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Effects of 15 years of conservation tillage on soil structure and productivity of wheat cultivation in northern China.

Introduction

China is one of the world's major dryland farming countries. Rainfed land (crop production without irrigation) is largely located in the 16 provinces of northern China, where 33 Mha of arid and semi-arid land represents 52.5% of the total national land area (Zhai and Deng 2000). These dryland farming areas have little rainfall (<750 mm/year), low winter temperatures, a short frost-free period (<150 days), and high evaporation (> 1500 mm/year). Current cropping systems rely on conventional mouldboard plough tillage but suffer from soil structural degradation, and poor fertility, resulting in low water use efficiency and decreased crop yields (Gao et al. 1999; Liu 2004).

Conventional tillage aggravates the problems of erosion by wind and water (Wang et al. 2000, 2001), which have serious impacts on the wider community via pollution, dust storms, and desertification (Rong et al. 2004; Li et al. 2005). Soil erosion has also been shown to deposit 456 Mt/year of sediments, including 5.08 Mt of organic matter and 0.3 Mt of N and P, into the Yellow River in Shanxi province alone. The loss of nutrients is equivalent to 25% of the total chemical fertiliser useage in Shanxi province (Zang and Gao 2003). The average desertification rate is approximately 2000 [km.sup.2] per year, and a total of 1.74 [Mkm.sup.2] land has already been lost, with desert accounting for about 18.1% of the total land area of China in 2004 (CSFA 2005). Zhang et al. (2004) reported that about 30 Mha of farming lands in northern China were seriously affected by the dust storms between March and May 2002.

More sustainable cropping systems, using conservation tillage to improve residue cover with minimum- or no-till, have been demonstrated in many environments. Such systems will be essential for the sustainable development of dryland farming in China.

Research in China has generally confirmed the improvements in productivity and sustainability achieved by conservation tillage. Mou et al. (1999) and Roldan et al. (2005) demonstrated that compared with conventional tillage, conservation tillage led to greater aggregate stability, more small soil pores, and fewer large pores. The effectiveness of no-tillage systems in controlling wind and water erosion, reducing soil loss, and increasing soil organic matter has been reported by Zang et al. (2003) and Zhou and Lu (2004). Conservation tillage also has been shown to increase crop yield and water use efficiency (Liao et al. 2002; Xue et al. 2005).

Most of these reports have been based on short-term experiments. More long-term systematic appraisals of conservation tillage systems in northern China are clearly required (Gao et al. 2003). This paper reports the outcomes of a conservation tillage project funded by the Australian Centre for International Agricultural Research (ACIAR) and the Chinese Ministry of Agriculture since 1992 in Shanxi province, northern China. This 15-year project has systematically compared the impact of long-term conservation tillage (i.e. no-tillage, full residue retention) with that of conventional tillage (mouldboard plough, all residue removed), in terms of soil bulk density, soil aggregation, porosity, soil fertility, soil available water, winter wheat yield, and water use efficiency.

Materials and methods

Site description

The experiment was conducted in Chenghuang village (37[degrees]32'-38[degrees]6'N, 112[degrees]4'-113[degrees]26'E), near the city of Linfen, situated in south-central Shanxi province from 1992 to 2006. Linfen is located in a semi-arid and semi-humid region, 360-500m above sea level on the loess plateau. Average annual temperature is 10-12[degrees]C with 130 frost-free days. Annual rainfall, concentrated from June to September, is about 500 mm and annual evaporation is 1800 mm. Figure 1 shows the annual mean monthly rainfall and temperature during the study from 1992 to 2006. According to the FAO soil classification system, the soil at the experimental site is Chromic Cambisol (FAO/UNESCO 1993), and according to the USDA texture classification system, the soil type is defined as silt loam (sand 23.1%, silt 43.3%, clay 33.6%) derived from loess soils, low in organic matter (0.9%) in the 0-400 mm depth and slightly alkaline (pH 7.9). The soil is deep (the maximum depth amounts to 100m) and well developed (Li and Gong 2002), with a saturated hydraulic conductivity of 19mm/h and a field water holding capacity of 26% (gravimetric).

