobacteria, Firmicutes, Myxococcota, Chloroflexi, Patescibacteria, Gemmatimonadota, Verrucomicrobiota, and Armatimonadota. The dominant fungi were Ascomycota, Basidiomycota, Rozellomycota, and Olpidiomycota. The bacterial and fungal diversity and structure of A. philoxeroides and A. sessilis showed the greatest difference between the pH 2.5 treatment and CK. Redundancy analysis showed that electrical conductivity (EC) and total phosphorus (TP) were the main factors changing the bacterial communities, and available phosphorus (AP), organic matter (OM), EC, and pH were the main factors changing the fungal communities. This study contributes to the microbial community structure of the invasive plant A. philoxeroides and provides a theoretical basis for studying the invasion mechanism of invasive plants under acid rain.

Nitric Acid Rain Increased Bacterial Community Diversity in North Subtropical Forest Soil

Nitric acid rain (NAR) seriously affects the biogeochemical cycles of forest communities’ ecosystems. However, the effects of NAR on the composition and diversity of the soil bacterial community remain unclear. In this study, a typical subtropical forest of Quercus acutissima was selected and simulated spraying of NAR at pH 2.5 (AR2.5), 3.5 (AR3.5), and 4.5 (AR4.5) was implemented to investigate the response of the forest soil bacterial communities to NAR. The results showed that the total number of OTUs of soil bacteria in AR2.5 and AR3.5 treatments was 1.11 and 1.23 times that in the control treatment without NAR (CK), respectively. Acidobacteria, Proteobacteria, and Actinobacteria were the dominant phyla in the subtropical forest, accounting for more than 80% of the community’s relative abundance. Concurrently, simulated NAR changed the relative abundance of Rhodanobacter significantly, which could be an indicator of soil bacterial community structure under NAR stress. Moreover, the Chao1, Shannon, and Simpson indices of strong acid rain treatments (i.e., AR2.5 and AR3.5) increased by 9.55%–22.5%, 3.6%–7.43%, and 0.15%–0.26%, respectively, compared to CK. Redundancy and correlation analysis illustrated that the phylum level structure of the bacterial community was significantly affected by soil total carbon, total nitrogen, and ammonium nitrogen. Our findings contribute to a better understanding of the effects of NAR on soil microbial communities and potential soil element cycling in north subtropical forests.

Corrosion behaviour of nickel based abradable coating used in aero engines

The corrosion potential of abradable nickel based coating used for aero engine components (for sealing and clearance control in compressor of aero engines) was explored and the effect of acid rain was observed for varied length of time. Coating plays an important role in aero engine manufacturing industry as it provides surface protection against corrosion, erosion, friction and wear when subjected to harsh conditions viz., high temperature and corrosive environment during service period. The coating deposition was done by using thermal plasma spray method. Porosity was observed to be high at critical plasma spray parameter (CPSP). Phase transformation and interlayer oxide formation were also observed during plasma spray coating. A comparative study was established between acid rain treated and untreated samples by evaluating the corrosion resistance potential, microstructural characterization, composition and phase analysis of coatings of treated samples using Scanning electron microscopy, X-ray diffraction and EDS. The weight gain of coating was also evident after acid rain treatment of up to ten weeks of immersion.

Effect of polyethylene microplastics and acid rain on the agricultural soil ecosystem in Southern China

The increasing microplastics (MPs) pollution and continuous acid rain coincide in many areas of the world. However, how MPs interact with acid rain is still unclear. Herein, we conducted a microcosm experiment to decipher the combined effect of polyethylene (PE) MPs (1%, 5%, and 10%) and acid rain (pH 4.0) on the agricultural soil ecosystem of Southern China, in which edaphic property, microbial community, enzymatic activity and CO2 emission were investigated. The results showed that PE MPs significantly decreased soil water retention and nitrate nitrogen content regardless of acid rain. Soil total nitrogen significantly decreased under the co-exposure of 10% PE MPs and acid rain. However, PE MPs did not alter soil microbial biomass, i.e., the content of microbial biomass carbon, total phospholipid fatty acids, with or without acid rain. 10% PE MPs and acid rain treatment significantly increased the activity of catalase and soil CO2 emission. PE MPs addition did not affect the temperature sensitivity (Q10) of soil CO2 emission regardless of acid rain. These findings suggest that MPs may interact with acid rain to affect soil ecosystems, thus underscoring the necessity to consider the interaction between MPs and ambient environmental factors when exploring the impact of MPs on the soil biodiversity and function.

