Impact of nutrient losses from agricultural lands on nutrient stocks in Dianshan Lake in Shanghai, China

Shou-bing WANG, Xiao-xue MA, Zheng-qiu FAN, Wei-qian ZHANG, Xiao-yong QIAN*,

1. Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, P. R. China

2. Shanghai Academy of Environmental Sciences, Shanghai 200233, P. R. China

Impact of nutrient losses from agricultural lands on nutrient stocks in Dianshan Lake in Shanghai, China

Shou-bing WANG1, Xiao-xue MA1, Zheng-qiu FAN1, Wei-qian ZHANG1, Xiao-yong QIAN*1,2

1. Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, P. R. China

2. Shanghai Academy of Environmental Sciences, Shanghai 200233, P. R. China

The water quality of Dianshan Lake in Shanghai Municipality, China, is impacted by nutrient losses from agricultural lands around the lake. In this study, nine types of agricultural land use were monitored in 2010 and 2011, and a correlation analysis between nutrient losses from agricultural non-point sources (NPS) and nutrient stocks in the lake was conducted over monthly and seasonal time periods. The results indicate that the monthly average concentration of total nitrogen (TN) ranged from 1.41 to 7.34 mg/L in 2010 and from 1.52 to 5.90 mg/L in 2011, while the monthly average concentration of total phosphorous (TP) ranged from 0.11 to 0.26 mg/L in 2010 and from 0.13 to 0.30 mg/L in 2011. The annual loss of TN from agricultural NPS was 195.55 tons in 2010 and 208.40 tons in 2011. The cultivation of water oat made the largest contribution to the loss of TN. The annual loss of TP was 44.58 tons in 2010 and 48.12 tons in 2011, and multi-vegetable cultivation made the largest contribution to the loss of TP. The results of correlation analysis show that the monthly stocks of TN and TP in the lake have a positive correlation with the monthly losses of TN and TP from agricultural NPS. According to the seasonal data, the stocks of TN and TP in the lake both have a much stronger correlation with the losses of TN and TP from agricultural NPS in summer than in other seasons. Agricultural NPS pollution control should be the main focus for the water resource conservation in this area.

agricultural non-point source pollution; nutrient losses; nutrient stocks; total nitrogen (TN); total phosphorus (TP); Dianshan Lake

1 Introduction

With rapid economic development, eutrophication occurring in estuaries and lakes has become a serious environmental problem all over the world. Nutrient losses from agricultural lands to the water environment have increased rapidly in comparison to losses from industrial and residential lands, and agricultural non-point source (NPS) pollution has been demonstrated to accelerate eutrophication of aquatic systems in many countries (De Wit and Bendoricchio 2003; Díaz, et al. 2012; Reungsang et al. 2007; Zhang et al. 2011). In China, the pollutants from agricultural production are the key factors in deteriorating water quality according to TheFirst China Pollution Source Census issued by Ministry of Environmental Protection in February 2010 (Liu et al. 2013). This is especially true in the Yangtze River Delta and the Taihu Lake Basin, where water eutrophication has been the primary focus of environmental protection since a severe blue algae bloom occurred in Taihu Lake in 2007 (Sun et al. 2013).

In many economically developed regions with abundant water resources, where the pollution of industrial and urban point sources has been controlled, such as the Yangtze River Delta, total nitrogen (TN) and total phosphorous (TP) losses from agricultural NPS have caused critical impacts on local water resources (Chen et al. 2009). To abate the eutrophication, pollution reduction measures should be implemented in agricultural catchments. In Jiangxi Province, an agroforestry system was developed to reduce nutrient losses (Wang et al. 2011). New research has demonstrated that agroforestry buffers can also significantly reduce runoff volume, and the transfer of sediment, TN, and TP to streams in the United States (Udawatta et al. 2011). However, their implementation is confounded by the fact that nutrient losses are not evenly distributed within different agricultural lands but show wide spatial variation according to soil characteristics, topography, climate, and agricultural practices (Mansikkaniemi 1982; Pionke et al. 1997; Parn et al. 2012). Furthermore, not all nutrients leaving the agricultural field will reach waters susceptible to eutrophication, and most nutrients may be retained during transport processes (Walling 1977; Prairie and Kal 1986; Arheimer and Brandt 1998). Meanwhile, nutrient losses from agricultural NPS can easily reach the water system in a flood plain with dense river networks in humid regions, causing water quality to deteriorate severely.