Winter wheat monoculture is common practice, providing an average yield of 2.05 t/ha before 1992 (Li et al. 1997), with sowing in September and harvesting in June.

Experimental design

Two tillage/straw treatments were evaluated: no tillage with all residue retained and standing stubble (NTSC); conventional tillage with complete residue removal (CT). NTSC normally consisted of no-till planting and fertilising in the last 10 days of September, spraying herbicide and insecticide in April, and harvest by combine harvester in the first 10 days of June (leaving 0.15-0.25-m-high standing wheat straw stubble). Chemical weed control was applied when necessary in the fallow period.

In the CT treatment all wheat straw was removed for fodder before ploughing to 0.20 m depth at the beginning of the fallow period, followed by tine tillage for seedbed preparation and manual broadcast of fertiliser before planting, normally in the last 10 days of September. Herbicide and insecticide spraying occurred in April, and harvest by combine harvester in the first 10 days of June.

[FIGURE 1 OMITTED]

The experiment was designed as a randomised block with 3 replications. Each plot was 9 m wide and 78 m long. Crop management followed local best practice using wheat variety Linfen 225 at a seeding rate of 225 kg/ha, and fertiliser applied to provide 150 kg N, 140 kg P, and 62 kg K (per ha in each case).

The 2BMF-11 no-till wheat planter developed by China Agricultural University (Fig. 2) was used with a 40 kW class tractor throughout the experiment. This machine used narrow-point openers and presswheels to place and firm seed and fertiliser at depths of 50 and 100mm, respectively. Residue clearance was maximised by mounting 5 openers on the front and 6 on the rear bar of the machine. For this experiment the machine was set to the 16-cm row spacing commonly used by local farmers, so operating width was 1.76 m.

Measurements

Rainfall

Rainfall was monitored throughout the experiment by a solar-powered automatic weather station (WeatherMaster[R] 2000-Environdata Pty Ltd, Qld, Australia).

Soil sampling and preparation

In August 2006, soil samples were collected from the plots of the 2 tillage/straw treatments. In each plot, one soil sample formed by 3 subsamples for aggregate stability and one soil sample consisting of 3 subsamples for organic matter, total N, and P determination were collected at 0-0.10 and 0.10-0.20 m depths, and one composite soil sample of 0-0.10 and 0.10-0.20 m depths, which was formed by 3 subsamples, was taken for soil porosity. Samples were taken using a trowel inserted into the soil at the lower level of each sampling depth to minimise compression and to obtain a representative sample of the soil.

Each soil sample was first passed through an 8-mm sieve by gently breaking apart the soil. Clods and aggregates > 8 mm were discarded, and the samples air-dried for 24 h in the laboratory before analysis.

[FIGURE 2 OMITTED]

Soil water stable aggregation

Soil water-stable aggregate distribution was determined by placing the soil sample on a nest of sieves, immersing directly in water, and agitating the sieves up and down 35 mm at 30 cycles/min for 15 min in water. Proportions of wet stable aggregates >2mm, 2-1 mm, 1-0.25 mm, and <0.25 mm were calculated, and micro-aggregates taken as those <0.25mm (Oades and Waters 1991). All the measurements were replicated 3 times.

Soil porosity

The mean pore effective diameter size was estimated at different moisture potentials based on a model of parallel cylindrical tubes using the equation:

d = 30/[[psi].sub.m] (1)

where d the equivalent pore diameter, is expressed in [micro]m, and [[psi].sub.m] is the absolute value of matric potential expressed in m of water. Hence, matric potential of -50 cm corresponds to pores of diameter 60 [micro]m. Soil porosity was classified as capillary porosity (<60 [micro]m) and aeration porosity (>60 [micro]m). Total porosity (TP) was calculated from bulk density and measured particle density (i.e. 2.6 mg/[m.sup.3]) (Sasal et al. 2006). All the measurements were replicated 3 times.