Impact of heavily polluted air and acid rain on mulberry silkworm and cocoon production of Bombyx mori Linn

The silk is the protein polymer spun into fibers by the silkworms (Bombyx mori L). It is a textile fiber highly appreciated for its outstanding properties (handle, luster, dye ability, comfort). In addition, silk has been recently investigated as a starting material for nontextile applications. In order to assess the effect of air pollution on the yield and quality of silk fibers, two locations of Indian locality had been chosen on the basis of the ambient air quality for rearing of mulberry silkworm, Bombyx mori L. Air pollution may be indicated by the higher acid rain. Air pollution caused by the burning of fossil fuels increase emissions of SO2, NO2, and CO2 which can ultimately lead to acid rain. Exposure of acid rain may affect the cultivation of mulberry and the quality of silkworm cocoon. The aim of this research was to determine the effect of mulberry plants that have been exposed to acid rain to the percentage of cocoon defects and type of defects in multivoltine races for tropical climates like C-nichi and Hosa mysore races of Bombyx mori L. Acid rain that to applied on mulberry plant for 5 weeks was a modification of acid rain that occurred in tropical area. A total of 400 individuals of newly hatched silkworm were used in early research. Larvae fed by mulberry leaves that had been treated with artificial rain water in different levels of pH, i.e., pH’s of 7.0 (well water as control), 6.0 (normal rain), and 5.0 (acid rain). Cocoon defects and type of cocoon defect were observed. Data were analyzed using multivariate ANOVA test. The results showed that the treatments had significant effect on defective cocoon percentage. Treatment of pH 7.0, 6.0, 5.0 had defective cocoons of 20%, 13%, 10%, respectively in race C-nichi 18% and 15% respectively in race Hosa mysore. Meanwhile, cocoon defect types in race C-nichi and Hosa mysore were outer defects consisting of printed, outside stained, malformed, very small, double cocoons, and inner defect consist of thin end and inside stained and luster of cocoons. As a whole, acid rain treatments to defective cocoon of Bombyx mori L. in C-nichi and Hosa Mysore races showed significant effect to defective cocoons.

Seasonality regulates the effects of acid rain on microbial community in a subtropical agricultural soil of Southern China

Acid rain alters soil carbon (C) cycling by influencing the soil microbial community structure and functions. However, the response of soil microbial communities to acid rain with time and underlying mechanisms are still poorly understood. Herein, we conducted a one-year intact soil core experiment to investigate the temporal changes of soil microbial community composition and C sources metabolism under acid rain (pH 5.0, pH 4.0, and pH 3.0) in an agricultural soil of southern China. We found that pH 3.0 acid rain increased the total, bacterial, gram-positive bacterial, and actinomycetal PLFAs at the early stage, but this effect diminished with time. Conversely, the gram-negative bacterial PLFAs contents were reduced under pH 3.0 acid rain at the later stage. Interestingly, pH 5.0 acid rain increased the total, bacterial, gram-positive bacterial, and actinomycetal PLFAs contents at the later stage. In addition, pH 3.0 and pH 5.0 acid rain treatments accordingly altered the soil microbial community structure at the early and later stage. However, acid rain did not change the microbial C sources utilization pattern. The principal response curve analysis revealed that the seasonal variation exerted a greater effect on the overall variance of soil microbial community structure than the acidity of acid rain. Our results demonstrate the asynchronous response of soil microbial community structure and function, which implies that the microbial functional redundancy may exist in the subtropical agricultural soil under acid rain.

