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Hydrology and Earth System Sciences, 7(3), Studies of acid (2003) deposition EGU and its effects in two small catchments in Hunan, China Studies of acid deposition and its effects in two small
Hydrology and Earth System Sciences, 7(3), Studies of acid (2003) deposition EGU and its effects in two small catchments in Hunan, China Studies of acid deposition and its effects in two small catchments in Hunan, China Nandong Xue 1,2, Hans Martin Seip 2, Bohan Liao 1 and Rolf D. Vogt 2 1 College of Resources and Environmental Sciences, Hunan Agricultural University, Changsha , P.R.China 2 Dept. of Chemistry, University of Oslo, P.O.Box 1033, Blindern, 0315 Oslo, Norway for corresponding author: Abstract Acid deposition and its effects were studied by analysing the chemistry in precipitation, stream water, soil water and soils in two catchments in Hunan. One site, Linkesuo (denoted LKS), is on the outskirts of Changsha, the provincial capital of Hunan, the other (Bailutang, denoted BLT) on the outskirts of Chenzhou in southern Hunan. Volume-weighted average ph values and sulphate concentrations in wet deposition were 4.58 (BLT) and 4.90 (LKS) and 174 µmol c L -1 and 152 µmol c L -1, respectively. Wet deposition of sulphate has been estimated as 4.3 gs m -2 yr -1 and 3.4 gs m -2 yr -1 at BLT and LKS, respectively. Estimates of the corresponding total depositions (dry + wet) are 6.1 gs m -2 yr -1 and 5.3 gs m -2 yr -1. In precipitation and throughfall, sulphate was the major anion and calcium the major cation. In stream and soil water, nitrate was slightly higher than sulphate on an equivalent basis and magnesium (Mg) not much lower than calcium (Ca). Important soil properties, such as soil ph, soil organic matter (SOM) content, exchangeable acidic cations, exchangeable base cations, effective cation exchange capacity (CECe), base saturation (BS), and aluminium (Al) and iron (Fe) pools, were determined for five forest soil profiles (consisting of four horizons) in each of the two catchments. The soils in BLT are generally more acid, have lower BS and higher Al and Fe pools than the LKS soils. The Al- and Fe-pools were generally higher in the topsoils (i.e. the O and A horizons) than in deeper soils (i.e. E and B horizons) especially at the most acidic site (BLT). There are significant correlations between Fe-pools and the corresponding Al-pools in both catchments except between the amorphous Fe ox and Al ox. Considering the long-term high deposition of sulphate, there is a risk of future ecological damage due to acidification, especially in the BLT catchment, although vegetation damage has yet to be observed in the catchments. This condition appears to be representative of a large part of Hunan. Keywords: acid deposition, soil acidification, catchment, Al-pools, Fe-pools, Hunan Introduction Acid deposition, associated with sulphur dioxide (SO 2 ) and nitrogen oxide (NO x ) emissions created during fossil-fuel combustion, gives rise to a serious impact on natural ecosystems. Although SO 2 and NO x emissions are declining in North America and Europe, they are rising in many developing countries across the world (Amann, 2001). However, damage from acid deposition still occurs in some regions of North America and Europe and may be increasing in other regions, in particular in south-eastern Asia (Kuylenstierna et al., 2001). Much information about effects of acid deposition has been obtained through catchment studies. Most of these are in Europe or North America and include Christophersen et al. (1982); Nilsson (1985); Neal et al. (1990); Probst et al. (1990); Driscoll et al. (1994); Dambrine et al. (2000); West et al. (2001); Likens et al. 2002). Recently there have also been such studies in China (Larssen et al. 1998, 2001; Zhao et al. 2001; Shao, 2002). Acid deposition in China, particularly south of the Yangtze, has increased substantially during the last decades. In 2000, 30% of China was characterised as acid deposition