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

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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

112 W. Cao et al. / Global and Planetary Change 47 (2005) 111–121 excessive application of inorganic fertilizers is considered 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 14800106 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

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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. Cao et al. / Global and Planetary Change 47 (2005) 111–121 mineralizable N was considered as the product of the peanuts, and 130 kg/ha for clover in a year. N fixation amount of organic matter, fraction of total N in by heterotrophic organisms in a paddy field might organic matter (3%), and annual mineralizable portion account for another N addition. However, widely of organic N (2%): adopted rice–soybean and rice–vegetable rotations and high rate of inorganic N fertilizer application N ¼ q V O N N ð1Þ m b s m e p suppressed non-legume N fixation. Biological N where Nm is mineralizable N (kg/ha), qb is bulk fixation and biological transformation of N by 3 density of specific soil (kg/m ), Om is organic matter organisms, therefore, were not taken into account in content of soil (%), Vs is volume of the top 30-cm soil this study. The estimate showed that about 0.2% of the 3 layer in 1 ha (=3000 m ), Nc is total N fraction of soil total N inputs was from legume fixation, compared organic matter (3%), and Np is annual mineralizable with a national average value of 5% in China and a portion of soil organic N (2%). world average value of 7% (Sheldrick et al., 2003). The estimate was based on a GIS soil map layer The quantity of N fixed by legume was relatively (Fig. 3), and the soil properties were obtained from a small due to the minimal sown areas of legume crops soil survey between 1986 and 1989 (Soil Survey in the river catchment. Group, 1991). Only arable and horticultural lands were Annual atmospheric N deposition in the river used for mineralization estimate due to frequent tillage, catchment was estimated based on recorded precip- and continuous additions of fresh organic matter (crop itation from a meteorological station and the N residue). Mineralization and denitrification over non- content in rainfall from a comprehensive survey on agricultural lands were thought to be equivalent. The acid rain since late 1980s. In the 1980s, acid rain was total mineralizable N was summed up according to the first reported in this area and then a comprehensive administrative boundary for counties and cities. survey was carried out. The survey showed nitrate The main source of biological N fixation is through was an important contributor to acid rain in the area the growing of legumes, such as soybean (Glycine (Tang et al., 1996; Yu et al., 1998). An average N max), peanuts (Arachis hypogaea), and clover (Astra- content of 0.89 mg/l was used to calculate the galus sinicus). Average rates of N fixation were atmospheric N deposition in the river catchment (Yu assumed to be 105 kg/ha for soybean, 112 kg/ha for et al., 1998). The N deposition over arable and horticultural lands was included for calculations. The total N deposition over forest, urban and other land uses was also calculated, and 25% was thought to be exportable due to immobilization and gaseous emission, e.g., plant consumption and denitrification. The total N in crop harvests was estimated as a summation of N in crop economic parts (Fujian Statistics Office, 1995), e.g., seeds and fruits. The total N content in seeds and fruits for different crops was collected from different sources (Lu et al., 2000; Cao and Zhu, 2000). The total N in vegetation was assumed to be 3.5 g/kg because production from diverse vegetable species was summed as an overall production rather than itemized in the yearbook. Fruits including citrus, longan, and banana have a wide range of N content, but 0.15 g/kg was assumed for the N harvest calculation in fruits (Table 2). Ammonia volatilization is an important pathway of N loss. Loss rate through ammonia volatilization primarily varies with fertilizer types (Burkart and Fig. 3. The mineralizable N in the Jiulong River catchment. James, 1999). Ammonium bicarbonate and urea were 中国科技论文在线 http://www.paper.edu.cn

