Effect of Land Use on Soil Erosion and Nutrients in Dianchi Lake Watershed, China

Effect of Land Use on Soil Erosion and Nutrients in Dianchi Lake Watershed, China

Pedosphere 25(1): 103–111, 2015 ISSN 1002-0160/CN 32-1315/P c 2015 Soil Science Society of China ° Published by Elsevier B.V. and Science Press Effec...

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Pedosphere 25(1): 103–111, 2015 ISSN 1002-0160/CN 32-1315/P c 2015 Soil Science Society of China ° Published by Elsevier B.V. and Science Press

Effect of Land Use on Soil Erosion and Nutrients in Dianchi Lake Watershed, China NIU Xiao-Yin1,2 , WANG Yan-Hua1 , YANG Hao1,∗ , ZHENG Jia-Wen2 , ZOU Jun1 , XU Mei-Na1 , WU Shan-Shan1 and XIE Biao1 1 College 2 College

of Geography Sciences, Nanjing Normal University, Nanjing 210023 (China) of Resource and Environmental Engineering, Shandong University of Technology, Zibo 255049 (China)

(Received April 1, 2014; revised November 14, 2014)

ABSTRACT Soil erosion and loss of soil nutrients have been a crucial environment threat in Southwest China. The land use and its impact on soil qualities continue to be highlighted. The present study was conducted to compare soil erosion under four land use types (i.e., forestland, abandoned farmland, tillage, and grassland) and their effects on soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) in the Shuanglong catchment of the Dianchi Lake watershed, China. There were large variations in the erosion rate and the nutrient distributions across the four land use types. The erosion rates estimated by 137 Cs averaged 2 133 t km−2 year−1 under tillage and abandoned farmland over the erosion rate of non-cultivated sites, and the grasslands showed a net deposition. For all sites, the nutrient contents basically decreased with the soil depth. Compared with tillage and abandoned farmland, grassland had the highest SOC and TN contents within 0–40 cm soil layer, followed by forestland. The significant correlations between 137 Cs, SOC and TN were observed. The nutrient loss caused by erosion in tillage was the highest. These results suggested that grassland and forestland would be beneficial for SOC and TN sequestration over a long-term period because of their ability to reduce the loss of nutrients by soil erosion. Our study demonstrated that reduction of nutrient loss in the red soil area could be made through well-managed vegetation restoration measures. Key Words:

137 Cs,

nutrient loss, soil organic carbon, total nitrogen, total phosphorus, vegetation restoration

Citation: Niu, X. Y., Wang, Y. H., Yang, H., Zheng, J. W., Zou, J., Xu, M. N., Wu, S. S. and Xie, B. 2015. Effect of land use on soil erosion and nutrients in Dianchi Lake watershed, China. Pedosphere. 25(1): 103–111.

INTRODUCTION Soil erosion and loss of soil nutrients are being repeatedly mentioned as a global threat to environment and agricultural productivity (Pimentel, 2006; Dur´an Zuazo and Rodr´ıguez Pleguezuelo, 2008). For instance, it is estimated that soil loss by erosion reaches an average value of 50 Mg ha−1 year−1 , reducing up to 48% of crop productivity (de la Rosa et al., 2000). The study of Bilgo et al. (2006) has shown that 770 kg C ha−1 year−1 and 58 kg N ha−1 year−1 lost because of erosion in the cultivated soils. Many factors impact the degree of soil erosion and the biogeochemical cycling of soil nutrients (Houben et al., 2006; Novara et al., 2011; Sharma et al., 2011). One important factor is land use change (Hontoria et al., 1999; Henry et al., 2013). The impacts of human-induced land use change on soil erosion have received increasing attention worldwide (Bakker et al., 2008; de Neergaard et al., 2008; Wang et al., 2009; Garc´ıa-Ruiz, 2010). Different land uses may lead to changes in soil erosion, as well as in ∗ Corresponding

author. E-mail: [email protected]