areas (annual volume weighted precipitation ph 5.6); in some areas acid deposition levels exceed the environmental damage level) (SEPA, 2000). Hunan is among the provinces in China most seriously affected by acid deposition. Acid rain was first reported in Changsha (the capital of Hunan) in 1972; it has since become one of the most seriously acid rain polluted cities in China, with an average volume weighted precipitation ph value below 4.5. In 2000, the annual average volume weighted ph was 3.53 and the acid 399 Nandong Xue, Hans Martin Seip, Bohan Liao and Rolf D. Vogt rain frequency (i.e. ph 5.6) was 90% (SEPA, 2001). Fortunately, Chinese public opinion and the authorities have realised the importance of this problem. According to the National tenth five-year Plan for Environmental Protection, the SO 2 emissions shall decrease by 10% from 2000 to 2005 (SEPA, 2002). Total SO 2 emissions in China decreased from 1995 to 1999 but have since probably increased (Li and Gao, 2002). Furthermore, building of taller chimneystacks to reduce local pollution increases long-range transport and thereby acid deposition in remote regions. Acid precipitation and its precursors affect human health, materials and ecosystems (Rodhe et al., 1995; SEPA, 2000). While surface water acidification is not thought to be a major problem in China, acidification of soil and soil water may affect terrestrial ecosystems seriously in many areas including parts of Hunan (Chen et al., 1993, 1995; Williams et al., 1995; Larssen et al., 1998; Liao et al., 1997; Zhao et al., 1994; Seip et al., 1999). The soil and soil water in many areas of the province are sensitive to acidification due to low weathering rates (Duan et al., 2002) and medium to small pools of exchangeable base cations (Liao and Li, 1989). Decrease in soil ph due to acid deposition occurs most rapidly when the cation exchange capacity is low and ph values are intermediate (5 6). Where soils already have a low ph, the change in soil ph may be small even if the acid deposition is high; instead soil water acidity and aluminium (Al)-concentration may increase. This will generally imply the most serious ecological threat. Following several research programmes, great progress has been achieved in understanding the soil acidification process in Hunan in recent decades. The studies include responses of Hunan forest soil to acidic input (Liao et al., 1998), mechanisms of sulphate adsorption in red soils in southern Hunan (Chen et al., 1993, 1995), buffering mechanism of Hunan soil (Liao and Li, 1989), and relative sensitivity of Hunan soils to acid deposition (Hao et al., 1999). However, the only other catchment scale studies on acid deposition in Hunan are conducted by the Integrated Monitoring Program on Acidification of Chinese Terrestrial Systems (IMPACTS, 2003). The study site, CaiJiaTang (CJT), is about 150 km SW of Changsha (Shao, 2002). Site descriptions Two acid sensitive catchments, Bailutang (BLT) and Linkesuo (LKS), were selected in Hunan province for detailed studies. The province of Hunan is located in the middle reaches of the Yangtze and lies between E and E and between 24.6 N and 30.5 N (see Fig.1). Its total area is km 2. The mountains and hills occupy more than 80% of the area and more than one-quarter of the terrain lies at a height of more than 500 m. Hunan province has a subtropical monsoon humid continental climate. The annual average temperature is º C, the non-frost period lasts for days. Annual sunshine is h, the annual average rainfall is between 1200 and 1700 mm with most rain in spring and summer. It is one of the rainiest regions in China. The forested BLT catchment (Fig. 2) is nearly 400 m a.s.l. at N, E about 20 km east of Chenzhou. BLT has, on average, 300 frost-free days a year. The annual average temperature is 16.9 C. The total annual rainfall is 1548 mm with the maximum precipitation occurring in spring and summer. The area of the catchment is about 0.15 km 2. The main tree species are Chinese fir (Cunninghemia lanceolota) and Masson pine (Pinus Fig. 1. Map of the Peoples Republic of China with Hunan Province indicated. 