Assessing dissolved inorganic nitrogen flux in the Yangtze River, China: Sources and scenarios
Introduction
There is currently much concern worldwide over the water quality entering coastal waters from major river systems, especially in relation to nitrogen (N) and phosphorous (P) (Li et al., 2007, Galloway et al., 2008). In the past half century increases in riverine nitrogen fluxes into estuaries have resulted in critical ecological issues (Bouwman et al., 2005, Seitzinger et al., 2005). This has directly led to hypoxia (low dissolved oxygen < 2 mL L− 1) in estuarine waters (Rabalais et al., 2007, Diaz and Rosenberg, 2008), where low dissolved oxygen has been damaging the food chain through a reduction in primary production (Helly and Levin, 2004, Dumont et al., 2005). Of note, occurrences of harmful algal blooms in estuarine waters appear more frequent than a decade ago (Backer and Mcgillicuddy, 2006, Liu et al., 2011).
The water in the Yangtze estuary is also overloaded with nutrients. According to Zhou et al. (2006) nitrogen is the dominant nutrient in the estuary, and it increased from 0.22 mg L− 1 in 1964 to 0.76 mg L–1 in 2004. Deng et al. (2010) calculated the annual loads of pollutants in the Yangtze estuarine water and their estimates indicate that in 2005 DIN reached nearly 1100 × 103 t and active phosphate 32 × 103 t. Both are far higher than the quality targets stipulated by the Ministry of Environmental Protection, China, which gives the allowable annual loads for the Yangtze estuary as 226 × 103 t DIN and 18 × 103 t active phosphate (Meng, 2008).
The sources of DIN in the Yangtze River basin have drawn the attention of many researchers using the Globe-NEWS model (Yan et al., 2010). On that basis, they predicted that the 2050 total nitrogen (TN) diffuse input would range from 5729 to 10,265 kg N km− 2 yr− 1, when DIN loads will be from 708 to 1228 kg N km− 2 yr− 1 (1.27–2.21 million tons), primarily driven by N-fertilizer and animal manure. Most previous studies (Shen et al., 2003, Hao et al., 2006, Yan et al., 2010) indicate that N-fertilizer has been the main source of DIN in the Yangtze because the application of N-fertilizer increased dramatically from the 1970s to the 1990s. However, the growth rate of N-fertilizer application dropped significantly after 2000. Because of the variety of estimates available, and the recent and ongoing changes to inputs within the catchment, we set out here to re-examine this issue and to specifically connect catchment inputs to the measured outputs at Datong hydrological gauging station, the downstream-most station located at the upstream end of tidal influence, some 600 km from the river mouth (Fig. 1).
In this study, we aim to use the time series of DIN concentrations as measured at Datong station in conjunction with an examination of the major sources of nitrogen in the Yangtze River basin to explain changes in the observed DIN concentrations at Datong. We have apportioned the contributions of each of the major sources (sewage, atmospheric wet deposition, animal manure and fertilizer) to the total DIN loads. Using future projections of the behavior of these main sources of DIN we set up scenarios to estimate DIN loads at 10 year intervals to 2050. This study provides the basis for better strategic planning for the management of nutrient levels in the Yangtze estuary by identifying where investment in nutrient load reduction will be most effective.
Section snippets
Database and methods
The Yangtze Water Conservancy Commission (YWCC) has published records of discharge and concentrations of ammonia, nitrate and nitrite at Datong station for the period 1959–1984. The YWCC recorded hydrochemical data at intervals of a few days. DIN concentrations were calculated by extracting the nitrogen component of their molarities, summing and multiplying by the molecular weight of nitrogen as follows: Conc.(DIN) = (Conc.(ammonia)/17 + Conc.(nitrite)/39 + Conc.(nitrate)/64) × 14. From this base,
Nitrogen inputs to the catchment and DIN concentration
The DIN concentration at Datong, as compiled for this study, remained relatively stable from the 1960s to 1980, mostly below 0.4 mg L− 1 (Fig. 2A). DIN then increased rapidly to the end of the 1990s, from 0.5 to nearly 1.5 mg L− 1. After 2000, DIN has remained steady in the range 1.6–1.8 mg L− 1, with the exception that in 2006 a high DIN concentration of 2.2 mg L− 1 was recorded in the Shanghai marine environment survey project data (Fig. 2A). This was mainly because 2006 was a very dry year with one of
Contributions of N sources to DIN in the Yangtze River
Principal components analysis using these N-source variables produces 4 factors as shown in Table 1. Factor 1 explains > 98% of the variance in DIN so it has been used to apportion the relative contributions of each of the 4 sources to total DIN loads. The four source variables constitute Factor 1 as:where F denotes N fertilizer, P denotes protein consumption, M denotes manure and A denotes atmospheric wet deposition. (n = 32, F = 112.791; p < 0.001).
