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137Cs tracing dynamics of soil erosion, organic carbon and nitrogen in sloping farmland converted from original grassland in Tibetan plateau
ARTICLE IN PRESS Applied Radiation and Isotopes 68 (2010) 1650–1655
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137
Cs tracing dynamics of soil erosion, organic carbon and nitrogen in sloping farmland converted from original grassland in Tibetan plateau
Nie Xiaojun a,n, Wang Xiaodan b, Liu Suzhen b, Gu Shixian b, Liu Haijun c a
School of Surveying and Land Information Engineering, Henan Polytechnic University, Jiaozuo 454000, China Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Conservation, Chengdu 610041, China c Institute of Water Resources Planning, Surveying, Design, and Research, Lhasa 850000, China b
a r t i c l e in f o
a b s t r a c t
Article history: Received 28 January 2010 Received in revised form 2 April 2010 Accepted 4 April 2010
There is a shortage of research concerning the relationships between land-use change, soil erosion, and soil organic carbon (SOC) and nitrogen (N) dynamics in alpine environments such as those found in the Tibetan plateau. In this paper, typical sloping farmlands converted from grassland 50 years ago in eastern Tibet were selected to determine dynamics of soil erosion, SOC, and total N associated with land-use change. Soil samples were collected from sloping farmland and control fields (grassland). The 137 Cs, SOC, total N contents, and soil particle size fractions were analyzed in these samples. As compared with the control fields, 137Cs, SOC, and total N inventories in the sloping farmlands decreased by 30%, 27%, and 33%, respectively. Meanwhile variations in the three parameters were enhanced in the sloping farmlands, with coefficients of variation (CVs) of 38%, 23%, and 20%, respectively, for 37Cs, SOC, and total N. In addition, SOC and total N inventories significantly decreased with increasing soil erosion in the sloping farmland. In a sloping farmland with a steep 241 gradient, the 137Cs inventory gradually increased along a downslope transect with its lowest value at 0 Bq m 2 in the top-slope position (0 m). The soil clay ( o0.002 mm) content in such an area increased with decreasing elevation (r ¼ 0.95, p¼ 0.001). Significant correlations between 137Cs and clay (r ¼ 0.92, p ¼ 0.003), SOC (r ¼0.96, p ¼0.001), or total N (r ¼ 0.95, p ¼0.001) were also found in the farmland. These results showed that converting alpine grassland to sloping farmland accelerates soil erosion, losses in SOC and N, and increases the soil’s spatial variability. The combined impacts of tillage and water erosion contributed a significant decrease in the soil’s organic carbon and N storages. Particularly in steep sloping farmlands, tillage erosion contributed for severe soil loss, but the soil redistribution pattern was dominated by water erosion, not tillage erosion, due to the lack of boundaries across the field patches. It was also found that 137 Cs, SOC, and total N moved along the same pathway within these sloping farmlands, resulting in net C and N losses during soil redistribution. The negative influences of land-use conversion from grassland to farmland in sloping areas in the Tibetan plateau should be addressed. & 2010 Elsevier Ltd. All rights reserved.
Keywords: 137 Cs technique Land-use change SOC Water erosion Tillage erosion Tibetan plateau
1. Introduction Tibet, known as the ‘‘World’s Ridge,’’ features one of the most unique regional ecology areas in the world. Numerous glaciers, lakes, wetlands, and large international rivers are distributed in the area, earning Tibet another distinction as the ‘‘Asian Tower.’’ The Tibetan plateau exerts a profound impact on eastern Asian geographical environments by providing an important barrier for the stability of climate systems in this region. It is also an important gene pool for global biological species and a key area for global biodiversity conservation. However, Tibetan ecosystems are fragile and sensitive to incompatible exogenic actions, such as
n
Corresponding author. Tel./fax: +86 391 3983693. E-mail address: [email protected] (N. Xiaojun).
0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.04.017
rapid industrial development, intensive agriculture, and global climate change. Alpine grassland is a major type of ecosystem in the Tibetan plateau, covering an area of 7.848 107 ha, which is approximately 65.6% of the land area of Tibet. This ecosystem plays important barrier functions in Tibet in terms of ecological security, such as climate regulation, water and soil conservation, and biodiversity maintenance. These functions, however, are weakening due to unreasonable land uses in the region. The decline in climate regulation and soil erosion control functions is especially severe. Soils under alpine grasslands are generally high in SOC content because of low temperature and well-developed vegetation as a result of natural succession (Institute of Soil Science, Academia Sinica, 1986), which contributes an important C pool in sustaining global C circulation balance. Previous studies showed that ecologically incompatible disturbances, such as overgrazing, cultivation, and global warming have resulted in
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decreasing large sections of organic C storage in alpine grassland soils (Kang, 1996; Cheng et al., 1997; Wu and Tiessen, 2002; Jian, 2002). Without effective measures to prevent these disturbances, the C sink function of alpine grasslands could be lost or transformed into a C source, further accelerating global warming. Investigations on soil erosion conducted in the ‘‘Three Rivers Watershed’’ (Jinsha River, Lancang River, and Nujiang River) showed that these regions are undergoing widespread soil erosion, especially in sloping farmlands, resulting in an increase in sediment content in rivers and a decrease in water quality (Zhong et al., 2008). To pursue economic profits, local residents reclaimed forests and grasslands, even those on steep slopes, and turned them into croplands (Zhou et al., 2005), which often brings major and negative influences on soil quality and function (Bossuyt et al., 1999; Dinesh et al., 2003; Elberling et al., 2003; Walker and Desanker, 2004; Merino et al., 2004; Guo and Gifford, 2002; Su et al., 2004; Islam and Weil, 2000; Li et al., 2007; Yang et al., 2008; Mostafa et al., 2008). Recognized as a primary soil quality constituent, soil organic matter is important in maintaining soil structure stability, aiding in the soil infiltration of air and water, promoting water retention, and enhancing soil fertility (Gregorich et al., 1994; Roose and Barthes, 2001; Zaujec, 2001). Reduction in SOC induced by cultivation change the distribution and stability of soil aggregates, thus increasing the likelihood of soil erosion (Cambardella and Elliott, 1993; Six et al., 2000). For example, Mostafa et al. (2008) reported that the conversion of grassland into cropland during an 18 year-period decreased SOC and total N by 50% each, and increased soil erodibility by 51% in the highlands of Iran. As such, soil erosion, accelerated by the expansion of sloping farmlands in the Tibetan plateau, could be a serious issue to the sustainable development of local agriculture. The satellite remote sensing technique is a major method used to investigate soil erosion in the Tibetan plateau (Zhong et al., 2008). However, it can only provide a qualitative description, not a quantitative evaluation. On the other hand, the Universal Soil Loss Equation (USLE) model can assess soil erosion quantitatively, but some parameters in the model, such as the rainfall factor (USLE-R) and soil erodibility factor (USLE-K), are difficult to determine due to the lack of long-term field observations. In comparison, the 137Cs technique can overcome the limitations associated with the above mentioned traditional approaches and has been proven effective in assessing soil erosion intensities and processes (Ritchie and McHenry, 1990; Walling and Quine,1990, 1991; Li et al., 2003; Zhang et al., 2004). Aside from 137Cs being a well-established tracer for soil erosion, it also provides a valuable tool for investigating medium-term erosion–carbon relationships. Recently, studies concerning 137Cs application to evaluate soil erosion (Yan et al., 2003) and erosion–carbon relationships (Li et al., 2003; Zhang et al., 2006a; Wei et al., 2008) were conducted in the alpine grasslands of Qinghai–Tibetan plateau. Unfortunately, the dynamics of soil erosion, SOC, and N in landuse change from grassland to sloping farmland remain unclear. Against this background, the current study aims to understand the effects of the conversion of grassland to sloping farmland on soil erosion, SOC, and total N, as well as to determine the erosion-induced soil redistribution pattern in sloping farmlands at an alpine site in Jinsha River Watershed, eastern Tibet. The results may provide insights for promoting ecological security in the Tibetan Plateau.
