Tillage Effect on Organic Carbon in a Purple Paddy Soil1

Pedosphere 16(5): 660-667, 2006 ISSN 1002-0160/CN 32-1315/P @ 2006 Soil Science Society of China Published by Elsevier Limited and Science Press

PEDOSPHERE ww.elsevier.com/locate/pedosphere

Tillage Effect on Organic Carbon in a Purple Paddy Soil*' HUANG Xue-Xia'>', GAO Ming', WE1 Chao-Fu', XIE De-Ti' and PAN Gen-Xing3 College of Resources and Environment, Southwest Agricultural University, Chongqing 4 00716 (China,). E-mail: huangxuexia20000yahoo. corn..cn College of Environmental Science and Technology, Guangzhou University, Guangzhou 510006 (China) College of Resources and Environment, Nanjing Agricultural University, Nanjing 21 0095 (China) (Received March 7, 2006; revised July 24, 2006)

ABSTRACT The distribution and storage of soil organic carbon (SOC) based on a long-term experiment with various tillage systems were studied in a paddy soil derived from purple soil in Chongqing, China. Organic carbon storage in the 0-20 and 0-40 cm soil layers under different tillage systems were in an order: ridge tillage with rice-rape rotation (RT-rr) > conventional tillage with rice only (CT-r) > ridge tillage with rice only (RT-r) > conventional tillage with rice-rape rotation (CT-rr). The RT-rr system had significantly higher levels of soil organic carbon in the 0-40 cm topsoil, while the proportion of the total remaining organic carbon in the total soil organic carbon in the 0-10 cm layer was greatest in the RT-rr system. This was the reason why the RT-rr system enhanced soil organic carbon storage. These showed that tillage system type was crucial for carbon storage. Carbon levels in soil humus and crop-yield results showed that the RT-rr system enhanced soil fertility and crop productivity. Adoption of this tillage system would be beneficial both for environmental protection and economic development. Key Words:

carbon sequestration, organic carbon storage, purple paddy soil, tillage system

The increase of carbon dioxide and methane in the atmosphere is a contributory factor to global warming and is thus related to the concern of potential global warming. Therefore, there is a growing interest in soil's effect to act as sink or source of carbon (Cai et al., 2003; Wang et al., 2001; Scott et aL, 2002; Peter, 1997; Lal, 1999). The global soil organic carbon (SOC) stock in 1 m depth is estimated to be 1500 Pg (Pan, 2000), which is double the atmospheric C02-C stock. Even a 0.1% change of soil storage will result in a change of atmospheric COz concentration around 1 pL L-l (Eswaren et ab., 1993). The Intergovernmental Panel on Climate Change (IPCC) has estimated recently that SOC losses have contributed t o about 30%-50% of the increased COZ in the atmosphere, of which 30%-50% resulted from cropland (IPCC, 1997). Purple paddy soils, derived mainly from sandstone with different degrees of weathering, have been the major cropland soils in Chongqing and Sichuan regions in China and soil factors influencing fertility have been studied extensively (Wei et al., 1996, 1997, 1998; Gao et al., 1996, 2000; Che e t al., 1990; Xu, 1993; Yang et aL, 1992; Zhou et al., 2000). However, there have been few studies concerning the effect of tillage on soil organic carbon sequestration, which refers t o the long-term storage of carbon in soil, under long-term field trials (Yang and Michelle, 1999; Slobodian et al., 2002). In this research, the variation in distribution and storage of soil organic carbon of a purple paddy under four different tillage systems were studied based on a long-term tillage-cropping experiment t o demonstrate the applicability of rational tillage system to C sequestration and sustaining productivity. MATERIALS AND METHODS The experimental site (30' 26' N, 106' 26' E, 230 m above sea level) is located at the farm of Southwest Agricultural University, Chongqing, China. The average annual temperature is 18.3 O C ; the ~~

*'Project supported by the National Natural Science Foundation of China (No. 40231016)

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TILLAGE EFFECT O N SOIL ORGANIC C