Leaching of simulated acid rain deteriorates soil physiochemical and mechanical properties in three agricultural soils

Acid rain is a long-standing environmental issue that affects ecosystem properties. Great attempts have been made to clarify how acid rain affects soil physiochemical and microbial properties, but soil mechanical properties in response to acid rain conditions have rarely been investigated. This microcosm experiment was conducted to study how simulated acid rain (SAR) with different acidity levels would affect soil mechanical properties and the associated physiochemical indicators of three agricultural soils (Humic Acrisol, Haplic Ferralsol and Rhodic Ferralsol) with different soil properties. The results showed that the soil plasticity index was significantly decreased by the strongly acidic SAR treatments (pH < 3.5) for all the three soils, while the soil penetration resistance was not significantly changed by any of the SAR treatments. Correlation and multiple linear regression analyses suggested that sand content with negligible changes under the SAR treatments contributed substantially to the stable soil penetration resistance. Furthermore, we observed that in comparison to the other two soils, the Haplic Ferralsol appeared to be more vulnerable to acid rain. Such a high sensitivity of the Haplic Ferralsol to the acid rain treatments might have been associated with its relatively poor soil structure and quality, which were indicated by a high sand content and low soil organic matter content. These results suggest that in comparison to high-quality soils, low-quality soils may have experienced greater stress under acid depositions, and the results highlight the necessity of studying in situ how soil mechanical properties would change under acid rain scenarios.

Simulated acid rain impact on growth, yield and leaf anatomy of Dioscorea rotundata (L.)

Abstract. Acid rain causes damage to crops and also has adverse effect on the soil and the environment. These have led to the abandonment of farmlands in affected areas. Southern Nigeria which is known for yam cultivation is subjected to acid rain resulting from gas flaring activities, importation of fairly used vehicles, power generation and industrial activities. The growth, yield and leaf anatomy response of Dioscorea rotundata (L.) exposed to simulated acid rain (SAR) of pH 2.0, 3.0, 4.0 and 6.0 (control) was investigated. Acid rain exposure caused morphological changes in plants including chlorosis, necrosis, and leaf folding. Acidic precipitation inhibited vine length, leaf number, chlorophyll content index and leaf area at pH 2.0; while growth was encouraged at pH 6.0. 22% of leaf surface area of D. rotundata L, was injured after simulated acid rain treatment at pH 2.0 while necrosis was absent at the (control) pH 6.0. Harvest index of D. rotundata was not significantly different (p&gt;0.05) between the pH treatment groups; however, was slightly higher at the (control) pH 6.0. Percentage dry matter partitioning of D. rotundata leaf and stem was not affected by the acid rain exposure; nevertheless, maximum percentage dry matter of tuber was obtained at pH 4.0 and pH 6.0. Transverse sections of D. rotundata leaves showed cuticle wax damage and rupture of epidermis at lower pH 2.0 and 3.0. Mesophyll degradation and cytoplasm depletion was also observed in the treated leaves. The plant leaves revealed dark tissues in necrotic areas which may be phenolic compounds secretion. However, it became clear that significant increases in the growth parameters considered occurred at pH 6.0 (control).

Impact of simulated acid rain on the composition of soil microbial communities and soil respiration in typical subtropical forests in Southwest China