400 Studies of acid deposition and its effects in two small catchments in Hunan, China group corresponding to Ferric Acrisol in the FAO system and its parent material is shale. Kaolinite is the dominant clay mineral; there are also small amounts of hydromica and biotite flakes. The main tree species, Chinese fir (Cunninghemia lanceolota), was planted after deforestation and slash-and-burn cultivation in the late 1960s. Since then, the soil has been practically undisturbed by land-use activities. Five soil plots were selected for detailed studies (Fig. 2). Fig. 2. Topographical map and sampling sites in BLT, near Chenzhou, and in LKS, near Changsha (The plots 1-10 are soil sampling sites; A, B, C and D are the sampling sites for stream water; plots 1 and 7 are soil water sampling sites. The sampling sites for wet deposition, bulk deposition and throughfall were set up near plots 3 and 7) massoniana) but there is also a range of broad-leaf trees and bushes. The soil is in the Chinese red soil group corresponding to Ferric Acrisol in the FAO system and its parent material is granite. Kaolinite is the dominant clay mineral; there are also small amounts of vermiculite and illite. The catchment was deforested on a large scale in the 1960s. The soil has been practically undisturbed by other land-use activities. Five soil plots were selected for detailed studies (Fig. 2). Changsha, at 28.2 N, E, about 100 m a.s.l., has a subtropical climate featuring relatively long hot summers and short cold winters, with annual average temperature of 17.5 ºC and annual average precipitation,1378 mm. The LKS catchment (0.12 km 2, about 100 m a.s.l, Fig. 2) is some 40 km east of Changsha. The soil is in the Chinese red soil ANALYSES AND METHODS Water was monitored through the two catchments, BLT and LKS. To collect soil water, lysimeters (Vogt et al., 2001), were installed in the A horizon at 20 cm depth in Plot 1 (BLT) and in plot 7 (LKS) close to the streams (Fig. 2). From 19 June 2000 to 19 June 2002, 54 wet deposition, 24 throughfall, 54 bulk deposition, 13 soil water and 12 stream water samples were collected from BLT and 58 wet deposition, 58 throughfall, 58 bulk deposition and 12 stream water samples were collected from LKS. Additional deposition measurements are available from Hunan Agricultural University (HAU), 10 km from LKS, but at a lower elevation (about 50 m.a.s.l.) where 58 wet deposition samples and 58 bulk deposition samples were collected during the same period. The wet-only deposition was collected in plastic buckets placed in the field only during precipitation events while buckets for collecting bulk deposition and throughfall samples were in the field continuously during the monitoring period. Soil water was sampled monthly. However, the lysimeters collected little or no water in dry periods. Stream water was sampled monthly but not in the dry season (usually from July to November). Precipitation volume was measured as soon as rain or snow stopped. All samples were stored in a refrigerator and analysed within a few weeks. The samples were filtered under vacuum through a 0.45 µm membrane prior to analysis. The ph values in water samples were measured with a ph electrode (Ag/AgCl) at 20ºC. The cations of calcium (Ca 2+ ), magnesium (Mg 2+ ), sodium (Na + ) and potassium (K + ) were analysed by Flame-AAS. Total monomeric Al (Al a ) in soil water and stream water samples was separated into a labile fraction (mainly inorganic spacies, Al i ) and a non-labile fraction (mainly organic species, Al o ). Al a was extracted and determined as described by Barnes (1975) and Driscoll (1984). Concentrations of Al o and Al i were calculated by the computer program ALCHEMI (Schecher and Driscoll, 1987). Ammonium (NH 4+ ) was determined spectrophotometrically after reaction with Nessler s reagent (K 2 HgI 4 ) (GB7479, 1989); nitrate (NO 3 ) was determined by a UV