annual rainfall is 1105.4 mm with 70% in summer May to September; the annual sunlight is 1276.7 h; and the time of no frost is about 334 d. The soil is a gray-brown purple Udic Cambisol, developed from purple parent material of Mesozoic sandstone formation. The original soil physical and chemical properties are as follows: pH (HzO), 7.1; total nitrogen, 1.74 g kg-'; total phosphorus, 0.75 g kg-'; total potassium, 22.7 g kg-'; available nitrogen, 120.1 mg kg-'; available phosphorus, 7.5 mg kg-'; available potassium, 71.1 mg kg-l; sand, 447.4 g kg-'; and clay content, 144.2 g kg-l. The experiment was initiated in 1990. Four tillage treatments were designed: 1) conventional tillage with rice only (CT-r) system, where regular tillage practices were used for rice with three times of plowing and harrowing annually, and after the rice harvest, the field was submerged with water; 2) ridge tillage with rice only (RT-r) system, where ridges (five in each plot) with top of 25 cm width were intervened with channels of 30 cm width and 35 cm depth, no tillage practices were performed and the field was submerged with water after the rice harvest; 3) ridge tillage with rotation of rice and rape (RT-rr) system, where ridges were made as in Treatment 2, but rape was cultivated after the rice harvest with the water level being maintained just to the channel bottom during the growth of the rape, and after the rape harvest, the field was submerged with water to cultivate rice; and 4) conventional tillage with rotation of rice and rape (CT-rr) system, where tillage was the same as in Treatment 1, but the field was drained after the rice harvest for rape cultivation, and submerged after rape harvest for rice cultivation. The experiment was under a completely randomized design with four replications each. Each plot was 20 m2 in size and 600 rice seedlings per plot were transplanted. The annual application of fertilizers was as follows: urea, 273.1 kg ha-' applied 2/3 as basal manure and 1/3 as topdressing; calcium superphosphate, 500.3 kg ha-l applied as basal; and potassium chloride, 150.1 kg ha-l applied 1/2 as basal and 1/2 as topdressing. The residues returned to the soil for the different tillage systems are shown in Table I. TABLE I Itesidues returned to the soil under different tillage systems Tillage systema)

Rice residues

CT-r RT-r RT-rr CT-rr

2 748.0-3 301.5 3 436.5-3 933.0 3 562.5-4024.5 2 697.0-3 532.5

Rape residues

Ruderal residues

kg ha-'

~~

0

0 742.5-940.5 768.0-987.0

year-' 1912.5-3 154.5 2 032.5-3 579.0 8 746.5-10 011.0 6 217.5-8 004.0

Total 4 660.5-6 456.0 5469.0-7 512.0 13051.5-14976.0 9682.5-12 523.5

~

a)CT-r: conventional tillage with rice only system; RT-r: ridge tillage with rice only system; RT-rr: ridge tillage with rotation of rice and rape system; CT-rr: conventional tillage with rotation of rice and rape system.

Soil samples from 0 to 20 cm depth were collected in 1990, 1993, 1995, 1997, and 1999, respectively. Soil sampling at depth intervals of &lo, 10-20, 20-30, and 30-40 cm was performed in 2000 using a soil drill. All soil samples were collected as a composite from 3-5 random sites in each plot after fall field operations. Laboratory analyses were conducted a t Southwest Agricultural University. Soil bulk density was measured using oven dried weight of a soil cylinder. Air-dried samples were ground to pass through a 0.25-mm sieve, and subsamples were analyzed for SOC using the potassium dichromate method, oxidation using 0.4 mol L-l KzCr204-HzS04 solution a t 170-180 "C (oil bath). The readily oxidized organic carbon content was determined using 0.2 mol L-' KzCrz04-H2S04 solution at 130-140 "C following a procedure described by Yuan (1963). A slightly modified procedure was used for studying the binding status of soil humus (Xiong, 1985). Storage of SOC was expressed as:

SOC = h x d x c where SOC is the organic carbon storage in a layer (kg m-'), h is the thickness of the soil (m), d is bulk density (g ~ m - ~ and ) , c is the organic carbon content (g kg-l). The proportion of the remaining soil organic carbon (ROC) was defined as:

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ROC =

SOC-EOC SOC

where EOC is the readily oxidized organic carbon content as mentioned above. The Duncan method was used for mean separation to test differences of SOC storage in 2000 in the 0-40 cm soil depth under different tillage systems with the significance defined at P < 0.05. RESULTS AND DISCUSSION Soil organic carbon

The trends of SOC for 0-20 cm depth from 1990 to 1999 are shown in Fig. 1. SOC increased from 1990 to 1999 by 51.3%, 38.1%, 89.0%, and 41.4%, for the CT-r, RT-r, RT-rr, and CT-rr systems, respectively. SOC under the RT-rr system was much greater than those under the CT-r, RT-r, and CTrr systems, although there was no significant difference among the latter three. This could be due to the greater amount of residues that were returned before rape transplantation under the RT-rr system (Table I). Furthermore, the changing curves showed a saturation trend of SOC under the systems other than RT-rr, which showed a still rapid increase in SOC. Overall, C sequestration under the tillage system of RT-rr had been acting profoundly, which could be an optional practice for reducing atmospheric COz by agriculture. 28.0

--

24.0

r

-

0

Y

.2 20.0

A x

0

0 v,

16.0 12.0 1990

1993

1996

CT-r RT-r RT-rr

CT-rr

1999

Year Fig. 1 Organic carbon (SOC) content (0-20 cm) from 1990 t o 1999 in a purple paddy soil at the research farm of Southwest Agricultural University, Chongqing, China, for four tillage systems: conventional tillage with rice only (CT-r), ridge tillage with rice only (RT-r), ridge tillage with rotation of rice and rape (RT-rr), and conventional tillage with rotation of rice and rape (CT-rr).

