Urbanization increases carbon concentration and pCO2 in subtropical streams

Elevated nitrogen loadings facilitate carbon dioxide emissions from urban inland waters

Carbon dioxide (CO2) production and emissions from inland waters play considerable roles in global atmospheric CO2 sources, while there are still uncertainties regarding notable nutrient inputs and anthropogenic activities. Urban inland waters, with frequently anthropogenic modifications and severely nitrogen loadings, were hotspots for CO2 emissions. Here, we investigated the spatiotemporal patterns of partial pressure of CO2 (pCO2) and CO2 fluxes (FCO2) in typical urban inland waters in Tianjin, China. Our observation indicated that pCO2 values were oversaturated in highly polluted waters, particularly in sewage rivers and urban rivers, exhibiting approximately 9 times higher than the atmosphere equilibrium concentration during sampling campaigns. Obviously, the spatiotemporal distributions of pCO2 and FCO2 emphasized that the water environmental conditions and anthropogenic activities jointly adjusted primary productivity and biological respiration of inland waters. Meanwhile, statistically positive correlations between pCO2/FCO2 and NH4+-N/NO3--N (p < 0.05) suggested that nitrogen biogeochemical processes, especially the nitrification, played a dominant role in CO2 emissions attributing to the water acidification that stimulated CO2 production and emissions. Except for slight CO2 sinks in waters with low organic contents, the total CO2 emissions from the urban surface waters of Tianjin were remarkable (286.8 Gg yr-1). The results emphasized that the reductions of nitrogen loadings, sewage draining waters, and agricultural pollution could alleviate CO2 emissions from urban inland waters.

Urbanization and weather dynamics co-dominated the spatial-temporal variation in pCO2 and CO2 fluxes in small montanic rivers draining diverse landscapes

Rivers have been widely reported as important CO2 emitters to the atmosphere. Rapid urbanization has a profound impact on the carbon biogeochemical cycle of rivers, leading to enhanced riverine CO2 evasions. However, it is still unclear whether the spatial-temporal patterns of CO2 emissions in the rivers draining diverse landscapes dominated by urbanization were stable, especially in mountainous areas. This study carried out a two-year investigation of water environmental hydrochemistry in three small mountainous rivers draining urban, suburban and rural landscapes in southwestern China, and CO2 partial pressure (pCO2) and fluxes (fCO2) in surface water were measured using headspace equilibrium method and classical thin boundary layer model. The average pCO2 and fCO2 in the highly urbanized river were of 4783.6 μatm and 700.0 mmol m-2 d-1, conspicuously higher than those in the rural river (1525.9 μatm and 123.2 mmol m-2 d-1), and the suburban river presented a moderate level (3114.2 μatm and 261.2 mmol m-2 d-1). It provided even clearer evidence that watershed urbanization could remarkably enhance riverine CO2 emissions. More importantly, the three rivers presented different longitudinal variations in pCO2, implying diversified spatial patterns of riverine CO2 emissions as a result of urbanization. The urban land can explain 49.6-69.1% of the total spatial variation in pCO2 at the reach scale, indicating that urban land distribution indirectly dominated the longitudinal pattern of riverine pCO2 and fCO2. pCO2 and fCO2 in the three rivers showed similar temporal variability with higher warm-rainy seasons and lower dry seasons, which are significantly controlled by weather dynamics, including monthly temperature and precipitation, but seem to be impervious to watershed urbanization. High temperature-stimulated microorganisms metabolism and riched-CO2 runoff input lead much higher pCO2 in warm-rainy seasons. However, it showed more sensitivity of pCO2 to monthly weather dynamics in urbanized rivers than that in rural rivers, and warm-rainy seasons showed hot moments of CO2 evasion for urban rivers. TOC, DOC, TN, pH and DO were the main controls on pCO2 in the urban and suburban rivers, while only pH and DO were connected with pCO2 in the rural rivers. This indicated differential controls and regulatory processes of pCO2 in the rivers draining diverse landscapes. Furthermore, it suggested that pCO2 calculated by the pH-total alkalinity method would obviously overestimate pCO2 in urban polluted rivers due to the inevitable influence of non-carbonate alkalinity, and thus, a relatively conservative headspace method should be recommended. We highlighted that urbanization and weather dynamics co-dominated the multiformity and uncertainty in spatial-temporal patterns of riverine CO2 evasions, which should be considered when modeling CO2 dynamics in urbanized rivers.