spectrophotometric method 401 Nandong Xue, Hans Martin Seip, Bohan Liao and Rolf D. Vogt (Skoog et al., 1998); bicarbonate (HCO 3 ) by Gran titration (Gran, 1950, 1952). Sulphate (SO 4 2 ) was titrated with a known amount of 0.02 M barium chloride (BaCl 2 ) in excess and titrating the surplus BaCl 2 with EDTA (di-sodium ethylene- diamine-tetra-acetate solution (APHA, 1985); and chloride (Cl ) was titrated by 0.1 M silver nitrate (AgNO 3 ) with potassium dichromate (K 2 Cr 2 O 7 ) indicator (Department of Chemistry Columbus State University, 2000). Soils from four generic horizons (O, A, E and B) at the selected plots (Figs. 2 and 3) were sampled at depths given in Table 1. After removing coarse fragments and root debris, the samples were air dried and passed through a 2 mm sieve prior to analyses. Chemical analyses were conducted partly at the Department. of Environmental Sciences, Hunan Agricultural University, and partly at the Department of Chemistry, University of Oslo. Soil ph H2O was measured using a Ross ph electrode (ISO, 1994). Exchangeable cations were extracted using an unbuffered BaCl 2 solution (Hendershot and Duquette, 1986). Metal cations (Ca 2+, Mg 2+, Na +, K +, Fe 3+, Mn 2+ and Al 3+ ) in the extracts were determined by ICP-AES (Inductively Coupled Plasma - Atomic Emission Spectrometry). The ph (BaCl 2 ) values measured with a Ross ph electrode gave H + in the extracts. The equivalent sum of exchangeable cations gave the effective cation exchange capacity (CEC e ). Soil organic matter (SOM) was determined by oxidation with a known amount of 0.8 M K 2 Cr 2 O 7 in excess and titrating the surplus K 2 Cr 2 O 7 with ferrous sulphate (FeSO 4 ) solution (Lu, 1999). Al and Fe-pools include exchangeable Al and Fe (Al ex and Fe ex ), weakly organically bound Al and Fe (Al c and Fe c ), strongly organically bound Al and Fe (Al p and Fe p ) and amorphous and organic Al and Fe (Al ox and Fe ox ). The data for exchangeable Al and Fe are generated in the CEC e determination described above. Al c and Fe c were extracted by cupric chloride (CuCl 2 ) electrolyte (Jou and Kamprath, ph value O-horizon A E B Fig. 3. Soil ph values (in H 2 O) found in 4 horizons at the 10 plots. Table 1. Soil properties and the Al- and Fe-pools (mean values and standard deviations) in soils from the two Hunan catchments Horizon Depth CECe BS AlS SOM Al-pools (cmol c kg -1 ) Fe-pools (cmol c kg -1 ) (cm)(cmol c (%)(%) (g kg -1 )Al ex Al c Al p Al ox Fe ex 10 3 Fe c 10 3 Fe p Fe ox kg -1 ) SITE BLT O ± ± ± ± ± ± ± ±8.9 20±5 50± ± ±5.5 A ± ± ± ± ± ± ± ±6.8 14±3 20±4 35.9± ±2.7 E ± ± ± ± ± ± ± ±4.2 13±3 30±6 28.4± ±2.2 B ± ± ± ± ± ± ± ±3.0 11±2 20±3 18.6± ±0.9 SITE LKS O ± ± ± ± ± ± ± ± ±0.8 19±5 8.0± ±4.0 A ± ± ± ± ± ± ± ± ±0.6 12±3 7.5± ±2.8 E ± ± ± ± ± ± ± ± ±0.5 8±1 7.6± ±2.4 B ± ± ± ± ± ± ± ± ±0.4 7±1 5.9± ±1.5 CEC e : effective soil cation exchange capacity; BS: base saturation; AlS; aluminium saturation; SOM: soil organic matter; Al ex and Fe ex : exchangeable Al and Fe, Al c and Fe c : weakly organically bound Al and Fe, Al p and Fe p : strongly organically bound Al and Fe; Al ox and Fe ox : amorphous and organic Al and Fe. 402 Studies of acid deposition and its effects in two small catchments in Hunan, China Table 2. Results for calibration samples. N is number of replicates N Na K Ca Mg Al NIVA (mg L 1 ) Calibration on ICP (mg L 1 ) Deviation (%) Calibration on AAS (mg L 1 ) Deviation (%) ). Al p and Fe p were extracted using a high ph (ph=10) sodium pyrophosphate solution (Van Reeuwijk, 1995). Easily available mineral bound Al ox and Fe ox were extracted with an acid ammonium oxalate H 2 C 2 O 4 /(NH 4 ) 2 C 2 O 4 solution (McKeague and Day, 1966). Fe and Al in the extracts were determined by ICP-AES. House standard water samples with matrix similar to the actual samples were analysed along with the actual samples to account for possible errors and drift in the instrument. On the ICP-AES, the standard sample was run for every 7th sample and for every 30th sample on the AAS. Standard