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Dryland cropping in central Queensland impacts on Soil Chemical Parameters.

Stuart Buck1 and Michael Braunack2

1 DPI&F, State Farm Rd, Biloela, QLD 4715. www.dpi.qld.gov.au Email stuart.buck@dpi.qld.gov.au
2
DPI&F, Hospital Rd, Emerald QLD 4720. www.dpi.qld.gov.au. Email michael.braunack@dpi.qld.gov.au

Abstract

When the Central Queensland Sustainable Farming Systems project commenced in 1997, soils in on-farm trials were characterised to provide a benchmark to assess change over time. At three on-farm trials, soils were also sampled from adjoining undisturbed areas (paired sites) to measure nutrient condition in a natural state. Samples were collected from two soils locally known as ‘Brigalow’ and ‘Open Downs’ but only soil chemical parameters were considered, restricting the assessment to changes in soil nutrition factors.

Management practices (tillage, traffic, crops grown) at each site have been relatively constant, and re-sampling (pH1:5 water, electrical conductivity1:5, chloride, nitrate-N, bicarbonate extractable P, exchangable cations, organic carbon (Walkley Black), sulphate-S and trace elements (DPTA Cu, Zn, Mn, Fe)) at the same depth intervals took place again in mid 2007.

Changes in organic carbon, phosphorus and sulphate-sulfur in the 0-10 cm layer occurred across all sites over the ten year period. Decreases in organic carbon were most evident at sites where initial organic carbon levels were highest. All sites recorded a decreasing trend in phosphorus levels (0-10cm), but significant changes were evident at only five sites. On the Open Downs sites, sulphate-sulfur levels remained static whereas small yet significant increases were measured at many of the Brigalow sites. Trace elements changed very little between sampling times.

This study indicates important depletion of many nutrients under the current farming system of zero tillage, controlled traffic, ‘opportunity’ cropping and fertiliser management. While indicating significant soil fertility rundown, the small number of samples collected and the short time frame of this study precludes an accurate assessment of the sustainability of the farming systems. As a baseline has been established to enable assessment in the future, further research is required to determine mechanisms to maintain, or improve soil fertility levels in central Queensland dryland cropping systems.

Key words

pH, soil carbon, nitrogen, sulfur, trace elements, soil testing, farming systems

Introduction

When the Central Queensland Sustainable Farming Systems project (CQSFS) commenced in 1997, soil samples were taken to characterise physical and chemical properties of each trial site and map soil variation, to determine site suitability for the intended trial program. This involved full soil characterisation, including soil chemical (eg pH, EC, plant nutrients), physical (eg clay content, water holding capacity) and morphology (eg soil description, horizon layers) assessments. This information has also been used to assist with the interpretation of trial results, particularly when fertiliser treatments have been imposed.

Surface soil samples were collected at initial site characterisation in 1997/1998 to provide a benchmark to assess change in soil parameters over time. At three sites samples were also collected from adjoining undisturbed sites to measure nutrient condition from a virgin state to estimate the impact of the dryland cropping system employed. The intention was not to develop sustainability indicators for the newly evolving farming systems, but to provide a measure of change in the measured parameters over time. However these parameters could be used as indicators and have been by many organisations. A comprehensive list of indicators for the grains industry has been provided by Dalal et al (1998).

Methods

The original characterisation (1997/98) of each study area consisted of one to three profile samples (0-150 cm) and up to 25 surface samples (0-10 cm) depending on the site area. Also, at sites 1, 6 and 11, samples were also taken in native vegetation adjacent to the study area. Sites that are currently used for grain production were re-sampled mid 2007 using the same techniques as the original sampling, and the site report maps and GPS coordinates recorded in 1997/98 were used to locate the original sampling points. However the poor accuracy of the GPS coordinates recorded in 1997/98 meant locations were determined from the site report maps. The sampling protocol for each site was: at 5 locations, 5 sub-samples were collected and bulked from 0-10cm samples near where the original samples were taken. For the soil parameters, pH, organic carbon, total nitrogen, EC, chloride, nitrate-N, phosphorus, exchangeable cations, sulphate-sulfur and trace elements were analysed using methods described in Rayment and Higginson (1992).