Development of a two-dimensional model to assess carbon dynamics and anthropogenic effects on CO2 emissions in the Tan river, southern China

Tidal rivers are key biochemical reaction channels along the land-ocean aquatic continuum, receiving carbon from wastewater and agricultural drains, which can considerably affect CO2 emissions. We developed a two-dimensional hydrodynamic and ecological model coupled with an inorganic carbon module along the Tan River in southern China. The simulations of and observations regarding discharge, temperature, total organic carbon (TOC), total inorganic carbon (TIC), and other common water quality variables were generally in good agreement. Based on the validated model, we employed statistical and scenario analyses to evaluate the carbon distribution, TOC and TIC budgets, and the imbalances induced by climatic and anthropogenic changes, providing insights into their potential greenhouse effect. The Tan River was consistently supersaturated with CO2 with an annual mean air-water CO2 emission flux (FCO2) of 226.1 ± 84.9 mmol m-2 d-1, and significant temporal and spatial variations of FCO2, TOC, and TIC were observed. Urban small streams tended to emit additional CO2 during wet seasons, and rural tributaries usually had an increase in TOC concentrations during the dry season. FCO2 was significantly positively correlated with air temperature and negatively correlated with total nitrogen, total phosphorus, and TOC. The annual riverine input of carbon to the urban river network was 17.37 Gg C yr-1, with 59.82% of TOC, and carbon output was 15.31 Gg C yr-1, with 66.87% of TOC. The retention rates for TOC and total carbon were 50.7% and 11.8% in the urban branch, respectively. Furthermore, warming and wastewater treatment could prevent urban river networks and downstream rivers from becoming carbon sources. Therefore, our findings suggest that riverine management strategies change the global CO2 release from tidal rivers and estuarine systems under climate change.

Increasing riverine export of dissolved organic carbon from China

River transport of dissolved organic carbon (DOC) to the ocean is a crucial but poorly quantified regional carbon cycle component. Large uncertainties remaining on the riverine DOC export from China, as well as its trend and drivers of change, have challenged the reconciliation between atmosphere-based and land-based estimates of China's land carbon sink. Here, we harmonized a large database of riverine in-situ measurements and applied a random forest model, to quantify riverine DOC fluxes (FDOC ) and DOC concentrations (CDOC ) in rivers across China. This study proposes the first DOC modeling effort capable of reproducing well the magnitude of riverine CDOC and FDOC , as well as its trends, on a monthly scale and with a much wider spatial distribution over China compared to previous studies that mainly focused on annual-scale estimates and large rivers. Results show that over the period 2001-2015, the average CDOC was 2.25 ± 0.45 mg/L and average FDOC was 4.04 ± 1.02 Tg/year. Simultaneously, we found a significant increase in FDOC (+0.044 Tg/year2 , p = .01), but little change in CDOC (-0.001 mg/L/year, p > .10). Although the trend in CDOC is not significant at the country scale, it is significantly increasing in the Yangtze River Basin and Huaihe River Basin (0.005 and 0.013 mg/L/year, p < .05) while significantly decreasing in the Yellow River Basin and Southwest Rivers Basin (-0.043 and -0.014 mg/L/year, p = .01). Changes in hydrology, play a stronger role than direct impacts of anthropogenic activities in determining the spatio-temporal variability of FDOC and CDOC across China. However, and in contrast with other basins, the significant increase in CDOC in the Yangtze River Basin and Huaihe River Basin is attributable to direct anthropogenic activities. Given the dominance of hydrology in driving FDOC , the increase in FDOC is likely to continue under the projected increase in river discharge over China resulting from a future wetter climate.

Diurnal and seasonal variations of pCO2 and fluorescent dissolved organic matter (FDOM) in different polluted lakes

This study aimed to assess pollution and daily-to-seasonal dynamics of the partial pressure of CO2 (pCO2) and CO2 degassing flux concerning the fluorescent dissolved organic matter (FDOM) from tropical lakes. A membrane-enclosed pCO2 sensor and water quality multimeter analyzer was deployed to continuously record daily and seasonal variations in pCO2 and CO2 degassing flux in three lakes in Savar, Dhaka. During both wet and dry seasons, all lake water was supersaturated with CO2 in contrast to the atmospheric equilibrium (~400 μatm). The pCO2 values in the lake water during the dry season were relatively low in comparison, and the pCO2 levels in the wet season were much higher due to external inputs of organic matter from watersheds and direct inputs of CO2 from soils or wetlands. The estimated water-to-air CO2 degassing flux in the different levels of polluted lakes varies with the pollution context. Study areas calculated the carbon flux and three lakes released respectively 86.75×107g CO2 year-1, 13.8×107g CO2 year-1, and 9.17×107g CO2 year-1. Three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy combined with parallel factor (PARAFAC) analysis was used to investigate the distributions of fluorescent components in DOM. EEM-PARAFAC analysis identified humic-like, fulvic-like, protein-like, and more tyrosine-like FDOM components and their environmental dynamics. Terrestrial DOM may provide inputs to the terrestrial humic-like component in the lake water. In contrast, the biological activity of plankton-derived FDOM is the most likely source for the autochthonous humic-like component. FDOM and DO concentrations have negative correlations with pCO2, indicating that when the FDOM and DO level is decreased, the amount of pCO2 values increases.