calibration samples from NIVA (an ISO-accredited chemical laboratory at the Norwegian Institute for Water Research in Oslo, Norway) for Ca 2+, Mg 2+, Na +, K + and Al 3+ determination were also analysed on both ICP-AES and AAS. The average discrepancies for calibration samples are given in Table 2. The relative deviations of Fe 3+ and Mn 2+ determination between ICP-AES and AAS were below 5%. All the results for other ions were less than one standard deviation from the mean. In the analyses of soil samples, the intra-laboratory precision was checked by determining values of three sample replicates. The standard deviations (SD) among replicates were below 7.0%. Chemical changes, especially in NH 4 + and NO 3, may take place in the field and during transport and storage (Richter and Lindberg, 1988). It took usually two to three hours to transport the samples from the catchment to the refrigerator in the laboratory. Therefore, concentrations for samples analysed immediately after sampling were compared with results for samples stored for three hours at room temperature; the NH 4 + values were about 7% lower and the NO 3 values about 5.4% higher than in samples analysed immediately. Storage in a refrigerator for three weeks resulted in values of NH 4 + and NO 3 about 9.5% lower; this was as if chloroform had been added before storage to prevent biological activity. The changes, while significant, do not distort the general picture and major conclusions. The uncertainties in the analyses were discussed by Liao et al., (1997, 1998) and by Larssen et al., 1998). Results PRECIPITATION CHEMISTRY The volume-weighted ph and concentrations of major ions in the precipitation at the two catchments are listed in Table 3. At the BLT catchment, the volume-weighted ph in wet deposition for the study period was All samples were acidic (ph 5.6). In the LKS catchment, the volumeweighted ph for the period was 4.90; 80% of the events, corresponding to about 78% of the total precipitation amount, were acidic. HAU, located closer to Changsha city centre, experienced considerably lower ph values and much higher concentrations of Ca 2+ and SO 4 2 in both wet deposition and bulk deposition. The sulphate concentration in the wet deposition is much higher than the nitrate concentration both at BLT and at LKS, implying that sulphate is the dominant anion and SO 2 the major acid rain precursor. At BLT, the volume-weighted sulphate concentration in wet deposition was 174 µmol c L 1 for the study period. With an annual precipitation amount of 1548 mm (based on average values for long-term observations in this area), the corresponding wet deposition was 4.26 gs m 2 yr 1. At LKS, the volume-weighted sulphate concentration was 152 µmol c L 1, and the annual precipitation amount 1378 mm (also based on long-term observations), giving a wet deposition of 3.37 gs m 2 yr 1. According to the IMPACTS study at the CJT site (Shao, 2002) dry deposition is important in Hunan catchments. Although difficult to quantify, for 2001 it has been estimated roughly. At the IMPACTS CJT catchment the average SO 2 concentration was approximately 20 ìg SO 2 m -3 (Shao, 2002). Hence, the average SO 2 concentrations in the LKS and BLT catchments are also assumed to be 20 µg SO 2 m 3. The deposition velocity of sulphur dioxide from the atmosphere onto plant foliage and other solid surfaces varies according to the surface. Zhao et al. (1994) estimated the average deposition velocity to be 0.29 cm s 1 in rural areas near Chongqing. However, Larssen et al. (1998) estimated the average deposition velocity to be 0.6 cm s 1 in a small catchment near Guiyang. On the assumption that the average 403 Nandong Xue, Hans Martin Seip, Bohan Liao and Rolf D. Vogt Table 3. The ph and chemical composition of deposition and soil water (volume-weighted averages) in the catchments and at HAU. Sites Sample types No. of ph Ca 2+ Mg 2+ Na + K NH 4 Al i SO 4 NO 3 HCO 3 Cl - Σ cations Σ anions AD sam
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