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Table 2 The total N content produced by a person is about The elemental N content for crops 1.86 kg/year in solid waste, and 4.0 kg/year in Crop N content in grain domestic sewage (Valiela et al., 1997; San Diego- (g/kg) McGlone et al., 2000). 54.6% of the total N was Rice 13.30 thought to be DIN (Shen et al., 2003). Therefore, the Wheat 19.40 total N from households was estimated using the Soybean 59.20 Peanut 44.00 registered inhabitants in the catchment and the above Maize 13.80 coefficients. Immigrants were thought to be equiv- Oilseed 37.80 alent to emigrants in the catchment. The amount of Potato 1.80 industrial wastes was obtained from the statistics Sugarcane (dry) 1.00 (Fujian Statistics Office, 1995), and the general N Tobacco 25.00 Vegetable 3.50 concentrations in industrial wastes were 17.41 mg/l Fruit 0.15 for total N and 34.81 mg/l for DIN (Shen et al., 2003). The total eroded areas were 11.15% (1434.1 km2) of the total lands, and the average soil erosion was the main N fertilizers applied in this area (Cao et al., 2387 t/km2. The total N in the soil averages 0.93 g/kg, 2003). Field tracer 15N experiments showed that more and 50% was thought to be DIN (Cao and Zhu, 2000). than 50% of the surface broadcast urea (about 54%) and ammonium bicarbonate (about 57%) in noncalcareous 2.4. N biogeochemical budgeting in the estuary paddy fields were lost through ammonia volatilization. The recovery rate (the percentage of nutrient input A major aim of the LOICZ is to quantitatively which is recovered as nutrient output in the crop) of urea assess the role of the coastal zone in the cycling of and ammonium bicarbonate in rice was only 27.5% and carbon and nutrients at regional and global scales 24.0%, respectively (Chen and Zhu, 1981). Recent field (Crossland et al., 2001; Talaue-McManusa et al., experiments showed that the ammonia loss rate through 2003; Wattayakorn et al., 2001). The LOICZ bio- volatilization from urea in a paddy filed varied from geochemical model (Gordon et al., 1996) provides a 7.52% to 13.67% of total applied N, depending on common assessment framework within which world- fertilization timing and crop-growing periods (Tian et wide coastal assessments in carbon and nutrient cycles al., 2001). In view of the above, annual average loss rate become possible and cost-effective. Detailed proce- of 15% was assumed for ammonia volatilization from dure and description of the model can be found at urea and 20% for ammonium bicarbonate. http://www.nioz.nl/loicz/info.htm. The linked water– N can be reduced to gaseous N forms under anoxic salt–nutrient model for N budget begins with water conditions, i.e., denitrification. This process has been and salt balance calculations in an estuary. The considered as another pathway of N loss in soil–plant riverine N input is a key variable to conduct the N systems. Denitrification is carried out by denitrifying budget in the estuary and to link the catchment N bacteria in soils and is favored in warm anaerobic balance model. The amount of riverine N input was conditions. Agricultural land use in the Jiulong River flow-weighted, and estimated by multiplying meas- catchment can be categorized into the paddy fields and ured concentrations of DIN in the river outlet from the other land uses (horticulture, rainfed upland for crops) three cruises by the relevant discharges. to reflect soil moisture and anoxic conditions based on a GIS land-use map layer. Although denitrification rate from laboratory incubations can provide a precise 3. Results and analyses value on a point site, scaling-up from such experiments to regional level may result in major errors (Sheldrick 3.1. The exportable N in the catchment et al., 2003). Therefore, 70 kg/ha was assigned as the N denitrification in paddy fields and 30 kg/ha for other The exportable N in the catchment is a potential land uses (Li, 1990). Similarly, arable and horticultural riverine N source, and can be partially transported lands were included in the denitrification estimate. through streamflow into the estuary system. Over the 中国科技论文在线 http://www.paper.edu.cn

