Effects of long-term repeated mineral and organic fertilizer applications on soil organic carbon and total nitrogen in a semi-arid cropland

Europ. J. Agronomy 45 (2013) 20–26

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European Journal of Agronomy journal homepage: www.elsevier.com/locate/eja

Effects of long-term repeated mineral and organic fertilizer applications on soil organic carbon and total nitrogen in a semi-arid cropland Zhou Zhengchao a,b,∗ , Gan Zhuoting c , Shangguan Zhouping b , Zhang Fuping a a b c

Department of Tourism and Environmental Sciences, Shaanxi Normal University, Xi’an 710062, PR China State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences, Yangling 712100, PR China Key Laboratory of Disaster Survey and Mechanism Simulation of Shaanxi Province, Baoji University of Arts and Sciences, Baoji, Shaanxi 721007, PR China

a r t i c l e

i n f o

Article history: Received 18 July 2012 Received in revised form 18 October 2012 Accepted 2 November 2012 Keywords: Soil organic carbon Soil N Fertilizer Winter wheat Semi-arid

a b s t r a c t To meet and the ever increasing need for food and mitigation of global climate changes, plenty of fertilizers have been used to increase crop yield in China, especially in semi-arid regions. In this study, we investigated the impacts of long-term fertilization on wheat yields, soil organic carbon (SOC) and soil nitrogen (N) in the semi-arid Loess Plateau, China. One fallow and eight winter wheat-wheat (Triticum aestivum L.) rotation cropping plots were selected for the field experiment from 1984 to 2010 in the semi-arid Loess Plateau, China. In total we conducted eight fertilization treatments including no fertilizer, mineral nitrogen fertilizer (N), mineral phosphate (P), cattle manure (M), N + P, N + M, P + M and N + P + M. In 2010, we collected three replicate soil samples from each plot to the depth of 100 cm from soil surface. Meanwhile, soil bulk density, SOC, total N, and mineral N (ammonium and nitrate), wheat grain and aboveground biomass yields in each plot were measured. We found that mineral fertilizers, especially those applied together with cattle manure, increased winter wheat grain and aboveground biomass yields dramatically. Moreover, wheat biomass was found to have significant correlation with SOC and soil total N in the 0–20 cm soil layer. We also found that SOC and soil N were highest in the topsoil layers (0–30 cm) than other layers and declined to the depth of 50 cm with insignificant changes from 50 to 100 cm in all treatments. Compared to the data in 1984, fertilizer application increased surface soil SOC content, especially for the N + P + M treatment after 26 years cropping and fertilization. However, changes in soil total N and mineral N differed from SOC with decreasing N in mineral-fertilized and fallow plots but increasing N in the M-fertilized plots. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Since the beginning of the last century, average global temperatures have risen significantly, causing drier summers, warmer winters, and extreme precipitation events (Lal, 2004). Climate change is generally ascribed to human activities which are intensifying emissions of greenhouse gases such as CO2 , CH4 and N2 O, of which CO2 is having the greatest effect (Ramaswamy, 2001). Because atmospheric CO2 is connected to many carbon pools (oceanic, geological and biotic), reducing carbon (C) emission and increasing C sinks are of great importance for global warming mitigation. Croplands cover more than 11% of the world’s land area and store about 1500 GT of C, of which more than 70% is stored in

∗ Corresponding author at: Department of Tourism and Environmental Sciences, Shaanxi Normal University, Xi‘an, Shannxi Province, 710062, PR China. Tel.: +86 29 85310525; fax: +86 29 85310524. E-mail address: [email protected] (Z. Zhengchao). 1161-0301/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.eja.2012.11.002

