Seasonal variations of nitrogen and phosphorus retention in an

Environ Sci Pollut Res (2010) 17:312–320 DOI 10.1007/s11356-009-0246-x

AREA 2 • AQUATIC BIOLOGY AND ECOLOGY • RESEARCH ARTICLE

Seasonal variations of nitrogen and phosphorus retention in an agricultural drainage river in East China Dingjiang Chen & Jun Lu & Hailong Wang & Yena Shen & Mark O. Kimberley

Received: 20 May 2009 / Accepted: 3 September 2009 / Published online: 1 October 2009 # Springer-Verlag 2009

Abstract Background, aim, and scope Riverine retention decreases loads of nitrogen (N) and phosphorus (P) in running water. It is an important process in nutrient cycling in watersheds. However, temporal riverine nutrient retention capacity varies due to changes in hydrological, ecological, and nutrient inputs into the watershed. Quantitative information of seasonal riverine N and P retention is critical for developing strategies to combat diffuse source pollution and eutrophication in riverine and coastal systems. This study examined seasonal variation of riverine total N (TN)

Responsible editor: Zhihong Xu Electronic supplementary material The online version of this article (doi:10.1007/s11356-009-0246-x) contains supplementary material, which is available to authorized users. D. Chen : J. Lu (*) Department of Natural Resources, College of Environmental Science and Natural Resources, Zhejiang University, KaiXuan Road 258#, Hangzhou 310029, Zhejiang Province, People’s Republic of China e-mail: [email protected] J. Lu China Ministry of Education Key Lab of Environment Remediation and Ecological Health, Zhejiang University, Hangzhou 310029, China H. Wang : M. O. Kimberley Scion, Private Bag 3020, Rotorua 3046, New Zealand Y. Shen Zhejiang Provincial Key Laboratory of Subtropical Soil and Plant Nutrition, Zhejiang University, Hangzhou 310029, China

and total P (TP) retention in the ChangLe River, an agricultural drainage river in east China. Methods Water quality, hydrological parameters, and hydrophyte coverage were monitored along the ChangLe River monthly during 2004–2006. Nutrient export loads (including chemical fertilizer, livestock, and domestic sources) entering the river from the catchment area were computed using an export coefficient model based on estimated nutrient sources. Riverine TN and TP retention loads (RNRL and RPRL) were estimated using mass balance calculations. Temporal variations in riverine nutrient retention were analyzed statistically. Results and discussion Estimated annual riverine retention loads ranged from 1,538 to 2,127 t year–1 for RNRL and from 79.4 to 90.4 t year–1 for RPRL. Monthly retention loads varied from 6.4 to 300.8 t month–1 for RNRL and from 1.4 to 15.3 t month–1 for RPRL. Both RNRL and RPRL increased with river flow, water temperature, hydrophyte coverage, monthly sunshine hours, and total TN and TP inputs. Dissolved oxygen concentration and the pH level of the river water decreased with RNRL and RPRL. Riverine nutrient retention ratios (retention as a percentage of total input) were only related to hydrophyte coverage and monthly sunshine hours. Monthly variations in RNRL and RPRL were functions of TN and TP loads. Conclusions Riverine nutrient retention capacity varied with environmental conditions. Annual RNRL and RPRL accounted for 30.3–48.3% and 52.5–71.2%, respectively, of total input TN and TP loads in the ChangLe River. Monthly riverine retention ratios were 3.5–88.7% for TN and 20.5– 92.6% for TP. Hydrophyte growth and coverage on the river bed is the main cause for seasonal variation in riverine nutrient retention capacity. The total input TN and TP loads were the best indicators of RNRL and RPRL, respectively. Recommendations and perspectives High riverine nutrient retention capacity during summer due to hydrophytic

Environ Sci Pollut Res (2010) 17:312–320

growth is favorable to the avoidance of algal bloom in both river systems and coastal water in southeast China. Policies should be developed to strictly control nutrient applications on agricultural lands. Strategies for promoting hydrophyte growth in rivers are desirable for water quality management. Keywords Aquatic plants . Diffuse source pollution . Eutrophication . Nonpoint pollution . Riverine . Total nitrogen retention . Total phosphorus retention

