Long-Term Trends in Acid Neutralizing Capacity under Increasing Acidic Deposition: A Special Example of Eutrophic Taihu Lake, China

Large Seasonal Variation in Nitrogen Isotopic Abundances of Ammonia Volatilized from a Cropland Ecosystem and Implications for Regional NH3 Source Partitioning

Ammonia (NH3) volatilization from agricultural lands is a main source of atmospheric reduced nitrogen species (NHx). Accurately quantifying its contribution to regional atmospheric NHx deposition is critical for controlling regional air nitrogen pollution. The stable nitrogen isotope composition (expressed by δ15N) is a promising indicator to trace atmospheric NHx sources, presupposing a reliable nitrogen isotopic signature of NH3 emission sources. To obtain more specific seasonal δ15N values of soil NH3 volatilization for reliable regional seasonal NH3 source partitioning, we utilized an active dynamic sampling technique to measure the δ15N-NH3 values volatilized from maize cropping land in northeast China. These values varied from -38.0 to -0.2‰, with a significantly lower rate-weighted value observed in the early period (May-June, -30.5 ± 6.7‰) as compared with the late period (July-October, -8.5 ± 4.3‰). Seasonal δ15N-NH3 variations were related to the main NH3 production pathway, degree of soil ammonium consumption, and soil environment. Bayesian isotope mixing model analysis revealed that without considering the seasonal δ15N variation in soil-volatilized NH3 could result in an overestimate by up to absolute 38% for agricultural volatile NH3 to regional atmospheric bulk ammonium deposition during July-October, further demonstrating that it is essential to distinguish seasonal δ15N profile of agricultural volatile NH3 in regional source apportionment.

Long-term changes in major ion chemistry-similarities and differences between inflow rivers and the lake in Taihu watershed, China

The major ion chemistry in the Taihu watershed has dramatically changed due to human disturbances; however, little is known about the similarities and differences in the responses of the inflow rivers and Taihu lake to the disturbances. Using historical (1950s-1970s) and recent (2018-2021) water chemistry data of inflow rivers and the lake, as well as socioeconomic and land use data, we explored the drivers for the major ion chemistry change and different responses of the inflow rivers and the receiving lake. The results indicated that, compared with 1950s-1970s, all the major ions and TDS in rivers and Taihu lake significantly increased (by 91% for Mg2+ and by 395% for Cl- in rivers; by 68% for HCO3- and 134% for Na+ in the lake); however, their increases in major ion composition presented a clear difference, i.e., although current dominant cation remained Ca in inflow rivers, the second dominant cation has shifted from Mg2+ (1950s-1970s) to Na+ (2018-2021) for rivers, while for the lake, the second dominant cation has become frequently Na+ (2018-2021), followed by Ca2+, indicating a clear salinization tendency. Furthermore, the change of some indicative ratio indices of inflow rivers and the lake in the past decades presented an apparent difference, i.e., the river systems had a higher increase rate in Ca2+/Mg2+ and SO42-/Cl- than the lake, while the lake had a higher increase in (Ca2+ + Mg2+)/HCO3-, TH/TA, and Cl-/Na+ than the river systems. Analyses indicated that increased human disturbances were the major driver for the similar increase in the TDS and major ions for both river systems and the lake, while the different algal biomass in the rivers and lake, the land use change, and declined hydrological connectivity in this watershed played important roles in the different alterations of the water chemistry indices. Comparison of major ion correlation change between the running and stagnant waters indicated a clear "lacunification" trend of inflow rivers in terms of water chemistry characteristics in this dense river-network region. Our work revealed the cause and effect of the fundamental water chemistry change in a rapid development region and will provide scientific basis for the integrated management and recovery in the watershed.

Shift of major driver for chemical weathering from the natural control to human dominance since 1980s in the Taihu watershed, China