the upper 100 cm of soils (Eswarran et al., 1993). Cropland soils can function as either sources or sinks for atmospheric CO2 (White et al., 2000; Follett et al., 2001; Leifeld and Fuhrer, 2009; Wang et al., 2009; Yu et al., 2012; Zhang et al., 2012) and play important roles in the global C cycle and balance. Carbon dynamics in cropland ecosystems are strongly influenced by various processes such as photosynthesis and decomposition which are affected by cultivation practices (Ganjegunte et al., 2005; Triberti et al., 2008; Zhang et al., 2012). Several studies have demonstrated that conversion cropland to forest or grassland increases soil organic carbon (SOC) storage (Leifeld and Fuhrer, 2009; Wang et al., 2009). However, since food demand is increasing, especially in developing countries which have over 850 million food-insecure people (Borlaug, 2007), little cropland can be converted to forest or grassland. Therefore, changes in cultivation practices will play a critical role in future management of soil carbon storage. Changes in cultivation practices also affect soil nitrogen (N), and it has been well documented that over 90% of N in most surface soil occurs in organic forms coupled with SOC (Nieder and Benbi, 2008).

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Soil N is often limited in semi-arid areas, and an understanding of the long-term SOC dynamics can contribute to N management in these areas (Vitousek et al., 1997; Wang et al., 2009). Growing interest in the potential of soils to sequester atmospheric CO2 has stimulated considerable efforts to monitor changes of soil organic carbon in cropland ecosystems globally (Triberti et al., 2008; Yu et al., 2012; Wiesmeier et al., 2012). The changes of soil carbon concentration or content in winter wheat-maize cropping system under long term fertilizer applications were widely reported in China (Cai and Qin, 2006; Gong et al., 2009; Hai et al., 2010; Zhang et al., 2009). However, changes in SOC and N concentrations and storage in continuous wheat-wheat cropping land, especially in the semi-arid area of Chinese Loess Plateau, have rarely been reported in long-term studies of effects of mineral and manure fertilizers (Gao et al., 2009). Data from such studies will be important for developing strategies for sustainable cropland management in the semi-arid area. Therefore, the objective of this study was to determine the effects of long-term mineral and manure fertilizers on SOC and soil N content in continuous wheat-wheat cropping land in the Chinese Loess Plateau.

2. Materials and methods 2.1. Study area The field experiment was established in 1984 at the Changwu State Key Agro-ecological Experimental Station (Changwu County, Shaanxi Province, China). The site (107◦ 40 E, 35◦ 12 N, 1220 m a.s.l.) has typical characteristics of the Loess Plateau region, north-west China, including a semi-arid climate with warm summers and very cold winters. The 50-year mean annual temperatures is 9.1 ◦ C and mean annual precipitation is 580 mm, about 70% falling between June and September. The driest months are May and June. At the beginning of the experiment (1984) the soil organic carbon (SOC), total N and bulk density in the top 20 cm of the soil were 6.5 g kg−1 , 0.62 g kg−1 , and 1.30 g cm−3 , respectively (Gao et al., 2009). The soil is classified as loam (Cumulic Haplustoll: USDA Soil Taxonomy System) developed from loess deposits.

2.2. Experimental design and treatments The experiment began in 1984 on land that had previously been cultivated with continuous winter wheat (Triticum aestivum L.) for at least 30 years. Thirty-six treatments with triplicates were designed as incompletely random blocks in the experiment, including bare fallow, continuous wheat-wheat, maize-maize (Zea mays L.), alfalfa-alfalfa (Medicago sativa L.), wheat-maize, and maize-peas (Pisum sativum L.) rotation with various fertilizer rates. The plot size was 10.3 m × 6.5 m, separated by 0.5 m strips. All plots have been managed as same in each year by the workers of Changwu National Key Agro-ecological Experimental Station from 1984 to 2010. The treatments selected for this present study are: bare fallow (no cropping) plots and eight continuous wheat-wheat rotations cropping with various fertilizer treatment plots. The fertilizer treatments were: no fertilizer (CK); mineral N fertilizer (N); mineral phosphorus fertilizer (P); cattle manure (M); mineral N plus P fertilizer (NP); mineral N fertilizer plus cattle manure (NM); mineral P fertilizer plus cattle manure (PM); and mineral N and P fertilizer plus cattle manure (NPM). Mineral nitrogen fertilizer used in this study was urea, and P was superphosphate. M was mainly cow excretion, containing 10.7 g C kg−1 and 1.16 g N kg−1 (equivalent to 87 kg N ha−1 year−1 at the application rate used). Fertilizer application details are given in Table 1. All fertilizers (including M) were applied as a single dose 5–7 days prior to sowing.