The relationship between soil respiration (SR) and microbial community structure (MCS) is relevant to changes in forest soil ecosystem stability and chemical cycling under acid rain. Simulated acid rain treatments of pH 4.5 (control), 4.0, 3.25 and 2.5 were applied to two forest stands in the Three Gorges Reservoir Area of Jinyun Mountain, Chongqing. We used phospholipid fatty acid (PLFA) analysis to observe the MCS in the 0–10 cm soil layer and measured SR in situ from January 2016 to December 2017. Additionally, we determined the effects of soil properties on the MCS and SR. Acid rain simulation significantly increased the fungal PLFA abundance and decreased the bacterial PLFA abundance in the mixed coniferous and broad-leaved forest (CF). However, in the evergreen broad-leaved forest (BF), the abundance of bacterial and fungal PLFAs did not differ significantly among treatments. Redundancy analysis (RDA) showed that significant changes in the MSC were mainly due to the C/N ratio, hydrolysable N content, content, fine root biomass and sucrase activity. Acid rain simulation in the CF and BF significantly inhibited SR, but the SR sensitivity to simulated acid rain differed among forests. In 2017, the annual mean SR in the CF under the pH 4.0, 3.25 and 2.5 treatments decreased significantly by 6.1%, 19.2% and 28.9%, but in the BF, SR decreased significantly by 25.6% only under pH 2.5. The structural equation model showed that the relationship between the MCS and the variation in SR was closer and more direct than that with soil nutrients. The microbial community structure was an important factor driving the response of soil respiration to acid rain.

Acid rain: an unsuspected factor predisposing Panzhihua airport landslide, China.

Understanding the effect of acid rain to landslides is crucial for a better landslide risk assessment. This work aims to reveal the unsuspected but key role of acid rain in Panzhihua airport landslide, China. Firstly, we propose a hypothesis that acid rain may aggravate the slaking behavior of mudstone at weak interlayer and make it more fragmented, eventually further reducing its shear strength and predisposing the Panzhihua airport landslide. Subsequently, mudstone samples are subjected to slaking durability test, respectively, using water with a pH of 7 and two dilute hydrochloric acid solution with pH of 5 and 3. Slaking durability index (Idn) is adopted aiming to quantitatively evaluate the impact of acid rain on the slaking. Moreover, the mechanisms of acid rain affecting the slaking behavior of mudstone are revealed by (1) analyzing cation compositions changes in different pH slaking fluid and (2) observing micro-structure change of mudstone-chip before and after acid rain treatment. Finally, three works are conducted as evidences to prove that acid rain indeed plays a key role in the occurrence of Panzhihua airport landslide, including (1) analysis of the link between the slaking behavior of mudstone and its shear strength, (2) comparison of cations between spring water at the edge of the toe of landslide and acid rain, and (3) comparison of mineral contents of mudstone samples collected from different locations. These findings have implications for comprehensively analyzing the formation mechanism of landslide in acid rain area (such as Europe, North America, and China).

Alginate Oligosaccharides Alleviate the Damage of Rice Leaves Caused by Acid Rain and High Temperature

Alginate oligosaccharides (AOS) are known for functions in regulating plant growth and stress resistance. This study investigated the damage on rice leaves caused by acid rain (AR) and high temperature (HT) simultaneously, as well as the alleviating effect of AOS on these stresses. The results show that plant biomass and antioxidant enzyme activities (AEAs) after AR treatment reduced more severely under HT conditions than normal-temperature conditions. Both AR and HT triggered the accumulation of reactive oxygen species (ROS) in rice leaves. The suppressing effects of AR and HT were individual in most cases, except for AEAs. Microscopic analysis showed that pH 2 AR and HT injured leaf epidermis, particularly the bulliform cells, the veins and interveinal regions. Spraying AOS resulted in a slight elevation of biomass, a significant increase in AEAs and a remarkable decline in ROS concentrations under HT conditions with AR. Besides, the chlorophyll a contents of a leaf after pH 2 AR plus AOS treatment under HT conditions remained 66.1% of that after pH 7 treatment under normal-temperature conditions. Moreover, AOS protected the integrity of leaf tissue even after pH 3 treatment. Taken together, the above results suggest that AR and HT inhibited AEAs, led to the accumulation of ROS and damaged rice leaf. However, foliar applying AOS enhanced AEAs, scavenged ROS, and thus alleviated the stress induced by HT and AR.

