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

Denitrification regulates spatiotemporal pattern of N2O emission in an interconnected urban river-lake network

Urban rivers are hotspots of N2O production and emission. Interconnected river-lake networks are constructed to improve the water quality and hydrodynamic conditions of urban rivers in many cities of China. However, the impact of the river-lake connectivity project on N2O production and emission remains unclear. This study investigated dissolved N2O and emission of the river-lake network in Wuhan City, China from March 2021 to December 2021. The results showed that river-lake connection greatly decreased riverine Nitrogen (N) concentration and increased dissolved oxygen (DO) concentration compare to traditional urban rivers. N2O emissions from the urban river interconnected with lakes (LUR: 67.3 ± 92.6 μmol/m2/d) were much lower than those from the traditional urban rivers (UR: 467.3 ± 1075.7 μmol/m2/d) and agricultural rivers (AR: 20.4 ± 15.3μmol/m2/d). Regression tree analysis suggested that the N2O concentrations were extremely high when hypoxia exists (DO < 1.6 mg/L), and TDN was the primary factor regulating N2O concentrations when hypoxia does not occur. Thus, we ascribe the low N2O emission in the LUR and AR to the lower N contents and higher DO concentrations. The microbial process of N2O production and consumption were quantitatively estimated by isotopic models. The mean proportion of denitrification derived N2O (fbD) was 63.5 %, 55.6 %, 42.3 % and 42.7 % in the UR, LUR, lakes and AR, suggested denitrification dominated N2O production in the urban rivers, but nitrification dominated N2O production in the lakes and AR. The positive correlation between logN2O and fbD suggested that denitrification is the key process to regulate the N2O production and emission. The abundance of denitrification genes (nirS and nirK) was much higher than that of nitrification genes (amoA and amoB), also evidenced that denitrification was the main N2O source. Therefore, river-lake interconnected projects changed the nutrients level and hypoxic condition, leading to the inhibition of denitrification and nitrification, and ultimately resulting in a decrease of N2O production and emission. These results advance the knowledge on the microbial processes that regulate N2O emissions in inland waters and illustrate the integrated management of water quality and N2O emission.

Watershed urbanization dominated the spatiotemporal pattern of riverine methane emissions: Evidence from montanic streams that drain different landscapes in Southwest China

Methane (CH₄) emissions from streams are an important component of the global carbon budget of freshwater ecosystems, but these emissions are highly variable and uncertain at the temporal and spatial scales associated with watershed urbanization. In this study, we conducted investigations of dissolved CH₄ concentrations and fluxes and related environmental parameters at high spatiotemporal resolution in three montanic streams that drain different landscapes in Southwest China. We found that the average CH₄ concentrations and fluxes in the highly urbanized stream (2049 ± 2164 nmol L^-1 and 11.95 ± 11.75 mmol·m^-2·d^-1) were much higher than those in the suburban stream (1021 ± 1183 nmol L^-1 and 3.29 ± 3.66 mmol·m^-2·d^-1) and were approximately 12.3 and 27.8 times those in the rural stream, respectively. It provides powerful evidence that watershed urbanization strongly enhances riverine CH₄ emission potential. Temporal patterns of CH₄ concentrations and fluxes and their controls were not consistent among the three streams. Seasonal CH₄ concentrations in the urbanized streams had negative exponential relationships with monthly precipitation and demonstrated greater sensitivity to rainfall dilution than to the temperature priming effect. Additionally, the CH₄ concentrations in the urban and semiurban streams showed strong, but opposite, longitudinal patterns, which were closely related to urban distribution patterns and the HAILS (human activity intensity of the land surface) within the watersheds. High carbon and nitrogen loads from sewage discharge in urban areas and the spatial arrangement of the sewage drainage contributed to the different spatial patterns of the CH₄ emissions in different urbanized streams. Moreover, CH₄ concentrations in the rural stream were mainly controlled by pH and inorganic nitrogen (NH₄⁺ and NO₃^-), while urban and semiurban streams were dominated by total organic carbon and nitrogen. We highlighted that rapid urban expansion in montanic small catchments will substantially enhance riverine CH₄ concentrations and fluxes and dominate their spatiotemporal pattern and regulatory mechanisms. Future work should consider the spatiotemporal patterns of such urban-disturbed riverine CH₄ emissions and focus on the relationship between urban activities with aquatic carbon emissions.

Land use and hydrological factors control concentrations and diffusive fluxes of riverine dissolved carbon dioxide and methane in low-order streams

We analyzed the impacts of land use/land cover types on carbon dioxide (CO₂) and methane (CH₄) concentration and diffusion in 1^st to 4^th Strahler order tributaries of the Longchuan River to the upper Yangtze River in China by using headspace equilibration method and CO₂SYS program. Field sampling and measurements were conducted during the dry and wet seasons from 2017 to 2019. The average of calculated CO₂ partial pressure (pCO₂, mean ± SD: 2389 ± 3220 μatm) by CO₂SYS program was 1.9-fold higher than the value (mean ± SD: 1230 ± 1440 μatm) 10 years ago in the Longchuan River basin, where the urban land area increased by a factor of 7 times. Further analysis showed that corrected pCO₂ by headspace method and dissolved CH₄ (dCH₄) decrease as the stream order and flow velocity increase. The pCO₂ and dCH₄ in the wet season was lower than that in the dry season. The explanatory ability of land use types on the variation of corrected pCO₂ and dCH₄ was stronger at the reach scale than at the riparian and catchment scales in two seasons. Urban land at reach scale further showed much higher explanation on corrected pCO₂ and dCH₄ than cropland, grassland and forest land in the wet season. The Longchuan River emits approximately 112.5 kt CO₂-C and 1.0 kt CH₄-C per year, being 1.7-fold of the total lateral export of dissolved inorganic and dissolved organic carbon (68.3 kt C y^-1). The findings highlight the scale effects of land use on the observed seasonality in dissolved carbon gases in low-order streams.