Environ. Sci. Technol. 2008, 42, 2328–2333
Historical Distribution and Partitioning of Phosphorus in Sediments in an Agricultural Watershed in the Yangtze-Huaihe Region, China HONG ZHANG AND BAOQING SHAN* State Key Laboratory on Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 10085, China
Received August 13, 2007. Revised manuscript received January 3, 2008. Accepted January 10, 2008.
Agricultural intensification developed rapidly in East China since the 1980s, which caused most of the lakes in these areas to become eutrophic at almost the same time. Less is known about the relationship between agricultural intensification and watershed water quality, as well as historical nutrients dynamics and lake eutrophication processes. The study area, a typical agricultural watershed with high-yield grain production characterized by multipond systems in China’s YangtzeHuaihe region, was selected to evaluate the effects of agricultural intensification on P sediment retention using 137Cs dated sediment cores. Experimental results showed that P kept increasing in the multipond sediments during the past decades, which could be attributed almost entirely to agricultural intensification. An inflection point appeared in the 1980s, before which TP showed no or only a slight increase (200 mg · kg-1). Thereafter, it increased dramatically (about 400 mg · kg-1 by the year 2004) due to the extensive application of phosphate fertilizers. The chemical reactive fraction, KCl-extracted phosphorus (KCl-P), accounted for only 0.3% or less of TP. However, NaOH-extracted inorganic P (NaOH-Pi), accounting for 13-46%, was the main factor causing TP to increase due to long-term P fertilization, whereas CaCO3-bound phosphorus (CasP), together with residual phosphorus (Res-P) stayed at a relatively stable level. The increasing TP contents indicated that there was a potential to overwhelm the sorption capacity of multipond systems causing continuous water quality deterioration in the watershed and downstream waters. It is implied that P export resulting from agricultural intensification and water deterioration should be taken into account during the formation of watershed management strategies for the water environment.
Introduction Intensified agricultural cultivation practice may cause water quality deterioration in a watershed, and accelerate eutrophication of downstream waters. In China, Taihu Lake and Chaohu Lake in the Yangtze-Huaihe region are famous for their eutrophication (1). Nonpoint source (NPS) pollution from agricultural land is an important factor (2) due to the high grain productivity which leads to nutrient loading on * Corresponding author phone: +86 10 6284 9817; fax: +86 10 6284 3541; e-mail:
[email protected]. 2328
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the lakes. It was found that NPS contributed 68% of the phosphorus and 74% of nitrogen loading on Chaohu Lake (3). Since the agriculture reform in the 1980s, agricultural cultivation in the region has intensified under pressure of the steady increase in population and relative scarcity of available land. Agricultural intensification involves the intensive use of high-yield crop varieties combined with tillage and irrigation, as well as industrially produced fertilizers, pesticides, and herbicides. In addition, the breeding of poultry is expanding year after year, and farmyard manure as well as domestic waste are often effused directly into adjacent waters. As a result, the nutrient loading on watersheds has increased considerably. Previous research always focused on the NPS mechanisms of generation and transportation (4), as well as on control methods in watersheds (5), such as multipond systems. However, there is a poor understanding of the relationship between recent agricultural intensification and water quality deterioration in a watershed. An improved understanding of the phosphorus dynamics response to anthropogenic changes is essential for the formation of effective management strategies of agricultural cultivation and lake eutrophication (6, 7). In the Yangtze-Huaihe region, multipond systems are a typical landscape structure, similar to India, Sri Lanka, and some central European countries (8).The traditional function of these artificial wetlands is to store rainwater to control flooding, prevent soil erosion, and supply water for irrigation in dry seasons (5). The water quality in the watershed is directly affected by the water quality in the ponds. Previous studies reported that 90% or more of suspended solids (SS) settle in multipond systems (9). The sediment particles reaching the pond floor are continuously covered by successive sediment layers, so sediments in the multipond system preserve valuable historical in situ information about the past condition of ponds and their environmental states. Measuring the radioactivity levels of 210Pb and/or 137Cs could provide a reconstruction of the chronology of the sediment core (10), thus allowing studies on the history of social and economic development (11–13), especially the P dynamics due to anthropogenic modification in recent decades. The Liuchahe watershed, located on the Northern bank of Chaohu Lake (Yangtze-Huaihe region, China), was selected as a case study to investigate the change of watershed P dynamics due to agricultural intensification. The watershed is typical of the agricultural areas with no industry (8). Longterm monitoring has shown that the water quality is declining (14). In this study, sediment cores were collected from ponds representing four land use types within the multipond system to reconstruct the nutrient history, to examine the possible effects of intensive cultivation on P dynamics in the Liuchahe watershed, and to further evaluate the change in water quality due to agricultural intensification during the past few decades.