Management practices between 1997 and 2007 have remained constant, where continuous cropping has been with zero tillage. Site 3 has been managed using a non-controlled traffic minimal tillage system, where 2 cultivations occurred up to sowing and stubbles were grazed. Site 5 was minimal till until 2002, there after zero till has been utilised. Table 1 summarises the soil type and management practices used at each site.

Table 1. Location, soil type and management of sites originally characterised and re-sampled in 2007.

Site

number

Location

Vegetation

Soil type1

Local Soil name

Management practices2

1

Jambin

Brigalow scrub

Brown or red Dermosol

Brigalow

ZT, CTF

2

Baralaba

Brigalow/Bauhinia

Self-mulch black Vertosol

Brigalow

ZT, CTF

3

Dysart

Brigalow

Self-mulch brown Vertosol

Brigalow

MT

4

Wowan

Brigalow

Self-mulch brown Vertosol

Brigalow

ZT, CTF

5

Theodore

Brigalow/ironbark

Self-mulch black Vertosol

Brigalow

MT then ZT, CTF

6

Gindie

Brigalow scrub

Self-mulch black Vertosol

Brigalow

ZT, CTF

7

Gindie

Brigalow scrub

Self-mulch brown Vertosol

Brigalow

ZT, CTF

8

Theodore

Brigalow/ironbark

Self-mulch black Vertosol

Brigalow

ZT, CTF

9

Fernlees

Open Downs

Self-mulch black Vertosol

Open Downs

ZT, CTF

10

Capella

Open Downs

Self-mulch black Vertosol

Open Downs

ZT, CTF

11

Capella

Open Downs

Self-mulch black Vertosol

Open Downs

ZT, CTF

1 Australian Soil Classification (Isbell, 1996)
2
Zero till (ZT); minimal till (MT); controlled traffic farming (CTF).

Statistical analysis

To look at differences between sampling times within sites, the mean and standard error of the difference was compared with the appropriate t-value to test if there is evidence that the change at the site was different from zero. To assess changes in parameters (eg organic carbon) between sites, the value for 1997-98 was subtracted from the 2007 value and tested by analysis of variance with soil as the treatment and sample within site as the block structure. Statistical analysis was only possible for samples collected from the 0-10cm layer.

Results

Soil types

Similar mean changes for each attribute occurred across the two main soil types (Brigalow v Open Downs). For example organic carbon in both soils decreased by a similar amount, even though Brigalow soils had on average higher organic carbon levels (data not shown).

Differences between sampling times within each site

Of the Brigalow soils only one showed a significant change (increase of 0.7) in pH (Table 2). Almost all sites (7 out of 8) showed decreases in chloride levels, with a mean decrease of 27 mg/kg (range 12 – 36mg/kg). Organic carbon significantly decreased at most sites (6 out of 8), with a mean decrease of 0.42% (range 0.13 – 0.86%) (Table 2). Organic carbon levels averaged 1.2% when measured in 1997-99, resulting in an average reduction of 35% to 2007. Total nitrogen levels didn’t follow the organic carbon trend, with only one site recording a significant drop (Table 2). Half of the Brigalow soils recorded an increase in sulphate-sulfur, however the increase was modest (mean 3.15 mg/kg (Table 2). Importantly two sites recorded large decreases (sites 6 & 7), with reductions of 33.5 kg/kg and 26.5 mg/kg of sulphate-sulfur respectively. Iron (Fe) levels significantly increased at three of the Brigalow soil sites, and at two of these the mean increase was dramatic (85 mg/kg and 67 mg/kg) (Table 2).

Table 2a. Change in soil attributes (0-10cm) from 1997 to 2007 for each site. Means highlighted in yellow are significantly different from zero so indicate an increase (+) / decrease (-) in the attribute over the period (P=0.05)

Soil

Site

pH ( se)

Cl ( se)

P ( se)

*OC ( se)

*TN ( se)

*SO4-S ( se)

   

-

mg/kg

mg/kg

%

%

mg/kg

Brigalow

1

-0.2

(0.36)

-12

(2)

-38

(11)

-0.24

(0.05)

-0.03

(0.01)

3.6

(1.2)

Brigalow

2

0.4

(0.13)

-13

(2)

-8

(4)

-0.10

(0.05)

0.00

(0.01)