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Table 1 Fertilizer application rates in each treatment (kg ha−1 ). Treatment

N application rate

P application rate

Fallow CK N P M NP NM PM NPM

0 0 120 0 0 120 120 0 120

0 0 0 39 0 39 0 39 39

a

M application rate (wet value)a 0 0 0 0 75,000 0 75,000 75,000 75,000

The water content was about 60–65% in weight.

Seed beds were tilled before September sowing of winter wheat in rows 20 cm apart, at a rate of 150 kg seed ha−1 in each of wheat cropping plot. Weeds were removed manually (in all treatment plots, including the fallow plots) as necessary, and plant protection measures (such as covering with netting and applying fungicides and pesticides) were applied as required. Wheat was harvested manually, cutting close to the ground, and all harvested biomass was removed from the plots in June each year. The weight of the harvested biomass was measured after air-drying.

2.3. Soil sampling and laboratory analysis Three replicate soil core samples (6 cm diameter) were collected from each treatment plot, to a depth of 100 cm, and divided into 15 sections (5 cm sections from 0 to 50 cm, and 10 cm sections from 50 to 100 cm). In total, 1215 soil samples were collected. The samples were sealed in plastic bags and transported to our laboratory for analysis. Soil bulk density () of a 0–20 cm core was measured separately for each plot using the method reported by Blake and Hartage (1986). Soil samples were air-dried and visible plant material was removed before screening with a 0.5 mm sieve. SOC was determined by the K2 Cr2 O7 –H2 SO4 oxidation method (Nelson and Sommers, 1982). Total N was measured by the Kjeldahl method (Bremner, 1996). Ammonium and nitrate were extracted by vigorously shaking the sample with 50 ml of 2.0 mol l−1 KCl for 30 min, then the extract was filtered and NH4 + nitrogen and NO3 − nitrogen concentrations were measured using a Skalar 5100 continuous flow spectrophotometer (Breda, The Netherlands) with cadmium reduction (at  = 540 nm) for nitrate, and cation complexation (at  = 660 nm) for ammonium.

2.4. Calculations and data analysis For each soil depth interval in each plot, SOC and total N were calculated using the equation: S = EC ×  × h × 10−1 where S is the element stock (Mg ha−1 ), EC is the element concentration (g kg−1 ),  is the bulk density (g cm−3 ), and h is the thickness of the soil layer (cm). SOC and N stock values for the 0–5-cm, 5–10-cm, 10–15-cm and 15–20-cm layers were summed to calculate SOC and N stocks in the top 20 cm of soil. ANOVAs were carried out with the SAS statistical software package (SAS Institute, Cary, USA) to test the treatment effects on bulk density, SOC, total N and harvested biomass.

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higher SOC and total N concentration in the upper 30 cm of soil layers (Fig. 2A and B). However, manure application did not demonstrate higher mineral N concentration. The between-treatment differences of mineral N concentration were not similar to SOC or total N. The higher mineral N concentration was only observed in the N, N + P + M and P treatments but not in the N + M and the N + P treatment (Fig. 2C). 4. Discussion

Fig. 1. Wheat grain yield (A) and aboveground biomass yield (B) under the indicated fallow, CK and fertilizer treatments. Bars represent standard errors (n = 3). Different letters above the bars indicate significant differences (p < 0.05) between treatments.

3. Results

Our results clearly support the hypothesis that long-term mineral and manure fertilizers application has great positive impact on SOC and soil N content in continuous wheat-wheat cropping land in the Chinese Loess Plateau. In this study, mineral fertilizer treatments significantly increased the grain and biomass yields of winter wheat, especially in conjunction with cattle manure applications, and that both grain and biomass yields correlated with SOC and total N concentrations (Figs. 1 and 3). We also found that SOC and total N were highest in the topsoil layers (0–30 cm), declined with depth to 50 cm, then increased slightly with depth from 50 to 100 cm (under all treatments) (Fig. 2). Similar results have been reported by other long term experiments conducted in semi-arid areas (e.g. Su, 2007; Wang et al., 2009; Gong et al., 2012; Pan et al., 2010).

