ARTICLE pubs.acs.org/est
Deposition and Leaching of Sulfur, Nitrogen and Calcium in Four Forested Catchments in China: Implications for Acidification Thorjørn Larssen,† Lei Duan,‡,* and Jan Mulder§ †
Norwegian Institute for Water Research, Gaustadalleen 21, 0349 Oslo, Norway State Key Laboratory of Environmental Simulation and Pollution Control, Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China § Department of Plant and Environmental Sciences, Norwegian University of Life Sciences, Box 5003, NO-1432 Ås, Norway ‡
ABSTRACT: Here we present the first detailed study on fluxes of sulfur (S), nitrogen (N), and major cations in Chinese subtropical forest catchments. Data are from four study sites, differing in inputs of atmospheric pollutants and sensitivity to acidification. Results show important differences from most sites in North America and Europe. Dry deposition of S, N, and calcium (Ca) is considerably larger than wet deposition in most cases causing deposition fluxes ranging from moderate to very high, both for acidifying compounds (S deposition 1.5-10.5 kiloequivalents per hectare and year (keq ha-1 yr-1); N deposition 0.4 to 2.5 keq ha-1 yr-1) and for alkaline compounds (Ca deposition 0.8 to 5.7 keq ha-1 yr-1). More than half of the input of acidity is neutralized by alkalinity associated with Ca deposition. Furthermore, the retention of incoming S and N is small in the soil root zone, but considerable in the deeper soils or riparian zone. Drainage water from the root zone of the soils at the two sites with the highest deposition show pronounced acidification. For the two sites with moderate deposition inputs, the root zones are retaining some of the incoming S and buffer some of the incoming acidity. The subsoils and the riparian zonesare strong sinks for N, S, and Ca. This is associated with substantial acid neutralization at all sites. These features are of major importance for the understanding of the longterm effects of acidification in China.
’ INTRODUCTION Acid rain is one of China’s major environmental problems.1 Annual emission of sulfur (S), primarily from the use of coal in energy production, was increasing rapidly until 2006, after which a small decline has been seen.1 Emission of nitrogen (N), partly from combustion sources (NOx) and partly from agricultural fertilizers and livestock (NH3), is still increasing.2,3 As a consequence, China has a relatively high deposition of S and N (in particular ammonium (NH4þ)); in some places the deposition fluxes are higher than in North America and Europe during the peak period for acid deposition (Table 1).4 An important difference between China and the much studied situation in North America and Europe, is the high level of calcium (Ca2þ) in the atmosphere (Table 1).4-6 In northern China base cations usually outweigh acid anions and the precipitation is typically slightly alkaline.5 In southern China, where the influence of alkaline dust from the desert areas in the north and northwest is smaller, the precipitation is typically acidic.4-6 The role of N, both as nitrate (NO3-) and ammonium (NH4þ) in acidification, has so far received little attention in China.9 However, N deposition can be relatively high, particularly due to high NH3 emissions from agriculture.10-12 As a result of intense use of N in agricultural fertilizers,13,14 increased lifestock production, and rapid increases in NOx emissions from combustion sources,2,3 N and N-cycling warrants further attention. How and to what extent the high deposition fluxes of S and N are affecting the forest ecosystems is largely unknown, although indications of reduced forest health has been reported.15 The fate of S, N, and Ca2þ in the catchment is crucial for future r 2011 American Chemical Society
acidification of soils and waters in China.4,16 Very few studies have discussed catchment input-output ion budgets in Chinese natural environments and there are several unknown factors regarding processes.4,17 Here we present annual fluxes for inputs (wet deposition and throughfall fluxes) and outputs (soil-water and streamwater) of major ions for four years (2001-2004) in four forested catchments in southern China receiving different loads of acid deposition. This is to our knowledge the first detailed element budget study in catchments in China, and probably in this kind of ecosystem in the world. Such data driven budget studies have proven powerful in explaining major chemical processes in more well studied systems elsewhere in the world, particularly in the traditional acid rain regions in the U.S. and Europe.18-20 The aim of this paper is to identify major processes driving the biogeochemical responses of forested, subtropical ecosystems in China under pressure from acid rain. This information is critical for future modeling and assessment of acid rain mitigation policy options in China.
