The effects of agricultural management on selected soil properties of the arable soils in Tibet, China

Catena 93 (2012) 1–8

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The effects of agricultural management on selected soil properties of the arable soils in Tibet, China Dan Zhang a, b, Zhonghao Zhou a, c, Bin Zhang a, b, Shuhan Du a, b, Gangcai Liu a,⁎ a Key Laboratory of Mountain Hazards and Earth Surface Processes, Chinese Academy of Sciences, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Resources, Chengdu, 610041, China b Graduate School of the Chinese Academy of Sciences, Beijing, 100039, China c Chongqing Bureau of Geology and Minerals Exploration, Chongqing, 402160, China

a r t i c l e

i n f o

Article history: Received 13 August 2011 Received in revised form 10 January 2012 Accepted 13 January 2012 Keywords: Arable land Soil property change Agricultural management Cultivation Tibetan plateau

a b s t r a c t Lhasa is a crucial agricultural region of the Tibetan plateau for local grain and vegetable supplies. Therefore, to sustain soil productivity, it is important to understand how agricultural management practices can cause changes in soil properties. Based on the information from the soil survey conducted in the late 1980s, we selected and sampled the following sites in the summer of 2007: 17 sites of the tillage (A) layer soils and 13 sites of soil profiles, including the tillage and subsoil layers from three types of arable land soils in Lhasa (alluvial soil, steppe soil, and meadow soil). At the same time, another 55 composite samples and core samples were taken from the grain-crop land, open vegetable land and greenhouse vegetable land of the alluvial soil. The selected soil properties were measured and compared to the soil survey data from the 1980s. The results showed that because of wind erosion and irrigation, the arable soils in the investigated area have become significantly more sandy (P b 0.05) since the late 1980s. Moreover, because of fertiliser application and acid precipitation, the soil pH and cation exchange capacity of the study soils are significantly lower (P b 0.05) than in the late 1980s, thus leading to soil acidification and lower soil fertility. Soil organic matter and the total nitrogen contents in the cultivated steppe soils and meadow soils increased, possibly because of manure addition and fertiliser use in the region. The soil organic matter and the total nitrogen content decreased in the alluvial soils, possibly due to an intensified cultivation; however, the available nitrogen and phosphorus increased significantly (P b 0.01), whereas potassium decreased significantly (P b 0.05). These changes were mainly attributed to the heavy use of nitrogen and phosphorus fertilisers and the infrequent use of potassium fertiliser. The changes in the A layer (tillage layer) were more apparent than in the other layers. This finding was especially evident in the vegetable land, where the changes are attributed to the agricultural management activities that often occur in this layer. The soil organic matter in the B layer increased significantly (P b 0.05) due to the accumulation of plant roots and the deposition of organic matter from the A horizon. For the same soil under different land use, the rank of the soil fertility was cropland b open vegetable land b greenhouse vegetable land, which further suggests that the changes in the soil properties were mainly due to the application of manure and the intensity of cultivation. © 2012 Elsevier B.V. All rights reserved.

1. Introduction To sustain soil productivity, it is important to understand how the change in both natural conditions and human activities can cause changes in soil properties. When considering the influence of natural conditions on the changes in soil properties, little attention has been paid to the influence of climate change. In the eastern region of Washington State in the United States, climate change has caused the semiarid shrub-steppe region to become hotter and drier, and the overall ⁎ Corresponding author at: Institute of Mountain Hazard & Environmental, Of Chinese Academy of Science, NO.9, Section 4 of Renming South Road of Chengdu, P.O. Box: 417, Postcode: 610041, China. Tel.: +86 28 85231287; fax: +86 028 85238973. E-mail address: [email protected] (G. Liu). 0341-8162/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2012.01.004

mean soil electric conductivity (EC), as well as ammonium and nitrate, increased with the elevation, whereas the soil pH decreased with increasing elevation (Smith et al., 2002). Climate change has also caused significant losses in soil organic matter (SOM) and total nitrogen (TN) in the soils of Qinghai–Tibet Plateau, China (Wang et al., 2007). Most of the existing research has been focused on how human activities influence soil properties. These studies showed that long-term cultivation can change the physical and chemical properties of soil. For example, after 60 years of farmland cultivation, the available phosphorus (P), total carbon (C), pH and clay contents in some California soils have increased significantly, whereas the hydraulic conductivity and sand content have decreased. However, a significant reduction of the soil quality has not been observed (Clerck et al., 2003). Malo et al. (2005) found that long-term cultivation (>80 years) in the northern