2. Materials and methods 2.1. Study area The study area lies in Jiangda County (301 210 -32 1 000 N, 971 21 -98 1530 E), at the upper reaches of the Jinsha River, eastern 0
1651
Fig. 1. Study area location.
Tibet Autonomous Region, China (Fig. 1). With an average altitude of 3650 m, the area is of typical alpine gorge geomorphology and has a semi-humid temperate climate. Temperature per annum averages 4.5 1C, with a maximum of 28 1C and a minimum of 15 1C. Annual precipitation is between 520 and 610 mm, and about 77.9–95.8% of the rain occurs between May and September. Annual evaporation reaches 1600–1700 mm. Land use types in the area mainly include forests, grasslands, and sloping farmlands. Most of the sloping farmlands had been converted from the original grasslands 50 years ago. Soils in the study area, for both grassland and farmland, are derived from slope materials of sandstone, limestone, metamorphic rock, and magmatic rock, and are classified as alpine meadow soil in Chinese soil taxonomy or as Calciudolls in US soil taxonomy. The dominant vegetation includes alpine meadow and alpine steppe, and the major crops are wheat (Triticum aestivum L.) and highland barley (Semen Avenae Nudae). 2.2. Soil sampling and laboratory analysis In the study area, sloping farmlands with an approximately 50year cultivation history are mainly distributed in the lower parts of hillslopes between a.s.l 3400 and 3900 m. The sloping farmlands are characterized by steep slopes (varying mainly from 101 to 251) and short lengths (varying mainly from 10 to 40 m). Within the 3400–3900 m elevation interval, seven sloping farmlands (i.e., 71, 111, 141, 161, 181, 191, and 241) at different locations were selected to conduct soil sampling for 137Cs and other physical and chemical determinations. Meanwhile, seven control fields were established for soil sampling, each in small patches of unbroken and irregular grasslands neighboring the selected sloping farmlands. The vegetation coverage was over 70% in the control fields. Except for the 241 sloping farmland, samples from the upper, middle, and lower positions of each of the other six farmlands were collected. In the 241 sloping farmland, seven soil sampling points were set at 5 m intervals along the down-slope transect and two replicates for each were collected. Unlike the sampling for sloping farmlands, soil samples for each control field were collected randomly in three replicates. All soil samples were collected to a depth of 23–37 cm depending on the soil thickness (i.e., up to the bedrock) using a 6.8-cm diameter hand-operated core sampler. The core sample was subsequently divided into two segments: the surface (0–20 cm) and the subsoil ( 420 cm). For the six sloping farmlands (i.e., 71, 111, 141, 161, 181, and 191), the three surface and subsoil subsamples from the upper, middle, and lower positions of each farmland were combined in bulk to create a composite sample. For the 241 sloping farmland, the two replicated soil cores were combined in
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Taking the 241 sloping farmland as a specific case, the soil erosion process can be tracked according to the 137Cs distribution along the down-slope transect. Compared with a 137Cs inventory of 1712 Bq m 2 in the control field, lower 137Cs inventories were found within the 241 sloping farmland (Fig. 2). This implies a net soil loss in steep sloping farmland. The 137Cs inventory increased along the down-slope transect, (Fig. 2), suggesting that soil loss gradually declined along this direction. In particular, 137Cs inventory in the top-slope position (0 m) was the lowest, with a value of 0 Bq m 2 (Fig. 2). This shows that tillage erosion induced the most severe soil loss in the positions. Soil clay (o0.002 mm) content increased with decreasing elevations in the farmlands (Fig. 3) and a significant negative correlation was found between the two factors (r ¼ 0.95, p¼ 0.001). Soil clay content was also significantly related to 137Cs inventory (r ¼0.92, p¼0.003). The
3909
1800 Elevation 137Cs
1600 1400
3903
1200
3900
1000
3897
800 600
3894
400 3891
200
3888
0 5
0
10 15 20 Horizontal length (m)
2.3. Statistical analysis Fig. 2.
One-way analysis of variance (ANOVA) was conducted to test the significance of differences found between the sloping farmlands and the control fields. Linear regression analysis was used to test the correlation between SOC, total N, and 137Cs.
inventory (Bq m-2)
3906
137Cs
bulk to create a composite sample. For the seven control fields, three surface-and subsoil subsamples from three random positions were combined in bulk to create a composite sample. Samples for the 137Cs reference could not be acquired because it was difficult to find an appropriate reference site in the alpine gorge geomorphology, that is, an area where soil erosion, deposition, and human disturbance had not occurred. Overall, 26 and 14 composite samples were collected from the sloping farmlands and control fields, respectively. Soil samples were air-dried, crushed, passed through a 2-mm mesh sieve to remove coarse material, and then divided into two, one for the measurement of 137Cs activity, and another for the measurements of SOC, total N, and soil particle-size fraction. Samples for the o2 mm fraction were sealed in a 200-cm3 (fl0l mm 25 mm) plexiglass box. Twenty-eight days later, 137 Cs activity of these samples was measured using a hyperpure lithium-drifted germanium detector linked to a LabSOCS (CANBERRA, USA) with a counting time of 36,000–86,400 s and a measurement precision of ca.74.7% (95% confidence). The original measurements of 137Cs activity, expressed in terms of per unit mass (Bq kg 1), were converted into the inventory (Bq m 2) by means of the total weight of the bulked core sample and the cross-sectional area of the sampling device. The SOC was determined using wet oxidation with K2Cr2O7, and the measurement of total nitrogen followed the classical Kjeldahl digestion method (Liu, 1996). Soil particle-size fractions were determined by pipette method following H2O2 treatment to destroy organic matter and dispersion of soil suspensions in Na-hexametaphosphate (Liu, 1996).
Elevation (m)
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137
25
30
Cs distribution across the 241 sloping farmland.