soil organic matter and nutrient concentration (Heshmati et al., 2012; Navas et al., 2012; Zhang, C. et al., 2013). 137 Cs content, nutrients and soil texture have been widely used as indicators of soil erosion (Zapata, 2003; Mabit et al., 2008). The 137 Cs method was used to characterize the decline in soil quality associated with tillage erosion including decreased soil organic matter (Li and Lindstrom, 2001; De Neergaard et al., 2008), and 137 Cs changes were highly correlated with soil organic carbon (SOC) changes (Li et al., 2006; Martinez et al., 2010). These results point to the applicability of using the 137 Cs technique to understand land use, erosion, and soil carbon interactions. Dianchi Lake is a tectonic lake situated in the central part of the Yunnan-Guizhou Plateau and along the watershed areas of the Yangtze River, the Pearl River, and the Red River. Due to the excessive population growth and the growing demands for cultivable lands, more and more farmlands were reclaimed around the lake, and the impacts of the cultivation on the lake ecosystem became more and more serious (Huang et

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ning County, Kunming City of China and located in southwestern Dianchi Lake (24◦ 280 –25◦ 280 N, 102◦ 370 – 102◦ 480 E). The Shuanglong catchment is a subcatchment of the Yangtze River. This area is characterized by a subtropical monsoon climate, with a mean annual rainfall of about 900 mm, most of which falls in intense storms from May to October. Month-averaged air temperature around this region varies between 7.4 ◦ C in January and 19.6 ◦ C in July. The mean annual air temperature is 14.8 ◦ C. The natural vegetation is dominated by broad-leaved evergreen forests, and the secondary vegetation is dominated by Burma pine and China Armand pine. Forest coverage in the Dianchi Lake basin is 48.9%. Due to the early development of economy and the serious artificial damage, the original vegetation has rarely existed. The main existing vegetation includes Pinus yunnanensis, Pinus armandii, Cyclobalanopsis glaucoides, Acacia mearnsii, sprouted shrub and shrubgrass. Artificial farmland vegetation includes rice, corn, wheat, bean, etc. In general, two crops are planted each year in this region. For the cropping system, the ricerape or rice-wheat was dominant in the past. Since the mid 1990s, rice field has been increasingly converted to more profitable cut flowers and vegetable crops. The soil parent material consists of limestone, sandstone residues, slope wash and alluvial diluvium and the soils mainly consist of red soil, yellow-brown soil and paddy soil.

al., 2007; Xiong et al., 2010). Improper land utilization is one of the dominant factor for non-point source pollution in Dianchi Lake watershed (Xiong et al., 2010). Nutrient losses resulting from soil erosion present one of the most important factors affecting productivity of the soils in these areas. At the small watershed scales, the information about spatial distribution characteristics of soil nutrients and relationship between soil nutrients and land use can provide some important basis for ecological restoration and reconstruction of degraded ecosystem. At present, few reports involve land use patterns impacting on soil nutrients in small watersheds of the red soil region of Southwest China. Herein we report on nutrient concentrations in soils at several sites in the watershed and assess the effects of four land use types (i.e., forestland, abandoned farmland, tillage, and grassland) on nutrient profiles. The objectives of our study were to: i) investigate how concentrations of nutrients varied with depth and land use in the small catchment; ii) compare the soil erosion between different land uses; and iii) study the effect of soil erosion on nutrient losses. This study will not only provide specific information on the effects of land use changes on soil erosion and nutrient distributions in red soil areas, but also give regionally based policy-relevant information on impacts of the national policy on land use change. MATERIALS AND METHODS Study area

Soil sampling and analyses

The land in the drainage area of Dianchi Lake is mainly of mountains, terraces and dammed river valleys. The total area of the drainage area is 2 920 km2 . Over the past 30 years, rapid population growth in the basin and economic development had changed the land use in the basin tremendously (Table I). This study was conducted in the Shuanglong catchment, in total comprising a 66 km2 area within Jin-

In August 2011, ten study sites were selected based on major land use types in this region, including two forestlands (FL), two abandoned farmlands (AF), three tillages (TL), and three grasslands (GL) (Fig. 1). All investigated soils developed from the same parent materials. The description of each land use is shown on Table II. Three 10 m × 10 m plots with similar slopes,

TABLE I Land use structure statistics of the Dianchi Lake watershed from 1974 to 2008 Land use

1974 Area m2

Farmland Forestland Grassland Bare area Water Construction land a) Cited

1 175.8a) 648.3 533.5 46.6 332.2 98.4

1988 Proportion

Area

% 41.5 22.9 18.8 1.6 11.7 3.5

m2

from Zhang, K. et al. (2013).