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Great Plains of the U.S. has caused significant reductions in available P (AP), available K (AK), pH, total C, organic C and total N (TN) in the surface soil layer (0–15 cm). In general, the organic matter, TN, total sulphur (S) and total phosphorus (TP) gradually decrease with an increasing cultivation time. In some cases, however, the available phosphorus increases with the cultivation time (Wang et al., 2002; Wen and Liang, 2001). Since 1980, the organic matter, nitrogen and phosphorus contents of farmland in the North China Plain have increased, whereas potassium (K) has decreased (Chen et al., 2006). In the semiarid Nigerian Savannah, Jaiyeoba (2003) reported that increasing the years of cultivation resulted in increased soil coarseness and a decrease in the water-stable aggregates in the 0–10 cm layer. At the same time, the organic matter, total nitrogen, available phosphorus, exchange cations and cation exchange capacity (CEC) declined. Similarly, Materechera and Mkhabela (2001) found that a continuous cultivation for 12 years resulted in a substantial decline in the soil organic C and N contents in southeastern Swaziland. Peter et al. (2006) suggested that cultivation has led to the degradation of both the physical and chemical properties of the soil in the Kali Basin, Hungary. However, Zhang et al. (2004) reported that the soil OM (organic matter), TN, available N (AN) and AP all increased significantly, whereas the AK decreased slightly after cultivation. However, mini- or no-tillage farming usually increases the soil organic matter in the top layer, improving the soil quality (Bayer et al., 2000; Halvorson et al., 2002; Karlen et al., 1994; Puget and Lal, 2005). Soil properties can be altered by the long-term manure application under continuous cropping (Jagadamma et al., 2008; Sebastia et al., 2007). The continuous application of chemical fertilisers, especially N fertiliser, results in a significant decrease in the soil organic carbon (SOC) content and a significant increase in the soil TN (Lee et al., 2009; Wang et al., 2008). However, the application of organic manure can lead to an increased SOC content and an improvement of certain soil properties (Freschet et al., 2008; Hati et al., 2007; Lee et al., 2009; Monaco et al., 2008; Shirani et al., 2002), most notably a decrease in the bulk soil density (Gill et al., 2009; Shirani et al., 2002) and an increase in the soil water retention capacity (Hati et al., 2007; Zhang et al., 2006), soil pH (Escobar and Hue, 2008; Odlare et al., 2008) and enzyme activity (Borken et al., 2002; Freschet et al., 2008). Meanwhile, a recent investigation demonstrated that an organic manure application can stimulate microbial activities more rapidly than changing chemical properties (Odlare et al., 2008). Furthermore, the change in chemical properties was more significant in the coarser soil organic fractions after the organic amendments (Sebastia et al., 2007). Lhasa contains intense human activity and is Tibet's political, economic and cultural centre. It is also Tibet's main agricultural production area. The grain and vegetable production in this area is an important part of the region's food supply. Understanding how human activities and natural conditions influence the changes in the physicochemical properties of the soil is important for the sustainable use and management of the farmland. However, few studies have reported on the recent changes in the soil properties of the alpine environment. Therefore, the objectives of this study were to use field sampling to explore the changes in the physicochemical properties of the typically cultivated soil and to explore the causes of those changes to sustain soil productivity in the region. 2. Materials and methods 2.1. Description of the study site Lhasa is located in the middle of the Tibet autonomous region, China (29° 14′26″–31° 03′47″ N, 89° 45′11″–92° 37′22″ E), and it has a land area of 29,518 km 2. The geological structure of Lhasa belongs to the Gangdise fold system, which includes the Linzhou-

