Modelling Agricultural Nitrogen Contributions to the Jiulong River Estuary and Coastal Water

中国科技论文在线 http://www.paper.edu.cn Global and Planetary Change 47 (2005) 111–121 www.elsevier.com/locate/gloplacha Modelling agricultural nitrogen contributions to the Jiulong River estuary and coastal water Wenzhi Cao*, Huasheng Hong, Shiping Yue Key Laboratory of Marine Environmental Science, Ministry of Education, Environmental Science Research Centre, Xiamen University, Xiamen, Fujian 361005, China Received 23 February 2004; accepted 29 October 2004 Abstract The geographical setting of the Jiulong River estuary determines that the estuary receives wastes from both riverine input and adjacent urban sewage. However, the dominant nitrogen (N) source remains unclear. A nutrient mass-balance model and a preliminary LOICZ (Land–Ocean Interactions in the Coastal Zone) biogeochemical model were linked to evaluate agricultural N contributions from the Jiulong River catchment to the estuary and coastal water. Results showed that agricultural N surplus was the largest N source in the catchment, contributing 60.87% of the total Nitrogen (N) and 68.63% of the dissolved inorganic nitrogen (DIN). Household wastes and other sources followed. Riverine DIN fluxes were about 20.3% of exportable DIN and 14.4% of exportable total N, but approximately 9.7% of DIN inputs, and 7.3% of total N inputs to the Jiulong River catchment. The model system clearly showed that agricultural and anthropogenic activities in the catchment were the major N sources of the estuary and coastal water, and riverine N fluxes from these sources substantially impacted the estuary and coastal water quality and biogeochemical processes. D 2004 Elsevier B.V. All rights reserved. Keywords: agricultural catchment; nitrogen balance; land–ocean interactions in the coastal zone; estuary 1. Background coastal catchments. In some regions where agriculture is productive, total nitrogen and total phosphorus The excessive use of commercial inorganic fertil- content in receiving water bodies increased over 10- izer for raising crop yield and meeting the demand of fold during the last two decades, and over 50% of the population growth in China has resulted in increased nitrogen (N) and phosphorus (P) were contributed by nutrient additions and subsequent losses from adjacent diffuse agricultural activities (Yan et al., 1999; Cao et al., 2003). In the Chaohu Lake, nutrients from * Corresponding author. Tel.: +86 592 2181907; fax: +86 592 agricultural diffuse sources accounted for 60% of 2180655. the total N and 63% of the total P in 1988 (Yan et al., E-mail address: [email protected] (W. Cao). 1999). Contamination in receiving water bodies by 0921-8181/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2004.10.006 转载中国科技论文在线 http://www.paper.edu.cn 112 W. Cao et al. / Global and Planetary Change 47 (2005) 111–121 excessive application of inorganic fertilizers is con- sidered very common in China (Li and Zhang, 1999). It is evident that the increases in nutrient losses and riverine nutrient loads have caused the eutrophication of many coastal and freshwater ecosystems (Nixon et al., 1995; Vitousek et al., 1997). The Jiulong River, with an annual average river flow of 14800Â106 m3, discharges to the coastal sea of Xiamen City through the Jiulong River estuary. The geographical setting determines that the estuary receives wastewater with high nutrient loads from both the Jiulong River catchment and urban (Xiamen City) sewage (Fig. 1). The potential N sources into the Jiulong River estuary and coastal water bodies include: (1) agricultural diffuse N from the Jiulong River catchment; (2) urban wastewater transported through the river from the two cities and six counties within the catchment (Fig. 2); and (3) urban sewage effluent from Xiamen City. The high nutrient loading from the above sources was directly responsible for water degradation in the estuary and coastal water. Since mid-1980s, some areas in the estuary and Fig. 2. Cities and counties in the Jiulong River catchment. coastal water have been deteriorated by eutrophication and excessive growth of benthic algae (Chen et al., link both catchment processes and estuarine, coastal 1993a; Hong et al., 1999). processes for: (1) calculating various exportable N Reversal of eutrophication requires the identifica- sources in the Jiulong River catchment, which tion of pollution sources and the reduction of nutrient potentially contributed to the estuary; (2) preliminarily input. However, the relative importance of N sources estimating N budget in the estuarine and coastal in the Jiulong River estuary and coastal water remains water; and (3) evaluating agricultural N contributions unclear. While much attention has been paid to the to the Jiulong River estuary and coastal water. nutrient loads from industrial sources (point sources), yet nutrient loads from agricultural sources in this area were often overlooked. Therefore, this study 2. Area descriptions, materials, and methods employed a comprehensive modelling approach to 2.1. Area descriptions The Jiulong River is the second largest river in Fujian Province, southeast China, with a catchment area of 14741 km2. The river catchment is politically administrated by six counties and two cities (Fig. 2). Of the land uses in the catchment, 12% is arable land, 7% horticultural, 66% forestland, 1% urban, and the remainder is others. The Jiulong River estuary is a typically subtropical system, with temperate climate and average annual rainfall of 1200 mm. The water temperature fluctuates from 13 to 32 8C and pH from 7.77 to 8.47, and primary productivity is relatively Fig. 1. Geographical setting of the Jiulong River estuary. high in the estuary (Yang and Hu, 1996). 