An experimental agricultural catchment of Wuchuan (area 9.4 km2), and a nested small catchment (area 1.7 km2) were selected to investigate nitrogen sources and export in stormflows. Results showed that the mean nitrogen input rate in 2002 was 471.5 kg ha−1 over the large catchment, and 421.4 kg ha−1 for the smaller. A substantial portion of the nitrogen that entered the catchment was not applied to surface soil of agricultural lands. Nitrogen from animal manure and domestic wastes was deposited partially in the residential lands of the three villages, or volatilized and nitrified. Spatially, a high nitrogen input rate was observed in the residential lands of the villages and the river terraces. The annual export rate was 28.8 kg ha−1 from the large catchment in stormflows in 2002, compared to a smaller value of 26.8 kg ha−1 for the nested one. The larger nitrogen export rate from the large catchment resulted from both the larger water flows and the higher concentration through the outlets. Understanding the nitrogen sources and export in these catchments could provide a base of knowledge for remediation of diffuse agricultural pollution.
The excessive use of commercial inorganic fertilizers for raising crop yield and meeting the demand of population growth in China has resulted in increased nutrient additions and subsequent losses from adjacent coastal catchments. In southeast China, anthropogenic nitrogen (N) inputs far exceeded N outputs in regional agricultural ecosystems, and the N overuse has resulted in N losses (Cao and Zhu, 2000). 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; Carpenter et al., 1998). In particular, increased N may be a concern with respect to drinking water supplies, especially with regard to methemoglobinemia in infants and concern about linkages to non-Hodgkin's lymphoma (Ward et al., 1996). Reversal of eutrophication and contamination caused by agricultural nutrient losses and development of remedial strategies require that nutrient sources be identified and their export from sources to receiving waterbodies be assessed (Cirmo and McDonnell, 1997; Mitchel, 2001; Worrall and Burt, 2001). The objectives of this study were to investigate N sources in the experimental catchments at both the farm and the catchment level and also to characterize N export from stormflows in the catchments.
Materials and methods
The experimental catchments at Wuchuan have areas of 9.4 km2 and 1.7 km2, respectively. The small one (catchment A) is nested within the large one (catchment B), and located at the upstream site of the same stream (Figure 1a). The stream is a tributary of the upper Jiulong River in Fujian province.
Catchment elevation varies between 5.0 m and 130 m. Land uses are very complex, consisting of forestry, horticultural, paddies, vegetables, residence, fishponds and other uses. A population of 4509 resides in three villages within catchment B. Rainfall is strongly influenced by the monsoon system. In 2002, recorded rainfall was 1624 mm, less than the annual average of 1720 mm. Rainfall recorded between July and September (wet season) was 1078 mm, accounting for about 66% of total rainfall in 2002.
Landform is characterized by rolling and undulate hills. The heavily weathered granite base has been dissected by small streams. Agriculture and horticulture are usually developed in the flat alluvial valley. Red earth and lateritic red earth are the main soil types in the catchment, with pH values ranging from 4.0 to 4.8 (mean value, 4.5).
Nitrogen source auditing
The N sources in the catchments include inorganic fertilizers, animal excrement, domestic wastes, atmospheric deposition, fishpond water discharges and sediments. The magnitude of N entering the catchment is not necessarily consistent with the flow of N to agricultural land (farms). Consequently, N sources auditing was carried out at the catchment scale and at the farm scale to characterize N sources and export in southeast China. The N sources within a catchment are described by equation 1, and within the farm by equation 2.
Inorganic N fertilizer data was obtained from regular visits and questionnaires. Ammonium bicarbonate and urea were main N fertilizers applied in this area (Cao et al., 2003). Estimates of the amount of elemental N produced in manure and wastes was made by using the population of residents, animals and elemental N content in the manure and wastes (Lu et al., 1996). Atmospheric N deposition in the river catchment was estimated based on the recorded rainfall from a meteorological station and the N concentration in rainfall from a comprehensive survey on acid rain since the late 1980s (Yu et al., 1998).
Fishponds were cleared once a year. The fishpond water was pumped out for irrigation, and the sediment was removed and applied to crops. Nitrogen from fishpond removal was estimated using the product of the water volume, the sediment amount and measured N content in discharges and sediments. Measured total mean N content was 9.02 mg l−1 from 21 discharges samples and 2.31 g kg−1 from 21 sediment samples. The water volume and the sediment amount were estimated using the product of surface areas of the fishponds and investigated water depth and sediment depth.
Nitrogen export measurements
Stormflows were measured every 0.5 h using a flowmeter installed at the catchment outlet. Water quality was monitored for each storm at outlet. Usually eight to ten water quality samples were taken manually for each storm. Sampling interval varied from 0.5 to 4 h depending on the rainfall duration. Total N was analyzed for each water sample. Standard colorimetric techniques were used to analyze total N. All samples were delivered to a laboratory and analyzed within 24 h.