’ MATERIALS AND METHODS Sites. Data are from four sites located in the subtropical region of southern China (Figure 1), all part of the same project, using the same monitoring protocol.4,21 All sites are located Received: May 8, 2009 Accepted: January 4, 2011 Revised: December 27, 2010 Published: January 20, 2011 1192
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Table 1. Annual Deposition Fluxes and Wet Deposition pH (Average 2001-2004) at the Four Chinese Study Sitesa,b
Chongqing (TieShanPing, TSP)
SO42-
Ca2þ
NH4þ
NO3-
keq ha-1 yr-1
keq ha-1 yr-1
keq ha-1 yr-1
keq ha-1 yr-1
pH
10.5
5.7
1.8
0.7
4.1
Hunan (CaiJiaTang, CJT)
3.7
2.9
1.5
1.0
4.4
Guizhou (LeiGongShan, LGS)
1.5
1.6
0.2
0.2
4.7
Guangdong (LiuXiHe, LXH)
2.1
0.8
0.4
0.4
4.4
Hubbard Brook (1979-1983)c
1.6
0.1
0.1
1.2
4.3
Lysina, Cz rep. ca. 1980d
2.8
0.5
0.6
0.4
4.2
a
Data for the Hubbard Brook site in the US and Lysina site in Czech Republic during the period of peak deposition (around 1980) are shown for comparison. b Deposition fluxes shown calculated based on throughfall measurements. From reference 4 c From ref 7. d From ref 8.
Figure 1. Map of China, showing the location of the four study sites.
within the so-called Acid Rain Control Zone, the target area for acid rain mitigation in China.6 The sites are forested catchments, with forest regeneration after large-scale clear felling around 1960. The soils (Haplic Acrisols) are acidic and representative for this part of China. Parent materials at all sites are sedimentary bedrocks, as sandstone and shale, except for the southernmost site (LiuXiHe; LXH), which is dominated by granite. The LXH site (area: 261 ha) is a broadleaved evergreen forest in Guangdong province, receiving moderately high deposition of S and N (Table 1). TieShanPing (TSP; 16 ha) is a pine forest located on a sandstone ridge outside Chongqing city, receiving the highest amounts of S
and N deposition among the sites. CaiJiaTang (CJT; 4 ha) is a mixed conifer-broadleaf forest situated within an agricultural region in Hunan province, receiving high S and N loadings. LeiGongShan (LGS; 6 ha) is a pine forest situated in a remote mountain region east in the Guizhou province; here, the deposition of S and N are the lowest among the studied sites. Sampling. Water chemistry was analyzed in precipitation, canopy throughfall, soil-water, and streamwater. Each site had one sampling station for precipitation (wet only collector) and four 10 m 10 m plots, each with four throughfall collectors and a set of ceramic suction lysimeters for soil-water collection at 1193
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site
compartment
2001
2002
2003
2004
TSP
precipitation
779
1251
1053
1082
soil depth) were used. The rationale for using this depth was that this is below almost all visible roots and hence the water flux is more close to the streamwater flux due to less impact from evapotranspiration. The relative concentration difference between the horizons have previously been shown to be small.17
throughfall stream
846 321
1382 760
1059 480
1316 725
’ RESULTS
precipitation
947
1553
1196
1262
throughfall
893
1382
1161
1146
stream
479
Table 2. Water Fluxes for All Sites, Years and Compartments (in mm yr-1)
CJT
LGS
LXH
945a,b
545a,b
893a,b
precipitation
1427
1700
1001
981
throughfall stream
1424 506
1973 1146
1239 908
1431 1020a,c
precipitation
na
1655
1621
1306
throughfall
na
1682
1759
1598
stream
na
1158a,d
1134a,d
915a,d
a
Due to malfunctioning limnigraph parts of the year at some sites, the streamwater flux was estimated based on other observational data. b Assumed to have the same ratio of streamwater flux to wet only deposition flux as observed at the TSP site. c Assumed to have the same ratio of streamwater flux to bulk deposition flux as in 2003. d Stream water flux assumed to be 0.7 times the observed bulk precipitation, based on ratio from literature.27
four depths.21 The throughfall collectors were placed in the corners of the plots.21 Throughfall and soil solution were collected weekly. The throughfall solutions from the four collectors at each of the four plots were bulked into one volume weighted sample prior to analysis. Tension (approximately 50 kPa) was applied to the lysimeters after sample collection and left until the next sampling one week later. Wet only precipitation was collected after each precipitation event and lumped into weekly samples prior to analyses. Stream water samples were collected and analyzed weekly. Throughfall and soil solution samples were collected weekly and lumped into 4-weekly samples prior to chemical analyses.21 Analyses. Chemical analyses of water samples were