Downstream carbon transport and surface CO2 evasion in the Hanjiang River Network and their implications for regional carbon budget.

Fluvial carbon fluxes have been increasingly recognized as important components of the global carbon budget. However, it is challenging to accurately quantify carbon fluxes in river networks; therefore, the role of carbon fluxes in the regional carbon budget remains poorly understood. The Hanjiang River Network (HRN) is located in a subtropical monsoon climate zone, and its material transport has a notable impact on the Changjiang River. In this study, it was hypothesized that the total fluvial carbon fluxes from the river network in the subtropical monsoon climate zone are dominated by vertical CO2 evasion and account for a large fraction of terrestrial net primary productivity (NPP) (e.g., 10 %) and fossil CO2 emissions (e.g., 30 %), which is roughly equivalent to the global average. Therefore, the downstream export of three carbon fractions and CO2 evasion were estimated in the HRN over the last two decades and the findings were compared with NPP and fossil CO2 emissions in the basin. The results suggest that approximately 2.14-6.02 Tg C year-1 (1 Tg = 1012 g) of carbon is exported in the HRN. Vertical CO2 evasion represents the largest destination at 1.22-5.34 Tg C year-1 or 68 % of the total fluvial carbon flux component, corresponding to 1.5 %-11 % of the fossil CO2 emissions. Downstream export of dissolved inorganic carbon is the second largest destination with a magnitude of 0.56-1.92 Tg C year-1. Downstream organic carbon export plays a relatively small role with a magnitude of 0.04-0.28 Tg C year-1. The findings also indicate that the offset of total fluvial carbon fluxes from terrestrial NPP is unexpectedly small (2.0 %-5.4 %). Data availability and the simplification of carbon processes introduced uncertainty; therefore, future research should incorporate a fuller representation of fluvial carbon processes and fractions to improve regional-scale carbon accounting.

Autochthonous sources and drought conditions drive anomalous oxygen-consuming pollution increase in a sluice-controlled reservoir in eastern China.

Freshwater reservoirs are an important type of inland waterbody. However, they can suffer from oxygen-consuming pollution, which can seriously threaten drinking water safety and negatively impact the health of aquatic ecosystems. Oxygen-consuming pollutants originate from both allochthonous and autochthonous sources, and have temporally and spatially heterogeneous drivers. Datanggang Reservoir, China, is located in a small agricultural watershed; it is controlled by multiple sluice gates. Anomalously high oxygen consumption indicators were observed in this reservoir in March 2021. Here, it was hypothesized that autochthonous sources were the primary drivers of oxygen-consuming pollution in the reservoir under drought conditions. Datasets of water quality, precipitation, primary productivity, and sediment were used to analyze water quality trends in the reservoir and inflow rivers, demonstrating the effects of allochthonous inputs and autochthonous pollution. No correlation was found between reservoir oxygen consumption indicators and allochthonous inputs; reservoir oxygen consumption indicators and chlorophyll-a concentration were significantly positively correlated (p < 0.05). Substantially lower precipitation and higher water temperature and pH (compared to historical levels) were also observed before the pollution event. Therefore, during this period the hydrological conditions, water temperature, pH, and other variables caused by short-term drought conditions may have facilitated phytoplankton growth in the reservoir. This contributed to a large increase in autochthonous oxygen-consuming pollutants, as reflected by the abnormally high indicators. Sediments contaminated with organic matter may also have been an important contributor. As the effects of environmental management and pollution control continue to emerge, exogenous pollutants imported from the land to reservoirs are currently effectively controlled. However, endogenous pollutants driven by a variety of factors, such as meteorology and hydrology, will likely become the main drivers of short-term changes in oxygen-consuming pollution in freshwater reservoirs in the foreseeable future.

Dominance of nitrous oxide production by nitrification and denitrification in the shallow Chaohu Lake, Eastern China: Insight from isotopic characteristics of dissolved nitrous oxide.