1 Introduction Loss of nitrogen (N) and phosphorus (P) from agriculture to streams or rivers has increased more rapidly than from industrial and residential land in many regions in recent years (Panagopoulos et al. 2007; Yang et al. 2007; Yin et al. 2007), commonly inducing eutrophication in rivers and coastal water bodies (Smith 2003; Gu et al. 2008). However, the level of N and P transported from a catchment to the river outlet can be decreased through riverine retention processes. Such riverine retention can occur as a result of organic matter burial in sediments, sediment sorption, plant and microbial uptake, and denitrification (Saunders and Kalff 2001; Sybil et al. 2002; Marcus and Kǒhler 2006). A variety of methods have been used to estimate total N (TN) and P (TP) retention in rivers, such as mass balance modeling (Behrendt and Opitz 2000; May et al. 2001; Lepistö et al. 2006; Marcus and Kǒhler 2006), experimental measurements (Takeda and Akira 2006), mechanism models (Brian et al. 1999; Andersson and Arheimer 2001), and statistical regressions (Saunders and Kalff 2001; Sybil et al. 2002). Though instantaneous river flow and nutrient concentration data are required to apply mass balance models, they allow the estimation of retention within a reach and a better understanding of the retention process at different temporal and spatial scales. There has been a wide range in the reported efficiency with which TN and TP are retained in river reaches. Riverine retention ratio (RRR; expressed as a percentage of total input TN or TP) can vary between 1–80% for TN systems (Sybil et al. 2002; Grizzetti et al. 2003; Grizzetti et al. 2005; Lepistö et al. 2006; Dierk and Michael 2008) and between 20–70% for TP (Svendsen and Kronvang 1993; Brian et al. 1999; Garnier et al. 2002; Grizzetti et al. 2003), because riverine TN and TP retention is labile and readily adapted to changed hydrological and pollutants' properties. It has been proposed that retention of TN and TP is inversely related to the ratio of water body depth to water travel time and to river flow (Behrendt and Opitz 2000; Sybil et al. 2002), and that TN and TP retention decreases with river slope (Brian et al. 1999). On the other hand,

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some studies have described a positive relation between TN retention load and input TN load (Windolf et al. 1996; Saunders and Kalff 2001), and TP retention has been found to increase with the abundance of submerged and emergent macrophytes (Marcus and Kǒhler 2006). In general, studies have addressed annual TN or TP removal from a river, but give no integrated information on the amounts of TN and TP retention on a seasonal scale. Moreover, there is no published information on riverine N and P retention in China. Therefore, the aim of this study was to provide a quantitative analysis of the environmental factors contributing to seasonal variations of riverine TN and TP retention in the ChangLe River, an agricultural drainage river in southeast China.

2 Materials and methods 2.1 Study river description The ChangLe River (120º35′56″–120º49′03″E, 29º27′98″– 29º35′12″N) is located in the southwest—Shaoxing area in Zhejiang Province, southeast China (Fig. 1). It drains a total area of 864 km2, flows about 70.5 km with 0.36% gradient, and is one the main tributaries of the Cao-E River that ultimately flows into the East China Sea. The ChangLe River has a sandy-gravel riverbed, and is 40–70 m wide with 0.2–2 m water levels. It has a long-term average annual flow of 18.4 m3 s–1 and flow quantity of 571×106 m3. The reach of the ChangLe River (from CL1 to CL3, Fig. 1) that concerns this study flows about 27.8 km, and the catchment includes seven towns with a total population of 373,500 in 2005. The main land uses in the catchment are agricultural, including rice paddies, dry land cropping, and plant nurseries, along with rural habitation. The ChangLe River is typical of agricultural drainage rivers in southeastern China that is being influenced by a subtropical monsoon with clearly defined rainy seasons and with intensive farming practices causing serious water pollution problems from diffuse agricultural sources. The annual average precipitation is 1,194 mm with monthly variations of 26–215 mm in 2004–2006 for the catchment recorded at the weather station in ShengZhou city. The rainfall mainly occurs in May and June and during the typhoon season (i.e., September). Water drainage from the river headstream (NanShan Reservoir, Fig. 1) is limited because it is the main local drinking water source, and rainfall is the main water source for the river. There are abundant hydrophytes in the ChangLe River, especially from April to October. According to various surveys, the composition of submerged and floating plant species in the ChangLe River does not change significantly and is dominated by species such as Eichhornia crassipes,

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Fig. 1 Sampling sites in the ChangLe River

Vallisneria natans (Lour.) Hara, Potamogeton crispus, Hydrilla, and Ceratophyllaceae. Moreover, the dominant riparian plant species, such as Phragmites australis, Typha orientalis Presl, and Juncus effusus L., are widely distributed in riparian buffers along the river, the area of these being typically about 15–20% of the reach area. 2.2 Sampling, analysis, and data collections 2.2.1 Sampling and chemical analysis Water quality at three sampling stations, which respectively represents upstream (CL1), middle (CL2), and downstream (CL3) sites on the ChangLe River, were monitored from January 2004 to December 2006 (Fig. 1). Dissolved oxygen (DO), water temperature, and temperature-corrected pH were measured using handheld Multi-parameter 350i SETs (The Merck Co Ltd, Germany) at the time of water sample collection. Flow velocity was measured in situ using a screw-type flow rate instrument (Chongqing Hydrology Instrument Co Ltd., China). From the bridges on the river,