Sobai Laya rotation layer, the eastern flank of the arcuate structure of middle Tibet and the western portion of the Yarlung Zangbo NEW direction structure. In this region, there is intense magnetic activity, and igneous rock, metamorphic rock and sedimentary rocks are widely distributed. The terrain in this area is higher in the northwest than it is in the southeast, and it has mountains alternating with valleys. The altitude of the main part of the area is above 4000 m, although the altitude is lower than 4000 m at the middle and lower reaches of the Lhasa River. The river valley accounts for only 9.4% of the land area, but it contains most of the arable land. Lhasa is in a temperate, semiarid, monsoon-climate plateau region with an annual mean temperature ranging from 1.5 °C to 7.8 °C, and it has an annual precipitation range of 340–600 mm. Sunny weather occurs during much of the year, with an annual average of more than 2800 hours (h) of sunshine. In the river valley, more than 85% of the year's precipitation occurs during June to September. This synchronisation of rainfall and temperature is beneficial to crop growth. From 1955 to 1989, the annual mean temperature in Lhasa was 7.58 °C, and the multi-year average precipitation was 421.9 mm, whereas from 1990 to 2007, the annual mean temperature in Lhasa was 8.62 °C, and the multi-year average precipitation was 482.0 mm. Thus, since the soil survey in the 1980s, the annual mean temperature has increased by 1.04 °C, and the annual mean precipitation has increased by 60.1 mm. The local climate has become warmer and more humid. The main vegetation in Lhasa is the natural vegetation that belongs to the mountain shrubby-grassland belt of southern Tibet, which includes alpine sparse vegetation, the alpine steppe, alpine meadow, subalpine meadow and mountain shrubby-grassland. The non-native vegetation consists of arid-land field crops, such as winter wheat, highland barley, maize, soybean, potato and pea (Wang et al., 2003), that are mainly located in the valleys. There are five main soil types that occupy 95% of the area: alpine frozen soil (Cambisols in FAO taxonomy; Aridisols in US taxonomy), alpine meadow soil (Humic Cambisols in FAO taxonomy; Cryaquoll in US taxonomy), alpine steppe soil (Calcic Chernozems in FAO taxonomy; Cryuborolls in US taxonomy), subalpine meadow soil (Humic Cambisols in FAO taxonomy; Typic Cryaquept in US taxonomy) and subalpine steppe soil (Calcic Chernozems in FAO taxonomy; Calcic Cryoboroll in US taxonomy). Arable soil covers approximately 1.2% of the Lhasa district area. Approximately 15% of the land in Tibet is arable and is mainly divided into 4 soil types: subalpine steppe soil accounts for 53.6% of arable land, alluvial soil (Eutriccambisols in FAO taxonomy, Fluvaquents in US taxonomy) accounts for 26.8%, subalpine meadow soil accounts for 12.7% and meadow soil (Cambisols in FAO taxonomy, Haplumbrepts in US taxonomy) accounts for 6.9%. Most of the arable land is manually irrigated grain cropland, open vegetable land and greenhouse vegetable land. 2.2. Methods of soil sampling Based on the sampling locations and other information – such as the land utilisation pattern, fertiliser application, irrigation and cultivation – in the Soil Survey Report (Qinghai-Tibet Plateau Comprehensive Scientific Expedition Team, CAS, 1985), and considering the representatives of arable land of more than 1 ha in both the 1980s and in 2007, 30 sample points (paired re-sample sites) were selected (Fig. 1). Of these sample points, 13 were soil profile sample sites, and the remaining 17 sites were tillage layer composite samples from the crop land. In this study, “cropland” refers to the land that is used for growing row crops (grains). In the 13 profile sites, 3 profile sites were sampled from the cropland of cultivated steppe soil, 4 were sampled from the cropland of alluvial soil and 6 were sampled from the vegetable land of alluvial soil. The soil samples were taken from every pedogenetic layer from A to C. For the composite sample sites, 5 samples came from the cultivated steppe soil, 8 came from the alluvial soil and 4 came from the cultivated

D. Zhang et al. / Catena 93 (2012) 1–8

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Fig. 1. Distribution of the farmland and soil paired sample (re-sampled) points in the study area.

meadow soil. Mixed samples were collected in the sampling area within a 50 m × 50 m plot. At the selected sampling sites, the soil samples were collected from a depth of 0 to 20 cm; each sample was obtained by mixing 3–5 sub-samples that were collected from 3 to 5 locations within the 50 m × 50 m plot. In addition to this sampling, we used the same method as mentioned above to take another 55 composite samples from the alluvial soils, 36 samples from the cropland, 8 samples from the open vegetable land and 11 from the greenhouse vegetable land. At the same time, the core sampling that was not performed during the 1980s was also conducted at these sampling sites. All of these samples (new samples) were collected in the summer of 2007, and the basic information about the sampling sites, such as the fertiliser use information and cultivation intensity, were also obtained by surveying the farmers. A global positioning system (GPS) apparatus and the recorded information of the last soil survey in the late 1980s, as well as the results from the farmers' survey, were used to identify the re-sampling sites. The corresponding samples from the 1980s (old samples) that were stored in airproof glassware were located in the storehouse of the Lhasa Agriculture Bureau in Lhasa. The soil samples from the 1980s were derived from the general survey of the Tibetan soil that was conducted from 1987 to 1991. The positions of these sampling sites were determined with the coordinates on topographic maps and information describing the positions.

pipette method. The soil organic matter was determined by oxidation with potassium dichromate in a heated-oil bath (wet combustion). The TN content was determined by the semi-micro-Kjeldahl method. The available N (alkaline hydrolytic nitrogen) was determined by the diffusion method. To determine the TP, the soil samples were digested with HClO4 and H2SO3, and the P content was determined using the colorimetric method. The AP was extracted with NaHCO3 and measured colorimetrically. The total K was determined using the HF and HClO4 digestion method. The plant-available K in the soil was extracted with 1 M NH4OAc and was determined by flame emission photometry. The pH was determined using a potentiometer. The CEC was determined by an acetic-ammonium exchange. 2.4. Data analysis All statistical analyses were performed using the SPSS program for Windows, version 11. The paired t-tests were conducted to determine any significant differences in the measured soil properties between the 1980s and 2007. Least significant difference (LSD) tests were performed to determine whether the variable means were significantly different. A statistical significance was accepted when the probability of the result, assuming the null hypothesis (P), was less than 0.05. The data of local fertiliser application were obtained from the report by the Statistics Bureau of Tibet in 2007. 3. Results

2.3. Methods of soil sample analysis 3.1. Variations in soil properties of the tillage layer Both the new and old samples were analysed at the same time. The basic chemical and physical properties of the test soils were determined by standard methods (Liu et al., 1996) using composite samples. The soil particle size distribution was measured using the