1 151.7 714.1 440.7 40.5 309.4 178.4

1998 Proportion

Area

% 40.6 25.2 15.5 1.4 10.9 6.3

m2 1 399.4 671.9 163.4 48.7 307.1 244.4

2008 Proportion

Area

Proportion

% 49.4 23.7 5.8 1.7 10.8 8.6

m2

% 41.9 23.5 7.1 1.8 10.9 14.7

1 187.6 666.8 202.1 50.4 310.2 417.6

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gradients, and altitudes were established at each land use and were considered true replicates.

or lost was estimated by comparison with the reference inventory for the area (906 Bq m−2 , established from 9 stable sites). A simplified mass balance model (Yang et al., 1998) was used to estimate soil erosion rates (t km−2 year−1 ) based on 137 Cs soil inventories (Bq m−2 ). TN and TP in the soil samples were determined after persulfate digestion using a UV-3600 spectrophotometer. The SOC was measured by a TOC-analyzer (TOC-L CSH CN200, Shimadzu, Japan). Statistical analysis

Fig. 1 Location of the study sites (S1–S10) in the Shuanglong catchment of southwestern Dianchi Lake, China.

Soil sampling was conducted using a soil auger (44 mm internal diameter). Soil samples of 40 cm depth were collected from four random points at each plot and divided at 5 cm intervals. All samples were airdried, crushed, and passed through a 0.15-mm sieve for SOC, total nitrogen (TN) and total phosphorus (TP) measurements. 137 Cs activity was assessed by γ-spectrometry (efficiency 62%, GWL-120-15, ORTEC, USA) coupled with a multi-channel analyzer using Gamma view software. The peak of 137 Cs used to determine activity was 661.7 keV, and 137 Cs mass activity (Bq kg−1 ) was converted to areal activities (Bq m−2 ) using the bulk density of the soil. The amount of 137 Cs that is gained

Data were reported as means±standard deviations. Statistical analyses were performed using SPSS 19.0 (SPSS Inc., Chicago, USA) with one-way analysis of variance (ANOVA) to examine the effects of land use type and soil depth on the concentrations of SOC, TN and TP. If significant effects were observed by ANOVA, a least significant difference (LSD) test at P < 0.05 was used. Correlation analysis was used to evaluate the relationships between soil variables. Regression analysis was conducted to obtain the relationships between the changes of SOC, TN and TP. Figures were drawn using ArcGis 9.3 and Excel 2003 software. RESULTS Soil

137

Cs distribution

The 137 Cs concentrations (Bq kg−1 ) in the soils of TL, AF, GL and FL are shown in Fig. 2. The distribution of 137 Cs with depth in the soil profile differed among the land use types. For TL and GL, the majority of 137 Cs contained in the top 15 cm of soil profile. For FL, 137 Cs concentration decreased exponentially with soil depth. For AF, a peak in 137 Cs concentration occurred within the 25–30 cm of the soil.

TABLE II Characteristics of ten sites under four land use types in the Shuanglong catchment of southeastern Dianchi Lake, China Land use

Site No.

Tillage

Abandoned farmland Grassland

Forestland a) Means±standard

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10

Altitude Latitude

m 1 984 2 037 2 025 2 035 2 044 2 182 2 098 1 965 2 125 1 999

deviations (n = 3).