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Table 3 agriculture and horticulture in the Jiulong River The agricultural N sources and losses in the catchment catchment strongly relied upon the anthropogenic N N Inputs and outputs N inputs, with much more inorganic N fertilizers used (t/year) than in other places. Surveys showed inorganic N Inputs Inorganic fertilizer 83398 fertilizer was applied at rates ranging from 150 to 300 Manure 18258 kg/ha in the catchment, and urea and ammonium Mineralization 9230 Atmospheric deposition 2728 bicarbonate were predominant (Cao et al., 2003). This Crop fixation 172 well explained that a significant portion of the total N Subtotal 113786 was lost to atmosphere through volatilization and Outputs Crop harvests 38909 denitrification from the catchment (Table 3). Overall, Volatilization 20970 the agricultural N surplus provided a dominant N Denitrification 9880 Subtotal 69759 source to the estuary and coastal water. Surplus N 44027 N balance studies at many spatial scales showed a surplus in China since 1975, and similarly, inorganic N fertilizers were considered as the largest input to catchment, agricultural N surplus was the largest N research regions (Lu et al., 1996; Yan et al., 1999; source, contributing 60.87% of the total N and Sheldrick et al., 2003). Historically, the N use for 68.63% of the DIN inputs, household wastes China was very low (Zhu, 1997). The N application (24.59% for total N and 18.73% for DIN) and other rates varied from 70 to 100 kg/ha and N was applied sources followed (Table 4). Of the agricultural N only as manure in the 1950s in the Lake Taihu. sources (Table 3), the total N from inorganic N However, since 1980s, the N application rate in the fertilizers was recorded in each county at level region became considerably high, and the current N ranging from 4251 to 29399 t with a total amount application varied from 500 to 800 kg/ha to different of 83398 t in the entire catchment (Fig. 4). Inorganic crops (Ellis and Wang, 1997). The ratio between N N fertilizers contributed 73% of the total N inputs and inputs and outputs (Table 4) in the Jiulong River manure 16%, comparing with the national average catchment was 1.63, consistent with 1.85 calculated level of 65% for inorganic fertilizers and 14% for by Lu et al. (2000) for the entire Fujian Province in manure in China (Sheldrick et al., 2003). Therefore, that period. However, Yan et al. (1999) obtained a

Fig. 4. Agricultural N inputs and outputs in the Jiulong River catchment. 中国科技论文在线 http://www.paper.edu.cn

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Table 4 groundwater, precipitation, and evaporation was The exportable total N (TN) and DIN sources and contributions in assumed to be zero. Here, (S +S )/2 was assigned the catchment syst ocn as salinity in the residual flow (SR), which is indicated N sources TN TN contribution DIN DIN contribution by Gordon et al. (1996). The salinity balance showed (t/year) (%) (t/year) (%) annual mixing exchange (VX) between the estuary and Household 17790 24.59 9611 18.73 the adjacent sea was 85109 m3/year and water Industrial 3666 5.07 1833 3.57 wastes exchange time (s) was approximately 2 days (Fig. 5). Atmospheric 3518 4.86 2990 5.83 Apparently, tides are the main force driving water deposition exchange with the adjacent sea. In the model, the Soil erosion 3334 4.61 1667 3.25 exchange water fluxes (VX) is critical to estimate N Agricultural 44027 60.87 35221 68.63 budget in the estuary. surplus Exportable N 72335 100 51322 100 The riverine DIN input (VQDINQ) was the largest N source of the estuary (10430 t/year). The DIN input (VoDINo) of 97.9 t/year from household wastes and higher ratio of 2.87 in a small agricultural watershed, industrial sewage from Xiamen City was included, and this may be explained by a higher percentage and the DIN in annual precipitation contributed 34.2 t/ (70%) of agricultural land use there than the value of year to the estuary. These DIN inputs together were 19% in the Jiulong River catchment. approximately balanced by the DIN exports in the residual flow (VRDINR=4676 t/year) and the DIN 3.2. N budget in the estuary exports in the mixing exchange with adjacent ocean (VX(DINocnDINsyst)=4648 t/year), with an internal 9 3 The river discharge (VQ) of 14.910 m /year was sink of DIN (DDINsyst) being 1238 t/year, and the N obtained from a flow gauge. The evaporation (VE), sink was approximately 11.9% of the riverine input calculated using the Penman equation (Penman, 1948), (Fig. 6). Moreover, a net efflux of DIN from the was 1360 mm (about 0.1109 m3/year over the estuary system indicated that the estuary was an N source of area of 85 km2), and was about 13% greater than the coastal sea (Fig. 6). annual average precipitation (VP) of 1200 mm. Assuming that phytoplankton was the dominant Assuming the groundwater supply to the estuary was primary producer in this estuarine system, the differ- zero, the annual residual water flux to the adjacent ence between observed DDIN and expected from the 9 3 ocean (VR) became 14.910 m . Averaged salinity decomposition of organic matter indicated the estuary in the estuary (Ssyst) and the adjacent ocean (Socn) was was a net nitrifying system. The expected DDIN is the obtained from the three cruises (Xiamen Survey product of dissolved inorganic phosphorus fluxes Office, 1996; Chen et al., 1993a; Hong et al., 1999; (Hong and Cao, 2001) in the estuary and molar ratio Hong and Cao, 2001), and salinity in the river flow, of phytoplankton (Gordon et al., 1996). Further