Anthropogenic acidification has become a concerned problem in the Taihu region; however, how it affected the regional weathering rate, especially at the different sub-watershed levels has hardly been studied. To reveal the impact of human activities on watershed weathering and water chemistry in Taihu sub-watersheds, historical water chemistry data (1950s-1970s) and recent water samples (2018-2021) of the local river systems, as well as sediment samples of Taihu lake were collected and analyzed, and a linear addition mass balance method was used to determine the weathering rate at the sub-watershed level. The results indicated that, compared with 60 years ago, the current weathering rate of carbonates and silicates in the Taoge water system (TG) was the highest, reaching 67.2 and 11.4 t·km^-2·a^-1, increasing by 4.1 and 2.7-folds, respectively; and meanwhile the carbonate and silicate weathering rates increased by 3.1 and 4.9-folds in the Nanhe water system (NH), and 5.2 and 3.4-folds in the Tiaoxi water system (TX), respectively. The increasing rate was significantly correlated to the atmospheric SO₂ concentration in different sub-watersheds and was affected by the sub-watershed lithology, e.g., TX had a higher increase rate of silicate weathering due to the wider distribution of silicates in this sub-watershed than the other two. The sediment evidence of Na/K and Ca/Al on the profile in different lake parts, which was influenced by different influx river systems, confirmed that the overall intensity of watershed weathering was higher in TG than in the TX sub-watershed and was higher in the recent decade than 50-60 years ago. The accelerated weathering rate was found to present a definite consistency with the social and economic development in the watershed. Combined analyses of the accelerated weathering rate in the watershed and sedimentation evidence indicated that the major driving force for the watershed weathering has shifted from carbonic acid under the natural condition to human-induced sulfuric acid since 1980s.

Accelerated carbonate dissolution caused by anthropogenic acidification - contrast of watershed soils to lake sediments in Taihu Region, China

Although Taihu watershed is an "acid-insensitive" region, anthropogenic acidification has greatly changed the water chemistry in Taihu Lake. However, how soil carbonates responded to the long-term human-induced acidification received less attention. In this work, we investigated soil carbonate concentrations from different land uses in the upstream of the lake and sediment carbonate profiles in the lake, to explore the linkage of carbonates dissolution in the land and sedimentation in the lake. The result showed that the wheat-rice surface soil, the most acidification-impacted by fertilization and acid deposition, had significantly lower pH than vegetable and wetland soils (p < 0.05). Meanwhile, the carbonate concentration in wetland soils, only impacted by acid deposition, was significantly higher than that in wheat-rice and vegetable soils (p < 0.05). The pH profile of fertilized soils, with an increasing trend from the surface to bottom, further indicated the acidifying effect of fertilization. Although the average soil pH across all land uses was 6.6 in the upstream of the lake, remaining carbonate buffering system, the significant carbonate decrease especially in surface soils evidenced the definite carbonate dissolution by acidification, which is cumulative and irreversible. Contrary to the topsoils, the sediment carbonate concentration presented an increasing trend from the depth of 15 cm (denoting around the early 1980s) to the surface, indicating that lake sediment is a major sink of carbonate Ca and Mg from the watershed, particular under an alkaline lake environment caused by frequent algae blooms in the past decades. In addition, Ca/Mg ratio in the sediment, having higher values in a higher pH environment, was quite different from the watershed soil pattern, suggesting different biogeochemical processes Ca and Mg underwent during their transportation and sedimentation. The effects of acidification-altered re-distribution of carbonate Ca and Mg and Ca/Mg ratio in the terrestrial and aquatic environments deserve wider considerations of ecosystem consequence.

Soil acidification and loss of base cations in a subtropical agricultural watershed

Soil acidification along with base cations loss degrades soil quality and is a major environmental problem, especially in agroecosystems with extensive nitrogen (N) fertilization. So far, the rates of proton (H⁺) production and real soil acidification (loss of base cations) remain unclear in subtropical agricultural watersheds. To assess the current status and future risk of soil acidification in subtropical red soil region of China, a two-year monitoring was conducted in a typical agricultural watershed with upland, paddy fields, and orchards where high N fertilizers are applied (320 kg N ha^-1 yr^-1). H⁺ production, neutralization and base cations losses were quantified based on the inputs (rainwater, inflow of water, and fertilizer) and outputs (outflow of water, groundwater drainage, and plant uptake) of major elements (K⁺, Ca²⁺, Na⁺, Mg²⁺, Al³⁺, NH₄⁺, NO₃^-, SO₄^2-, Cl^-, and H⁺). The result showed that total H⁺ production in the watershed was 5152 mol_c ha^-1 yr^-1. N transformation was the most important H⁺ source (68%), followed by excess plant uptake of cations (25%) and H⁺ deposition (7%). Base cations exchange and weathering of minerals (3842 mol_c ha^-1 yr^-1) dominated H⁺ neutralization, followed by SO₄^2- adsorption (1081 mol_c ha^-1 yr^-1), while H⁺ and Al³⁺ leaching amounted to 431 mol_c ha^-1 yr^-1, only. These results state clearly that despite significant soil acidification, the acidification of surface waters is minor, implying that soils have buffered substantially the net H⁺ addition. As a result of soil buffering, there was abundant loss of base cations, whose rate is significantly higher than the previously reported weathering rate of minerals in red soils (3842 vs 230-1080 mol_c ha^-1 yr^-1). This suggests that the pool of exchangeable base cations is being depleted in the watershed, increasing the vulnerability of the watershed, and posing a serious threat to future recovery of soils from acidification.