24◦ 350 4100 N 24◦ 360 5300 N 24◦ 350 3900 N 24◦ 360 800 N 24◦ 360 5300 N 24◦ 360 5800 N 24◦ 350 4200 N 24◦ 350 1400 N 24◦ 360 5800 N 24◦ 350 4500 N

Longitude

102◦ 320 400 E 102◦ 310 5400 E 102◦ 310 5400 E 102◦ 320 1100 E 102◦ 310 5800 E 102◦ 310 1000 E 102◦ 310 4500 E 102◦ 320 5200 E 102◦ 310 2400 E 102◦ 320 4600 E

Slope Soil

5 12 9 12 11 18 34 5 16 14

Clay

Silt

Sand

18.57±3.09a) 18.92±2.29 19.73±1.86 18.97±3.91 19.16±3.00 23.87±0.93 18.81±1.61 27.02±2.81 15.60±2.21 23.65±4.98

% 76.46±2.76 4.96±4.63 77.29±2.64 3.79±2.29 78.57±0.89 1.70±1.19 67.37±2.40 13.66±4.83 56.44±2.96 24.41±5.95 72.70±2.10 3.42±1.85 78.56±1.29 2.63±1.36 69.16±0.92 3.82±3.73 81.68±2.13 2.72±0.93 60.84±10.08 15.51±10.76

pH 4.73±0.16 4.29±0.24 5.08±0.12 4.56±0.12 5.14±0.26 4.61±0.10 4.76±0.12 5.18±0.29 4.41±0.12 5.01±0.20

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Fig. 2 137 Cs concentrations in soil profiles and total 137 Cs concentrations (calculated from 137 Cs activity and bulk density of soil) at 0–20 and 20–40 cm soil depths under different land uses of tillage (TL), abandoned farmland (AF), grassland (GL), and forestland (FL) in the Shuanglong catchment of southwestern Dianchi Lake, China.

The ranges of soil 137 Cs mass activity were quite larger among the sites and affected by the land use types (t-test, P < 0.001; Table III), where AF had the lowest 137 Cs value and GL had the highest one. For all the land uses, the 137 Cs content was higher in the upper layer (0–20 cm) than in the 20–40 cm layer (except AF) (Fig. 2). The total inventory of 137 Cs (Bq m−2 ), integrated to the depth of 0–40 cm, differed among the land uses (Table III). TABLE III Results of one-way analysis of variance for the effects of land use and soil depth on soil 137 Cs, soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) distributions Item

Degree of F value freedom 37 Cs

Land use 3 Soil depth 7

SOC

TN

TABLE IV 137 Cs

activities and soil erosion rates under different land uses of tillage (TL), abandoned farmland (AF), grassland (GL), and forestland (FL) in the Shuanglong catchment of southwestern Dianchi Lake, China Land use

137 Cs activity

137 Cs inventory

Soil erosion rate

TL AF GL FL

Bq kg−1 0.65±0.31a) 0.45±0.34 3.92±0.70 0.60±0.02

Bq m−2 330.4±157.5 173.2±88.3 1 354.4±475.5 271.2±51.4

t km−2 year−1 2 536.9±258.4 1 729.7±535.4 −242.8±235.2 928.5±145.0

a) Means±standard

deviations (n = 3).

TP

13.561*** 19.534*** 28.645** 20.129*** 0.455 2.567* 1.302 2.009

*, **, ***Significant at P < 0.05, P < 0.01 and P < 0.001 levels, respectively.

Estimated erosion levels ranged from −242.8 to > 2 500 t km−2 year−1 (Table IV). The sites under TL and AF ranged up to the highest and the FL site had on average the lowest erosion rates. Only GL showed a net soil deposition, which was different from the natural erosion without anthropogenic interference and its 137 Cs inventory exceeded the reference site background of 906 Bq m−2 . Soil organic carbon and total nitrogen

Fig. 3 Soil organic carbon (SOC) in different soil layers under different land uses of tillage (TL), abandoned farmland (AF), grassland (GL), and forestland (FL) in the Shuanglong catchment of southwestern Dianchi Lake, China.