In recent decades, most lakes in Eastern China have suffered unprecedented nitrogen pollution, making them potential "hotspots" for N2O production and emission. Understanding the mechanisms of N2O production and quantifying emissions in these lakes is essential for assessing regional and global N2O budgets and for mitigating N2O emissions. Here, we measure isotopic compositions (δ15N-N2O and δ18O-N2O) and site preference (SP) of dissolved N2O in an attempt to differentiate the relative contribution of N2O production processes in the shallow, eutrophic Chaohu Lake, Eastern China. Our results show that the bulk isotope ratios for δ15N-N2O, δ18O-N2O, and SP were 5.8 ±â€¯3.9‰, 29.3 ±â€¯13.4‰, and 18.6 ±â€¯3.2‰, respectively. More than 76.8% of the dissolved N2O was produced via microbial processes. Findings suggest that dissolved N2O is primarily produced via nitrification (between 27.3% and 48.0%) and denitrification (between 31.9% and 49.5%). In addition, isotopic data exhibit significant N2O consumption during denitrification. We estimate the average N2O emission rate (27.5 ±â€¯26.0 µg N m-2 h-1), which is higher than that from rivers in the Changjiang River network (CRN). We scaled-up the regional N2O emission (from 1.98 Gg N yr-1 to 4.58 Gg N yr-1) using a N2O emission factor (0.51 ±â€¯0.63%) for shallow lakes in the middle and lower region of the CRN. We suggest that beneficial circumstances for promoting complete denitrification may be helpful for reducing N2O production and emissions in fresh surface waters.

Enhanced nitrogen imbalances in agroecosystems driven by changing cropping systems in a coastal area of eastern China: from field to watershed scale.

Agricultural activities exacerbate nitrogen (N) imbalances in the agroecosystem by disturbing the N inputs and outputs, yet the influence of changes in cropping systems on the N balance of agroecosystems remains unclear. In this study, at the field scale, we calculated the N balance of four cropping systems, (1) traditional crops with traditional crop rotation (G-G), (2) vegetables with traditional crop rotation (V-G), (3) vegetables with vegetable rotation (V-V), and (4) greenhouse vegetables (GHV); then analyzed the influence of changes in cropping systems from 1995 to 2015 on the N balances in the agroecosystems in sub-watersheds of the Dagu River. The results indicate that N balances were higher in GHV, V-V, and V-G than G-G, due to significantly higher inputs of N fertilizers and lower N use efficiency (NUE) in vegetable cultivation compared to traditional crops. Driven by economic benefits between 1995 and 2015, V-G, V-V, and GHV replaced G-G in a considerable number of cultivation areas in the sub-watersheds. These changes resulted in an increase of 109.9-170.1% in the N balance in the agroecosystem in the sub-watersheds between 1995 and 2015. In the entire watershed, the total N surplus contribution by V-V, V-G, and GHV increased from 39.3% to 79.1% between 1995 and 2015. These findings suggest that increased vegetable cultivation contributed to the increased risk of N pollution in agricultural production. Thus, there should be a focus on the management of cropping systems to control N loss from agricultural lands.

Nitrogen pollution and sources in an aquatic system at an agricultural coastal area of Eastern China based on a dual-isotope approach.

Nitrogen (N) pollution of water courses is a major concern in most coastal watersheds in eastern China with intensive agricultural production. We use hydrogeological and dual-isotopic approaches to analyze the N concentrations, pollution, transformations, and sources of surface water and groundwater in an agricultural watershed of the Jiaozhou Bay (JZB) area. Results showed that dissolved total N (DTN) concentrations in sub-rivers (SRs) ranged from 6.0 to 25.3 mg N L-1 in the dry season and 9.1-26.7 mg N L-1 in the wet season, which indicated a positive relationship with the percentages of agricultural land. Meanwhile, the dominant dissolved N species in SRs changed from nitrate (NO3-, 64-100%) to dissolved organic N (DON, 52-77%) from the dry season to the wet season and the increased DON concentrations showed a positive relationship with the planted proportions of vegetable production systems. The NO3- concentrations of groundwaters ranged from 10.6 to 121.4 mg N L-1, which were over the limit for drinking water by the World Health Organization. Isotopic analysis indicated that most NO3- originated from the microbiological conversion via nitrification, whereas the deletion of denitrification was insignificant in this area. The results of the stable isotope analysis in R mixing model showed the contributions of potential NO3- sources which were in order of manure fertilizers (20.6-69.0%) > soil organic matter (19.5-53.2%) > chemical fertilizers (5.5-34.3%) > atmospheric deposition (1.3-18.8%). This study suggests that the management of crop productions and reasonable manure fertilizer application should be implemented to protect the quality of aquatic systems in the JZB area.

Carbon and Nitrogen Burial and Response to Climate Change and Anthropogenic Disturbance in Chaohu Lake, China.