Soil organic matter, total N, and P

Organic matter (SOM) of air-dried soil samples was determined by dry combustion. Total N and P were determined using the Kjeldahl digestion method and HCl[O.sub.4]-[H.sub.2]S[O.sub.4] digestion methods, respectively. All the measurements were replicated 3 times.

Earthworms

In August 1992, 1998, and 2006, one sample was taken from each of the plots by excavating a block of soil (1 [m.sup.2] by 0.3 m deep) and fine hand-sieving to observe and count earthworms.

Bulk density

In each plot, 3 random soil samples were taken using a 54-mm-diameter steel core sampling tube, manually driven into 0.20 m depth. The soil cores were divided into 2 depths: 0-0.10 and 0.10-0.20 m, then weighed wet, dried at 105[degrees]C for 48 h, and weighed again to determine bulk density.

Yield and water use efficiency

Winter wheat yields were determined by manual harvesting, threshing, and air-drying grain from three 1-[m.sup.2] areas taken at random in each plot.

Evapotranspiration (ET) is usually calculated using the formula:

ET = (P + I + [S.sub.g]) - D - [R.sub.f] - [DELTA]W (2)

where P is growing seasonal rainfall (mm), I is irrigation (mm), [S.sub.g] is groundwater contribution to plant-available water (mm), D is downward drainage out of the root-zone (mm), [R.sub.f] is surface runoff (mm), and [DELTA]W is change of soil water content (mm).

In this experiment, I, [S.sub.g], and D were ignored because there was no irrigation, the groundwater contribution from a watertable 50 m below the surface was negligible, and drainage out of the root-zone need not be considered in this area (Huang et al. 2005). Surface water runoff ([R.sub.f]) is normally small under the conditions of this experiment (Kang et al. 2000), but might reasonably be regarded as a treatment effect. [DELTA]W was taken as the difference between the initial and final water content of the 1.00 m soil profile during the growing period.

Water use efficiency (WUE) was calculated as the winter wheat yield (kg/ha) divided by the growing-season evapotranspiration (mm):

WUE = Yield/ET (3)

Statistical analysis

The SPSS analytical software package was used for all statistical analyses. Mean values were calculated for each of the measurements, and ANOVA was used to assess the effects of conservation tillage on the measured variables. When this indicated a significant F-value (P < 0.05), multiple comparisons of annual mean values were made on the basis of the least significant difference (l.s.d.).

Results and discussions

Bulk density

Soil bulk density is a first approximation of potential changes in soil structure with improved management (Arshad et al. 1999). Bulk density to 0.20 m depth under both NTSC and CT was approximately 1.24 Mg/[m.sup.3] at the start of the experiment after harvesting in 1992, and mean treatment values found between then and 2006 are illustrated in Fig. 3.

Mean soil bulk density from 1993 to 2006 for NTSC and CT was 1.36 and 1.31 Mg/[m.sup.3], respectively, and NTSC showed slightly higher soil bulk density. During the first 6 years of this experiment (1993-1998), soil bulk density to 0.20 m depth was significantly less in the CT treatment (P < 0.05), demonstrating the increase in bulk density which occurred in the no-till treatments, probably caused by wheel traffic. From 1999 to 2004, however, mean soil bulk densities of NTSC and CT were similar (1.37 and 1.36 Mg/[m.sup.3]), and in 2005 and 2006, bulk density in NTSC was slightly less than that in CT. Bulk density of soil under NTSC, while initially greater than that of soil under CT, became similar after about 8 years, suggesting that the traffic effect on bulk density has been negated by other changes in soil condition, such as improved soil organic carbon, increased biotic activity, and improved structure (Karlen et al. 1994).