SOC concentration varied with soil depth and land use type (Fig. 3). There was a decreasing trend of SOC with soil depth at 5 cm intervals in all land use types. For TL, SOC was significantly greater in the 0–20 cm

soil layer than in the 20–40 cm soil layer (Table V). For AF, SOC was the greatest in the 0–10 cm soil layer. For GL, SOC was significantly greater in the 0–20 cm soil layer. For FL, SOC was significantly greater in the 0–

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10 cm soil layer. Among all the land use types, the AF generally had significantly smaller SOC in the 0–40 cm soil layer compared with other three land use types (ttest, P < 0.001). The one-way ANOVA indicated that land use and soil depth exhibited a significant effect on SOC concentration (Table III). TABLE V Soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) at the 0–20 and 20–40 cm soil depths for different land uses of tillage (TL), abandoned farmland (AF), grassland (GL), and forestland (FL) in the Shuanglong catchment of southwestern Dianchi Lake, China Land use TL

AF

GL

FL

Soil depth cm 0–20 20–40 0–40 0–20 20–40 0–40 0–20 20–40 0–40 0–20 20–40 0–40

SOC

TN

24.23±1.21a) ab) 15.51±5.44b 19.85±3.33Bc) 13.32±4.95a 4.04±0.71b 8.68±2.83C 46.01±5.42a 31.08±4.76b 38.54±5.09A 26.54±7.93a 14.43±2.42b 20.49±5.17B

g kg−1 3.02±0.24a 2.33±0.61b 2.67±0.43B 1.53±0.36a 0.84±0.12b 1.19±0.24C 4.31±0.45a 3.18±0.43b 3.75±0.44A 2.04±0.52a 1.44±0.24b 1.74±0.38C

TP

0.74±0.09a 0.50±0.27b 0.63±0.18A 0.29±0.04a 0.23±0.03a 0.26±0.04B 0.43±0.04a 0.35±0.03a 0.39±0.04B 0.34±0.09a 0.27±0.13a 0.30±0.11B

a) Means±standard

deviations (n = 3). b) Means followed by the same lowercase letter within each column for each land use are not significantly different at P < 0.05. c) Means followed by the same uppercase letter within each column for 0–40 cm depth are not significantly different at P < 0.05.

The effects of soil depth and land use type on TN were similar to those on SOC (Fig. 4). Consistent with SOC, the TN of all land uses was significantly greater in the 0–20 cm soil layer than in the 20–40 cm soil layer (Table V). The AF had relatively smaller TN with an average value of 1.19 g kg−1 (0–40 cm) compared with the other land use types. For GL, TN was greatest in the 0–40 cm soil layer. Additionally, there was a trend of larger TN accumulated in the 20–40 cm for GL. The distribution of TN at the 0–25 cm depth was relatively uniform for TL. The TN concentration decreased as the soil depth increased from 0 to 40 cm soil layer, although no significant difference was found (Table III). Significant differences were found between four land uses for TN (t-test, P < 0.01; Table III). The land use types significantly affected SOC and TN contents. The relationship between SOC and TN among land uses can be modeled using a linear function (Table VI). The rank of correlations between SOC and TN for the four land use types was as follows: FL > AF > GL > TL.

Fig. 4 Soil total nitrogen (TN) in different soil layers under different land uses of tillage (TL), abandoned farmland (AF), grassland (GL), and forestland (FL) in the Shuanglong catchment of southwestern Dianchi Lake, China. TABLE VI Regression between soil organic carbon (Y , g kg−1 ) and total nitrogen (X, g kg−1 ) for different land uses Land use

Regression equation

F value

R2

Tillage Abandoned farmland Grassland Forestland

Y = 9.8147X − 0.6158 Y = 13.091X − 0.5955

97.53 371.58

0.8745*** 0.9841***

Y = 8.4358X + 0.6681 Y = 16.383X − 0.6918

108.99 998.14

0.8934*** 0.9862***

***Significant at P < 0.001.

Total phosphorus The TP concentrations varied within a relatively narrow range with few exceptions. It basically exhibited a trend of decreasing nutrient content with soil depth (Fig. 5). Among all sites, soil TP content average 0.63 g kg−1 for TL, 0.26 g kg−1 for AF, 0.39 g kg−1

Fig. 5 Soil total phosphorus (TP) in different soil layers under different land uses of tillage (TL), abandoned farmland (AF), grassland (GL), forestland (FL) in the Shuanglong catchment of southwestern Dianchi Lake, China.