Although many studies have been conducted on the relationship between nutrient losses from agricultural NPS and water quality, most of them have focused on the impact of nutrient losses from the watershed on nutrient concentrations upstream and downstream of a certain point (Vega et al. 1998; Wang et al. 2008; Chen et al. 2009). For flood plains with dense river networks influenced by daily tides, it is difficult to use conventional methods to study the impact of nutrient losses from agricultural NPS on nutrient stocks in the lake. There are few studies on the relationship between nutrient losses from agricultural NPS and nutrient stocks in lakes in an agricultural system. Dianshan Lake is one of the most important drinking water sources of Shanghai Municipality, and accounts for more than 50% of the drinking water supply of the city. However, the previous studies lack agricultural NPS analysis in the Dianshan Lake area. This paper first introduces nutrient losses from different types of agricultural land use and variations of nutrient concentration in Dianshan Lake, and then describes a correlative analysis between the nutrient losses from agricultural NPS and nutrient stocks in Dianshan Lake.

2 Materials and methods

2.1 Study area

Dianshan Lake, which is the largest freshwater lake in Shanghai Municipality, with anarea of 63 km2and an average depth of 2 m, sits in the Taihu Lake Basin and the Yangtze River Delta (Fig. 1). The lake connects with the Huangpu River and the Wusong River, and is influenced by tides from the Yangtze Estuary (Cheng et al. 2010). Much of the Dianshan Lake area is covered with slopes with gradients of less than 2%. The lake has a subtropical moist monsoon climate, with an annual average temperature of 15.5°C, an annual average wind speed of 3.7 m/s, and an annual average amount of sunlight of 2 371.7 h/year. The annual average precipitation and evaporation of the lake are 1 037.7 mm and 900 mm, respectively. Water in Dianshan Lake comes mainly from surface runoff and precipitation, and the lake retention time is about 29 days. There are more than 70 rivers flowing in or out of Dianshan Lake. Most of the inflowing water, about 67%, comes from Taihu Lake through the western ports. Water flows out of Dianshan Lake mainly through the eastern Lanlu Port, and then flows further east, finally entering the East China Sea through the Huangpu River (Wang and Dou 1998). The Dianshan Lake area in Shanghai, including Zhujiajiao Town, Jinze Town, and Liantang Town in Qingpu District, is the main study area for examination of agricultural NPS.

Fig. 1 Location of Dianshan Lake area in Shanghai

This area, which is a sub-catchment of the Taihu Lake Catchment, is a water source conservation zone within the Shanghai Municipality. The area is defined as an agriculture-dominated area, and industrial development is limited, with the breeding of livestock and poultry being forbidden. Based on the local characteristics of land use, agricultural lands, such as paddy fields and aquaculture ponds, have been primarily developed and have become the main sources of agricultural NPS pollution in this area. Anotherimportant factor that influences the hydrodynamics and pollutant transportation in the study area is tides, which occur about every 12 hours (Kang 2012). In the study area, agricultural lands consist of paddy fields, dry fields, and aquaculture ponds, with proportions of 49.30%, 21.19%, and 29.51%, respectively. The paddy fields can be divided into five plantation types, including paddy-wheat rotation, paddy-rapeseed rotation, paddy-green manure rotation, paddy-water oat rotation (water oat is a native aquatic economic crop), and water oat monoculture; the dry fields can be divided into two plantation types, including multi-vegetable cultivation and fruit cultivation; and the aquiculture ponds can be divided into two breeding types, including fish breeding and shrimp breeding.