20.5%
3.1. Soil erosion dynamics In each of the seven sloping farmlands, 137Cs inventory was lower than that of its control field (Table 1). The 137Cs inventory averaged 1263 Bq m 2 (SE7178) in the seven farmlands, showing a 30% decrease compared to the mean of 1798 Bq m 2 (SE767) in the control fields. What’s more, coefficient of variation (CV) in 137Cs inventory was nearly 4 times as great in the sloping farmlands (38%) as in the control fields (10%). These results suggest that the conversion from alpine grassland to sloping farmland remarkably accelerated soil erosion intensity and increased its spatial variation.
Table 1 Comparisons of the profile contents of
137
Grasslands
137
y = -0.0022 x + 8.5644
19.5%
r = -0.95, p = 0.001
19.0% 18.5% 18.0% 17.5% 17.0% 3888
3891
3894 3897 3900 Elevation (m)
3903
3906
Fig. 3. Relationship of clay content to elevation in the 241 sloping farmland.
Cs, SOC and total N (TN) between sloping farmland and control fields (grassland).
Slope gradient Sloping farmlands
Clay content (%)
20.0% 3. Results
Cs (Bq m 2) SOC (kg m 2) TN (kg m 2) 137 Cs (Bq m 2) SOC (kg m 2) TN (kg m 2)
241
191
181
161
141
111
71
8407 219 4.227 0.33 0.40 70.03 1712 6.03 0.54
809 3.73 0.40 1881 6.59 0.64
1689 5.10 0.51 1976 6.70 0.65
1331 3.63 0.39 1438 5.72 0.54
677 2.90 0.31 1813 6.80 0.70
1621 5.08 0.49 1876 6.75 0.62
1870 5.72 0.56 1901 6.88 0.67
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7.00
3.2. SOC and N dynamics
5.00
3.3. Relationship of SOC and N to soil erosion In the seven sloping farmlands, SOC and total N inventories decreased with decreasing 137Cs inventory (Fig. 5). A significant and positive correlation was also found between SOC and 137Cs (r ¼0.88, p¼0.01) and total N and 137Cs (r ¼0.91, p ¼0.004). These relationships show that storage of SOC and N decreases with increasing soil erosion in sloping farmlands. In the 241 sloping farmland, SOC and total N showed a distribution pattern similar to 137Cs across the entire slope, resulting in significant and positive correlations between them (r ¼0.96, p ¼0.001 between SOC and 137Cs; r ¼0.95, p¼0.001 between total N and 137Cs). The results are consistent with previous studies by Ritchie and McCarty (2003), Zhang et al. (2006b), and Wei et al. (2008), which suggest that erosioninduced soil redistribution exerts remarkable impact on the spatial dynamics of SOC and N.
6.00
0.60 SOC Total N
0.55
5.00
0.50
4.50
0.45
4.00
0.40
3.50
0.35
3.00
0.30
2.50
0.25
2.00
0.20 0
5
10
15
20
25
30
Downward slope (m) Fig. 4. Distribution of SOC and total N across the 241 sloping farmland.
Total N (kg m-2)
SOC (kg m-2)
5.50
6.00 SOC (kg m-2)
Similar to the results found for 137Cs inventory, contents of SOC and total N in each sloping farmland were lower than those in their respective control fields (Table 1). The inventories of SOC and total N averaged 4.34 (SE70.37) and 0.44 kg m 2 (SE70.03) in the seven farmlands, respectively, showing a significant decrease of 27% (po0.001) and 33% (p ¼0.001) compared to means of 6.49 kg m 2 SOC (SE70.16) and 0.62 kg m 2 total N (SE70.02) for their respective control fields. In addition, large variations in SOC (CV, 23%) and total N (CV, 20%) were found in the seven sloping farmlands, while small variations were found (CV: 7% for SOC; 10% for total N) in the control fields. These results suggest that the conversion from alpine grassland to sloping farmland induces obvious losses of SOC and N, and also encourages great spatial variations. The distribution patterns of SOC and total N were consistent across the 241 sloping farmland, that is, their contents gradually increased along the down-slope transect (Fig. 4). A significantly positive correlation (r¼0.99, p o0.001) between the farmlands was observed. Compared to those in the control fields (SOC, 5.72 kg m 2; total N, 0.54 kg m 2), decreases in SOC and total N inventories were most serious in the top-slope positions (0 m) of the farmlands, with a decrease of 47% in SOC and 48% in total N. The SOC and total N inventories in the bottom-slope position (30 m) were highest within the sloping farmland, but decreases of 7% and 8% were still found in the SOC and total N inventories, respectively, compared to those in the control field.
0.80 SOC Total N
0.70 0.60
4.00 0.50 3.00 0.40
2.00
Total N (kg m-2)
relationships between elevation, 137Cs, and soil clay indicate that soil fine particles are selectively transported by water erosion.
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0.30
1.00 0.00
0.20 677
809
840 137Cs
1331
1621
1689
1870
inventory (Bq m-2)
Fig. 5. SOC and total N pools versus
137
Cs inventory along the downward slope.
4. Discussion Land-use change from grassland to farmland in sloping areas intensified soil erosion, which is mainly attributed to two reasons. One, after the conversion from original grassland to sloping farmland, tillage can destroy the initial soil structure and expose organic-rich soils to open air, thus accelerating soil organic matter mineralization and reducing SOC content. Consequently, reductions in SOC result in increased susceptibility of soil to erosion (Cambardella and Elliott, 1993; Six et al., 2000; Mostafa et al., 2008). Another is the impact brought about by tillage erosion. The 137 Cs inventory of 0 Bq m 2 in the top-slope position (0 m) of the 241 sloping farmland confirms severe soil loss induced by tillage erosion. In this type of erosion, soils located upslope are transported entirely downward and then deposited in bottom slope positions. Accordingly, water erosion risks increase, especially when slope runoff gathers at deposited sites. Conversion from grassland to sloping farmland in the study area significantly reduced the contents of SOC and total N. Similar research with consistent results have also been reported in other climate regions (Guo and Gifford, 2002; Wu and Tiessen, 2002; Su et al., 2004; Li et al., 2007; Yang et al., 2008; Mostafa et al., 2008). Li et al. (2007) reported that SOC decreased by 29–41% 28 years after the conversion of an alpine grassland to a cropland in the northeastern fringe of the Tibetan plateau (an altitude of 2960 m). In this study, about 50 years of cultivation resulted in a decrease of 27% in SOC. The low decrease in SOC compared to that observed by Li et al. (2007) could be attributed to a lower SOM mineralization ratio under high altitudes ( 43400 m) and weak tillage intensity in the study area. Spatial variability observed for soil erosion, SOC, and total N in sloping farmlands is thought to be attributed to the combined effects of water erosion and tillage erosion. From the control fields, CV values of 137Cs (10%), SOC (7%), and total N (10%) were small under the influence of only water erosion. After land-use conversion, however, the combination of tillage erosion and water erosion increased the difference in soil erosion (CV, 38%) between the different sloping farmlands, also causing SOC and total N inventories to decrease with increasing soil erosion. These resulted in large spatial variations in SOC (CV, 23%) and total N (CV, 20%). Schumacher et al. (1999) also reported that the combination of tillage erosion and water erosion induces larger variability in soil productivity than a single erosion can in agricultural sloping landscapes. In the 241 sloping farmland, 137Cs inventory gradually increased along the down-slope transect, with the lowest value at 0 Bq m 2 in the top-slope position (0 m). Soil clay ( o0.002 mm) content increased with decreasing elevation.
