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Organic matter fractions and carbon mineralisation in perennial pasture leys on erodible silt soil

Yuying Shen, Lu Chen, Jing Yang, Mingming Wang and Zhibiao Nan

College of Pastoral Agriculture Science and Technology, Lanzhou University , 730020 China; www.lzu.edu.cn
Email: yy.shen@lzu.edu.cn , zhibiao@lzu.edu.cn

Abstract

Soil organic matter decomposition is related to the terrestrial carbon cycle. Soil organic matter fraction responded more rapidly to land use, disturbance, vegetation type, and other factors than bulk organic matter. Organic C fraction mineralisation was assessed in seven pasture leys in erodible soil on the Loess Plateau. Leys of bromegrass (Bromus inermis) clover (Trrifolium repens ), crowtoe (Lotus corniculatus.) lucerne (Medicago sativa CV. longdong and CV. Saditi), sainfoin (Onobrychis viciaefolia) and sweet pea (Lathyrus odoratus), were established at Qingyang experimental station of Lanzhou University in 2002. Dissolved organic carbon (NPOC) showed significant difference among the pasture leys. Total carbon content (TC) for seven pasture leys ranged between 19.7-25.8 g/kg, with the highest in clover. Organic carbon content for all seven species accounted to over 50% of TC, TC and TOC/TC were significantly lower in the 5-10 cm layer then 0-5 cm layer, TOC/TC ratio declined about 20% in 5-10 cm. Legume species leys had similar labile carbon distribution in 0-5 and 5-10 cm soil layers. Bromegrss had a highest C mineralisation rate among the seven species.Grass contributed more orxdizable C ,but it was easily break down and release to CO2. It is concluded that legumes have higher potential to store more organic carbon than grass.

Key Words

carbon cycle, clover, forage, Loess Plateau

Introduction

Soil organic matter is highly related to the soil chemical, biological and physical properties. Soil organic matter fraction responds more rapidly to changes in land use, disturbance, vegetation type, and other factors than bulk SOM (Post and Kwon 2000, Leifield & Knabner 2005, Yoo et al 2006). Identifying fractions that are sensitive enough to track short-term changes in both SOM quantitative and quality is a first step in developing fertility management for a sustainable farming system (Marriott & Wander, 2006). It was found that different organic carbon fractions have different degrees of oxidisibililty (Chan et al 2001). The mineralisation of soil organic C plays an important role in plant nutrition (Neza et al 2001), however, few studies have focused on the plant affects on soil microbial activity, which can be tested as carbon mineralisation of soil. The objectives of this study were to determine soil organic matter fraction differences among perennial pasture leys; and carbon mineralisation in an edodible soil on the Loess Plateau.

Material and Methods

Site and sampling

The study was carried out on a Heilu soil in Gansu Province, China (latitude 3540′ N, loggitude 10751′ E, 1297 m asl) where average annual precipitation is 562 mm. In 2002, leys of bromegrass (Bromus inermis),clover (Trrifolium repens ), crowtoe (Lotus corniculatus.) two lucernes (Medicago sativa CV. longdong and CV. Saditi) , sainfoin (Onobrychis viciaefolia ) and sweet pea (Lathyrus odoratus) were established on 3 m5 m plots in a randomised complete block with 4 replications. In August of 2007, soil samples from 0-10 cm soil were obtained.

Measurement

Dissolved organic carbon (DOC), total organic carbon (TOC) and total carbon (TC) were determined by combustion using liquid TOC ( Elementar Analysensysteme GmbH). Organic carbon fraction was assessed by a modified Walkley and Black (Chan et al 2001).

Incubation experiment

The 100 g fresh soil samples (< 2 mm) were incubated in 1 L jars at 25C for 45 days. Evolved CO2 was collected in 60 ml traps with 25 ml 0.1 N NaOH solution on days 1, 2, 3, 4, 6, 8, 10, 13, 17, 20, 25, 33, 39, 45. Soil moisture was periodically adjusted to field capacity (Neza et al 2001).

Results

Disolved organic carbon, organic carbon and total carbon

DOC in top soil showed a significant difference among the pasture leys. Total carbon ranged between 19.69-25.75 g/kg, with the highest in clover. The ratio of TOC/TC under all 7 species was under 0.5 and OC had generally high values under the pasture leys. DOC, TC and TOC/TC in the 5-10 cm layer were significantly lower than in 0-5 cm layer, for example, TOC/TC ratio was between 0.43 to 0.48, about 20% lower than that 5-10 cm (Table 1).