Higher Sensitivity of Soil Microbial Network Than Community Structure under Acid Rain

Acid rain (AR), as a global environmental threat, has profoundly adverse effects on natural soil ecosystems. Microorganisms involved in the nitrogen (N) cycle regulate the global N balance and climate stabilization, but little is known whether and how AR influences the structure and complexity of these microbial communities. Herein, we conducted an intact soil core experiment by manipulating the acidity of simulated rain (pH 7.5 (control, CK) vs. pH 4.0 (AR)) in subtropical agricultural soil, to reveal the differences in the structure and complexity of soil nitrifying and denitrifying microbiota using Illumina amplicon sequencing of functional genes (amoA, nirS, and nosZ). Networks of ammonia-oxidizing archaea (AOA) and nirS-carrying denitrifiers in AR treatment were less complex with fewer nodes and lower connectivity, while network of nosZ-carrying denitrifiers in AR treatment had higher complexity and connectivity relative to CK. Supporting this, AR reduced the abundance of keystone taxa in networks of AOA and nirS-carrying denitrifiers, but increased the abundance of keystone taxa in nosZ-carrying denitrifiers network. However, AR did not alter the community structure of AOA, ammonia-oxidizing bacteria (AOB), nirS-, and nosZ-carrying denitrifiers. Moreover, AR did not change soil N2O emissions during the experimental period. AOB community structure significantly correlated with content of soil available phosphorus (P), while the community structures of nirS- and nosZ-carrying denitrifiers both correlated with soil pH and available P content. Soil N2O emission was mainly driven by the nirS-carrying denitrifiers. Our results present new perspective on the impacts of AR on soil N-cycle microbial network complexity and keystone taxa in the context of global changes.

Idiosyncratic responses of microbial communities and carbon utilization to acid rain frequency in the agricultural and forest soils

The level of acidity in rainfall can alter soil nutrient cycling (e.g., carbon [C] cycling), while the effect of changing acid rain frequency on soil C cycling and involved microbial communities remains largely understudied. Here, we manipulated a laboratory intact soil core experiment to test the effect of acid rain frequency (0, 30% and 100%) on soil CO2 emission, microbial communities, and microbial metabolism of various carbon sources in the agricultural and forest soils in southern China. We found idiosyncratic responses of soil microbial communities and carbon utilization to acid rain frequency in two different soil ecosystems. The high acid rain frequency (100%) substantially reduced soil microbial biomass, particularly that of gram-negative bacteria in the forest soil, while this effect was not observed in the agricultural soil. Moreover, acid rain frequencies (30% and 100%) significantly reduced the microbial utilization of amines/amides in the agricultural soil, while they did not alter the carbon sources utilization in the forest soil. However, surprisingly, both the acid rain treatments did not affect the soil CO2 emission in the agricultural and forest soils, suggesting a complex relationship between soil microbial biomass or abundance and CO2 emission. Our results suggest that the agricultural and forest soils have idiosyncratic responses to changing acid rain frequency, probably because of differences in their biological and physicochemical properties.

Effects of simulated acid rain on the photosynthetic physiology of Acer ginnala seedlings

Extensive areas in China have been receiving high levels of acid rain. Acid rain affects plant growth by reducing the chlorophyll content, destroying leaf structure, and hindering photosynthesis. Acer ginnala Maxim. has a high economic, ornamental, and medicinal value. To clarify the acid rain resistance mechanism of maple trees, the chlorophyll content, gas exchange parameters, and chlorophyll fluorescence parameters were measured in the leaves of 2-year-old Acer ginnala seedlings 0, 15, and 30 days after simulated acid rain at pH 5.6, 5.0, 4.5, 4.0, 3.5, and 2.5. The results showed that the relative content of chlorophyll gradually decreased with the increasing acidity. The net photosynthetic rate (Pn) decreased with the increasing acidity, and the difference among the treatments was significant except for between pH 5.0 and the control. The stomatal conductance at pH &lt; 5.0 was significantly lower than that of the control, and the intercellular carbon dioxide (CO2) concentration of the leaves with the pH &lt; 4.5 treatments was significantly lower than that of the control. As the acidity increased, both Fv/Fm and Fv/F0 (Fv, variable fluorescence; Fm, maximum fluorescence; F0, initial fluorescence) decreased. The difference in the photosynthetic performance index based on the absorbed light energy (PIabs) between the pH 5.0 treatment and the control was not significant during the three periods after the simulated acid rain stress, whereas in the other treatments, this index was significantly lower than that of the control. Our results indicated that under mild acid rain (pH &gt; 4.0) stress, the chlorophyll content, gas exchange parameters, and chlorophyll fluorescence parameters of Acer ginnala changed little, whereas in the other treatments, especially the severe acid rain (pH &lt; 3.0) treatment, these indices showed significant changes that had a severe impact on plant growth and development.