2.2 Experiment design

In 2010 and 2011, sampling of water quality and nutrient losses was conducted 24 times. Water quality samples were taken from 15 sampling sites in different parts of the lake at the end of each month (NEPAC 1993). Nutrient samples of the agricultural lands were taken from nine monitoring sites, which represented nine typical agricultural production modes (two types of dry fields, five types of paddy fields, and two types of aquaculture ponds) (Fig. 2). Nutrient losses usually occur after each rainfall or water exchange. Every monitoring site for each plantation type of paddy fields and dry fields had three paddocks with the same area of 20 m2(4 m×5 m), as shown in Fig. 3(a). Each paddock was connected with a runoff collection bucket by a collection pipe. After runoff had been collected in the runoffcollection bucket, the water volumes and concentrations of TN and TP in the runoff were monitored. Every monitoring site for two breeding types of aquaculture ponds had three enclosures with the same area of 667 m2, as shown in Fig. 3(b). There was a pump in each enclosure for water. After discharge, the water level in every enclosure and concentrations of TN and TP in the discharged water were monitored. All the agricultural measurement at the monitoring sites followed the local conventional tillage and management modes. Precipitation was monitored with a rain gauge at every monitoring site after rainfalls.

Fig. 2 Monitoring sites of Dianshan Lake area in Shanghai

Fig. 3 Schematic design of monitoring sites

2.3 Sampling and analysis

Samples from the lake and agricultural lands were analyzed using the following methods. For measurement of TN, 10 mL of nutrient sample were digested by alkaline potassium peroxodisulphate under 120 °C for 30 minutes, an then 1 mL of HCl and non-ammonia water were added to the sample, respectively, when the volume of the sample was 25 mL. TN in the samples were measured with a UV spectrophotometer at wavelengths of 220 and 275 nm. Finally, TN concentration was calculated based on the standard curve (NEPAC 2002). For measurement of TP, 25 mL of nutrient sample were digested with potassium peroxodisulphate at 120 °C for 30 minutes, and then 1 mL of ascorbic acid solution and 2 mL of molybdate solution were added to the sample. After 15 minutes, TP of the samples was measured with a spectrophotometer at a wavelength of 700 nm. Finally, TP concentration was calculated based on the standard curve (NEPAC 2002). Every sample was analyzed within 24 hours after sampling.

The nutrient stocks were calculated from the monthly mean water volume and nutrient concentration, and a correlation analysis between nutrient stocks in the lake and nutrient losses from agricultural NPS was conducted using Pearson correlation analysis with SPSS 13.0.

3 Results

3.1 Monthly precipitation in Dianshan Lake area

As shown in Fig. 4, the monthly precipitation in the Dianshan Lake area ranged from 21.5 to 293.9 mm in 2010 and from 10.7 to 311.2 mm in 2011, and the annual precipitationwas 1 071.8 mm in 2010 and 1 268.9 mm in 2011. The precipitation in the area concentrated in summer and spring during the plum rain season, which accounted for more than 50% of the total annual precipitation.

Fig. 4 Daily and monthly precipitation in Dianshan Lake area in 2010 and 2011

3.2 Nutrient losses from agricultural lands in Dianshan Lake area

The annual TN losses from agricultural NPS were 195.55 tons in 2010 and 208.40 tons in 2011. The TN losses from water oat cultivation, which contributed the most to the TN losses of the whole area, were 52.85 tons in 2010 and 52.52 tons in 2011. The annual TP losses were 44.58 tons in 2010 and 48.12 tons in 2011. The TP losses from multi-vegetable cultivation, which contributed the most to the TP losses of the whole area, were 18.79 tons in 2010 and 20.72 tons in 2011 (Table 1).

Table 1 Nutrient losses from different agricultural lands in Dianshan Lake area

3.3 Monthly variation of TN and TP in Dianshan Lake

The monthly average concentration of TN in Dianshan Lake ranged from 1.41 to 7.34 mg/L in 2010 and from 1.52 to 5.90 mg/L in 2011, while the monthly average concentration of TP ranged from 0.11 to 0.26 mg/L in 2010 and from 0.13 to 0.30 mg/L in 2011 (Fig. 5). TN concentrations appeared generally lower from July to October than in other months, and TP concentrations followed the opposite pattern, appearing higher from February to March and from June to September than in other months.