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The distributions of 137Cs and clay across the whole slope indicate that both tillage erosion and water erosion determined soil redistribution pattern in these short and steep sloping farmlands. Furthermore, according to the clay distribution, water erosion could be mainly responsible for soil redistribution. This inference, however, does not agree with previous studies conducted in the hilly areas of the Sichuan Basin in southwestern China (Zhang et al., 2006b; Ge et al., 2007; Zhang et al., 2008). In these studies, tillage erosion played a primary role in the soil redistribution pattern of steep slopes (slope lengths r30 m), and the entire transport of tillage erosion does not induce an increase in clay content with decreasing elevation. This inconsistency can be explained by the fact that there are hardly any boundaries across the slope. Even if there had been any, these boundaries would have been quite loose and present as dwarf heaps of gravel in the study area. Without the effective protection of boundaries across the slope, plus the fact that sloping farmlands lie in lower regions of the long hillslope, runoff from up the slope would maintain a single path and proceed straight downward during the water erosion process. Hence, the entire slope of the farmland suffers obvious water erosion. On the other hand, in hilly areas of the Sichuan Basin, the slope runoff across steep hillslopes is discontinuous because of the set-up of boundaries between fields. Thus, runoff within a field finds difficulty in forming in the upper regions of the slope, gathering only in the lower regions. In fact, soil redistribution in steep sloping farmlands in hilly areas of the Sichuan Basin is not induced by water erosion but by tillage erosion. As such, building boundaries between fields may help reduce soil erosion in the agricultural sloping landscapes of the Tibetan plateau. Significant and positive correlations between SOC, total N, and 137 Cs were found in the 241 sloping farmland. This result is in line with the previous studies (Schumacher et al., 1999; Thapa et al., 2001; Ritchie and McCarty, 2003; Li and Lindstrom, 2001; Heckrath et al., 2005; Zhang et al., 2006a, 2006b; Ge et al., 2007; Ritchie et al., 2007; Zhang et al., 2008; Wei et al., 2008; Mabit et al., 2008), suggesting the same pathway during soil redistribution. As there exist strong correlations between SOC, N, and 137Cs, the 137Cs technique can provide a useful tool for extensive research on the dynamics of soil erosion, SOC, and N stocks in the Tibetan plateau.
5. Conclusions In the alpine gorge regions of eastern Tibet, the conversion from alpine grassland to sloping farmland was found to have accelerated soil erosion and resulted in significant losses of SOC and N. The combined impacts of tillage and water erosion in sloping farmlands induced increase in spatial variability in soil erosion, SOC, and total N, and significantly decreased SOC and total N storages with increasing soil erosion. Particularly in steeply sloping farmlands, tillage erosion contributed to severe soil losses. However, the soil redistribution pattern was dominated by water erosion, not tillage erosion, due to lack of boundaries across the field patches. Moreover, 137Cs, SOC, and total N moved along the same pathway within these sloping farmlands, resulting in net C and N losses during soil redistribution. Building boundaries could be an effective solution in minimizing soil erosion, organic carbon, and N losses in such areas. Future research on the relationships between land-use changes, soil erosion, and SOC and N contents in the plateau could be conducted using the 137Cs technique to better understand alpine soil dynamics.
Acknowledgements We acknowledge the Science and Technology Project of China (2007BAC06B06, 2007BAC06B08) and the Program for Science and Technology Innovation Talents at the Universities of Henan Province (2008HAST1T004) for the research funds they provided our team. We would also like to thank Dr. Zhang Junyan for her assistance in editing this paper and two local farmers for their assistance with soil sample collection. References Bossuyt, B., Diels, J., Deckers, J., 1999. Changes following cultivation and reforestation of loess soils in central Belgium: consequences for future land use. The land 3, 151–166. Cambardella, C.A., Elliott, E.T., 1993. Carbon and nitrogen distributions in aggregates from cultivated and grassland soils. Soil Sci. Soc. Am. J. 57, 1071– 1076. Cheng, G., Li, P., Zhang, X., Guo, D., Li, S., 1997. Influences of Climatic Changes on Snow Cover, Glaciers and Frozen Soils in China. Gansu Cultural Publishing House, Lanzhou, pp. 22–56 (in Chinese). Dinesh, R., Chaudhuri, S.G., Ganeshamurthy, A.N., Dey, C., 2003. Changes in soil microbial indices and their relationships following deforestation and cultivation in wet tropical forests. Appl. Soil Ecol. 24, 17–26. Elberling, B., Toure, A., Rasmussen, K., 2003. Changes in soil organic matter following groundnut–millet cropping at three locations in semiarid Senegal. West Africa Agric. Ecosyst. Environ. 96, 37–47. Ge, F.L., Zhang, J.H., Su, Z.A., Nie, X.J., 2007. Response of changes in soil nutrients to soil erosion on a purple soil of cultivated sloping land. Acta Ecol. Sin. 27, 459–463. Gregorich, E.G., Carter, M.R., Angers, D.A., Monreal, C.M., Ellert, B.H., 1994. Towards a minimum data set to assess soil organic matter quality in agricultural soils. Can. J. Soil Sci. 74, 367–385. Guo, L.B., Gifford, R.M., 2002. Soil carbon stocks and land use change: a meta analysis. Global Change Biol. 8, 345–360. Heckrath, G., Djurhuus, J., Quine, T.A., Van, O.K., Govers, G., Zhang, Y., 2005. Tillage erosion and its effect on soil properties and crop yield in Denmark. J. Environ. Qual. 34, 312–323. Institute of Soil Science, Academia Sinica, 1986. The Soil Atlas of China. Cartographic Publication House, Beijing, pp. 31–32. Islam, K.R., Weil, R.R., 2000. Land use effects on soil quality in tropical forest ecosystems of Bangladesh. Agric. Ecosyst. Environ. 79, 9–16. Jian, N., 2002. Carbon storage in grasslands of China. J. Arid Environ. 50, 205–218. Kang, X., 1996. The features of climate change in the Qinghai–Tibetan Plateau region in the past 40 years. J. Glaciol. Geocryol. 18 (Suppl.), 281–288. Li, X.G., Li, F.M., Rengel, Z., Zhan, Z.Y., Bhupinderpal, S., 2007. Soil physical properties and their relations to organic carbon pools as affected by land use in an alpine grassland. Geoderma 139, 98–105. Li, Y., Lindstrom, M.J., 2001. Evaluating soil quality-soil redistribution relationship on terraces and steep hillslope. Soil Sci. Soc. Am. J. 65, 1500–1508. Li, Y., Poesen, J., Yang, J.C., Fu, B., Zhang, J.H., 2003. Evaluating gully erosion using137Cs and 210Pb/137Cs ratio in a reservoir catchment. Soil Till Res. 69, 107–115. Liu, G.S., 1996. Soil Physical and Chemical Analysis and Description of Soil Profiles. Chinese Standard Press, Beijing, pp. 123–166 (in Chinese). Mabit, L., Bernard, C., Makhlouf, M., Laverdie re, M.R., 2008. Spatial variability of erosion and soil organic matter content estimated from 137Cs measurements and geostatistics. Geoderma 145, 245–251. Merino, A., Perez-Batallon, P., Macias, F., 2004. Responses of soil organic matter and greenhouse gas fluxes to soil management and land use changes in a humid temperate region of southern Europe. Soil Biol. Biochem. 36, 917–925. Mostafa, E., Mehdi, E., Majid, B., Hamed, F., Mahhboub, S., 2008. Effects of land use change on selected soil physical and chemical properties in north highland of Iran. J. Appl. Sci. 8, 496–502. Ritchie, J.C., McCarty, G.W., 2003. 137Cesium and soil carbon in a small agricultural watershed. Soil Till Res. 69, 45–51. Ritchie, J.C., McCarty, G.W., Venteris, E.R., Kaspar, T.C., 2007. Soil and soil organic carbon redistribution on the landscape. Geomorphology 89, 163–171. Ritchie, J.C., McHenry, J.R., 1990. Application of radioactive fallout cesium-137 for measuring soil erosion and sediment accumulation rates and patterns: a review. J. Environ. Qual. 19, 215–233. Roose, E., Barthes, B., 2001. Organic matter management for soil conservation and productivity restoration in Africa: a contribution from Francophone research. Nutr. Cycling Agroecosyst. 61, 159–170. Schumacher, T.E., Lindstrom, M.J., Schumacher, J.A., Lemme, G.D., 1999. Modeling spatial variation in productivity due to tillage and water erosion. Soil Till Res. 51, 331–339. Six, J., Paustian, K., Elliott, E.T., Combrink, C., 2000. Soil structure and organic matter: I. Distribution of aggregate-size classes and aggregate-associated carbon. Soil Sci. Soc. Am. J. 64, 681–689.