3.1.1. Soil particle size distribution The sand (2–0.02 mm), silt (0.02–0.002 mm) and clay (b0.002 mm) contents of the newly sampled tillage layer soils (0–20 cm) for the three

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types of farmland soils in Lhasa City are different from the data obtained in the soil survey conducted in the late 1980s. On average, the sand content significantly increased by 12.6%, whereas the silt and clay contents decreased by 5.0% and 7.6%, respectively. The results showed that the cultivated tillage layer (0–20 cm) in Lhasa City underwent a trend of sandification since the late 1980s, with the cultivated steppe soil showed the most obvious difference (Fig. 2). Irrigation and wind erosion (Zhao et al., 2006) might be the main causes of sandification because farmland irrigation often causes water erosion, and wind erosion also occurs in this region (Chen, 2007; Zhang, 2003). 3.1.2. Soil pH and CEC The soil pH of the farmland within the tillage layer (0–20 cm) in Lhasa City exhibited a trend of acidification, decreasing by an average of 0.48, which is a marked decrease when compared with the soil pH measured in the 1980s. The order of pH change was meadow soil > steppe soil > alluvial soil (Fig. 3). The pH of the meadow soil and steppe soil declined significantly by 0.94 and 0.54, respectively. Compared with the results of the soil survey from the 1980s, the CEC of the arable tillage layer (0–20 cm) of the three soils in Lhasa City was significantly different in 2007. The CEC was reduced by an average of 1.7 cmol·kg− 1, whereas the CEC of the meadow soil increased significantly by 1.7 cmol·kg− 1 (Fig. 3). 3.1.3. Soil organic matter Between the recent measurements and the survey in the 1980s, there was no significant difference in the soil organic matter (SOM) of the farmland tillage layer (0–20 cm), although the latest survey demonstrated an average increase of 1.2 g·kg − 1, and the SOM increased by 2.5 g·kg − 1 and 4.5 g·kg − 1 in the steppe soil and meadow soil, respectively. In contrast, the SOM in the alluvial soil decreased by 1.6 g·kg − 1 (Table 1). 3.1.4. Soil nutrients Compared with the survey conducted in the 1980s, all of the total nutrient contents declined in the alluvial soil (Table 1). The total nitrogen and potassium contents decreased significantly, by 0.2 g·kg − 1 and 1.0 g·kg − 1, respectively, in the alluvial soil, whereas they increased in the other two soil types. Of all of the soils, the alluvial soil had the greatest decrease in total phosphorus (a 0.1 g·kg − 1 decrease), whereas the farming steppe soil displayed no change. As a whole, the total soil potassium content decreased slightly by 0.2 g·kg − 1.

Fig. 2. Change in the soil particle-size distribution of the farmland tillage layer in the study area. Vertical bars are means ± standard deviations, followed by a and b letter indicate a significant change between sampling period at P b 0.05 and at P b 0.01, respectively, and by no letter indicates no a significant change.

Fig. 3. Change of the soil pH and the CEC of the farmland tillage layer in the study area. Vertical bars are means ± standard deviations, followed by a and b letter indicate a significant change between sampling period at P b 0.05 and at P b 0.01, respectively, and by no letter indicates no a significant change.

With regard to the change in the available nutrients (Table 1), the mean soil available nitrogen and phosphorus contents of the three types of the farmland soils showed significant increase of 29.8 mg·kg − 1 and 4.6 g·kg − 1, respectively. The order of the increase in available nitrogen was alluvial soil > meadow soil > steppe soil. However, the available potassium in all of the three soil types showed a significant decrease, with an average reduction of 34.7 mg·kg − 1 for the three types of farmland soils. The order of the reduction was meadow soil > steppe soil > alluvial soil. 3.2. Variation of soil properties within a profile 3.2.1. Steppe soil In the steppe soil, where the crops have been cultivated over a long period of time, a pH decrease was observed, with the biggest pH decrease occurring in the B and C layer. It was also observed that cultivation has decreased CEC in all of the layers and that the reduction was greater in the B and C layers than in the A layer (Fig. 4). The total nitrogen content in the A and B layers declined, but the soil organic matter increased significantly in the B layer, and the total K slightly increased in the A layer. The total P did not change in the C and B layers (Table 2). The available nitrogen and available phosphorus increased in all layers of the profiles, but the available potassium decreased significantly. Except for the available phosphorus, the rank of these changes (increase or decline) was A layer > B layer > C layer (Table 2). 3.2.2. Alluvial soil For the alluvial soil used as cropland, the CEC changed in the opposite direction of the pH changes in every soil layer: the soil pH increased slightly in the A layer and declined in the B layer, whereas the CEC decreased, as expected, in the A layer and increased in the B layer (Fig. 5). The SOM increased significantly in the B layer. Except for phosphorus in the A layer, the total nutrient content decreased (Table 3). In general, the changes in the available nutrients were similar to those in the steppe soil (Table 2). Compared with the cropland, the vegetable land of the alluvial soil has been planted with vegetables for over 10 years and has shown substantial changes in its soil properties. Except for the soil in the B layer, the alluvial vegetable land soil showed a decrease in soil pH and an increase in CEC, both of which were developments that followed a trend opposite that of the alluvial cropland soil (Fig. 5). The changes in the SOM, total nutrients and available nutrients showed