performed in four local laboratories following the same methods, using the same equipment and following the same protocol for quality assurance and control. Sulfate (SO42-), NO3-, Ca2þ, and NH4þ (as well as other major ions) were analyzed by ion chromatography following standard methods (ISO 14911 for cations and ISO 10304-1 for anions). The laboratories participated in the EMEP intercomparison program 22 and all data have passed internal quality control routines (by means of satisfactory agreement between measured and calculated conductivity as well as acceptable ion balance).23 Soil chemical analyses were done in one laboratory. Cation exchange capacity (and base saturation) and adsorbed SO42- was analyzed following refs 24 and 25, respectively. Prior to analysis, laboratory quality was controlled in a soil analysis intercomparison study.26 Flux Calculations. The fluxes of ions in each compartment were calculated from ion concentration and water flux (Table 2) for each sample and added up to annual values. For soil solution, where water flux measurements do not exist, the water flux for streamwater was used. For the calculation of solute fluxes in the soil, concentrations from the deepest lysimeters (approx 50 cm
Deposition Fluxes. Wet Only Deposition. The average wet only deposition fluxes of SO42- at the four sites were relatively large, at average values from 0.9 to 1.9 keq ha-1 yr-1 at the different sites (Figure 2). The corresponding NO3- fluxes were smaller, with averages for the different sites from 0.2 to 0.5 keq ha-1 yr-1 (Figure 3). NH4þ and Ca2þ were both important cations in wet deposition with fluxes from 0.4 to 0.9 keq ha-1 yr-1 for NH4þ and 0.6 to 0.8 keq ha-1 yr-1 for Ca2þ. Hþ fluxes also contributed substantially to the ion balance in the wet deposition (Figure 2). Volume weighted annual average pH values in wet deposition ranged from 4.1 at TSP to 4.7 at LGS (Table 1). Throughfall Deposition. At two of the sites (TSP and CJT, i.e. the two sites with the largest deposition fluxes), the increase in solute fluxes from wet only to throughfall deposition was striking (Figures 2 and 3). For SO42-, the increase was more than 5-fold at TSP and between two and three times at CJT. The mean annual deposition flux of SO42-, measured in throughfall over the four years, was large at TSP (10.5 keq ha-1 yr-1). Also at CJT the SO42- flux was large, but only about 1/3 of the flux at TSP (Table 1). At LGS and LXH the throughfall deposition flux of SO42- was more moderate at 1.5 and 2.1 keq ha-1 yr-1, respectively. These values are similar to peak depositions reported for Europe and North America around 1980 (Table 1). Also LGS and LXH had an increase in SO42- flux from wet only to throughfall, although the fluxes less than doubled. The throughfall deposition fluxes of Ca2þ were also large at TSP and CJT, and the relative increase from wet only to throughfall was about 9-fold at TSP and 5-fold at CJT (Figure 2). LGS also had a substantial increase in the Ca2þ flux from wet only to throughfall, but at LXH there was no change. At TSP, the Hþ flux increased from wet only to throughfall deposition, whereas at the three other sites the Hþ flux decreased between these two compartments (Figure 2). Total N deposition (the sum of NH4þ and NO3-) was approximately the same at TSP and CJT at 2.5 keq ha-1 yr-1, and considerably smaller at LGS (0.5 keq ha-1 yr-1) and LXH (0.8 keq ha-1 yr-1) (Figure 3). The relative increase in the flux of N species from wet only deposition to throughfall was around two times at TSP and CJT, but did not show much change at LGS and LXH. The contribution of NH4þ relative to NO3- to the total N deposition was different at the four sites: At TSP the NH4þ flux accounted for 70% of the total N flux, whereas this was 60% at CJT. At LGS and LXH, NH4þ, and NO3- contributed about equally to the total N deposition flux. Solute Fluxes in the Soil. At TSP and CJT (the two sites with the largest deposition), the fluxes of SO42- in soil-water were approximately the same as in the throughfall deposition, whereas at LGS and LXH, the average flux of SO42- in soil-water was considerably smaller than the flux in throughfall deposition. At TSP and CJT, there were some differences from year to year where the fluxes increased from throughfall to soil solution in relatively wet years (2002 and 2004; Figure 2). 1194