3.1. Winter wheat grain and aboveground biomass yields 4.1. Effect of long term fertilizer application on soil C Compared with unfertilized treatment, wheat grain and aboveground biomass were markedly increased in all fertilized treatments except for the P only treatment (Fig. 1). As expected, the N + P + M treatment showed the highest wheat grain and aboveground biomass yields, increasing them by 69% and 62% relative to the CK treatment, respectively. However, the lowest wheat grain and aboveground biomass yields were observed in the P only treatment (Fig. 1). Higher increasing of wheat grain and aboveground biomass yields were observed for the manure applied plot, compared to N and P fertilizer treatment plots (Fig. 1). The same phenomenon was also showed in N + P, N + M and P + M treatments. These means manure could bring higher increasing of wheat grain and aboveground yields than N and P. As expected, fertilizer mixed application could get higher grain and aboveground biomass yields (Fig. 1). Compared to the CK treatment plot, the total increase of wheat grain and aboveground yields in N and M treatment were 1507 kg ha−1 and 3975 kg ha−1 , respectively. However, the increase of grain and biomass yield in mineral N fertilizer plus cattle manure (N + M) up to 2407 kg ha−1 and 5302 kg ha−1 , respectively, compared to CK treatment (Fig. 1). 3.2. SOC, total N and mineral N concentrations in soil Soil organic carbon, total N and mineral N concentrations in soil profiles under each of the fallow and fertilizer-cropping treatments are shown in Fig. 2. Under each of the nine treatments SOC, total N and mineral N concentrations were highest in the 0–10 cm soil layer, declined with depth to the 45–50 cm layer, and then slightly increased with depth from 50 to 100 cm. Significant differences in SOC and total N between treatments were mainly observed in the 0–30 cm soil layers (Fig. 2A and B). In these layers, SOC and total N were higher under all of the fertilizer-cropping treatments than under CK (especially under the N + P + M treatment, which resulted in near-doubling of both variables). In contrast, the differences were much smaller in the lower (30–100 cm) soil layers among different treatment. All the treatments with cattle manure application, compared to others without applied cattle manure treatments, were observed

In arable soil, C input to soil was mainly determined by crop biomass including the aboveground biomass and root biomass (Triberti et al., 2008). However, removal of aboveground biomass as cooking fuel or for animal bedding was a common agricultural practice in the study area, therefore the existence of a significant linear relationship between SOC concentrations and C input from residues (Duiker and Lal, 1999) may be the best explanation for the lower SOC increasing (0.42 Mg C ha−1 in the wheat-wheat rotation cropping without fertilized) after 26 years of cropping than others previously reported (Brye et al., 2002; Wang et al., 2009; Gong et al., 2012). In this study, the accumulation rate of soil C content ranged from 0.016 Mg C ha−1 yr−1 (in CK treatment) to 0.373 Mg C ha−1 yr−1 (in the N + P + M treatment), respectively, after 26 years continuous cropping (Table 2). These results showed that fertilizer application could accelerate the accumulation of soil C. These findings were consistent with a large body of data indicating that fertilizer treatment generally increases the SOC and N contents of arable soils (Glendining & Powlson, 1995; Brye et al., 2002). However, accumulation rate of soil C content, even in the N + P + M treatment, are lower than reported previously. Jenkinson et al. (1992) reported that the SOC increase rate was 1.7 Mg ha−1 yr−1 for an experimental plot at Rothamsted treated with 144 kg fertilizer N ha−1 yr−1 . One explanation for this is that in our study area the annual precipitation (580 mm) is lower and evaporation higher (1440 mm) than at Rothamsted (710 mm and 599 mm, respectively), which means the loess soil is drier causing slower decomposition. Meanwhile, the turnover time of 26 years at our site was about 50% higher than that calculated (about 16 years) for Rothamsted. 4.2. Changes of soil N The soil N concentration (total N and mineral N) were positively related to SOC, variations in SOC could explained 88% and 21% of the observed variation in total N and mineral N concentration (Fig. 4a and b). Compared to CK treatment, the total N soil content was increased with fertilizer application, especially with cattle manure