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Table 1 The change in the soil organic matter and the nutrient content of the farmland tillage layer in the study area. Soil type

Steppe soil Alluvial soil Meadow soil Arable land

Total (g·kg− 1)

SOM (g·kg− 1) 2.5 ± 0.5 − 1.6 ± 0.3 4.5 ± 1.3a 1.2 ± 0.5

Available (mg·kg− 1)

N

P

K

N

P

K

0.1 ± 0.0a − 0.2 ± 0.0a 0.4 ± 0.1b 0.1 ± 0.0

0.0 ± 0.0 − 0.1 ± 0.0 − 0.1 ± 0.0 − 0.0 ± 0.0

0.4 ± 0.1 − 1.0 ± 0.2b 1.4 ± 0.3b − 0.2 ± 0.1

19.3 ± 4.2b 37.0 ± 5.9b 27.0 ± 7.3b 29.8 ± 5.8b

4.8 ± 1.9b 4.5 ± 1.6b 4.3 ± 2.0b 4.6 ± 1.9b

− 49.9 ± 10.1b − 20.9 ± 3.3b − 54.0 ± 13.0b − 34.7 ± 8.8b

Values are the means ± standard deviations; values followed by the letter a or b indicate a significant change between the two sampling periods at P b 0.05 and at P b 0.01, respectively; the absence of a letter indicates that there was no significant change.

thus indicating that the pH decreased (Miller et al., 2001). Most researchers (Lesturgez et al., 2006; Pierson-Wickmann et al., 2009; Rodriguez et al., 2008; Tarkalson et al., 2006) assert that this is a result of the long-term fertilisation because N fertilisers contain ammonium or other organic forms of N, thus resulting in a nitrification that releases H + and causes a loss of basic cations. Furthermore, cultivation also increases humus mineralisation, thus releasing more H + (Malo et al., 2005). In contrast, the application of organic manure can increase the soil pH and the CEC due to the release of ammonia (NH3) from the mineralisation of the organic N compounds (Escobar and Hue, 2008; Odlare et al., 2008). Therefore, the change in the soil pH and the CEC mainly depends on the type of manure applied. Our results showed that the soil pH and the CEC of the farmland tillage layer (0–20 cm) in Lhasa, Tibet have decreased, which is consistent with most reports from the other studies. The escalated use of fertilisers (Fig. 6) is likely one of the main factors that caused these observed decreases. The reduction in the soil pH and the CEC could also be caused by the acidification of precipitation in this area (Falkengren-Grerup, 1987; Pierson-Wickmann et al., 2009). Zhang et al. (2005) found that the pH values of the precipitations between 1987 and 1999 in the Lhasa region decreased from 8.36 to 7.5, and the content of the acidic ions (NO3− and SO42 −) in the rainwater increased significantly (Table 4). These factors might also cause soil acidification because there are significant correlations between the sulphur (S) deposition loads and SO42 − adsorption and between the pH of precipitation and the soil pH (Barton et al., 1994). In contrast, the CEC of the meadow soil increased (Fig. 3). Soil CEC is related to soil colloids (organic matter and mineral colloids as the carrier of cations). Thus, a soil with a higher organic matter content usually also has a high CEC. Table 1 shows that the amount of organic matter in the tillage layer of the meadow soil was greater than the amount of organic matter in the other two soil types. Because the meadow soil is usually fully covered by abundant herbaceous plants, and the synthetic amounts of organic matter are greater while the amount of decomposition is smaller, there is a larger overall accumulation of organic matter in this soil type. However, after cultivation, the litter of the introduced vegetation and their dead roots decompose, thus creating a large amount of organic matter in the A layer. Therefore, the accumulation of organic matter in the A layer of the soil is greater than in the other layers. The declines in the pH and the CEC were more obvious in the B and C layers (Fig. 4). The pH of the cultivated steppe soil decreased not only due to the soil parent materials and land use patterns but

Fig. 4. Comparison of the change of the soil pH and the CEC in the genetic horizon of the cultivated steppe soil. Vertical bars are means ± standard deviations, followed by a and b letter indicate a significant change between sampling period at P b 0.05 and at P b 0.01, respectively, and by no letter indicates no a significant change.

a trend (increase or decrease) similar to the cropland of the alluvial soil (Table 3). Except for the SOM, the extent of the change in the A layer, however, was greater than that in the B layer, especially for the change in the available nutrients. As in the cropland, a considerable amount of SOM also accumulated in the B layer of the vegetable land soil. In general, there was an obvious accumulation in the soil organic matter in the B layer, as would be expected in farmland under longterm cultivation. 4. Discussion 4.1. Soil chemical properties Previous studies (Cao, 2008; Jaiyeoba, 2003; Malo et al., 2005; Wang et al., 2008) have shown that, due to the removal of large quantities of biomass with a high ash alkalinity from the soils, soil pH and CEC decrease as a result of long-term cultivation. It was observed that the exchangeable calcium (Ca) and magnesium (Mg) decreased with time throughout the soil profile at the Glensaugh Research Station in Northeast Scotland, whereas the exchangeable H ions (H +) increased,