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Su, Y.Z., Zhao, H.L., Zhang, T.H., Zhao, X.Y., 2004. Soil properties following cultivation and non-grazing of a semi-arid sandy grassland in northern China. Soil Till Res. 75, 27–36. Thapa, B.B., Cassel, D.K., Garrity, D.P., 2001. Animal powered tillage translocated soil affects nutrient dynamics and soil properties at Claveria. Philipp. J. Soil Water Conserv. 56, 14–21. Walker, S.M., Desanker, P.V., 2004. The impact of land use on soil carbon in Miombo Woodlands of Malawi. For. Ecol. Manage. 203, 345–360. Walling, D.E., Quine, T.A., 1990. Calibration of caesium-137 measurements to provide quantitative erosion rate data. Land Degradation Rehabil. 2, 161–175. Walling, D.E., Quine, T.A., 1991. The use of caesium-137 measurements to investigate soil erosion on arable fields in the UK: potential applications and limitations. J. Soil Sci. 42, 147–165. Wei, G.X., Wang, Y.B., Wang, Y.L., 2008. Using 137Cs to quantify the redistribution of soil organic carbon and total N affected by intensive soil erosion in the headwaters of the Yangtze River, China. Appl. Radiat. Isot. 66, 2007–2012. Wu, R.G., Tiessen, H., 2002. Effect of land use on soil degradation in alpine grassland soil. China Soil Sci. Soc. Am. J. 66, 1648–1655. Yan, P., Dong, G.R., Zhang, X.B., Zou, X.Y., 2003. Application of caesium-137 technique on wind erosion in Gonghe Basin, Qinghai Province(II)—137Cs reference inventory and 137Cs model for wind erosion estimation. J. Desert Res. 23, 391–397 (in Chinese).
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Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
137
Cs tracing dynamics of soil erosion, organic carbon and nitrogen in sloping farmland converted from original grassland in Tibetan plateau
Nie Xiaojun a,n, Wang Xiaodan b, Liu Suzhen b, Gu Shixian b, Liu Haijun c a
School of Surveying and Land Information Engineering, Henan Polytechnic University, Jiaozuo 454000, China Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Conservation, Chengdu 610041, China c Institute of Water Resources Planning, Surveying, Design, and Research, Lhasa 850000, China b
a r t i c l e in f o
a b s t r a c t
Article history: Received 28 January 2010 Received in revised form 2 April 2010 Accepted 4 April 2010
There is a shortage of research concerning the relationships between land-use change, soil erosion, and soil organic carbon (SOC) and nitrogen (N) dynamics in alpine environments such as those found in the Tibetan plateau. In this paper, typical sloping farmlands converted from grassland 50 years ago in eastern Tibet were selected to determine dynamics of soil erosion, SOC, and total N associated with land-use change. Soil samples were collected from sloping farmland and control fields (grassland). The 137 Cs, SOC, total N contents, and soil particle size fractions were analyzed in these samples. As compared with the control fields, 137Cs, SOC, and total N inventories in the sloping farmlands decreased by 30%, 27%, and 33%, respectively. Meanwhile variations in the three parameters were enhanced in the sloping farmlands, with coefficients of variation (CVs) of 38%, 23%, and 20%, respectively, for 37Cs, SOC, and total N. In addition, SOC and total N inventories significantly decreased with increasing soil erosion in the sloping farmland. In a sloping farmland with a steep 241 gradient, the 137Cs inventory gradually increased along a downslope transect with its lowest value at 0 Bq m 2 in the top-slope position (0 m). The soil clay ( o0.002 mm) content in such an area increased with decreasing elevation (r ¼ 0.95, p¼ 0.001). Significant correlations between 137Cs and clay (r ¼ 0.92, p ¼ 0.003), SOC (r ¼0.96, p ¼0.001), or total N (r ¼ 0.95, p ¼0.001) were also found in the farmland. These results showed that converting alpine grassland to sloping farmland accelerates soil erosion, losses in SOC and N, and increases the soil’s spatial variability. The combined impacts of tillage and water erosion contributed a significant decrease in the soil’s organic carbon and N storages. Particularly in steep sloping farmlands, tillage erosion contributed for severe soil loss, but the soil redistribution pattern was dominated by water erosion, not tillage erosion, due to the lack of boundaries across the field patches. It was also found that 137 Cs, SOC, and total N moved along the same pathway within these sloping farmlands, resulting in net C and N losses during soil redistribution. The negative influences of land-use conversion from grassland to farmland in sloping areas in the Tibetan plateau should be addressed. & 2010 Elsevier Ltd. All rights reserved.
Keywords: 137 Cs technique Land-use change SOC Water erosion Tillage erosion Tibetan plateau
1. Introduction Tibet, known as the ‘‘World’s Ridge,’’ features one of the most unique regional ecology areas in the world. Numerous glaciers, lakes, wetlands, and large international rivers are distributed in the area, earning Tibet another distinction as the ‘‘Asian Tower.’’ The Tibetan plateau exerts a profound impact on eastern Asian geographical environments by providing an important barrier for the stability of climate systems in this region. It is also an important gene pool for global biological species and a key area for global biodiversity conservation. However, Tibetan ecosystems are fragile and sensitive to incompatible exogenic actions, such as
n
Corresponding author. Tel./fax: +86 391 3983693. E-mail address: [email protected] (N. Xiaojun).