In the soil profile taken in 2000, organic carbon in the top layer was highest, and it decreased rapidly with increasing soil depth (Table 11).This could be attributed to both root biomass and returned residues incorporated into the topsoil (Tolbert e t aL, 2002; Angers et al., 1995). Top layer (0-10 cm) SOC content under different tillage systems was in the order: RT-rr > CT-r > RT-r > CT-rr (Table 11).The calculated SOC storage in 0-40 cm layer also varied in the same order with significant difference between the tillage treatments. Apparently, tillage had profound effect on the SOC storage in the purple paddy soil, being coincident with the amount of residue incorporation under the different tillage systems in the topsoil (Table I). Many scientists have proposed that maintaining or enhancing SOC storage would be a practical way to mitigate the greenhouse effect through carbon sequestration under better managed tillage system (Bayer e t al., 2000; Bruce e t al., 1999; Ah1 et al., 1998; Alvarez e t al., 1998; Borin, 1997). Soil amended with green manure could not only increase the usable C source for soil microorganisms, but also enhance soil organic matter content (Ni et aL, 2004); and there are many ongoing researches regarding the C sequestration (Zhao et al., 2005; Xie et al., 2004; Li, 2004; Sun et al., 2003; Jin e t al., 2001; Peng et al., 2004; Pan, 1999; Eve, 2002).

TILLAGE EFFECT ON SOIL ORGANIC C

663

TABLE I1 Organic carbon (SOC) storage of the studied purple paddy soil under four tillage systems measured in 2000 Tillage system”)

Soil depth

Soil bulk density

SOC content

SOC storage Per Iayer

RT-rr

RT-r

CT-rr

CT-r

cm 0-10 10-20 20-30 30-40 0-10 10-20 20-30 30-40 0-10 10-20 20-30 30-40 0-10 10-20 20-30 3C-40

g cm-3 0.94 1.13 1.17 1.12 0.98 1.04 1.02 1.09 0.95 1.14

1.12 1.15 0.77 0.86 0.93 1.11

g kg-l

29.79 26.25 22.97 18.41 22.78 18.82 17.69 17.00 20.05 18.93 17.96 13.26 22.78 21.73 19.53 15.75

2.80 2.97 2.69 2.06 2.23 1.96 1.80 1.85 1.91 2.16 2.01 1.53 1.75 1.87 1.82 1.75

Total Mean

Standard error

kg m-2 10.52 ab)

0.33

7.85 b

0.39

7.60 b

0.19

7.19 c

0.20

a)CT-r: conventional tillage with rice only system; RT-r: ridge tillage with rice only system; RT-rr: ridge tillage with rotation of rice and rape system; CT-rr: conventional tillage with rotation of rice and rape system. b)Values in a column followed by the same letter are not significantly different (P < 0.05) using Duncan’s multiple range test.

Proportion of total remaining organic carbon The oxidation stability of organic matter has been considered as one of the indices for evaluating soil fertility. It reflects the enrichment and balancing activities of SOC, and indicates the effect in moisture thermal conditions under management practices (Yuan, 1963; Xu and Yuan, 1995). The proportion of total remaining organic carbon to total soil organic carbon (ROC/SOC) under different tillage systems is shown in Table 111. Generally, ROC/SOC in topsoil was usually over 50% (Xu and Yuan, 1995). However, this value ranged from 34% to 49% in the case studied here (Table 111). This indicated a relatively active nature of the accumulated SOC in the studied purple paddy soil. There were no significant differences in ROC/SOC among the different layers from topsoil, being similar to that of the paddy soils of Taihu Lake Region (Li et al., 2000). Nor there was difference in this ratio under the different tillage systems although the RT-rr system exerted a higher pool of oxidizable SOC in proportion to the total SOC. Organo-mineral complexity Tillage systems had a prominent effect on the binding status of soil humus (Table IV). Content of loosely bound humus carbon in the 0-20 cm layer under the RT-rr system was 14.5% higher than that under the RT-r system, 39.5% higher than that under the CT-r system, and 53.2% higher than that under the CT-rr system. In turn, the tightly bound humus carbon under the RT-rr system was 1.4% lower than that under the RT-r system, 7.2% lower than that under the CT-rr system, and 19.1% lower than that under the CT-r system. However, there was no much difference in the content of stably bound humus carbon between different tillage systems in 0-20 and 20-40 cm layers. As the ratio of loosely bound to tightly bound humus carbon was accepted as a criteria for detecting soil fertility (Xiong, 1985), the RT-rr system could be considered also as an option for enhancing soil fertility of purple paddy soil.

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et al.