Water stable aggregates

Soil aggregation is an important variable influencing soil structure and soil erosion (Eldridge and Leys 2003). Table 1 illustrates treatment effects on aggregate wet stability in 2 size classes and for 2 treatments at 0-0.10 and 0.10-0.20 m depths.

Significant (P < 0.05) treatment differences can be seen in the size distribution of water-stable soil aggregates. In long-term no-till soil, the percentage of water-stable aggregates of the largest size class (>2 mm) was approximately twice that in ploughed soil in both 0-0.10 and 0.10-0.20 m depths. Similarly, the percentage of water-stable aggregates of the smallest size class (<0.25) was greater in ploughed soil. Macro-aggregates constituted 58.6% and 53.5% of 0-0.10 and 0.10-0.20 m depths, respectively, of no-till soil, compared with 45.1% and 47.4% for ploughed soil.

[FIGURE 3 OMITTED]

These data are consistent with the increase in aggregation occurring as a result of greater biological activity in no-till soil, demonstrated by Tisdall and Oades (1982), and with a reduction in breakdown of surface soil aggregates as a result of residue protection of the soil surface and the absence of tillage (Oyedele et al. 1999). These findings are similar to the results of Peixoto et al. (2006).

Soil porosity

Tillage usually increases total porosity (Roseberg and McCoy 1992), but there were no significant differences between the porosity of no-tillage and conventional ploughed plots after 15 years of continuous treatment. Mean aeration porosity was slightly greater in the ploughed treatment, and mean capillary porosity (Table 2) was slightly greater in the no-till treatment.

Soil organic matter, total N, and P

Soil organic matter, total N, and P were significantly (P < 0.05) different between treatments after 15 years (Table 3). Improvements in mean soil organic matter and fertility levels of both soil layers of no-till were the range 10-30%, with the exception of phosphates in the 0.10-0.20 m depth, which declined. Effects were larger in the upper soil layer. The results show that over the longer term, no-tillage with all residues retained is effective in improved soil organic matter, total N (Roldan et al. 2005), and P (Rhoton 2000) in the upper soil layer, compared with the plough and residue removed.

Soil health

Earthworm numbers are often regarded as an index of soil biological activity and health, and the soil biota are particularly important in no-till systems because of their ability to modify the soil physical environment and assist in nutrient cycling (Chan 2001). Mean data in Table 4 show that there were no earthworms in the experimental plots to the depth of 0.30 m at the start of the experiment (in 1992). Six years later, there were 5 earthworms/[m.sup.2] in no-till treatments, and by 2006 the mean population was 19 earthworms/[m.sup.2], while there were still none in the ploughed plots. This improvement in earthworm population under no till has been commented on by many authors (e.g. Mele and Carter 1999; Chan and Heenan 2006; Reeleder et al. 2006), and could be explained by more natural soil surface conditions (residue) and reduced soil disturbance.

Soil water storage

Table 5 shows the soil water storage (0-0.20 m) at the time of winter wheat planting for different tillage/straw treatments. The mean soil water storage (0-0.20 m) in CT plots from 1993 to 2006 was 35.9mm, while in NTSC plots it was higher, approximately 38.9 mm. In the dry years of 1998, 2000, and 2005, particularly, soil water storages in NTSC were 47.2, 38.0, and 28.5 mm, and in CT plots were 40.2, 29.6, and 24.4 mm, respectively, representing a mean improvement of 20.9% in the no-till treatment.

Winter wheat yield

Winter wheat yields in 2 tillage/straw treatments fluctuated widely from year to year (Table 6). Mean yield for no-till management was generally greater than that for conventional ploughing treatment, and yield differences between treatments were significant in 6 out of 14 years (P < 0.05). It is interesting to note that the mean yield advantage of no till was relatively small (9.2%) in the first 5 years of the experiment, but this increased to a mean value of 24.5% in the subsequent 9 years.