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for GL and 0.30 g kg−1 for FL (Table V). For all sites, the concentrations of TP in the upper soil layer (0–20 cm) were higher than those in the 20–40 cm layer (Table V). There was a significant difference in TP content between TL, AF, GL and FL (t-test, P < 0.001; Table III). The depth had no significant effect on TP content (t-test, P > 0.05; Table III) Relationships between

137

Cs and soil nutrients

According to the soil erosion rate and soil nutrient content, the nutrient loss was estimated. The SOC, TN and TP losses were calculated using the following equation: Lx = Ex ·Cx

(1) −2

where Lx is the nutrient loss for land use x (t km year−1 ), Ex is the 137 Cs-derived erosion rate for land use x (t km−2 year−1 ), and Cx is the nutrient concentration for land use x (g kg−1 ). TL exhibited the highest nutrient losses, approximately up to 50.48, 6.85 and 1.60 t km−2 year−1 for SOC, TN and TP, respectively. The nutrient losses of AF for SOC, TN and TP were 19.03, 2.53 and 0.55 t km−2 year−1 , respectively. By contrast, the lowest nutrient losses was observed in FL, with 18.34, 1.58 and 0.28 t km−2 year−1 for SOC,

TN and TP, respectively. The correlations between137 Cs and nutrients for all sites are shown in Fig. 6. SOC and TN had a significant positive relationship with 137 Cs activity for the soils at 0–20 cm depth from all land uses, which indicates that variations of SOC and TN were associated with soil erosion and deposition in the surface soil. There was no significant correlation between 137 Cs and TP in this study. All changes in 137 Cs and nutrients among land uses can be modeled using the linear function (Fig. 6). DISCUSSION Effect of land use type on soil erosion In the study area, 137 Cs activity in the soil was affected by land use. This result is consistent with some previous studies (de Neergaard et al., 2008; Henry et al., 2013). For all land uses, except AF, the sampling of the 0–20 cm layer yielded much higher 137 Cs activities than that of the 20–40 cm depth. The average soil 137 Cs activities of land uses (less than 2 Bq kg−1 , except for GL) were lower than those previously reported (Porto et al., 2003; Yang et al., 2006; Junge et al., 2010). According to Mabit (2008), 137 Cs levels were affected by many factors such as terrain, climatic

Fig. 6 Relationships between 137 Cs activity and soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) in the soils for all the study sites at a depth of 0–20 cm.

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condition, vegetation, etc. In this study, the general differences in 137 Cs levels among land uses were most likely due to a combination of plant cover and slope gradient. Estimated erosion rates ranged from 1 729 to up to 2 537 t km−2 year−1 at the sites under three land use types, except for GL (Table III). The result essentially agrees with those in a previous study conducted in Dianchi Lake watershed (Zhang et al., 2008), indicating that soil erosion basically belongs to the light erosion in intensity. TL exhibited the highest soil erosion rate, followed by AF and FL. TL is more susceptible to erosion because of frequent cultivation of the soils and the vegetation is often removed before crops are planted. For AF, land abandonment is followed by natural vegetation regeneration, which results in decreased soil erosion. Compared with TL and AF, the FL has a relatively smaller soil erosion rate. In the stable forest ecosystem, where soil is protected by vegetation, the erosion rate is relatively low. In addition, we found that GL showed a net soil deposition, which probably reflects a strong influence of local geomorphology and vegetation cover. These results indicate that land use type is clearly a key factor controlling the intensity of soil erosion.