0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.04.017
rapid industrial development, intensive agriculture, and global climate change. Alpine grassland is a major type of ecosystem in the Tibetan plateau, covering an area of 7.848 107 ha, which is approximately 65.6% of the land area of Tibet. This ecosystem plays important barrier functions in Tibet in terms of ecological security, such as climate regulation, water and soil conservation, and biodiversity maintenance. These functions, however, are weakening due to unreasonable land uses in the region. The decline in climate regulation and soil erosion control functions is especially severe. Soils under alpine grasslands are generally high in SOC content because of low temperature and well-developed vegetation as a result of natural succession (Institute of Soil Science, Academia Sinica, 1986), which contributes an important C pool in sustaining global C circulation balance. Previous studies showed that ecologically incompatible disturbances, such as overgrazing, cultivation, and global warming have resulted in
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decreasing large sections of organic C storage in alpine grassland soils (Kang, 1996; Cheng et al., 1997; Wu and Tiessen, 2002; Jian, 2002). Without effective measures to prevent these disturbances, the C sink function of alpine grasslands could be lost or transformed into a C source, further accelerating global warming. Investigations on soil erosion conducted in the ‘‘Three Rivers Watershed’’ (Jinsha River, Lancang River, and Nujiang River) showed that these regions are undergoing widespread soil erosion, especially in sloping farmlands, resulting in an increase in sediment content in rivers and a decrease in water quality (Zhong et al., 2008). To pursue economic profits, local residents reclaimed forests and grasslands, even those on steep slopes, and turned them into croplands (Zhou et al., 2005), which often brings major and negative influences on soil quality and function (Bossuyt et al., 1999; Dinesh et al., 2003; Elberling et al., 2003; Walker and Desanker, 2004; Merino et al., 2004; Guo and Gifford, 2002; Su et al., 2004; Islam and Weil, 2000; Li et al., 2007; Yang et al., 2008; Mostafa et al., 2008). Recognized as a primary soil quality constituent, soil organic matter is important in maintaining soil structure stability, aiding in the soil infiltration of air and water, promoting water retention, and enhancing soil fertility (Gregorich et al., 1994; Roose and Barthes, 2001; Zaujec, 2001). Reduction in SOC induced by cultivation change the distribution and stability of soil aggregates, thus increasing the likelihood of soil erosion (Cambardella and Elliott, 1993; Six et al., 2000). For example, Mostafa et al. (2008) reported that the conversion of grassland into cropland during an 18 year-period decreased SOC and total N by 50% each, and increased soil erodibility by 51% in the highlands of Iran. As such, soil erosion, accelerated by the expansion of sloping farmlands in the Tibetan plateau, could be a serious issue to the sustainable development of local agriculture. The satellite remote sensing technique is a major method used to investigate soil erosion in the Tibetan plateau (Zhong et al., 2008). However, it can only provide a qualitative description, not a quantitative evaluation. On the other hand, the Universal Soil Loss Equation (USLE) model can assess soil erosion quantitatively, but some parameters in the model, such as the rainfall factor (USLE-R) and soil erodibility factor (USLE-K), are difficult to determine due to the lack of long-term field observations. In comparison, the 137Cs technique can overcome the limitations associated with the above mentioned traditional approaches and has been proven effective in assessing soil erosion intensities and processes (Ritchie and McHenry, 1990; Walling and Quine,1990, 1991; Li et al., 2003; Zhang et al., 2004). Aside from 137Cs being a well-established tracer for soil erosion, it also provides a valuable tool for investigating medium-term erosion–carbon relationships. Recently, studies concerning 137Cs application to evaluate soil erosion (Yan et al., 2003) and erosion–carbon relationships (Li et al., 2003; Zhang et al., 2006a; Wei et al., 2008) were conducted in the alpine grasslands of Qinghai–Tibetan plateau. Unfortunately, the dynamics of soil erosion, SOC, and N in landuse change from grassland to sloping farmland remain unclear. Against this background, the current study aims to understand the effects of the conversion of grassland to sloping farmland on soil erosion, SOC, and total N, as well as to determine the erosion-induced soil redistribution pattern in sloping farmlands at an alpine site in Jinsha River Watershed, eastern Tibet. The results may provide insights for promoting ecological security in the Tibetan Plateau.
2. Materials and methods 2.1. Study area The study area lies in Jiangda County (301 210 -32 1 000 N, 971 21 -98 1530 E), at the upper reaches of the Jinsha River, eastern 0
1651
Fig. 1. Study area location.
Tibet Autonomous Region, China (Fig. 1). With an average altitude of 3650 m, the area is of typical alpine gorge geomorphology and has a semi-humid temperate climate. Temperature per annum averages 4.5 1C, with a maximum of 28 1C and a minimum of 15 1C. Annual precipitation is between 520 and 610 mm, and about 77.9–95.8% of the rain occurs between May and September. Annual evaporation reaches 1600–1700 mm. Land use types in the area mainly include forests, grasslands, and sloping farmlands. Most of the sloping farmlands had been converted from the original grasslands 50 years ago. Soils in the study area, for both grassland and farmland, are derived from slope materials of sandstone, limestone, metamorphic rock, and magmatic rock, and are classified as alpine meadow soil in Chinese soil taxonomy or as Calciudolls in US soil taxonomy. The dominant vegetation includes alpine meadow and alpine steppe, and the major crops are wheat (Triticum aestivum L.) and highland barley (Semen Avenae Nudae). 2.2. Soil sampling and laboratory analysis In the study area, sloping farmlands with an approximately 50year cultivation history are mainly distributed in the lower parts of hillslopes between a.s.l 3400 and 3900 m. The sloping farmlands are characterized by steep slopes (varying mainly from 101 to 251) and short lengths (varying mainly from 10 to 40 m). Within the 3400–3900 m elevation interval, seven sloping farmlands (i.e., 71, 111, 141, 161, 181, 191, and 241) at different locations were selected to conduct soil sampling for 137Cs and other physical and chemical determinations. Meanwhile, seven control fields were established for soil sampling, each in small patches of unbroken and irregular grasslands neighboring the selected sloping farmlands. The vegetation coverage was over 70% in the control fields. Except for the 241 sloping farmland, samples from the upper, middle, and lower positions of each of the other six farmlands were collected. In the 241 sloping farmland, seven soil sampling points were set at 5 m intervals along the down-slope transect and two replicates for each were collected. Unlike the sampling for sloping farmlands, soil samples for each control field were collected randomly in three replicates. All soil samples were collected to a depth of 23–37 cm depending on the soil thickness (i.e., up to the bedrock) using a 6.8-cm diameter hand-operated core sampler. The core sample was subsequently divided into two segments: the surface (0–20 cm) and the subsoil ( 420 cm). For the six sloping farmlands (i.e., 71, 111, 141, 161, 181, and 191), the three surface and subsoil subsamples from the upper, middle, and lower positions of each farmland were combined in bulk to create a composite sample. For the 241 sloping farmland, the two replicated soil cores were combined in
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Taking the 241 sloping farmland as a specific case, the soil erosion process can be tracked according to the 137Cs distribution along the down-slope transect. Compared with a 137Cs inventory of 1712 Bq m 2 in the control field, lower 137Cs inventories were found within the 241 sloping farmland (Fig. 2). This implies a net soil loss in steep sloping farmland. The 137Cs inventory increased along the down-slope transect, (Fig. 2), suggesting that soil loss gradually declined along this direction. In particular, 137Cs inventory in the top-slope position (0 m) was the lowest, with a value of 0 Bq m 2 (Fig. 2). This shows that tillage erosion induced the most severe soil loss in the positions. Soil clay (o0.002 mm) content increased with decreasing elevations in the farmlands (Fig. 3) and a significant negative correlation was found between the two factors (r ¼ 0.95, p¼ 0.001). Soil clay content was also significantly related to 137Cs inventory (r ¼0.92, p¼0.003). The
3909
1800 Elevation 137Cs
1600 1400
3903
1200
3900
1000
3897
800 600
3894
400 3891
200
3888
0 5
0
10 15 20 Horizontal length (m)