Regression analysis of yield and rainfall for each treatment demonstrated significant correlations (De Vita et al. 2007). This is illustrated in Fig. 4, which demonstrates the relatively small difference between treatments in the 4 wet years (rainfall >500mm), and much greater difference in the 10 dry years (rainfall <500mm). Considering these dry 10 years on their own, the mean yield of no-till winter wheat was 24.5% greater than that from ploughed plots and the maximum difference of 56.1% occurred in 2000 (328 mm annual rainfall).

[FIGURE 4 OMITTED]

The positive effect on crop yield of conservation tillage found in this study is consistent with the results of Radford et al. (1995) and Basamba et al. (2006). The significant contribution under conservation tillage management can be attributed to increased soil water storage, combined with the changes in soil bulk density, water stability of aggregates, capillary porosity, and fertility.

Water use efficiency

The WUE of no-till and conventional tillage ranged from 9.8 and 9.1 kg/ha x mm to 23.1 and 19.1 kg/ha x mm, respectively, and the WUE in no tillage was significantly higher than that in conventional tillage in 9 out of 14 years (at P=0.05). The maximum difference of 5.9 kg/ha x mm (23.1 v. 17.2 kg/ha x mm) occurred in the dry year of 2002 (417 mm annual rainfall).

These results demonstrated that, during the experimental period from 1993 to 2006, especially in the dry years, conservation tillage was considerably more efficient in converting available water into crop yield, whereas in ploughed plots, the poor soil structure caused by excessive tillage decreased soil water storage and winter wheat yield, thereby resulting in low water use efficiency.

Conclusions

Continuous long-term (15 years) conservation tillage practice in northern China provided evidence consistent with an improvement in soil structure and biological activity resulting from no tillage. This included significantly increased aggregate stability in the larger size classes, and greater capillary porosity (pores of diameter <60 [micro]m). There was no evidence of change in long-term bulk density (compared with traditional tillage) but improvements in the mean values of soil nutrients and soil water storage at 0-0.20m soil depth. Crop yield and WUE were 19.1% and 17.6% significantly greater for no-till than for ploughing practice, respectively. Our data demonstrated that conservation tillage was a significant improvement for current farming in dryland farming areas, and more long-term research on the relationships between conservation tillage, soil structure, productivity, and environment conditions is required in northern China.

Acknowledgments

This work was financed by Australian Centre for International Agricultural Research (ACIAR) and Ministry of Agriculture, China. We are grateful to Mr Deng Jing and Zhou Wanrong for managing the field experiment. Also thanks to all the postgraduate students working in Conservation Tillage Research Centre, MOA, who provided their input to this work.

Manuscript received 9 January 2007, accepted 5 June 2007

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Hongwen Li (A,B), Huanwen Gao (A), Hongdan Wu (A), Wenying Li (A), Xiaoyan Wang (A), and Jin He (A)

(A) Department of Agricultural Engineering, China Agricultural University, PO Box 46, Beijing 100083, China.

(B) Corresponding author. Email: lhwen@cau.edu.cn
Table 1. Soil wet stable aggregate size classes (mm) at 0-0.10
and 0.10-0.20 m depths (%) Values within a column in the same
depth followed by the same letters are not significantly
different (P < 0.05)

Soil Treatment Aggregate size classes
depth
(m) > 2 2-1 1-0.25 < 0.25

0-0.10 NTSC 16.0a 25.Oa 17.6a 41.4a
 CT 8.0b 21.Oa 16.1a 54.9b

0.10-0.20 NTSC 20.8a 27.0a 5.7a 46.5a
 CT 9.56 23.0a 14.9b 52.6a

Soil Macro- Micro-
depth aggregates aggregates
(m) > 0.25 < 0.25

0-0.10 58.6 41.4
 45.1 54.9

0.10-0.20 53.5 46.5
 47.4 52.6

Table 2. Soil porosity ([cm.sup.3]/100[cm.sup.3])
at 0-0.20m depth under NTSC and CT treatments

Values within a column followed by the same
letters are not significantly different
(P < 0.05)