Z. Zhengchao et al. / Europ. J. Agronomy 45 (2013) 20–26 -1

Total N concentration / g kg

Soil organic C concentration / g kg 0

3

6

9

12

0

10

10

20

20

30

30

A

Fallow CK N P NP M NPM PM NM

60 70 80 90

0.6

0.9

(C.V. range) (2.4-12.7) (0.2-17.2) (4.1-22.0) (4.4-23.5) (2.3-11.6) (0.4-26.1) (3.4-18.2) (1.4-17.3) (2.0-20.3)

-1

1.2

1.5

B

40 Depth / cm

Depth / cm

50

0.3

0

15

0

40

23

50 Fallow CK N P NP M NPM PM NM

60 70 80 90

(C.V. Range) (3.0-20.3) (1.2-20.2) (2.4-15.6) (4.5-32.0) (3.7-18.5) (4.5-23.7) (2.1-15.9) (2.4-24.8) (0.5-12.1)

100

100

-1

M ineral N / mg kg 0

10

20

30

0 10 20 30

Depth / cm

40

C

50 60 70 80 90

100

Fallow CK N P NP M NPM PM NM

(C.V. range) (12.3-20.3) (10.1-19.3) (14.5-32.6) (9.0-17.8) (11.9-23.7) (17.8-31.6) (11.2-19.6) (12.4-20.3) (13.8-22.4)

Fig. 2. SOC (A), total N (B) and mineral N (3) soil profiles after 26 years of the treatments.

used (Table 3). This result was similar to the previous study conducted by Triberti et al. (2008). The larger amounts of soil N can be explained by a root-decay process (Wang et al., 2009). In this study, in order to protect the experimental plots, the biomass of wheat roots was not measured, but Gao et al. (2009) demonstrated that it correlates significantly with the aboveground biomass yield. Thus, the winter wheat roots biomass would increase with increasing of

aboveground biomass under fertilizer application in this present study (Fig. 1), and therefore result higher N concentration in fertilized plots compared to CK treatments (Fig. 2A and B; Table 3). As expected, the soil N concentration (total N and mineral N) were positively related to SOC. However, there were also some differences. The variations in SOC could explained 88% and 21% of the observed variation in total N and mineral N concentration (Fig. 4a

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5000

5000

a

4000

4000

3000

Grain yield / kg.ha -1

y = 536.54 x - 2052 .7 R 2 = 0.78 p<0.05

-1

Grain yield / kg.ha

b

2000

3000

2000

1000

y = 4798 .3x - 937.46 R 2 = 0.72 p<0.05

1000

0 0

0.0

0

5000

2

4

6

8

10

12

10000

-1

Aboveground biomass yield / kg.ha

Grain yield / kg.ha -1

2000

1000

5

0

1.0

1.2

d

8000

6000

4000

y = 1358 .3x - 5483 .5 R 2 = 0.92 p<0.05

2000

10

15

20

25

0

30

10000

-1

e Aboveground biomass yield / kg.ha

8000

6000

4000

y = 12116 x - 26 36.4 R 2 = 0.84 p<0.05

2000

4

6

8

10

12 -1

M ineral N / mg.kg 10000

2

Soil organic C concentration / g.kg

-1

-1

0.8

0

0

Aboveground biomass yield / kg.ha

0.6

Total N concentration / g.kg

c

3000

0.4

-1

Soil organic C concentration / g.kg-1

4000

0.2

0

f

8000

6000

4000

2000

0

0.0

0.2

0.4

0.6

0.8

1.0 -1

Total N concentration / g.kg

1.2

0

5

10

15

20

25

30

-1

M ineral N / mg.kg

Fig. 3. Relationships between yields (grain and biomass) and SOC, Total N and mineral N (averaged 0–20 cm soil concentration).