Table 1 DOC, TOC,TC concentrations before incubation in soil samples from seven different pasture leys

 

DOC
mg/kg

TOC g/kg

TC g/kg

TOC/TC

DOC
mg/kg

TOC
g/kg

TC
g/kg

TOC/TC

Species

0-5 cm

5-10 cm

Bromegrass

40.6

13.3

22.6

0.59

28.6

8.2

17.7

0.46

Clover

39.4

15.1

25.8

0.58

32.2

8.3

18.5

0.45

Crowtoe

36.3

10.4

20.4

0.51

34.0

7.6

17.8

0.43

Lucerne Cv. Longdong

36.7

11.4

21.0

0.54

32.4

8.6

18.1

0.47

Lucerne Cv. Saditi

36.3

10.9

19.8

0.55

33.7

7.5

16.4

0.46

Sainfoin

39.6

13.8

23.1

0.59

38.1

8.2

17.3

0.48

Sweet Pea

33.8

10.2

19.7

0.51

29.1

7.9

17.5

0.45

Lsd0.05

3.5

2.6

2.8

0.04

2.8

0.6

0.64

0.02

Oxidisible organic carbon

The most labile organic carbon under the seven pasture leys accounted for 73%-8 % of total organic carbon. Less than 10% of organic carbon was the heavy fraction. Grass and legumes with shallow roots had the lowest oxidisable organic carbon, Bromegrass had significantly higher medium oxidisable carbon than the other legume species. Lucenre and sainfoin with deep root systems had a relatively similar labile carbon in the 0-5 and 5-10 cm layers (Table 2).

Table 2 Percentage of fractions of organic carbon to total TOC from soils under different pastures leys using a modified Walkley-Black method

 

Fraction 1
%

Fraction 2
%

Fraction 3
%

Fraction 1
%

Fraction 2
%

Fraction 3
%

Leys

0-5 cm

5-10 cm

Bromegrass

73.0

23.4

3.6

64.7

28.7

6.6

Clover

77.7

14.8

7.5

71.4

19.6

9.0

Crowtoe

87.4

2.2

10.5

78.1

6.4

15.5

Lucerne Cv. Longdong

79.9

14.1

6.0

77.41

13.3

9.3

Lucerne Cv. Saditi

78.3

13.9

7.9

77.94

19.8

2.3

Sainfoin

78.9

10.2

10.9

72.99

18.0

9.0

Sweet Pea

81.1

11.8

7.1

81.50

10.9

7.6

Lsd0.05

10.3

9.1

7.5

6.71

7.18

6.5

**, significant at P<0.01; ***, significant at P<0.001; ns, not significant at P<0.05I have NO IDEA what is diferent from what from this table
Fraction 1:
Organic carbon oxidisible under acid : aqueous solution ratio of 0.5:1;Fraction 2:difference in oxidisible organic carbon extracted between using acid/aqueous solution of 0.5 :1 and 1:1 ; Fraction 3: oxidisible organic carbon extracted between acid : aqueous solutions of 1:1 and 2:1

CO2 evolution and C mineralisation

Within the 39 days incubation, bromegrass released the greatest amount of CO2-C among the seven perennial leys in both 0-5 cm and 5-10 cm layers, the lowest CO2 was found under crowtoe leys (1.01 mg/g soil), suggesting that grass had higher potential than legumes to release CO2..

Figure 1. Soil carbon mineralisation in 0-5 cm layer under seven different pasture leys (bars stand for l.s.d.

Figure. 2. Soil carbon mineralisation in 5-10 cm layer under seven different pasture leys(bars stand for l.s.d.

Conclusion

In the Loess Plateau, organic carbon content in 0-5 cm soil layer from five-years pasture leys was over 50% of total carbon content. Pernnial Bromegrass had higher DOC content and oxidiable organic carbon fraction than lucerne, sainfoin, and it also had higher potential to realease CO2. Legumes may have higher a potential to store carbon than grasses in these soils.

Acknowledgement

This research was funded by“973” program (National Basic Research Program of China ) (2007-CB106804) and ACIAR project- LWR/2006/191.

References

Chan KY., Bowman A and Oates A(2001). Oxidisible organic carbon fractions and soil quality changes in an Oxic Paleustalf under different pasture leys. Soil Sci Soc Am J. 166(1),61~67.

Leifield J., Kogel –Knabner I.(2005) Soil organic matter fraction as early indicator carbon stock chengeds under different land-use. Geoderma. 124: 143-155

Marriott EE., Wander M(2006). Qualitative and quantitative differnces in particulate organic matter fractions in organica and conventional framing systems. Soil Biology & Biochemistry 38:1527-1536

Neza S., Martnez-Yrzara A, Brqueza A and Garca-Oliva F(2001). Carbon mineralizationmineralisation in the southern Sonoran Desert. Acta Oecologica. 22,269~276.

Post WM and Kwon KC(2000). Soil carbon sequestration and land-use change process and potential. Global Change Biology. 6,317~327.

Yoo GY., Spomer LA and Wander MM (2006). Regulation of carbon mineralizationmineralisation rates by soil structure and water in an agricultural field and a prairie-like soil. Geoderma.135:16-25

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