Table 2 The change in soil organic matter and the nutrient content in the steppe soil profile. Soil layer

SOM (g·kg− 1)

A B C

2.5 ± 4.3 7.6 ± 3.5a 0.7 ± 0.6

Total (g·kg− 1)

Available (mg·kg− 1)

N

P

K

N

P

K

− 4.1 ± 6.9 − 2.8 ± 5.2 0.1 ± 0.0

0.1 ± 0.1 0.0 ± 0.0 0.0 ± 0.0

2.4 ± 2.0 0.8 ± 0.7 0.4 ± 0.3

40.8 ± 16.7b 24.2 ± 1.1b 9.0 ± 3.1

5.2 ± 6.4a 8.2 ± 0.7a 8.2 ± 0.9a

− 76.4 ± 62.0b − 54.4 ± 81.7a − 40.8 ± 35.5a

Values are the means ± standard deviations; values followed by the letter a or b indicate a significant change between the two sampling periods at P b 0.05 and at P b 0.01, respectively; the absence of a letter indicates that there was no significant change.

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application amount of per unit area£¨kg.ha-1£©

6

8 pH (crop) pH (vegetable) CEC (crop) CEC (vegetable)

b

changed value

6

4 a

2

0

-2

-4

a

A

B

200 180

Nitrogen £¨N£© Phosphate£¨P2O5£©

160

Potassium£¨K2O£©

140 120 100 80 60 40 20 0 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008

year

C

soil layer Fig. 6. Chemical fertiliser consumption from 1990 to 2006 in Lhasa. Fig. 5. Comparison of the change of the soil pH and the CEC (unit in cmol·kg− 1) in the genetic horizon between the cropland and the vegetable-land of the alluvial soil. Vertical bars are means ± standard deviations, followed by a and b letter indicate a significant change between sampling period at P b 0.05 and at P b 0.01, respectively, and by no letter indicates no a significant change..

also due to the long-term cultivation that causes the litter of the harvested crops and the dead roots to accumulate in the tillage (A) layer. To a certain extent, the increase in the organic matter alleviated the decline in the pH (Wang et al., 2008). Additionally, due to the eluviations of rainfall and irrigation, some acidic substances were leached from the A layer into the subsoil, which subsequently caused the decrease in the pH of the B and C layers. In Tibet, the CEC of the natural soils is controlled by the organic matter, but due to the long-term cultivation of the steppe soil, the CEC was affected by the soil utilisation patterns and fertilisation practices. In the B and C layers of the steppe soil, the greater decrease in the CEC might be attributed to the removal of potassium by the crops, general basic cation loss and soil acidification due to long-term cultivation (Dai et al., 1998; Matschonat and Vogt, 1997; Zalewska, 2008). In the A layer of both the cropland and the vegetable land (Fig. 5), the pH of the alluvial soil changed little. However, the CEC changed significantly because the alluvial soil is relatively fertile and is intensely cultivated in Tibet. The small change in the pH of the alluvial soil might have been influenced by the soil parent materials, land use patterns, and fertilisation practices. Fig. 5 shows that the pH in the tillage layer has changed, demonstrating an increase in the pH of the A layer of the cropland and a decline in the vegetable land. Such change might be attributed to the intensive farming of the alluvial soil, while the farming intensity of the cropland was notably less than that of the vegetable land. The CEC in the A layer of the cropland decreased, whereas it increased in the vegetable land. Along with intensive land management and cultivation practices, an increase of soil

clay content in the tillage layer might have led to an increase in the CEC in the vegetable land. However, the general cropland practices of long-term cultivation usually call for the application of less fertiliser (both chemical and organic) in cropland soils than in vegetable land. Because of this practice, the organic matter content decreased, which led to a decline in the CEC. 4.2. Soil organic matter and nutrients Over the last 20 years, more agricultural soils in China show a trend of increasing SOM and total nitrogen (Cao, 2008; Zhang et al., 2004). In other regions of China, an increased SOM accumulation after cultivation was also reported (Zhong, 2009). Nevertheless, many studies have shown that the SOM and total nitrogen decreased because of long-term cultivation (Jaiyeoba, 2003; Koch and Stockfisch, 2006; Malo et al., 2005; Materechera and Mkhabela, 2001; Peter et al., 2006) and chemical fertiliser application (Wang et al., 2008). In our present study, the changes of SOM in the steppe soil and meadow soil agree with the increasing tendency of the SOM, possibly due to the practice of combining the application of manure and chemical fertilisers (Hati et al., 2007; Monaco et al., 2008) in the Lhasa region. In the alluvial soil, the decreasing tendency in total nitrogen is attributed to both the intensified cultivation (Abu Muhammad Shajaat, 2006; Zhong, 2009) and to the climate change in the Qinghai–Tibet Plateau that has caused significant losses in SOM and TN (Wang et al., 2007) in the region. The total phosphorus in the farmland soil in Lhasa showed an overall decreasing trend, which differed from the increasing trend for the phosphorus content (Cao, 2008; Zhang et al., 2004). The available phosphorus content increased gradually, which is in agreement with the results of Cao (2008) and Zhang et al. (2004), but differs