Fig. 5. Water and salt budgets for the Jiulong River estuary (water flux in 109 m3/year and salt in 109 psu-m3/year). 中国科技论文在线 http://www.paper.edu.cn

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Fig. 6. The DIN budget for the Jiulong River estuary (DIN Flux in t/year). stoichiometric calculations in the similar way for net 9.7% of the DIN inputs, and 7.3% of the total N inputs system metabolism, which is the difference between to the Jiulong River catchment. These results were organic carbon production and respiration, showed that generally consistent with the findings that less than the Jiulong River estuary was an autotrophic system, 30% of the total N inputs to a catchment were and apparently a net producer of organic material. exported to the oceans with surface runoff in streams The LOICZ biogeochemical model (Gordon et al., (Howarth et al., 1996; Boyer et al., 2002; Van 1996) facilitates rapid coastal assessments within the Breemen et al., 2002). However, much less riverine project bThe role of the coastal ocean in the disturbed N exports here likely showed that given the high N and undisturbed nutrient and carbon cyclesQ, which inputs, both the N consuming rate by crops and was jointly sponsored by LOICZ and the United gaseous N losses were also high in the catchment. Nations Environment Programme, and supported by Global Environment Facility. The model has been 3.3. Discussion and scenario analyses used to build a worldwide nutrient cycle database in estuary and coastal areas. Estimates of riverine N Little research is available on quantitative assess- fluxes in some Chinese rivers were highly variable ment of agricultural N contributions to an estuary depending on locations, river runoff, and sampling and coastal water, due to a traditional view that N seasons (Table 5). The riverine N fluxes calculated from industrial sources was the major polluter to (10430 t/year) in this study were located between the rivers. The recent estimates of DIN fluxes from two estimates (Chen et al., 1993b; Zhang, 1996), but some Chinese rivers (Table 5) usually exceeded the closed to the higher one (Zhang, 1996). This amount old ones, except the Pearl River (Huang et al., of N was about 20.3% of the exportable DIN and 2003), showing a general trend of increase. In the 14.4% of the exportable total N, but approximately Changjiang River, Zhang (1996) attributed this increase in riverine DIN fluxes to the increase in chemical fertilizers uses, compared to the previous Table 5 estimate by Edmond et al. (1985). However, as Estimates of riverine DIN fluxes in some Chinese rivers Zhang (1996) already indicated, there was no direct Rivers Drainage Fluxes Areal yield Sources evidence to relate the riverine N exports to area (km2) (t/year) (kg/km2 year) agricultural activities. The more recent research by Jiulong 14741 6000 407 Chen et al. Shen et al. (2003) showed that the precipitation in River (1993b) the Changjiang River catchment was the major N 12600 855 Zhang (1996) sources of riverine N fluxes. These calculations Changjiang 1808500 625800 346 Zhang (1996) 1746000 965 Shen et al. showed a high temporal and spatial variability of N (2003) sources in a catchment. Our models clearly indicated Pearl River 442585 369600 835 Zhang (1996) that agricultural activities and domestic wastes in the 179293 405 Huang et al. catchment were the major N sources of the estuary (2003) and coastal water. 中国科技论文在线 http://www.paper.edu.cn