2.3. Statistical analysis Fig. 2.
One-way analysis of variance (ANOVA) was conducted to test the significance of differences found between the sloping farmlands and the control fields. Linear regression analysis was used to test the correlation between SOC, total N, and 137Cs.
inventory (Bq m-2)
3906
137Cs
bulk to create a composite sample. For the seven control fields, three surface-and subsoil subsamples from three random positions were combined in bulk to create a composite sample. Samples for the 137Cs reference could not be acquired because it was difficult to find an appropriate reference site in the alpine gorge geomorphology, that is, an area where soil erosion, deposition, and human disturbance had not occurred. Overall, 26 and 14 composite samples were collected from the sloping farmlands and control fields, respectively. Soil samples were air-dried, crushed, passed through a 2-mm mesh sieve to remove coarse material, and then divided into two, one for the measurement of 137Cs activity, and another for the measurements of SOC, total N, and soil particle-size fraction. Samples for the o2 mm fraction were sealed in a 200-cm3 (fl0l mm 25 mm) plexiglass box. Twenty-eight days later, 137 Cs activity of these samples was measured using a hyperpure lithium-drifted germanium detector linked to a LabSOCS (CANBERRA, USA) with a counting time of 36,000–86,400 s and a measurement precision of ca.74.7% (95% confidence). The original measurements of 137Cs activity, expressed in terms of per unit mass (Bq kg 1), were converted into the inventory (Bq m 2) by means of the total weight of the bulked core sample and the cross-sectional area of the sampling device. The SOC was determined using wet oxidation with K2Cr2O7, and the measurement of total nitrogen followed the classical Kjeldahl digestion method (Liu, 1996). Soil particle-size fractions were determined by pipette method following H2O2 treatment to destroy organic matter and dispersion of soil suspensions in Na-hexametaphosphate (Liu, 1996).
Elevation (m)
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137
25
30
Cs distribution across the 241 sloping farmland.
20.5%
3.1. Soil erosion dynamics In each of the seven sloping farmlands, 137Cs inventory was lower than that of its control field (Table 1). The 137Cs inventory averaged 1263 Bq m 2 (SE7178) in the seven farmlands, showing a 30% decrease compared to the mean of 1798 Bq m 2 (SE767) in the control fields. What’s more, coefficient of variation (CV) in 137Cs inventory was nearly 4 times as great in the sloping farmlands (38%) as in the control fields (10%). These results suggest that the conversion from alpine grassland to sloping farmland remarkably accelerated soil erosion intensity and increased its spatial variation.
Table 1 Comparisons of the profile contents of
137
Grasslands
137
y = -0.0022 x + 8.5644
19.5%
r = -0.95, p = 0.001
19.0% 18.5% 18.0% 17.5% 17.0% 3888
3891
3894 3897 3900 Elevation (m)
3903
3906
Fig. 3. Relationship of clay content to elevation in the 241 sloping farmland.
Cs, SOC and total N (TN) between sloping farmland and control fields (grassland).
Slope gradient Sloping farmlands
Clay content (%)
20.0% 3. Results
Cs (Bq m 2) SOC (kg m 2) TN (kg m 2) 137 Cs (Bq m 2) SOC (kg m 2) TN (kg m 2)
241
191
181
161
141
111
71
8407 219 4.227 0.33 0.40 70.03 1712 6.03 0.54
809 3.73 0.40 1881 6.59 0.64
1689 5.10 0.51 1976 6.70 0.65
1331 3.63 0.39 1438 5.72 0.54
677 2.90 0.31 1813 6.80 0.70
1621 5.08 0.49 1876 6.75 0.62
1870 5.72 0.56 1901 6.88 0.67
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7.00
3.2. SOC and N dynamics
5.00
3.3. Relationship of SOC and N to soil erosion In the seven sloping farmlands, SOC and total N inventories decreased with decreasing 137Cs inventory (Fig. 5). A significant and positive correlation was also found between SOC and 137Cs (r ¼0.88, p¼0.01) and total N and 137Cs (r ¼0.91, p ¼0.004). These relationships show that storage of SOC and N decreases with increasing soil erosion in sloping farmlands. In the 241 sloping farmland, SOC and total N showed a distribution pattern similar to 137Cs across the entire slope, resulting in significant and positive correlations between them (r ¼0.96, p ¼0.001 between SOC and 137Cs; r ¼0.95, p¼0.001 between total N and 137Cs). The results are consistent with previous studies by Ritchie and McCarty (2003), Zhang et al. (2006b), and Wei et al. (2008), which suggest that erosioninduced soil redistribution exerts remarkable impact on the spatial dynamics of SOC and N.
6.00
0.60 SOC Total N
0.55
5.00
0.50
4.50
0.45
4.00
0.40
3.50
0.35
3.00
0.30
2.50
0.25
2.00
0.20 0
5
10
15
20
25
30
Downward slope (m) Fig. 4. Distribution of SOC and total N across the 241 sloping farmland.