 Aeration Capillary
Treatment Total porosity porosity
 porosity (>60[micro]m) (<60[micro]m)

NTSC 43.02a 34.31a 8.71a
CT 44.89a 36.45a 8.44a

Table 3. Soil organic matter (%), total N (g/kg), and
P (g/kg) under NTSC and CT treatments at 0-0.10 m and
0.10-0.20 m depths

Values within a column followed by the same
letters are not significantly different
(P < 0.05)

 Soil
Soil depth organic
(m) Treatment matter Total N Total P

0-0.10 NTSC 1.822a 0.668a 0.738a
 CT 1.356b 0.553b 0.645b

0.10-0.20 NTSC 1.202a 0.541a 0.608a
 CT 0.991b 0.415b 0.644b

Table 4. Mean earthworm population
(number/[m.sup.2]) under NTSC and CT
treatments to the depth of 0.30 m

Treatment 1992 1998 2006

NTSC 0 5 19
CT 0 0 0

Table 5. Soil water storage (mm) at winter wheat
planting time of NTSC and CT at 0-0.20 m soil depth

Values within a column followed by the same letters
are not significantly different (P < 0.05). Samples
were taken before planting during 1993 to 2006 at
Linfen. IR, Increasing ratio of NTSC to CT

Treatment 1993 1994 1995 1996

NTSC 30.0a 36.0a 28.3a 48.9a
CT 27.6a 37.2a 28.0a 49.2a
IR (%) 8.7 -3.2 1.1 -0.6

Treatment 1997 1998 1999 2000

NTSC 52.0a 47.2a 38.6a 38.0a
CT 50.0a 40.2b 35.8a 29.6b
IR (%) 4.0 17.4 7.8 28.4

 2001 2002 2003 2004

NTSC 40.0a 36.0a 37.6a 56.2a
CT 34.6b 34.2a 35.4a 52.4a
IR (%) 15.6 5.3 6.2 7.3

 2005 2006

NTSC 28.5a 27.5a
CT 24.4b 24.3b
IR (%) 16.8 13.1

Table 6. Winter wheat yields (kg/ha) and water use
efficiencies (kg/ha.mm) under NTSC and CT treatments
from 1993 to 2006

Values within a row followed by the same letters
are not significantly different (P < 0.05)

 [DELTA]W
 (mm) Yield WUE

Year P (mm) NTSC CK NTSC CK NTSC CK

1993 192.4 -74.1 -78.7 2985a 2548a 11.2a 9.4b
1994 199.4 -43.8 -44.7 3162a 3002a 13.0a 12.3a
1995 198.3 -58.1 -59.1 2513a 2342a 9.8a 9.1a
1996 137.4 -32.1 -43.5 3865a 3455a 22.8a 19.1b
1997 175.1 -79.0 -80.3 4142a 3908a 16.3a 15.3a
1998 209.3 -56.8 -35.3 3060a 2495b 11.5a 10.2a
1999 67.6 -77.7 -70.1 2644a 2148b 18.2a 15.6b
2000 76.1 -56.8 -43.6 2578a 1652b 19.4a 13.8b
2001 169.2 -56.5 -48.5 3814a 2917b 16.9a 13.4b
2002 137.3 -45.8 -37.1 4230a 3000b 23.1a 17.2b
2003 162.9 -39.9 -42.0 3975a 3360a 19.6a 16.4b
2004 269.5 -51.9 -50.3 5175a 4605a 16.1a 14.4b
2005 100.9 -86.4 -51.4 3765a 2650b 20.1a 17.46
2006 207.1 -48.1 -44.9 4696a 4410a 18.4a 17.5a
Mean 164.5 -57.6 -52.1 3614 3035 16.9 14.4
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Article Details
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Author:Li, Hongwen; Gao, Huanwen; Wu, Hongdan; Li, Wenying; Wang, Xiaoyan; He, Jin
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:9CHIN
Date:Aug 1, 2007
Words:5289
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