Fig. 5 Variations of monthly average TN and TP concentrations in Dianshan Lake in 2010 and 2011

The monthly average water level in Dianshan Lake ranged from 2.38 to 2.94 m in 2010 and from 2.28 to 3.12 m in 2011. The water volume could be calculated according to the water level (Chen and Li 2008), and the corresponding monthly average water volume ranged from 1.03 × 108to 1.37 × 108m3in 2010 and from 1.01 × 108to 1.55 × 108m3in 2011 (Table 2).

Table 2 Water levels and water volumes in Dianshan Lake in 2010 and 2011

4 Discussion

4.1 Relationship between nutrient losses from agricultural NPS and nutrient stocks in Dianshan Lake

The correlation analyses between monthly nutrient losses from agricultural NPS and nutrient stocks in Dianshan Lake are shown in Fig. 6. The monthly TN stock in the lake had a positive correlation with the monthly TN loss from agricultural NPS, and the monthly TP stock in the lake had a stronger positive correlation with the monthly TP loss from agricultural NPS. This may be due to the different N:P ratio in the lake as well as from agricultural NPS (Fig. 7). When the nutrients were discharged to the lake from agricultural NPS, the TN was easily utilized by the phytoplankton in the lake. However, phytoplankton utilized less phosphorus than nitrogen in the lake. Therefore, the N:P ratio from agricultural NPS was much lower than that in the lake (Kim et al. 2007).

Fig. 6 Relationships between monthly average TN and TP losses from agricultural NPS and TN and TP stocks in Dianshan Lake in 2010 and 2011

Fig. 7 Variations of monthly average N:P ratio in lake and from agricultural NPS in 2010 and 2011

4.2 Relationship between seasonal nutrient losses from agricultural NPS and nutrient stocks in Dianshan Lake

According to mean temperature differences, there are four seasons in the Dianshan Lake area, including spring (March to May), summer (June to August), autumn (September to November), and winter (December to February). The correlation analyses between seasonal nutrient losses from agricultural NPS and nutrient stocks in Dianshan Lake are shown in Fig. 8 and Fig. 9. The TN and TP stocks in the lake both had stronger correlation with the TN and TP losses from agricultural NPS in summer than in other seasons, which may be due to low self-purification ability, steady temperature, and stable phytoplankton quantity in summer (Ruan and Wang 1993; You 1997; Shi et al. 2005; Yang et al. 2009). In summer, the effects of self-purification were worse than other seasons, with the result that TN and TP could not be utilized in a timely manner in Dianshan Lake. However, TN remained at low level during summer. This may be because of the fact that algae grows rapidly when TN concentration is around 3.5 mg/L, and TN concentration has no significant effect on algae growth when TN concentration exceeds 3.5 mg/L. TN concentration in the lake in the summer of 2010 and 2011 was below 3.5 mg/L, which resulted in TN being utilized sufficiently by algae (Chen and Li 2010).

Fig. 8 Relationships between TN loss from agricultural NPS and TN stock in Dianshan Lake in different seasons in 2010 and 2011

Fig. 9 Relationships between TP loss from agricultural NPS and TP stock in Dianshan Lake in different seasons in 2010 and 2011

5 Conclusions

Nutrient losses from different types of agricultural land use and variation of nutrient concentration in Dianshan Lake were monitored in this study. The relationship between the nutrient losses from agricultural NPS and nutrient stocks in Dianshan Lake was analyzed. The main findings of the study are as follows:

(2) The monthly TN and TP losses from agricultural NPS were both positively correlated with the monthly TN and TP stocks in the lake in 2010 and 2011, and, especially in summer, the correlation coefficients were higher than in other seasons. Therefore, control of the pollution caused by local agricultural activities should be the main focus of study in the future.

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(Edited by Fang WANG)

This work was supported by the Project of the Shanghai Science and Technology Committee (Grants No. 08DZ1203200 and 08DZ1203205).

*Corresponding author (e-mail: qianxy@saes.sh.cn)

Received Sep. 29, 2013; accepted Sep. 16, 2014