Total N (kg m-2)
SOC (kg m-2)
5.50
6.00 SOC (kg m-2)
Similar to the results found for 137Cs inventory, contents of SOC and total N in each sloping farmland were lower than those in their respective control fields (Table 1). The inventories of SOC and total N averaged 4.34 (SE70.37) and 0.44 kg m 2 (SE70.03) in the seven farmlands, respectively, showing a significant decrease of 27% (po0.001) and 33% (p ¼0.001) compared to means of 6.49 kg m 2 SOC (SE70.16) and 0.62 kg m 2 total N (SE70.02) for their respective control fields. In addition, large variations in SOC (CV, 23%) and total N (CV, 20%) were found in the seven sloping farmlands, while small variations were found (CV: 7% for SOC; 10% for total N) in the control fields. These results suggest that the conversion from alpine grassland to sloping farmland induces obvious losses of SOC and N, and also encourages great spatial variations. The distribution patterns of SOC and total N were consistent across the 241 sloping farmland, that is, their contents gradually increased along the down-slope transect (Fig. 4). A significantly positive correlation (r¼0.99, p o0.001) between the farmlands was observed. Compared to those in the control fields (SOC, 5.72 kg m 2; total N, 0.54 kg m 2), decreases in SOC and total N inventories were most serious in the top-slope positions (0 m) of the farmlands, with a decrease of 47% in SOC and 48% in total N. The SOC and total N inventories in the bottom-slope position (30 m) were highest within the sloping farmland, but decreases of 7% and 8% were still found in the SOC and total N inventories, respectively, compared to those in the control field.
0.80 SOC Total N
0.70 0.60
4.00 0.50 3.00 0.40
2.00
Total N (kg m-2)
relationships between elevation, 137Cs, and soil clay indicate that soil fine particles are selectively transported by water erosion.
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0.30
1.00 0.00
0.20 677
809
840 137Cs
1331
1621
1689
1870
inventory (Bq m-2)
Fig. 5. SOC and total N pools versus
137
Cs inventory along the downward slope.
4. Discussion Land-use change from grassland to farmland in sloping areas intensified soil erosion, which is mainly attributed to two reasons. One, after the conversion from original grassland to sloping farmland, tillage can destroy the initial soil structure and expose organic-rich soils to open air, thus accelerating soil organic matter mineralization and reducing SOC content. Consequently, reductions in SOC result in increased susceptibility of soil to erosion (Cambardella and Elliott, 1993; Six et al., 2000; Mostafa et al., 2008). Another is the impact brought about by tillage erosion. The 137 Cs inventory of 0 Bq m 2 in the top-slope position (0 m) of the 241 sloping farmland confirms severe soil loss induced by tillage erosion. In this type of erosion, soils located upslope are transported entirely downward and then deposited in bottom slope positions. Accordingly, water erosion risks increase, especially when slope runoff gathers at deposited sites. Conversion from grassland to sloping farmland in the study area significantly reduced the contents of SOC and total N. Similar research with consistent results have also been reported in other climate regions (Guo and Gifford, 2002; Wu and Tiessen, 2002; Su et al., 2004; Li et al., 2007; Yang et al., 2008; Mostafa et al., 2008). Li et al. (2007) reported that SOC decreased by 29–41% 28 years after the conversion of an alpine grassland to a cropland in the northeastern fringe of the Tibetan plateau (an altitude of 2960 m). In this study, about 50 years of cultivation resulted in a decrease of 27% in SOC. The low decrease in SOC compared to that observed by Li et al. (2007) could be attributed to a lower SOM mineralization ratio under high altitudes ( 43400 m) and weak tillage intensity in the study area. Spatial variability observed for soil erosion, SOC, and total N in sloping farmlands is thought to be attributed to the combined effects of water erosion and tillage erosion. From the control fields, CV values of 137Cs (10%), SOC (7%), and total N (10%) were small under the influence of only water erosion. After land-use conversion, however, the combination of tillage erosion and water erosion increased the difference in soil erosion (CV, 38%) between the different sloping farmlands, also causing SOC and total N inventories to decrease with increasing soil erosion. These resulted in large spatial variations in SOC (CV, 23%) and total N (CV, 20%). Schumacher et al. (1999) also reported that the combination of tillage erosion and water erosion induces larger variability in soil productivity than a single erosion can in agricultural sloping landscapes. In the 241 sloping farmland, 137Cs inventory gradually increased along the down-slope transect, with the lowest value at 0 Bq m 2 in the top-slope position (0 m). Soil clay ( o0.002 mm) content increased with decreasing elevation.
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The distributions of 137Cs and clay across the whole slope indicate that both tillage erosion and water erosion determined soil redistribution pattern in these short and steep sloping farmlands. Furthermore, according to the clay distribution, water erosion could be mainly responsible for soil redistribution. This inference, however, does not agree with previous studies conducted in the hilly areas of the Sichuan Basin in southwestern China (Zhang et al., 2006b; Ge et al., 2007; Zhang et al., 2008). In these studies, tillage erosion played a primary role in the soil redistribution pattern of steep slopes (slope lengths r30 m), and the entire transport of tillage erosion does not induce an increase in clay content with decreasing elevation. This inconsistency can be explained by the fact that there are hardly any boundaries across the slope. Even if there had been any, these boundaries would have been quite loose and present as dwarf heaps of gravel in the study area. Without the effective protection of boundaries across the slope, plus the fact that sloping farmlands lie in lower regions of the long hillslope, runoff from up the slope would maintain a single path and proceed straight downward during the water erosion process. Hence, the entire slope of the farmland suffers obvious water erosion. On the other hand, in hilly areas of the Sichuan Basin, the slope runoff across steep hillslopes is discontinuous because of the set-up of boundaries between fields. Thus, runoff within a field finds difficulty in forming in the upper regions of the slope, gathering only in the lower regions. In fact, soil redistribution in steep sloping farmlands in hilly areas of the Sichuan Basin is not induced by water erosion but by tillage erosion. As such, building boundaries between fields may help reduce soil erosion in the agricultural sloping landscapes of the Tibetan plateau. Significant and positive correlations between SOC, total N, and 137 Cs were found in the 241 sloping farmland. This result is in line with the previous studies (Schumacher et al., 1999; Thapa et al., 2001; Ritchie and McCarty, 2003; Li and Lindstrom, 2001; Heckrath et al., 2005; Zhang et al., 2006a, 2006b; Ge et al., 2007; Ritchie et al., 2007; Zhang et al., 2008; Wei et al., 2008; Mabit et al., 2008), suggesting the same pathway during soil redistribution. As there exist strong correlations between SOC, N, and 137Cs, the 137Cs technique can provide a useful tool for extensive research on the dynamics of soil erosion, SOC, and N stocks in the Tibetan plateau.
5. Conclusions In the alpine gorge regions of eastern Tibet, the conversion from alpine grassland to sloping farmland was found to have accelerated soil erosion and resulted in significant losses of SOC and N. The combined impacts of tillage and water erosion in sloping farmlands induced increase in spatial variability in soil erosion, SOC, and total N, and significantly decreased SOC and total N storages with increasing soil erosion. Particularly in steeply sloping farmlands, tillage erosion contributed to severe soil losses. However, the soil redistribution pattern was dominated by water erosion, not tillage erosion, due to lack of boundaries across the field patches. Moreover, 137Cs, SOC, and total N moved along the same pathway within these sloping farmlands, resulting in net C and N losses during soil redistribution. Building boundaries could be an effective solution in minimizing soil erosion, organic carbon, and N losses in such areas. Future research on the relationships between land-use changes, soil erosion, and SOC and N contents in the plateau could be conducted using the 137Cs technique to better understand alpine soil dynamics.
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