Z. Zhengchao et al. / Europ. J. Agronomy 45 (2013) 20–26

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Table 2 SOC and SSOC (storage of soil organic carbon) accumulation in the upper 20 cm of soil between 1984 and 2010 under each of the treatments. Treatment

Fallow CK N P NP M NPM PM NM

1984

2010

 (g cm−3 )

SOC (g kg−1 )

SSOC (Mg ha−1 )

 (g cm−3 )

1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3

6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5

16.9 16.9 16.9 16.9 16.9 16.9 16.9 16.9 16.9

1.34 1.28 1.28 1.28 1.27 1.28 1.20 1.26 1.21

± ± ± ± ± ± ± ± ±

SOC (g kg−1 )

0.03c 0.04b 0.05b 0.06b 0.03b 0.04b 0.07a 0.05b 0.06a

5.89 6.76 7.08 7.03 8.30 10.26 11.08 10.55 10.39

± ± ± ± ± ± ± ± ±

SSOC changes (Mg ha−1 )

Annual Cinput (Mg C ha−1 yr−1 )

−1.11 0.42 1.20 1.10 4.18 9.37 9.69 9.68 8.24

0.043 0.016 0.046 0.042 0.161 0.361 0.373 0.372 0.317

SSOC (Mg ha−1 )

0.53d 0.54c 0.46c 0.48c 0.32b 1.42a 0.63a 0.54a 0.90a

15.79 17.31 18.1 18.00 21.08 26.27 26.59 26.58 25.14

Note: different superscript letters indicate significant differences (p < 0.05) between treatments.

1.2

40

a

b

35

y = 1.142 8x + 5.902 8 R 2 = 0.21 n=13 5

30 Mineral N / mg.kg -1

Total N concentration / g.kg

-1

1.0 0.8 0.6 0.4 y = 0.092 1x - 0.069 4 R 2 = 0.88 n=135

0.2

25 20 15 10 5

0.0

0 0

5

10

Soil organic C concentration / g.kg

15

0

-1

5

10

15

Soil organic C concentration / g.kg-1

Fig. 4. Relationships between SOC and total N (a) and mineral N (b) in the soil profiles after 26 years of the treatments.

Table 3 Total N accumulation in the upper 20 cm of soil between 1984 and 2010 under each of the treatments. Treatment

Fallow CK N P NP M NPM PM NM

1984

2010

N stock change (Mg ha−1 )

 (g cm−3 )

Total N (g kg−1 )

N stock (Mg ha−1 )

 (g cm−3 )

1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3

0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62

1.61 1.61 1.61 1.61 1.61 1.61 1.61 1.61 1.61

1.34 1.28 1.28 1.28 1.27 1.28 1.20 1.26 1.21

± ± ± ± ± ± ± ± ±

0.03c 0.04b 0.05b 0.06b 0.03b 0.04b 0.07a 0.05b 0.06a

Total N (g kg−1 ) 0.52 0.54 0.55 0.57 0.72 0.95 1.02 0.91 0.87

± ± ± ± ± ± ± ± ±

0.077f 0.046ef 0.018ef 0.115e 0.055d 0.105ab 0.052a 0.051bc 0.031c

N stock (Mg ha−1 ) 1.39 1.38 1.41 1.21 1.45 2.43 2.45 2.29 2.11

−0.22 −0.23 −0.20 −0.40 −0.16 0.82 0.84 0.68 0.50

Note: Different superscript letters indicate significant differences (p < 0.05) between treatments.

and b), namely, the difference of mineral N between treatments were not strictly similar to the difference of total N between treatments (Fig. 2B and C). A possible explanation is that mineral N is more labile than SOC and total N, and can be absorbed by wheat roots or can be rapidly lost through leaching or volatilization processes.

surface soil C (Zhou and Shangguan, 2007; Wang et al., 2009). Soil N, especially mineral N, is transferred from subsurface to surface through roots absorbing. All of the C and N contained in roots will be released and contained in surface soil when the roots die. Thus, the SOC and soil N content increased in surface soil.