The use of Biolog Eco microplates to compare the effects of sulfuric and nitric acid rain on the metabolic functions of soil microbial communities in a subtropical plantation within the Yangtze River Delta region

China has recently undergone rapid economic development. However, it is the third-largest acid rain region in the world after Europe and North America, and the acid rain problem in the Yangtze River Delta region is particularly serious. Over recent years, the type of acid rain in the Yangtze River Delta region has changed from sulfuric acid rain (SAR) into nitric acid rain (NAR) because of SO2 and NOX emission controls. This has made ecosystems more complicated. In this study, we used Biolog Eco microplates to investigate the impact of simulated SAR and NAR on soil microbial community function over a year, beginning in March 2015. The results showed that the long-term simulated acid rain treatments reduced the average well color development (AWCD) of the carbon sources in soil microbial communities. Secondly, the effects of acid type on the utilization of polymers and amino acids increased in the later stages of the simulated acid rain treatments. Furthermore, the differences in the utilization of carbon sources caused by the simulated acid rain treatments were mainly due to soil microorganism utilization of L-serine, Tween40, D-mannitol, and N-acetyl-D-glucosamine. Finally, the physical and chemical properties of the soil were not significantly correlated with the utilization of the six C groups. Our results suggested that the two types of acid rain had different effects on soil microbial community function. When the type of acid rain changed, the stability of the ecosystem was challenged and the risk of ecosystem disruption greatly increased.

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.

Acid rain mediated nitrogen and sulfur deposition alters soil nitrogen, phosphorus and carbon fractions in a subtropical paddy

Agricultural ecosystems are globally important sinks of carbon and other nutrient elements. In China, acid rain events affect about 0.62 million km2, representing about 6.4% of total land area; however, the impacts of acid rain mediated nitrogen (N) and sulfur (S) depositions on soil organic carbon (SOC) fractions and nutrient stocks in paddy soils and implications for yield production under climate change are unclear. We conducted a field experiment during two annual crop seasons to determine the effects of simulated acid rain on SOC fractions and nutrients in a subtropical paddy in China. Acid rain treatments comprised solutions of HNO3 + H2SO4 to simulate N and S deposition at pH levels of 4.5, 3.5, and 2.5. The results showed that content of soil C fractions varied with acid rain pH. Acid rain led to increased SOC concentrations and decreased the ratios of soil labile organic carbon (LOC): SOC and dissolved organic carbon (DOC): SOC concentrations, independently of crop season and growth stage. Soil salinity was positively associated with SOC suggesting that higher levels of salinity inhibit C decomposition favoring SOC accumulation. Treatment effects of acid rain on soil microbial C and N depended on crop growth stage. Concentration of Fe2+ was positively correlated with DOC in early and late paddy soils under acid rain, possibly as a result of Fe2+ retention by DOC, when Fe3+ is reduced to Fe2+. Acid rain led to increases in soil TN, TP, available N, NH4+ concentration, and SOC, and decreases in ratios of DOC: SOC, indicating decreases in soil biological activity and mineralization processes. Increases in dead rice plant biomass under acid rain were consistent with the increase in soil N and P concentrations, due to reduced nutrient uptake, and higher levels of total SOC.