residue input into the soil and the relatively severe soil erosion. At a depth of 0–40 cm, the SOC and TN contents of TL were higher than those of AF, which may be attributed to the topographic feature and farming practice. Liang et al. (2012) reported that the application of N and P fertilizers contributed to the increased SOC and TN contents of the tillage to a certain extent. A recent research has also shown that the SOC and TN contents in the cultivated fields were higher than those in the older abandoned fields, because the recovery of the natural vegetation after a long-term period of abandonment equalized the nutrient conditions in the cultivated fields that received regular additions of manure (Navas et al., 2012). The SOC and TN contents in the surface soil were usually significantly different from those observed in the lower soil layers for all land use types (Table IV). The SOC varied from 13.32–46.01 g kg−1 in the upper soil (0–20 cm) to 4.04–31.08 g kg−1 in the 20–40 cm layers. The TN varied from a range of 1.53 to 4.31 g kg−1 in the upper soil (0–20 cm) to a range of 0.84 to 3.18 g kg−1 in the 20–40 cm layers. This difference probably can be attributed to vegetation coverage, the amounts of litter fall and roots, and human disturbance (McCarty et al., 1998; DeGryze et al., 2004). Liu et al. (2005) reported that high residue inputs in the surface soil may contribute to the increased SOC and TN contents. Other researches have demonstrated that the surface soil is more active for SOC and TN sequestration (Lenka et al., 2012). The results from this study demonstrated that the effect of land use types on SOC and TN contents were significant, suggesting that reconversion of tillage into forestland and grassland will improve the SOC and TN contents in soil.

Effects of land use type and soil depth on SOC and TN In the present study, the highest SOC content was observed in GL, followed by FL, TL and AF. GL exhibited the highest TN content at the 0–40 cm soil depth, followed by TL, FL and AF. The content of TN varied with soil depth and land use type, similar to that of SOC. Our results are consistent with the earlier findings (Schilling et al., 2009; Fu et al., 2010; Zhang, C. et al., 2013), suggesting that SOC and TN are highly related. In the current study, GL exhibited the highest SOC and TN contents at 0–40 cm compared with the other land uses. Wei et al. (2009) revealed that the profile distribution of fine roots agrees with the distributions of organic C and total N, suggesting that the extensive fine root system may be responsible for the higher SOC and TN contents in the natural grassland. Thus, the root system may be the major contributor to organic matter and nutrients in grasslands. The vegetation and plant litter, combined with the minimal soil disturbance in the GL, form an almost continuous covering layer that protects the soil from erosion. FL also showed higher SOC and TN contents in the 0–40 cm layer, which suggests that this vegetation cover type may be favorable for the improvement of C and N contents of the soil layers. However, the lowest SOC and TN in AF are attributed to the reduced

Effects of land use type and soil depth on TP In the present study, soil TP was found to differ significantly among land use patterns. However, soil TP did not vary systematically with depth. This result is similar to the previous research (Schilling et al., 2009; Martinez et al., 2010). TP was larger in tillage than the other land use types. Ekholm and Lehtoranta (2012) pointed that soil TP could be influenced by some factors such as parent material, fertilizer, and human activities. In this study, the higher content of TP observed in TL is mainly attributed to the application of fertilizer. Although soil P content is smaller, it can remain in the soils for a longer time compared to N, and consequently can be accumulated in rivers and lakes, causing eutrophication. Many researches have pointed that aquatic eutrophication is considered to be linked with