5. Conclusions 4.3. Distribution of C and N in soil profile In this present study, the larger amounts of SOC and soil N were observed in the surface soil layers (0–40 cm depth) (Fig. 2A–C), these can be explained by a root-decay process. Most of the plant roots are located in the surface soil and build up a main source of

The fertilizer treatments, especially cattle manure applications, significantly increased winter wheat yields (grain and aboveground biomass) and both SOC and total N contents in the examined loess plateau soil. In addition, SOC stocks were significantly correlated with crop biomass yield. Thus, cropping and fertilizer treatment

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could promote C sequestration in soils of the semi-arid region of north-west China, which supports a population of approximately 80 million, and contains ca. 1.3 million ha of arable land. Inherent soil fertility is low, and N levels in all soils in the region are particularly low. Thus, since the 1980s large amounts of synthetic fertilizer have been applied to arable soils in the region, and both soil productivity and SOC storage have significantly improved. The presented results should contribute to our understanding of the effects of cropping and fertilizer treatments on soil C sequestration, and thus facilitate predictions of C changes in this and other cropland ecosystems, which should provide some valuable information for improving agricultural practices, food security and soil quality, while reducing atmospheric CO2 concentrations and mitigating global warming. Acknowledgements This work was supported by the fundamental Research Funds for the Central University (GK201103003), the Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China (131025) and the Natural Science Foundation of Shaanxi (2011JQ5005). References Blake, G.R., Hartage, K.H., 1986. Bulk density. In: Klute, A. (Ed.), Methods of Soil Analysis. Part 1-Physical and Mineralogical Methods. ASA and SSSA, Madison, WI, pp. 363–375. Borlaug, N., 2007. Feeding a hungry world. Science 318, 359. Bremner, J.M., 1996. Nitrogen-total. In: Sparks, A.L., Page, A.L., Helmke, P.A., Loeppert, R.H., Soltanpour, P.N., Tabatabai, M.A., Johnston, C.T., Sumner, M.E. (Eds.), Methods of Soil Analysis. Part 3. Chemical Methods. SSSA and ASA, Madison, WI, pp. 1085–1121. Brye, K.R., Gower, S.T., Norman, J.M., Bundy, L.G., 2002. Carbon budgets for a prairie and agroecosystems: effects of land use and interannual variability. Ecological Applications 12, 962–979. Cai, Z.C., Qin, S.W., 2006. Dynamics of crop yields and soil organic carbon in a long term fertilization experiment in the Huang-Huai-Hai Plain of China. Geoderma 136, 708–715. Duiker, S.W., Lal, R., 1999. Crop residue and tillage effects on carbon sequestration in a Luvisol in central Ohio. Soil & Tillage Research 52, 73–81. Eswarran, H., Van den Berg, E., Reich, P., 1993. Organic carbon in soils of the world. Soil Science Society of America Journal 57, 192–194. Follett, R.F., Kimble, J.M., Lal, R., 2001. The potential of U.S. grazing lands to sequester soil carbon. In: Follett, R.F., Kimble, J.M., Lal, R. (Eds.), The Potential of U.S. Grazing Lands to Sequester Carbon and Mitigate the Greenhouse Effect. Lewis Publishers, Boca Raton, FL, p. 442. Ganjegunte, G.K., Vance, G.F., Preston, C.M., Schuman, G.E., Ingram, L.J., Stahl, P.D., 2005. Soil organic carbon composition in a northern mixed-grass prairie: effects of grazing. Soil Science Society of America Journal 69, 1746–1756. Gao, H.Y., Guo, S.L., Liu, W.Z., Che, S.G., 2009. Effects of fertilizer on wheat yield and soil organic carbon accumulation in rainfed loessial tablelands. Plant Nutrition and Fertilizer Science (in Chinese with English abstract) 15 (6), 1333–1338.

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