TABLE 111 Distribution of readily oxidized (EOC) and remaining (ROC) organic carbon in soil layers under four studied tillage systems Tillage system”) CT-r

RT-r

RT-rr

CT-rr

Soil depth

SOC

EOC

ROC

ROC/SOC

cm 0-10 10-20 20-30 30-40 0-10 10-20 20-30 30-40 0-10 10-20 20-30 30-40 0-10 10-20 20-30 30-40

22.78 21.73 19.53 15.75 22.78 18.82 17.69 17.00 29.79 26.25 22.97 18.41 20.05 18.93 17.96 13.26

g kg-l 14.10 12.89 11.88 10.27 12.22 10.99 10.90 9.38 18.78 15.17 11.88 10.35 12.32 11.35 10.34 7.88

8.68 8.84 7.65 5.48 10.56 7.83 6.79 7.62 11.01 11.08 11.09 8.06 7.73 7.58 7.62 5.38

38.10 40.68 39.17 34.79 46.36 41.60 38.38 44.82 36.96 42.21 48.28 43.78 38.55 40.04 42.43 40.57

%

^)CT-r: conventional tillage with rice only system; RT-r; ridge tillage with rice only system; RT-rr: ridge tillage with rotation of rice and rape system; CT-rr: conventional tillage with rotation of rice and rape system. TABLE IV Contents of different forms of soil humus carbon under four studied tillage systems Tillage systema) CT-r RT-r RT-rr CT-rr

Soil depth

Loosely bound humus carbon

Stably bound humus carbon

Tightly bound humus carbon

Loosely boundltightly bound humus carbon

cm 0-20 20-40 0-20 20-40 0-20 20-40 0-20 20-40

4.91 3.56 5.98 4.83 6.85 4.69 4.47 5.63

g kg-’ 2.36 1.53 2.47 3.32 2.59 3.46 2.54 3.18

6.12 4.25 5.21 5.09 5.14 5.57 5.51 4.48

0.802 0.838 1.148 0.949 1.332 0.842 0.811 1.257

a)CT-r: conventional tillage with rice only system; RT-r: ridge tillage with rice only system; RT-rr: ridge tillage with rotation of rice and rape system; CT-rr: conventional tillage with rotation of rice and rape system.

Crop yield

Because yield was an integrated result of many factors both of inherent soil fertility and environmental conditions, crop yields varied from one year to the other under a single tillage system (Fig. 2), being highest in 1996 and lowest in 1993 except for under the RT-rr system in 1990. However, in a single year, crop yield varied with the tillage treatments and the highest yield was found under the RT-IT system, followed by the CT-rr, RT-r, and then CT-r systems. Thus, the ridge culture-rice and rape system was beneficial for enhancing soil productivity.

Soil organic C sequestration and crop productivity Agricultural soil is generally considered to be a source of greenhouse gases and proper management could make the soil act as a sink (Tristram and Wilfred, 2002; Follett, 2001; Paustian et al.: 2000; Ogle et aL, 2004). Evidence from many studies shows that SOC storage decreases due to cultivation (Clapp

665

TILLAGE EFFECT ON SOIL ORGANIC C

16

714

2 12 2 10 "

+

CT-r

W

RT-r

?

A RT-rr

5

a 6

x CT-rr

E .-c u 4 >

2 0 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Year Fig. 2 Rice yields of four tillage systems: conventional tillage with rice only (CT-r), ridge tillage with rice only (RT-r), ridge tillage with rotation of rice and rape (RT-rr), and conventional tillage with rotation of rice and rape (RT-rr).

e t al., 2000; Campbell et aL, 1996; Duiker and Lal, 1999; Hao et aL, 2001; Halvorson et al., 2002; Mann, 1986). Davidson and Ackerman (1993) reported a 30% decrease in SOC after 20 years of cultivation. While agricultural management practices affected cropland SOC levels, proper management would have profound effect in enhancing carbon sequestration and the mitigation potential for greenhouse gases (Shrestha et aL, 2002; Deen and Kataki, 2003). Decreasing tillage intensity and changing single-cropping into crop rotation have been adopted as options for increasing the SOC (Johnson e t al., 1995). When rotation of rice and rape under the ridge culture system (RT-rr) was practiced, the soil could be de-intensified with tillage and enhanced with organic matter input from returned residues. In other words, from the results mentioned above, SOC input to the soil increased and decomposition and oxidation of the organic matter decreased, resulting in an increased SOC storage. Therefore, the tillage system of RT-rr studied here could be recommended as an efficient tillage practice for increasing the carbon sequestration in purple soils in Sichuan Basin, China. From the economic viewpoint, the RT-rr system would also give additional benefits of decreasing field labor work with increasing crop yield. Thus, the use of the ridge culture-rice and rape (RT-rr) system was beneficial for both environmental protection and economic development.

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