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NO3- fluxes in soil-water was larger than the throughfall input flux at all sites. The annual average sum of the NH4þ and NO3fluxes in throughfall deposition was similar to that in soil solution, although there were considerable differences from year to year. Average annual Hþ and Al fluxes in the soils were elevated at TSP only (Figure 2). At all other sites, soil-water was slightly acidic and Al concentrations (and thus Al fluxes) were small. Stream Water Fluxes. A general trend for all sites is that the fluxes of solutes are smaller in streamwater than at the 50 cm depth soil solution. This decrease is more pronounced at TSP and CJT than at LGS and LXH. The relative loss of ions is generally larger for NO3- than for Ca2þ and SO42-. At all sites there was considerable acid neutralization of water when comparing soil solution with streamwater fluxes of Aluminum (Al) and Hþ (Figure 2). The total sink of ions from deposition to streamwater is substantial. For SO42-, the annual fluxes in streamwater averaged 33% of the flux in throughfall for TSP and CJT, and 16-18% for LGS and LXH on average. For N, between 14% (LXH) and 34% (LGS) of the deposited N reached the stream. The Ca2þ fluxes in streamwater were between 36% (LXH) and 61% (CJT) of throughfall deposition.
’ DISCUSSION
Figure 2. Fluxes of SO42- (note different x-axis for the sites) , Ca2þ, inorganic monomeric aluminum (Ali), and Hþ in wet deposition (marked WO), throughfall (TF), soil-water (SS) and streamwater (W) at the four sites. The horizontal lines in each diamond box show the median, the range of the boxes is (SD and the square inside each box shows the average. The numbers indicate fluxes for the individual years (1 represents 2001, 2 represents 2002, etc.). The size (height) of the diamond illustrate the variation between plots and between years; the numbers show the variation between years only.
At TSP the average Ca2þ fluxes in soil-water were smaller than throughfall deposition. At CJT and LGS there were no clear differences between the Ca2þ fluxes in throughfall and soil solution, although the differences in soil solution fluxes between years were large. At LXH the Ca2þ flux in throughfall was considerably smaller than in soil-water. The annual average NH4þ fluxes in soil-water were close to zero at the four sites (Figure 3). By contrast, the annual average
Deposition. All sites have considerably greater fluxes of SO42- in throughfall than in wet deposition, due to dry deposition. The elevated dry deposition of S is a combination of particulate deposition (SO42- associated with Ca2þ or NH4þ) and gaseous SO2 deposition. At TSP the pH was smaller in throughfall than in wet only deposition, which can be explained by considerable deposition of gaseous SO2 (or H2SO4 in fog). At the three other sites the pH was greater in throughfall than in wet only deposition, indicating the relative importance of particulate deposition of alkaline particles (e.g., CaCO3). Probably, much of the dry deposition of SO42- at these sites is in the form of neutral CaSO4 and (NH4)2SO4.28 The relative importance of gaseous SO2 in dry deposition is larger at TSP probably because the site is located in close vicinity of large S emission sources in Chongqing city (15 km west of TSP). With the exception of the LXH site (with broad leaved forest), all sites also show a substantial increase in the Ca2þ flux from wet deposition to throughfall. We attribute the increase to dry deposition, rather than canopy leaching since it is much larger than what is generally found as canopy leaching.29 It is also wellknown that the concentration of Ca2þ-containing particles in the Chinese atmosphere is high many places.5 For the N species, the change in the fluxes from wet deposition to throughfall is less obvious and shows different patterns for the different sites. This relatively large increase in NH4þ-N and NO3--N flux in throughfall at TSP and CJT can be explained by elevated rates of N dry deposition. The other two sites (LGS and LXH) have more moderate N deposition loads and any increasing effect of dry deposition seems to have been counteracted by N uptake processes in the canopy.12 Soil. The changes in SO42- fluxes from throughfall to soilwater are distinctly different between the four sites. At the two high deposition sites (TSP and CJT), the input and output of the soil seem to balance over the four year period. At LGS and LXH there appears to be a SO42- sink in the upper soil. The pools of adsorbed SO42- in the soils at LGS and LXH are small to 1195
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Table 3. Adsorbed SO42- and Exchangeable Ca2þ Pools in the Soil (Upper 50 cm) site adsorbed SO42- pool (keq ha-1) exchangeable Ca2þ pool (keq ha-1)
Figure 3. Fluxes of NO3-, NH4þ and the sum of inorganic nitrogen (Nsum). See caption for Figure 2 for explanations.