Unravelling the spatiotemporal variation of pCO2 in low order streams: Linkages to land use and stream order

Headwater streams make the majority of cumulative stream length in a river basin, carbon dioxide (CO₂) emission from headwater (low order) streams is thus an essential component. Anthropogenic activities in headwater areas such as land use change and land use practices can strongly modify terrestrial carbon and nutrient input, which could affect the level of partial pressure of dissolved carbon dioxide (pCO₂) and CO₂ degassing from streams. However, there are large uncertainties in estimates due to the lack of data in subtropical rivers of rapidly developing rural regions. The spatiotemporal variation and driving factors of the pCO₂ and CO₂ degassing from low-order streams remain to be explored. In this study, we assess multi-spatial scale effects of land use on pCO₂ dynamics in seven headwater tributary rivers in Central China during 2016, 2017 and 2018 in rainy and dry seasons. Our results reveal that the stream pCO₂ level consistently increases as the stream order increases from 1 to 3 under apparent seasonal variations. Riverine pCO₂ is positively related to the percentage of urban land and cropland surrounding the river segments, but is negatively related to the percentage of forest land. The stream pCO₂ is more closely correlated with the 1000 and 2000 m diameters of circular buffers at upstream sampling sites than the circular buffers with 100 and 500 m diameters. There exist significant relationships of pCO₂ with the concentrations of TN, TP, DO, and DOC in the low-order streams. The partial redundancy analysis quantifies the relative importance of anthropogenic land uses, natural factors and water chemical variables in mediating stream pCO₂, showing that influences of anthropogenic land uses (urban and cropland) on pCO₂ decrease, with a percentage role of 34%, 14%, and 4% in the 1st-, 2nd- and 3rd-order streams, respectively. The impact of nutrients on pCO₂, however, increases as the stream order increases. Urban influence on stream pCO₂ also decreases as stream order increases. Our study highlights the effect of land use/land cover types and stream order on riverine pCO₂ and provides new insight into estimating CO₂ emission in headwater streams. Future studies are needed on the linkage between riverine CO₂ degassing and stream orders under changing land use conditions.

Income inequality and environmental degradation in Egypt: evidence from dynamic ARDL approach

Over the past four decades, the Egyptian economy has suffered from both income inequality and environmental degradation. This dual problem raises the question about the nature of the relationship between inequality and the environment in a developing country like Egypt. In this regard, the study aims to examine the impact of income inequality on carbon emissions during the period 1975-2017. The analysis considers the ability of the political economy approach compared to the Keynesian trade-off approach to explain the inequality-environment relationship in Egypt. To do this, the novel dynamic autoregressive distributed lags approach is employed to capture the short-run and long-run relationships and to overcome the complications associated with the structure of the widely used autoregressive distributive lags model. The findings show that the relationship between inequality and CO₂ emissions is not a trade-off. Rather, inequality leads to environmental deterioration in the long run, but there is no significant effect in the short run. In the long run, a 1% rise in the Gini coefficient increases CO₂ emissions by 2.28%. These results support the political economy approach in explaining the inequality-environment nexus. Hence, the economic development policies adopted in Egypt during the past four decades have led to a negative impact on the environment. The study advises economic policy makers in Egypt to adopt income redistribution policies to reduce the severity of income inequality. Improving income distribution has a positive effect on the environment in Egypt.

pCO2 and CO2 evasion from two small suburban rivers: Implications of the watershed urbanization process

Rivers are widely reported as CO₂-emitting hotpots and are attracting increasing attention worldwide. However, less attention has been given to the CO₂ emission from the suburban rivers which are experiencing rapid watershed urbanization and increasing anthropogenic stress. Here, two small suburban rivers in Southwest China were studied, and seasonal sampling campaigns with high spatial resolution were carried out to explore the characterization of partial pressure (pCO₂) and CO₂ efflux and their possible controls. The results showed that, the pCO₂ and estimated CO₂ fluxes from the two suburban rivers ranged from 37 to 6466 μatm (mean of 1293 ± 1126 μatm) and -72-1569 mmol·m^-2·d^-1 (mean of 185 ± 240 mmol·m^-2·d^-1), respectively. And, both of them exhibited disproportionately high variability and acted as strong CO₂ emitters to the atmosphere. The pCO₂ in the two suburban rivers showed significant spatial variability, with urban sections having 2-2.5 times higher values than exurban sections, and, the urban land use proportion in the basins accounted for 35%-67% of such spatial variation in pCO₂. The sewage-dominated urban tributaries had much higher pCO₂ and acted as an obvious exciter to the high pCO₂ in urban sections of suburban rivers. Carbon and nutrients concentrations also accounted for the spatial variation in pCO₂ and fCO2 in the two suburban rivers, and acted as good indicators. The seasonal variation in pCO₂, with the highest values in autumn and lowest values in spring, was controlled by the precipitation dilution effect and seasonal temperature as well as the boosted primary production at several urban sites. We highlighted that small suburban rivers showed disproportionally high spatial variability in pCO₂ and CO₂ fluxes in their limited basin areas due to the development of urbanization, and could be used as a good model for studying the complex impacts of anthropogenic disturbances on river carbon biogeochemical processes.