中国科技论文在线 http://www.paper.edu.cn W. Cao et al. / Global and Planetary Change 47 (2005) 111–121 113 2.2. Model systems Jiulong River catchment. The agricultural N surplus, defined as the difference between N inputs (inorganic The comprehensive modelling approach offers a fertilizers, animal manure, mineralization, legume linkage between the nutrient balance at the catchment crop fixation and atmospheric deposition) and outputs scale and the estuary biogeochemical budget model. (crop harvests, denitrification and volatilization), was The model system includes an N balance model in the potentially available for river transport (exportable). catchment and the LOICZ (Land–Ocean Interactions in In this study, 80% of the total agricultural N surplus the Coastal Zone) biogeochemical model in the estuary. was thought to be DIN. The catchment N balance model coupled with a The amount of inorganic N fertilizers, such as urea digital database and a Geographical Information and ammonium bicarbonate, was obtained from the System (GIS) was developed to evaluate contributions Fujian Statistics (Fujian Statistics Office, 1995). The of agricultural N surplus to the Jiulong River catch- estimate of the total N in animal manure produced in ment. N can be immobilized, denitrified, stored, and each county was made using animal population from exported in a variety of forms in a catchment. Much the Fujian Statistics (Fujian Statistics Office, 1995) work has been done to investigate the relationship and general elemental N content in manure (Lu et al., between the total N inputs to a catchment and riverine 1996). The product of excreted N and animal N exports (e.g., Howarth et al., 1996; De Wit, 2000; population in each animal class was summed up, Boyer et al., 2002; Van Breemen et al., 2002; Shen et whereas 25% of total animal excrements (Yan et al., al., 2003). In this study, the exportable N by a river 1999) was assumed to apply as fertilizers in the refers to the part of the total N inputs to a catchment Jiulong River catchment (Table 1). that reaches soil and water (streamflow and ground- Potentially mineralizable N from the soil organic water) system, and partially exports to a final river matter pool constitutes a substantial source of N; it is outlet (estuary or lake). The exportable N sources in particularly true in places where soil organic matter the catchment include human wastes, industrial wastes, content is high and climate is favorable. Mineraliza- agricultural N surplus, soil erosion, and wet deposition. tion in paddy soils has been investigated by many The N budget in the Jiulong River estuary was researchers (e.g., Cai et al., 1981; Yan et al., 1999) estimated according to the LOICZ (Land–Ocean and many estimate models have been developed. Soil Interactions in the Coastal Zone) biogeochemical organic matter content is likely to be a key factor model, which stoichiometrically linked water–salt– influencing mineralization due to the high soil nitrogen budget together (Gordon et al., 1996). Among moisture in paddy fields in most of the time. N the input parameters for the model, river discharge was mineralization was estimated as a function of organic obtained from measurements from a river gauge. matter content for each soil map unit using Eq. (1) Climatic data for the evaporation estimate in 1995 (Burkart and James, 1999). The amount of organic was from a meteorological station in Xiamen City. The matter in a hectare of soil was calculated as the water, salinity, and dissolved inorganic nitrogen (DIN) product of the bulk density, organic matter content in the estuary were mainly obtained from three cruises and soil volume of the top 30-cm layer. The amount of in February, May, and October in 1995. During each cruise, 12 water samples from the estuary to the adjacent sea were collected for N content and salinity Table 1 analysis (Xiamen Survey Office, 1996; Chen et al., The N content in animal manure 1993a; Hong et al., 1999; Hong and Cao, 2001), and Animal class Elemental N content Excrements (g/kg) (kg/year per unit) the data was used to calculate N budget in the estuary. Pig 24.0 2200 Cattle 18.9 9000 2.3. N balance in the catchment Goat 22.0 50 Rabbit 23.0 12 Agricultural N surplus, household wastes, indus- Chicken 29.4 5 trial wastes, soil erosion and atmospheric deposition Duck 10.0 10 were included for the exportable N estimates in the Goose 4.1 14 中国科技论文在线 http://www.paper.edu.cn 114 W.

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