Lakes are a crucial component of the global carbon and nitrogen cycle. As a trend of enhanced human activities and climate change, the mechanisms of burial remain poorly understood. In this study, diverse biogeochemical techniques were applied to analyze the temporal variation of organic carbon and nitrogen burial rates in Chaohu Lake. The results showed that burial rates have ranged from 9.39 to 35.87 g C m-2 yr-1 for carbon and from 1.66 to 5.67 g N m-2 yr-1 for nitrogen since the 1860s. The average rates were 19.6 g C m-2 yr-1 and 3.14 g N m-2 yr-1 after the 1970s, which were significantly higher than the rate before the 1970s, showing an increasing trend. The decrease of C/N ratios as well as organic matter δ13C values indicates that the major organic matter source in sediment has been algal production since the 1970s. The increase of δ15N values indicated that the promotion in productivity was stimulated by nutrient input from sewage and agricultural runoff. The burial rates of organic carbon and nitrogen were significantly positively related to socio-economics and temperature, indicating that Chaohu Lake will become an increasing carbon and nitrogen pool under conditions of enhanced human activities and intensive precipitation.

Tracking nitrate sources in the Chaohu Lake, China, using the nitrogen and oxygen isotopic approach.

The Chaohu Lake is highly polluted and suffers from severe eutrophication. Nitrate is a key form of nitrogen that can cause water quality degradation. In this study, hydrochemical and dual isotopic approaches were utilized to identify the seasonal variation of nitrate sources in the Chaohu Lake and its inflowing rivers. The average nitrate concentrations ([NO3-]) of the lake and its inflowing rivers were 89.9 and 140.8 µmol L-1, respectively. The isotopic values of δ15N-NO3- and δ18O-NO3- in the lake ranged from - 1.01 to + 16.67‰ and from - 4.39 to + 22.20‰, respectively. The two major rivers had distinct isotopic compositions, with average δ15N-NO3- values of + 17.52 and + 3.51‰, and average δ18O-NO3- values of + 2.71 and + 7.47‰ for the Nanfei River and Hangbu River, respectively. The results show that soil organic ammonium and urban wastewater discharge were the main sources of nitrate in the Chaohu Lake, and nitrate assimilation was an important process affected [NO3-] and isotopic compositions, especially in the western Chaohu Lake. The elevated [NO3-] and δ15N-NO3- values in the western Chaohu Lake indicate the high influence of human activities. Urban wastewater discharge was the primary nitrate source in the Nanfei River and soil organic ammonium was the main source in the Hangbu River. Although nitrate from direct atmospheric deposition was low, its strong flushing effect can substantially improve riverine [NO3-] and nitrate loading from terrestrial ecosystem. The relatively high nitrate fluxes from the Hangbu River indicate that nitrogen loading from agricultural watershed is unneglectable in watershed nitrogen sources reduction strategies.

Heavy metals in estuarine surface sediments of the Hai River Basin, variation characteristics, chemical speciation and ecological risk.

The Hai River Basin (HRB) is considered to be one of the most polluted areas in China due to the high regional population density and rapid economic development. The estuaries of the HRB, which receive pollutants from terrestrial rivers, may subsequently suffer potential pollution and result in ecological risk of heavy metals. Six heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn) were measured in estuarine surface sediments from 10 estuaries of the HRB to investigate their variation characteristics and ecological risks. The spatial difference of Cr, Ni, Pb, and Zn in sediments was higher than that of the rest two elements. The Yongdingxin Estuary (YDX) and Ziyaxin Estuary (ZYX) in the Northern Hai River System (NHRS) were the most severe in terms of heavy metal contamination. According to the Risk Assessment Code (RAC) classification, Cd associated with the exchangeable and carbonate fraction (the average of 21.3 %) indicated medium risk to high risk. More than 50 % of Cr, Cu, Ni, and Zn on average were associated with the residual fraction. Based on the sum of the first three fractions (exchangeable and carbonate + reducible + oxidizable), the mobility order of these heavy metals was Cd >Pb > Zn ≈ Cu > Ni > Cr. Compared to the background values of cinnamon soil, the potential ecological risk index (RI) values ranged from 25.6 to 168, with an average of 91.2, indicating a low ecological risk in estuarine sites of the HRB. Cd and Pb were the dominant contributors to the toxic-response factor (45.8 and 25.5 %, respectively). The results give insight into the different control measures pertaining to heavy metal pollution and risk for both relatively clean estuaries and urban seriously polluted areas, respectively, for the formation of protect strategies of aquatic environment in the HRB.