Disruptions and re-establishment of the calcium-bicarbonate equilibrium in freshwaters

During recent decades, increasing anthropogenic activities have affected natural ionic composition, including the strongest and most common relationship between ionic concentrations in the majority of natural global freshwaters, i.e., the Ca²⁺-ANC (acid neutralizing capacity) equilibrium. Using long-term monitoring data and MAGIC modelling, we evaluated effects of major present environmental stressors (synthetic fertilizers, liming, acidic deposition, forest disturbances, and climate change) on the Ca²⁺-ANC equilibrium. We evaluated the effects for three different types of terrestrial ecosystems, a circumneutral lowland agricultural catchment, two acid sensitive mountain forest catchments differing in forest health, and one acid sensitive alpine catchment. All catchments are in a region with the world-largest changes in fertilizing rates and acidic deposition in the 20^th century, with increasing impacts until the late 1980s, and their subsequent abrupt, dramatic decreases. These strong changes resulted in a substantial disruption, followed by continuing re-establishment of the Ca²⁺-ANC relationship in all study waters. The shape of the disruption and the following re-establishment of its new value were dependent on the intensity, duration, and combination of stressors, as well as on catchment characteristics (bedrock composition, soil amount and composition, vegetation status, and hydrology). We conclude that a new equilibrium may deviate from its natural value due to the (1) legacy of fertilizing, acidic deposition and liming, affecting the soil Ca²⁺ pools, (2) forest disturbances and management practices, and (3) climate change.

The bioavailability and contribution of the newly deposited heavy metals (copper and lead) from atmosphere to rice (Oryza sativa L.)

Despite the global importance of atmospheric heavy metal input into agricultural soils, research has primarily focused on the amount of the depositions with limited attention given to the risk of the newly deposited heavy metals. To understand the remobilization of the newly deposited copper (Cu) and lead (Pb) from the atmosphere and explore the metals' mobility and bioavailability to rice (Oryza sativa L.), a soil transplant experiment was conducted in three areas along a gradient of atmospheric depositions. Approximately 61% of the Cu and 76% of the Pb depositions tended to be present in potentially mobile fractions. The soil retention of newly deposited Cu and Pb presented as higher mobile fractions than these in the original soil. The newly deposited Cu and Pb in soils only accounted for 0.34-8.7% and 0.07-0.29% of the total soil Cu and Pb pools, but they contributed 30-84% and 6-41% in rice tissues, respectively. A major implication of these findings is that once the heavy metal is deposited, it may be reactivated in soils and transported to aerial parts or foliar uptake into plant tissues, emphasizing the important role of the newly deposited Cu and Pb in contributing to the edible parts of crops.

Characterization of phosphorus accumulation and release using diffusive gradients in thin films (DGT) - linking the watershed to Taihu Lake, China

Taihu Lake is well known internationally due to its algae bloom, and phosphorus fertilization and accumulation in the watershed soil played an important role in the lake eutrophication. To explore the linkage between soil and sediment legacy P, the soils from four representative land uses in the upstream watershed and sediments from typical lake areas were sampled and analyzed for P species. Meanwhile, the DGT technology was used to characterize the labile P in the soil and sediment and its release dynamics. The results indicated that Taihu Lake was the major total phosphorus sink (TP = 481.7 ± 97 mg/kg) and wheat land the major reactive phosphorus stock (RP = 37.3 ± 9 mg/kg) in the watershed. The DGT-P dynamic with the deployment time (t) presented a downward exponential curve (f(t) = a × t^-b, b > 0) and the regression parameters implied the initial P stock (a) and release rate (b) of labile P. Although the result of the extract based method was statistically correlated to the DGT measured P (p < 0.05), the DGT result had advantages over the traditional method due to its in-situ measurement and kinetic characterization of available P. Since the regression parameters reflected combined factors that impact the interaction between solid and solution P, the soil or sediment properties such as pH and organic matters need to be further studied in details, although they presented a significant correlation with DGT-P in this work. Our work provides a new insight in the rapid assessment of the size and resupply capacity of the existing legacy P stocks.