Results and discussion
Overall, the area-weighted, mean input rate of N over catchment B was 471.5 kg ha−1 (Table 1), higher than the rate of 421.4 kg ha−1 over catchment A. However, the mean application rate of N over the farms within catchment B and catchment A was 341.6 and 435.7 kg ha−1, respectively. Over the catchments inorganic N fertilizers were the largest single source input (> 56%, Table 1), followed by manure application (> 14.9%), domestic wastes (> 4.4%), and atmospheric wet deposition (> 3%). The input patterns into the farms remained similar within the catchments.
A significant discrepancy between N inputs to catchment B and N applied to the farms within the catchment B showed that the spatial patterns of N inputs varied considerably. A substantial portion of N entering the catchment was not applied to surface soil of the farms. In fact, this N was mainly derived from animal manure and domestic wastes (Table 1), which was deposited partially in the residential lands of three villages, or emitted through volatilization and nitrification.
A slight difference of N inputs between the catchment A and the farms within the catchment A was observed, due to a fact that the N from fishpond discharges and sediments was internally recycled, rather than a new N source in the catchment A, but re-applied to farm crops. Therefore, this source of N was taken into account when the N sources were audited in the farms within catchment A.
Higher N input rate (kg ha−1) in catchment B, compared to the rate in catchment A, showed that although residential lands occupied a small portion of the total area, the N deposition (> 800 kg ha−1) in the three villages contributed substantially to the N inputs to catchment B, further effectively increasing the N input rate there (Figure 1b).
The spatial patterns of N inputs in the catchments verified the above results from data analysis. A high N input rate was observed in the residential lands of three villages and the river terraces (Figure 1b). Traditionally, animal manure and domestic wastes were applied as organic fertilizers to crops. However, due to the huge population and very limited land resources, which exacerbated the farmers' low incomes, the conventional farming benefit has become minimal. Farmers in China have been forced to seek off-farm employment and benefits. A negative consequence is that only small portion of animal manure and domestic wastes are currently used as organic fertilizers. The remainder is often deposited onto residential land, because there are no facilities available for water treatment and waste disposal in rural areas. The development in Chinese rural areas nowadays has been substantially hindered by farmers' lower incomes and degraded environmental quality.
Nitrogen export in stormflows
Nitrogen export in each stormflow was quantified by multiplying measured N concentration by the flow rate during the storm event. Nitrogen export rate in stormflows was aggregated to a monthly scale (Figure 2), owing to the difficulties in distinguishing flows between intermittent rainfall events. The monthly N export rate in stormflows varied from almost zero to 17.6 kg ha− 1 in catchment B, with a summation of 28.8 kg ha− 1 in 2002. In catchment A, the rate varied from nearly zero to 15.1 kg ha− 1, with a summation of 26.8 kg ha− 1 in 2002.
Seasonal N export patterns showed that approximately 95% of the N export in stormflows occurred during July to September in the catchments, when rainfall intensity was very large and the rainfall was about 66% of the total rainfall in 2002 (Figure 2). More or less, the rainfall was typhoon related in the period. These results were in agreement with the findings that overall nutrient yield was generally accounted for by the few extreme flood events in a given year (Novotny and Chesters, 1989).
The mean N export in stormflows accounted for approximately 6.1% of the total N inputs to catchment B, but about 6.4% the total N inputs to catchment A in 2002. The slight difference showed that N in catchment A is more susceptible to transport because much higher percentages of inorganic fertilizers were applied there (Table 1).
A higher riverine N export rate (28.8 kg ha− 1) from catchment B showed that more N was available from erosion, and this was verified by our N sources auditing. Nitrogen deposited in the residential lands of the three villages increased the N susceptibility to erosion and transport in the large catchment. In fact, higher total N concentration and higher water flows through the outlet of catchment B (Table 2) resulted in much higher riverine N exports. Catchment B played a more critical role in N transport, due to the higher N availability and higher water flows there. Understanding the N sources and export in these catchments could provide a knowledge base for remediation of diffuse agricultural pollution.
With industrial source pollution under control, the environmental implication of this study is that diffuse agricultural pollution is becoming one of the major environmental concerns in southeast China (Cao et al., 2005). The magnitude and variety of N input to a catchment can exert a significant influence on N export in stormflow. Environmental quality of residential villages, and integrated nutrient management in agricultural catchments are crucial to control nutrient pollution in receiving waterbodies. To mitigate diffuse agricultural pollution, facilities for water treatment and waste disposal must be built and further implementation of integrated nutrient management in Chinese rural areas is essential.
The authors acknowledge the funding for this study from the Natural Science Foundation, China (40301045).