moderate, and equivalent to only between two and 10 years of SO42- deposition in throughfall. Adsorbed SO42- ranges from 1.5 to 28.5 keq ha-1 across all four sites (Table 3), whereas the SO42- throughfall fluxes range from 1.5 to 10.5 keq ha-1 yr-1. Hence SO42- adsorption does not seem an important long-term SO42- sink in the upper 50 cm of the soil, at least not at the sites where the deposition flux is high (TSP and CJT). However, adsorption and desorption may still be important for controlling the short-term fluctuation of the SO42- flux, such as the observed year to year variation. Currently, at LGS the adsorbed pool of SO42- in the soil may be building up; the site has received considerably less SO42- deposition over time than the other sites. However, S emissions, and hence the long-range atmospheric transport and deposition of S, has increased very rapidly in Southwest China within the past decade or two.1 The changes in the flux of Ca2þ in the soil solution differ at the four sites. At two of the sites (CJT and LGS), the Ca2þ flux is roughly the same in throughfall deposition and soil solution and hence there are no major sinks or sources (although there is a noticeable variation between years, that may be explained by cation exchange). At the two other sites (TSP and LXH) there is a sink of Ca2þ in the soil. The sink at LXH is limited and may be attributed to vegetation uptake; the size of the sink is approximately
TSP
16.3-28.5
10.5-31.9
CJT
5.4-10.6
12.7-61.6
LGS
1.5-21.5
12.1-222.7
LXH
1.8-10.9
0.8-1.5
0.5 keq ha-1 yr-1, which can be a typical uptake rate for such a system.16 At TSP, however, the major Ca sink is too large to be explained by plant uptake (the Ca2þ uptake by forest trees at TSP has been estimated to 0.25-1.10 keq ha-1 yr-1 16). The considerable loss of dissolved Ca2þ from percolation water in the soil occurs simultaneously with a large mobilization of Al and to a lesser extent Hþ, while the anion flux increases, primarily due to the increase in NO3- (presumably due to nitrification of NH4þ). A possible mechanism can be cation exchange; if this is the case, the soil is actually building up the exchangeable Ca2þ pool and hence base saturation. The size of the exchangeable Ca2þ pool in the upper 50 cm of soil (Table 3) was equal to one or two decades of the annual sink. Thus, with the estimated Ca2þ pool, the hypothesized process with increased exchangeable Ca2þ can not have been ongoing for a long time. A recent increase in Ca2þ deposition in the Chongqing area is not unlikely given the speed of development in this region (and in China in general), which is associated with a rapid increase in emission sources like construction, dust, road traffic, and combustion. For total N, the average fluxes for the four years are approximately similar in soil solution as in throughfall deposition. This suggests minor N retention rates in the soils of the four catchments. There are however larger variations from year to year in the soil solution; the fluxes are larger in the wet years, which can be attributed to mineralized N from litterfall 12,30 at high moisture conditions (driven by rainfall). The flux of NH4þ is low in the soil solution (with the exception of one year at LXH) and hence most NH4þ derived from throughfall deposition and mineralization of organic matter is nitrified. Thus, the NO3- flux increased from throughfall to the soil-water at all sites for most years.30 Stream. The overall element budgets (i.e., the difference between throughfall deposition flux and streamwater output) show considerable retention of S, N and Ca in all four catchments. Interestingly, most of this retention occurs in the deeper soil or in the riparian zone, that is, after percolation water has left the upper 50 cm of soil but before the water leaves the catchment as streamwater at the weir. As to the cause of this we can only speculate until new data becomes available, but the fact that it occurs at all sites suggests that it is a general