Table 3 The change in the soil organic matter and the nutrient content in the alluvial soil profile. Arable land use type

Soil genetic layer

SOM (g·kg− 1)

Crop land

A B C A B C

− 0.4 ± 0.2.7 6.2 ± 2.4b − 0.7 ± 6.2 − 0.1 ± 0.9 4.2 ± 4.4a − 1.8 ± 1.7

Vegetable land

Total (g·kg− 1)

Available (mg·kg− 1)

N

P

K

N

P

K

− 4.3 ± 7.3 0.4 ± 0.2 − 0.1 ± 0.3 − 3.6 ± 7.9 − 3.8 ± 7.3 − 0.1 ± 0.5

0.1 ± 0.3 − 0.3 ± 0.2 − 0.4 ± 0.2 0.7 ± 0.2 0.4 ± 0.2 0.1 ± 0.3

− 1.2 ± 4.4 3.0 ± 7.8 5.2 ± 9.1 − 1.1 ± 3.5 − 0.9 ± 3.1 − 0.0 ± 2.6

26.6 ± 16.5b 42.8 ± 6.6b − 0.6 ± 2.1 135.9 ± 71.9b 35.3 ± 14.6b − 16.1 ± 11.3a

− 1.4 ± 1.1 6.4 ± 3.5a 5.9 ± 5.6a 42.7 ± 19.9b 22.3 ± 34.1b 22.6 ± 23.6b

− 32.6 ± 27.8b − 15.7 ± 14.5a − 6.5 ± 5.4 − 43.0 ± 54.8b − 19.2 ± 31.6b − 16.6 ± 18.8b

Values are the means ± standard deviations; values followed by the letter a or b indicate a significant change between the two sampling periods at P b 0.05 and at P b 0.01, respectively; the absence of a letter indicates that there was no significant change.

D. Zhang et al. / Catena 93 (2012) 1–8

7

Table 4 Comparison of the precipitation chemical properties in two periods of the 20th century in Lhasa. Period

pH

K+

Na+

Ca2 +

Mg2 +

NH4 +

CL−

NO3−

SO42 −

HCO3−

1987–1988 1997–1999

8.4 7.5

14.8 5.14

89.0 11.2

150.3 197.4

5.7 10.9

21.9 14.3

21.7 9.7

2.0 6.9

2.5 5.2

288.9 101.2

The ion content unit is μeq·L− 1, these data were obtained from Zhang et al. (2005).

from the results of Jaiyeoba (2003) and Malo et al. (2005), which showed that the available phosphorus decreased after long-term cultivation. The total potassium did not decrease significantly, especially for the steppe soil and the meadow soil (Table 1). This finding indicates that the total potassium content in the soils was high. The weathering of the parent materials in the studied region might provide sufficient K to compensate for the crop removal of K because the parent soil material has a high potassium content (Liu and Gao, 2005). However, due to the infrequence application of potassium fertiliser, the available potassium decreased significantly (Table 1); this finding is in line with the results of Zhang et al. (2004), Malo et al. (2005), and Cai and Qian (2004). The increase in the available nitrogen and the available phosphorus, as well as the reduction in available potassium, is a result of unbalanced fertilisation (Rodriguez et al., 2008; Tarkalson et al., 2006). Fig. 6 shows that the nitrogen and phosphorus levels were apparently higher than the potassium level in areas treated with a chemical fertiliser. The trend of soil property changes in the soil profile sites are similar to the other sites, but the changes in the A layer (the tillage layer) were more apparent than in the subsoil layers (especially in the vegetable land), which reflects that the main farming activities, such as cultivation and fertiliser application, occurred in this layer (Liu and Zhang, 2006). In the study area, the organic matter of the B layer of the farmland increased significantly (Tables 2 and 3) because of the accumulation of plant roots and other organic matters in the layer (Bruun et al., 2007; Buurman and Jongmans, 2005). The changes in the soil properties were mainly attributable to the management of activities such as cultivation and fertiliser application, a conclusion which is further evinced from the land utilisation patterns. In the study region, the cropland area has decreased and the vegetable fields have increased significantly over the last 20 years (Chu et al., 2006). This change could result in a series of changes in soil properties (Chu et al., 2006; Liao et al., 2009). In this study, the physical and chemical properties of the soil in the cropland, the open vegetable land and the greenhouse vegetable land of the same alluvial soil were measured. The results (Table 5) showed that the order of the fertility levels of the different soils was cropland b open