Experimental Section Study Area and Background. The case study was conducted in the Liuchahe watershed by Chaohu Lake (Figure 1). The watershed has an area of 691 hm2, with a typical landscape of a multipond system. In this area, the mean annual temperature is 15.5 °C and the long-term average precipitation is 940 mm. The land use pattern for the watershed is 45% rice cultivation, 26% dry land farming, 16% forest, 7% village, and 6% ponds (5). The ponds are classified into five types according to their location and characteristics: hill ponds (HP, located in foothills); farmland ponds (FP, within nonirrigated farmland); rice ponds (RP, within rice fields); 10.1021/es0720208 CCC: $40.75
2008 American Chemical Society
Published on Web 02/22/2008
FIGURE 1. Maps of Liuchahe watershed near Chaohu Lake showing the study area; also indicated are the core positions at the sampling sites. village ponds (VP, near or within a village), and stream ponds (SP, within the Liuchahe stream). Most ponds have a small surface area, ranging from 100 to 10 000m2 (5). There are only a small number of hill ponds in the watershed with most of them surrounded by forests, not subjected to agricultural intensification. Thus, samples in this study were obtained from four ponds as representations of FP, RP, VP, and SP. Sediment Sampling and Dating. The sampling work was carried out in December 2004 using a self-made core sampler device similar to the gravity corer reported in ref 15, with a diameter of 80 mm and a length of 150 cm. The sediment columns were divided into 1 cm slices in sequence. Samples were initially air-dried to reduce the water content before transferred into an oven to dry at 40 °C, then passed through a 100-mesh sieve for homogeneity before analysis. Sediment profiles were established by measuring the specific activity of 137Cs, a unique tracer for sedimentation from atmospheric nuclear weapons tests in the 1950s and 1960s, since there were no natural sources of 137Cs in the environment (16). Global fallout of 137Cs began in 1954, peaking from 1963 to 1964, and then decreasing since this maximum. Unique events, such as the Chernobyl accident in April 1986, could cause regional dispersal of measurable 137Cs that affect the total global deposition budget (17). This yearly pattern of fallout can be used to develop a chronology of sediment cores by determining activities in sediments by gamma ray spectrometry and using the year 1964 as a marker. Laboratory Analysis. Total phosphorus (TP) of sediment samples was analyzed by the HClO4sH2SO4 digestion method (18). The P fractionation was determined following procedure of van Eck as modified by Moore and Reddy (19, 20). Repeated measurements of standard samples have shown the P analytical accuracy was 10%. The P fractions extracted were exchangeable P (KClsP, extracted by 1 M KCl), aluminum/ iron-bound P (NaOH-Pi, extracted by 0.1 M NaOH, spectrophotometry directly), organic-bound P (NaOHsPo, extracted by 0.1 M NaOH, the difference between digestion and no-digestion), calcium-bound P (CasP, extracted by 0.5 M HCl), and residual P (Res-P, calculated by the difference with TP). The determination of organic matter (OM) was carried out following the method of residual titration of K2Cr2O7 (18). For P and OM fraction analysis the variations
FIGURE 2. Records of artificial fallout of 137Cs activities in four sediment cores from FP (a), RP (b), VP (c), and SP (d) in the multipond system of the Liuchahe watershed by Chaohu Lake. The deepest depths of these four peaks of 137Cs activity are taken to correspond to the record of a fallout maximum from atmospheric testing of nuclear weapons (1964). within replicates were