phenomenon. Sorption in deep soils may be one mechanism removing SO42-, Ca2þ and other elements from water before entering the stream. In addition, reduction processes involving S and N may play an important role in the riparian zones of the catchments, where groundwater emerges in areas with ample supply of labile organic matter from the vegetation. Formation of sulphides and denitrification, resulting in strong decreases in SO42- and NO3- concentrations, are both acid neutralizing processes and result in a significant increase in pH and a decrease in Al flux when comparing soilwater and streamwater. Due to charge balance constraints, the removal of the dominant mobile anions from groundwater also causes a decrease of the Ca2þ flux. It should also be noted that the 1196
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Environmental Science & Technology concentration of potentially toxic Al decreases in this process. Further research of the mechanisms of the removal of S, N, and particularly of Ca2þ and Al, and where this occurs in the catchments, is underway. Subtropical versus Boreal and Temperate Regions. Some of the main trends in changes in ion fluxes from deposition via soil solution to streamwater reported here for subtropical Chinese forests differ from those reported for boreal and temperate forests in North-America and Europe: In a review of data from 21 forested research catchments in North-America and Europe, most sites (18 of 21) had larger S export with steamwater than input with deposition and all sites had larger Ca export than input.18 In some regions considerable S retention has been reported, which was attributed to sulfate adsorption in the upper soil horizons.31 For N, considerable catchment retention, is commonly observed also in North-America and Europe.18 The subtropical Chinese catchments differ from those in temporal and boreal areas with respect to two main characteristics: First, they have experienced an increase in atmospheric input of Ca2þ, which may drive cation exchange in acid soils, such that soil acidity (Hþ and Al3þ) declines, while the acidity of soil-water increases. In the long run, the gradual increase in the soil’s base saturation will cause a long-term increase in the pH of soilwater. Second, all investigated Chinese subtropical sites show a large retention of S, N, and Ca in the deeper soils or riparian zones. Probably, this is due to reduction processes, which are likely to be most pronounced in systems with hot and wet summers, characteristic for much of South China.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (þ86-10)62771403; fax: (þ86-10)62773597; e-mail:
[email protected].
’ ACKNOWLEDGMENT We are grateful for the financial support of the Norwegian Agency for Development Cooperation (grant CHN-0030; IMPACTS project) and of the Norwegian Research Council (grant 193725/S30). We are grateful for the contribution of many of the IMPACTS partners, in particular Zhao Dawei and Zhang Dongbao from Chongqing Institute of Environmental Science and Monitoring, Xiang Renjun and Chang Yi from Hunan Research Institute of Environmental Protection Science, Xiao Jinsong and Peng Xiaoyu from Guizhou Research Institute of Environmental Protection Science, Zhang Jinhong and Zhang Zhanyi from Guangzhou Research Institute of Environmental Protection. We greatly acknowledge the in depth comments and detailed suggestions from four anonymous reviewers. ’ REFERENCES (1) MEP. Report on the State of the Environment in China 2008; Ministry of Environmental Protection (MEP) of China: Beijing, 2009, in Chinese. (2) Tian, H.; Hao, J. Current status and future trend of nitrogen oxides emissions in China. Abstr. Pap., Am. Chem. Soc. 2003, 226, U564– U564. (3) Zhang, X.; Zhang, P.; Zhang, Y.; Li, X.; Qiu, H. The trend, seasonal cycle, and sources of tropospheric NO2 over China during 1997-2006 based on satellite measurement. Sci. Chin. Ser. D 2007, 50, 1877–1884.
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