Acid deposition induced base cation loss and different responses of soils and sediments in Taihu Lake watershed, China

Acid deposition and algae blooms have resulted in great changes in the water chemistry of Taihu Lake; however, there have been few calculated results to describe these processes. Here we used a mass balance model to estimate base cation losses and evaluate the model applicability in this intensively human-impacted watershed based on a long-term database (1985-2015). The results showed that carbonate weathering induced Ca²⁺ and Mg²⁺ losses in the watershed were responsible for the increased ion net reaction (INR) of Ca²⁺and Mg²⁺ in the lake. While the increase of K⁺ and Na⁺ were not appropriate to provide independents check on the mass balance model because they generally entered the lake as human discharges, not reflecting change of the geochemical process in the watershed as the watershed dominant bedrock is carbonate but not silicate. Acid deposition in Taihu region caused decrease in pH, lime potential, and different Ca, Mg species of surface soils. Our field measurements of sediments in the two lake parts showed that the sediment lime potential was significantly higher in the algae dominated lake area than in the hydrophyte-dominated area due to the in-lake alkalinization by algae blooms, indicating that algae blooms played a significant role in the acidification resistance. Meanwhile, the measured lime potential of the watershed soils was lower than that of the sediment, implying a potential risk of acidification in the watershed. This research helps understand the accelerated interactions between human activities and natural geochemical processes and accelerated water chemistry change at the watershed level.

Rapid evaluation of leaching potential of heavy metals from municipal solid waste incineration fly ash

Municipal solid waste incineration fly ash is directly landfilled after solidification in the industry. The rapid evaluation of contaminant leaching is required before the landfill of fly ash. In order to reduce the time to evaluate the effect of solidification, a set of rapid evaluation method was developed through the determination of characteristic index, heavy metal leaching analysis, principal component analysis, and mathematical model construction. It was found that f-CaO, acid neutralizing capacity, pH and soluble calcium were negatively correlated with heavy metal leaching. The soluble chlorine was positively correlated with heavy metal leaching. The effect of each feature indicators on heavy metal leaching was evaluated using principal component analysis and mathematical analysis software R.3.4.4. Furthermore, R.3.4.4 was used to detect the optimal model and the excess probability formula by stepwise linear regression and logistic regression analysis method. By introducing the measured value of feature indicator into the excess probability formula, the rate of excess-standard of heavy metals leaching can be preliminarily determined. Based on the above ideas, a rapid detection and evaluation system could be developed according to the local leaching standards and the components of fly ash selected locally.

δ15N and nutrient stoichiometry of water, aquatic organisms and environmental implications in Taihu lake, China

Nitrogen pollution has become a worldwide problem and the source identification is important for the development of pertinent control measures. In this study, isotope end members (rain, nitrogen fertilizer, untreated/treated sewage), and samples (river water discharging to Taihu lake, lake water, aquatic organisms of different trophic levels) were taken during 2010-2015 to examine their δ¹⁵N values and nutrient stoichiometry. Results indicated that phytoplankton (primary producers), which directly take up and incorporate N from the lake water, had a similar δ¹⁵N value (14.1‰ ± 3.2) to the end member of treated sewage (14.0‰ ± 7.5), and the most frequently observed δ¹⁵N value in the lake water was 8-12‰, both indicating the dominant impact of the sewage discharge. Relationship analysis between N isotope value of nitrate and nitrate concentration indicated that different N cycling existed between the algae-dominated northwest lake (NW) and the macrophyte-dominated southeast lake (SE), which is a result of both impacts of river inputs and denitrification. Our nutrient stoichiometry analysis showed that the lake water had a significantly higher N:P ratio than that of algae (p < 0.05), suggesting that N is available in excess relative to the amount demanded by the algae. The long-term trend of the socio-economic development in the watershed further confirmed that the rapid population increase and urbanization have resulted in a great change in the N loading and source proportion. We suggest that although P control is necessary in terms of eutrophication control, N pollution control is urgent for the water quality and ecological recovery for Taihu lake.

Detection of Organochlorine Pesticides in Contaminated Marine Environments via Cyclodextrin-Promoted Fluorescence Modulation

The development of practical and robust detection methods for pesticides is an important research objective owing to the known toxicity, carcinogenicity, and environmental persistence of these compounds. Pesticides have been found in bodies of water that are located near areas where pesticides are commonly used and easily spread to beaches, lakes, and rivers; affect the species living in those waterways; and harm humans who come into contact with or eat fish from such water. Reported herein is the rapid, sensitive, and selective detection of four organochlorine pesticides in a variety of water sources across the state of Rhode Island using cyclodextrin-promoted fluorescence detection. This method relies on the ability of cyclodextrin to promote analyte-specific fluorescence modulation of a high quantum yield fluorophore when a pesticide is in close proximity, combined with subsequent array-based statistical analyses of the measurable changes in the emission signals. This system operates with high sensitivity (low micromolar detection limits), selectivity (100% differentiation between structurally similar analytes), and general applicability (for different water samples with varying salinity and pH as well as for different water temperatures).