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soil erosion, since primary production in waters is assumed to be enhanced by the P carried away by eroded soil (House et al., 1998; Carpenter, 2005). The research of Barbosa et al. (2009) indicated that P concentration in sediment and runoff was controlled by tillage system. A previous research revealed that the transported P from the agricultural area was more compared to that from other areas due to P-fertilizer application and conventional tillage practices (Heshmati et al., 2012). Although there was no correlation between 137 Cs and TP in the whole studied area, a significant positive correlation between 137 Cs and TP was observed in TL (r = 0.7485, P < 0.01), indicating that TP loss is significantly accompanied by soil erosion in TL. Mengel and Kirkby (2001) reported that TP loss by erosion was 0.438 and 1.288 kg ha−1 year−1 in no-tillage and conventional tillage, respectively. In the current study, the estimated loss of TP from TL was up to 1.60 t km−2 year−1 and both the concentration and loss of TP were highest in TL. Compared with the other land uses, the high TP loss in TL was probably the most serious risk of non-point water source pollution at watershed scale. Implications for land management Our results supported the hypothesis that land use types significantly affected soil erosion rate, as well as SOC, TN and TP distributions in soils. The SOC, TN and TP contents in the top layer were higher than those in the deep layers. Results from this study demonstrated that the reconversion of tillages into forestlands and grasslands would improve the SOC and TN contents in the topsoil. Natural grasslands had the highest SOC and TN contents in the 0–40 cm layer and the lowest soil erosion rate. Therefore, natural grasslands might be the optimal choice for SOC and TN sequestration. Forestlands should also be promoted for their great contribution to SOC and TN distributions and their relatively smaller erosion in the study area. It is suggested to reduce erosion and associated environmental problems by protecting soil quality and managing land use. The results of this study provide useful information for decision makers and planners to take sustainable land use management and soil conservation measures in the area. CONCLUSIONS In this study, soil erosion and the variations in SOC, TN and TP contents were evaluated under different land use types in the Shuanglong catchment of

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the Dianchi Lake of China. Land use types had significant effects on the erosion rate and the contents of nutrients. Significant differences in erosion rate were observed among land uses, following the order of TL > AF > FL > GL. The contents of SOC, TN and TP in the top soil layer were higher than those in the deep layer. Compared with tillages and abandoned farmlands, the degrees of soil erosion for grasslands and forestlands were also smaller, accordingly, the inventories of SOC and TN were higher. Significant correlations were observed between SOC, TN and 137 Cs. The nutrients loss caused by erosion was the highest under tillage. This indicated that grasslands and forestlands had a potential for preventing further SOC and TN losses induced by erosion. Therefore, grasslands and forestlands might be the optimal choice for SOC and TN sequestration in the Dianchi Lake watershed. ACKNOWLEDGEMENTS This study was supported by the National Natural Science Foundation of China (Nos. 41273102, 41030751 and 41273103). We would like to express our thanks to Mr. G. Berson from Shandong University of Technology, China for his help in polishing the language of this manuscript. The authors also thank the anonymous reviewers and editors for their valuable comments and suggestions on the manuscript. REFERENCES Bakker, M. M., Govers, G., van Doorn, A., Quetier, F., Chouvardas, D. and Rounsevell, M. 2008. The response of soil erosion and sediment export to land-use change in four areas of Europe: The importance of landscape pattern. Geomorphology. 98: 213–226. Barbosa, F. T., Bertol, I., Luciano, R. V. and Gonzalez, A. P. 2009. Phosphorus losses in water and sediments in runoff of the water erosion in oat and vetch crops seed in contour and downhill. Soil Till. Res. 106: 22–28. Bilgo, A., Serpanti´ e, G., Masse, D., Fournier, J. and Hien, V. 2006. Carbon, nitrogen, and fine particles removed by water erosion on crops, fallows and mixed plots in Sudannese Savannas (Burkina Faso). In Roose, E. J., Lal, R., Feller, C., Barth` es, B. and Stewart, B. A. (eds.) Soil Erosion and Carbon Dynamics. CRC Press, Boka Raton. pp. 146–166. Carpenter, S. R. 2005. Eutrophication of aquatic ecosystems: Bistability and soil phosphorus. P. Natl. Acad. Sci. USA. 102: 10002–10005. DeGryze, S., Six, J., Paustian, K., Morris, S. J., Paul, E. A. and Merckx, R. 2004. Soil organic carbon pool changes following land-use conversions. Glob. Change Biol. 10: 1120–1132. de la Rosa, D., Moreno, J. A., Mayol, F. and Bons´ on, T. 2000. Assessment of soil erosion vulnerability in western Europe and potential impact on crop productivity due to loss of soil depth using the ImpelERO model. Agr. Ecosyst. Environ. 81: 179–190. de Neergaard, A., Magid, J. and Mertz, O. 2008. Soil erosion from shifting cultivation and other smallholder land use in

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