Table 5 Comparison of the physical and chemical properties in the alluvial soil under different utilisation patterns. Item

Crop land (n = 36)

Open vegetable land (n = 6)

Greenhouse vegetable land (n = 11)

Bulk density (g·cm− 3) Structure coefficient (%) pH SOM (g·kg− 1) CEC (cmol·kg− 1) Total N (g·kg− 1) Total P (g·kg− 1) Total K (g·kg− 1) Available N (mg·kg− 1) Available P (mg·kg− 1) Available K (mg·kg− 1)

1.3 ± 0.1a 55.6 ± 16.6a 7.6 ± 0.6a 21.7 ± 5.2a 9.7 ± 2.7a 1.3 ± 0.3a 0.8 ± 0.1a 24.2 ± 2.5a 121.3 ± 24.0a 16.5 ± 7.5a 84.0 ± 27.4a

1.3 ± 0.1a 57.5 ± 7.0a 6.9 ± 1.2a 23.2 ± 3.7ab 11.4 ± 4.8a 1.6 ± 0.3ab 1.1 ± 0.3b 21.8 ± 2.0b 200.8 ± 68.9b 56.2 ± 42.6a 88.0 ± 14.7a

1.3 ± 0.1a 66.3 ± 16.9a 6.1 ± 1.0b 29.1 ± 11.7b 10.5 ± 4.0a 1.9 ± 0.6b 1.5 ± 0.6c 22.6 ± 2.2ab 184.0 ± 70.2b 122.7 ± 67.5b 102.2 ± 77.0a

Values are the means ± standard deviations; values followed by different letters within the rows are significantly different between land use patterns at P b 0.05.

vegetable land b greenhouse vegetable land. These results are due to the different intensities of cultivation and fertilisation. Compared to the cropland, the soil pH, alkaline phosphatase, and total potassium in the vegetable land were notably lower, whereas the organic matter, total nitrogen, total phosphorus, available nitrogen, available phosphorus, and available potassium were substantially higher. According to our survey, the order of the fertiliser application rates (organic fertiliser and chemical fertiliser) and cultivation intensity was greenhouse vegetable field > open vegetable field > grain field. The application of both organic fertilisers and nitrogen fertilisers was higher in the vegetable fields, which resulted in an increase of SOM and nitrogen and an improvement of other soil properties (Hati et al., 2007; Lee et al., 2009; Monaco et al., 2008; Shirani et al., 2002). 5. Conclusion In conclusion, the arable soils in Lhasa have undergone sandification since the late 1980s. The soil pH and the CEC decreased remarkably, which led to soil acidification and lower soil fertility. The soil organic matter and total nitrogen in the cultivated steppe soil and meadow soil increased slightly, but they declined in the alluvial soil. The soil available nitrogen and phosphorus contents increased significantly, whereas the soil available potassium content decreased substantially; these changes were mainly attributed to the high application rates of nitrogen and phosphorus fertilisers and the low input of potassium fertilisers. Especially in the vegetable land, the changes in the soil properties of the A layer (the ploughed layer) were more apparent than in the subsoil layers. The soil organic matter in the B layer increased notably. For the various soil utilisation patterns of the same soil type, the rank of the soil fertility was croplandb open vegetable land b greenhouse vegetable land, which again indicates that the changes in the soil properties were mainly attributable to organic fertiliser (manure) application and the extent of cultivation. Acknowledgements The authors thank the anonymous referees for their comments and are grateful for the financial support provided by following projects: The Knowledge Innovation Program of the Chinese Academy of Sciences (Grant no. KZCX2-YW-QN314) and the One Hundred Young Persons Project of the Institute of Mountain Hazards and Environment, Chinese Academy of Sciences (Grant no. SDSQB-2010-02). References Abu Muhammad Shajaat, A., 2006. Induced intensification, land use/land cover changes and land degradation in Bangladesh. Advances in Earth Science 21, 183–190. Barton, D., Hope, D., Billett, M.F., 1994. Sulphate adsorption capacity and pH of upland podzolic soils in Scotland: effects of parent material, texture and precipitation chemistry. Applied Geochemistry 9, 127–139. Bayer, C., Mielniczuk, J., Amado, T.J.C., Martin-Neto, L., Fernandes, S.V., 2000. Organic matter storage in a sandy clay loam Acrisol affected by tillage and cropping systems in southern Brazil. Soil and Tillage Research 54, 101–109. Borken, W., Muhs, A., Beese, F., 2002. Changes in microbial and soil properties following compost treatment of degraded temperate forest soil. Soil Biology & Biochemistry 34, 403–412. Bruun, S., Christensen, B.T., Thomsen, I.K., Jensen, E.S., Jensen, L.S., 2007. Modeling vertical movement of organic matter in a soil incubated for 41 years with 14C labeled straw. Soil Biology and Biochemistry 39, 368–371. Buurman, P., Jongmans, A.G., 2005. Podzolisation and soil organic matter dynamics. Geoderma 125, 71–83.

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