Scenarios for future changes in DIN concentration
Of note, the sewage treatment technology in China is not highly developed. As Wang et al. (2006) reported, physical settlement was applied in most sewage treatment plants (about 85%) in China, and this can only remove 10% of the nitrogen in sewage. The remaining (15%) plants applied biological treatment technology, which can remove 35% of the nitrogen in sewage. On average, only about 13.75% of nitrogen in sewage is being removed now. Unfortunately, in the Yangtze River basin, no treatment
Conclusion
Nitrogen loads in the Yangtze River have increased considerably in the period since China's transition to a market-based economy that began in the late 1970s so that they now exceed the levels stipulated by the Ministry of Environmental Protection, China. DIN concentrations are also now very close to the level at which the water becomes unsuitable for use as source water in water treatment plants where the end use of the water is for human consumption. This is of particular concern as large
Acknowledgments
This research is supported by the Australian Research Council (grant no: P110103381), The Creative Research Groups of the China Natural Science Foundation (40721004), and ESPA (grant no. NE/J001902/1).
References (26)
- et al.
Study on the total water pollutant load allocation in the Changjiang (Yangtze River) Estuary and adjacent seawater area
Estuarine, Coastal and Shelf Science
(2010) - et al.
Seasonal changes in nitrogen and phosphorus transport in the lower Changjiang River before the construction of the Three Gorges Dam
Estuarine, Coastal and Shelf Science
(2008) - et al.
Long-term variations in dissolved silicate, nitrogen, and phosphorus flux from the Yangtze River into the East China Sea and impacts on estuarine ecosystem
Estuarine, Coastal and Shelf Science
(2007) - et al.
Harmful algal bloom
Oceanography
(2006) - et al.
Exploring changes in river nitrogen export to the world's oceans
Global Biogeochemical Cycles
(2005) - et al.
Spreading dead zones and consequences for marine ecosystems
Science
(2008) - et al.
Global modeling of the fate of nitrogen from point and nonpoint sources in soil, groundwater and surface water
Global Biogeochemical Cycles
(2003) - et al.
Concentrations of nitrogen and phosphorus and nutrient Transport to estuary of the Yangtze River
Environmental Science
(2001) - et al.
Global distribution and sources of dissolved inorganic nitrogen export to the coastal zone: results from a spatially explicit, global model
Global Biogeochemical Cycles
(2005) - et al.
The environmental reach of Asia
Annual Review of Environment and Resources
(2008)
A method of emission of non-point source pollution load in the large-scale basins of China
Acta Scienitae Circumstantiae
Global distribution of naturally occurring marine hypoxia on continental margins
Deep-sea Research I
Wastewater Treatment
Cited by (31)
Trends in nutrients in the Changjiang River
2023, Science of the Total EnvironmentHigh importance of coupled nitrification-denitrification for nitrogen removal in a large periodically low-oxygen estuary
2022, Science of the Total EnvironmentAgricultural practices regulate the seasonality of groundwater-river nitrogen exchanges
2022, Agricultural Water ManagementFate of dissolved inorganic nitrogen in turbulent rivers: The critical role of dissolved oxygen levels
2022, Environmental PollutionExploring the effect of floodgates operation systems on water environmental capacity in a regulated river network of Wuxi, China
2021, Journal of Cleaner Production