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The riverine N fluxes was about 20.3% of the catchment and estuary might lead to uncertainties in exportable DIN and 14.4% of the exportable total N in the estimates. the Jiulong River catchment, and this yielded 9324 t/ A more reliable estimate of N cycles in a catchment year (DIN in residual and mixing water) of the DIN and estuary needs process-based models to integrate exports to the coastal sea, and 1238 t/year of the DIN catchment and estuarine processes. Such models (e.g., sink in the estuary. If much more chemical fertilizers Mortazavi et al., 2000; Donner, 2004) can quantify N were used, higher amount of the riverine N exports fate and transport in each ecological component, and would be expected. This increase would be balanced evaluate involved processes in N cycles. However, by much more N efflux to the coastal sea and higher N extensive measurements are still necessary to validate sink in the estuary. The increase would definitely exert these models. an influence on estuary and coastal biogeochemistry, Despite the above uncertainties in the N budgeting and exacerbate eutrophication in the ecosystems. This in the catchment and estuary, the model system still might not be the case, but nutrient exports from the provides a valuable insight into the N contributions of Jiulong River catchment substantially impacted estu- agricultural and anthropogenic activities to the Jiulong ary and coastal water quality and biogeochemical River estuary and coastal water. processes. Contributions of agricultural N to the estuary and coastal systems from the agricultural catchment had been previously overlooked. 4. Conclusions

3.4. Uncertainty analyses The N mass-balance model in the Jiulong River catchment and the LOICZ biogeochemical model The model system is subject to considerable were linked to evaluate agricultural N contributions uncertainties because of extreme complexity of N to the Jiulong River estuary and coastal water. The cycles in a catchment and estuary. model system clearly indicated that agricultural and Landscape plays an important role in N retention anthropogenic activities in the Jiulong River catch- and transport in a catchment. Wetlands, ponds, lakes, ment were the major N sources of the estuary and and riparian vegetation can effectively reduce diffuse coastal water. N exports from the agricultural and N losses to surface waters. Direct efflux of N from anthropogenic activities have substantially impacted mainly point sources is not in contact with the soil/ the estuary and coastal water quality and biogeo- groundwater system, whereas indirect N emissions chemical processes. Best management practices and from diffuse sources reaches the surface water via the landscape management that aiming at the reduction of soil/groundwater system. Therefore, landscape can diffuse nutrient losses via surface water can be further decrease actual N flow to surface water by efficiently applied to nutrient management in the immobilization, such as denitrification and retention. catchment, estuary, and coastal water. Losses by denitrification in landscape soils are highly uncertain. Although denitrification rate from laboratory incubations can provide a precise value on Acknowledgements a point site, scaling-up from such measurements to a catchment can result in major errors (Sheldrick et al., The authors acknowledge the funds for this study 2003). An average rate of denitrification uniformly from the Natural Science Foundation (40301045), used for all soils and land uses could not reflect spatial China. In particular, we acknowledge START and variability of the processes. Volatilization of inorganic APN, which provided financial support and selected fertilizers and mineralization inputs varies with many this work for oral presentation at the International factors, and the estimates of volatilization and Young Scientists’ Global Change Conference, held in mineralization in the catchment might yield some Trieste, Italy, 16–20 November 2003. We would like errors. Monitoring of riverine nutrient fluxes was very to acknowledge Dr S.V. Smith and Dr C. Crossland weak both in time and space (Zhang, 2002), and for their scientific assistance in LOICZ-UNEP East inadequate monitoring and scarcity of data in the Asia Workshop, 12–14 June 2000, Hong Kong. We 中国科技论文在线 http://www.paper.edu.cn

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