water samples were collected about 30 cm below the water surface in 2.5 L polyethylene jars once per month between 9AM and 2PM for chemical analysis. These samples were preacidified with H2SO4 in the field (15 ml of concentrated H2SO4 per sample) for chemical analysis (Wei et al. 2002). TN concentration was measured using the persulfate digestion-UV spectrophotometric method, while TP was measured following persulfate digestion using the ammonium molybdate spectrophotometric method in a laboratory up to 6 h after sampling according to National Standard Methods of China (GB 11893-89, Wei et al. 2002). 2.2.2 Estimation of hydrophyte coverage A combination of visual hydrophyte coverage percentage (Irena 2002) and wet or dry hydrophyte biomass (kilogram or gram per square; Anderson and Kalff 1986) was employed to estimate hydrophyte abundance in the ChangLe River. Hydrophyte coverage of the river bed (the dimensionless portion of area coverage as a percentage) was used to describe hydrophyte abundance and was measured in a 20-m


reach (located after CL2, Fig. 1) in April, June, and August 2004 and in April, May, August, and October 2005–2006. This 20-m reach was selected because it contained areas with high, average, and low hydrophyte coverage in different locations. Firstly, the lengths and widths of hydrophyte clusters (including submerged and floating plant species) were measured using a plastic tape. Then, each cluster was taken as a center dividing the 20-m reach into several regular sections, the areas of these being measured using a plastic tape. Finally, coverage percentage was calculated as the ratio of the hydrophyte cluster to the section area, with an overall average of this coverage percentage across all sections within the 20-m reach being taken as the hydrophyte coverage percentage. 2.2.3 Basic data set collection Data on TN and TP sources (human population, quantity of poultry, and chemical fertilizer use) in the ChangLe watershed was collected for each village in 2004–2006. Appropriate nutrient generation coefficients for human and livestock poultry, and nutrient export ratios for domestic waste, livestock–poultry waste and different fertilizers from the varying land uses in the catchment, were obtained from the literature and used to estimate nutrient export loads from land into the ChangLe River (Chen et al. 2009). The monthly river flows and rainfall in 2004–2006 and other descriptive data applying to the ChangLe River were supplied by the Zhejiang Provincial Government Hydrology Office. Correlation analysis and the least significant differences (LSD) test at the 0.05 level of significance were performed using Data Processing System for Practical Statistics software (Tang and Feng 1997). 2.3 Estimation of riverine TN and TP retention Riverine TN and TP retention loads within the reach were calculated by mass balance, based on the measured load difference between the inlet loads (i.e., nutrient loads at CL1) and outlet loads (i.e., nutrient loads at CL3) and the estimated catchment area export load (i.e., EL, including loads from point and diffuse sources) along the reach (Behrendt and Opitz 2000; May et al. 2001). These calculations are described by the following mass balance equations: RNRL ¼ Nbeginning þ NEL Nend RPRL ¼ Pbeginning þ PEL Pend ; where the subscript “beginning” denotes loads the beginning of the reach, the subscript “EL” denotes export loads from the catchment along the reach, and the subscript “end” denotes output loads at the end of the reach. Inlet (i.e.,

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Nbeginning and Pbeginning) and outlet (i.e., Nend and Pend) nutrient loads were calculated by multiplying nutrients concentrations with flows. The retention TN and TP ratio (%) was calculated using RNRL 100= Nbeginning þ NEL and RPRL 100= Pbeginning þ PEL , respectively. The annual catchment area nutrient export loads (including field applications, livestock, and domestic pollutants) that entered the river (i.e., EL) were computed using an export coefficient model (Johnes 1996). In the ChangLe watershed, point source pollution (including waste water treatment plants and industrial sewage outlets) was negligible with annual discharge of 0.2 t for TN, while TP could not be detected in sewage outlets in 2004–2006. In order to estimate monthly RNRL and RPRL, monthly ELs estimates were required. Because rainfall enhances the N and P exports from land to river by leaching or by surface and subsurface runoff, leading to increased transfer, especially during rainstorms (Grayson et al. 1996; May et al. 2001), it may account for seasonal variations of diffuse sources pollution (Grizzetti et al. 2005). This idea is also supported by the fact that there was a significant correlation between rainfall and TN or TP concentration in three sampling sites in ChangLe River in 2004–2006 (Table 1). Therefore, monthly ELs were estimated by allocating annual levels among months in proportion to monthly rainfall. Monthly NEL and PEL estimates varied from 15.4– 414.8 t TN month-1 and 2.2–16.7 tTP month-1, respectively.

3 Results and discussion 3.1 Temporal variations of riverine nutrient retention In the ChangLe River, annual RNRL ranged from 1,537.6 to 2,126.8 t year–1 while RPRL ranged from 79.4 to 90.4 t year–1 during the 2004–2006 period of the study. This suggests that EL and upstream input load (i.e., total input load) was subject to significant riverine retention processes,

Table 1 Correlations between monthly measurements of total nitrogen (TN) or total phosphorus (TP) concentration and monthly rainfall in the ChangLe River during 2004–2006 Nutrient

Rainfall (mm) CL1 (n=36)

CL2 (n=36)

CL3 (n=36)

TN (mg L−1)

0.70**

0.72**

0.71**

TP (mg L−1)

0.69**

0.58**

0.60**

CL1, CL2, and CL3 denote the sampling site at upstream, middle, and downstream locations, respectively ** p