4.2

(0.7)

Brigalow

3

0.3

(0.07)

-32

(2)

-13

(6)

-0.61

(0.14)

-0.01

(0.01)

-1.5

(0.8)

Brigalow

4

0.0

(0.25)

-12

(2)

-3

(9)

-0.25

(0.06)

-0.02

(0.01)

3.0

(0.5)

Brigalow

5

0.3

(0.07)

10

(3)

2

(2)

-0.13

(0.03)

-0.01

(0.00)

1.8

(0.2)

Brigalow

6

0.7

(0.03)

-20

(0)

-26

(6)

-0.86

(0.04)

-0.02

(0.01)

-33.5

(0.4)

Brigalow

7

0.4

(0.40)

-13

(3)

-25

(6)

-0.43

(0.00)

0.00

(0.01)

-26.5

(2.0)

Brigalow

8

0.7

(0.17)

-36

(6)

-11

(5)

-0.02

(0.04)

0.01

(0.00)

-2.0

(0.6)

Open downs

9

-0.2

(0.04)

   

-7

(2)

-0.37

(0.06)

-0.01

(0.00)

   

Open downs

10

0.2

(0.12)

-22

(3)

-23

(4)

-0.42

(0.11)

-0.01

(0.00)

-0.7

(0.4)

Open downs

11

-0.1

(0.08)

-18

(2)

-8

(2)

-0.08

(0.07)

-0.01

(0.00)

1.0

(0.4)

*OC-Organic cargon, TN-total nitrogen, SO4-S- sulphate-sulfur

Table 2b. Change in soil attributes (0-10cm) from 1997 to 2007 for each site. Means highlighted in yellow are significantly different from zero so indicate an increase (+) / decrease (-) in the attribute over the period (P=0.05)

Soil

Site

K ( se)

Cu ( se)

Zn ( se)

Mn ( se)

Fe ( se)

   

meq/100g

mg/kg

mg/kg

mg/kg

mg/kg

Brigalow

1

-0.17

(0.53)

-0.02

(0.16)

-0.20

(0.28)

63.8

(24.8)

85.2

(11.3)

Brigalow

2

0.06

(0.08)

0.44

(0.10)

0.16

(0.11)

4.7

(2.8)

8.8

(1.5)

Brigalow

3

0.07

(0.11)

0.16

(0.10)

0.03

(0.08)

1.1

(0.9)

1.0

(0.8)

Brigalow

4

   

0.44

(0.19)

0.18

(0.14)

42.5

(21.1)

67.6

(2.8)

Brigalow

5

   

-0.03

(0.05)

0.35

(0.21)

4.1

(3.1)

0.7

(1.4)

Brigalow

6

-0.36

(0.06)

-0.60

(0.20)

-0.25

(0.05)

-25.5

(8.4)

-17.9

(0.9)

Brigalow

7

-0.33

(0.11)

-0.45

(0.55)

-0.25

(0.05)

4.6

(6.8)

3.8

(13.3)

Brigalow

8

0.00

(0.12)

0.20

(0.09)

0.24

(0.09)

-3.5

(1.7)

0.7

(0.8)

Open downs

9

   

0.06

(0.07)

-0.06

(0.06)

-11.8

(1.9)

   

Open downs

10

-0.35

(0.09)

0.03

(0.02)

-0.09

(0.04)

2.0

(1.4)

4.1

(0.9)

Open downs

11

-0.06

(0.05)

0.09

(0.03)

0.07

(0.08)

8.1

(1.7)

7.1

(0.3)

Similar results were obtained for the Open Downs soils. Chloride levels dropped an average of 20 mg/kg (range 18 – 22 mg/kg) and organic carbon dropped by an average of 0.4% (range 0.37 – 0.42%) (Table 2). Organic carbon levels averaged 0.9% when measured in 1997-99, resulting in a 44% decrease to 2007. Even though organic carbon levels dropped by 44%, total nitrogen levels only dropped by an average of 0.01%, equating to 20% lower in 2007. Phosphorus levels also dropped between the sampling periods, with a mean drop of 12.6mg/kg (range 7 – 23mg/kg) (Table 2). Like the Brigalow soils iron (Fe) levels also increased, with an average increase of 7.3mg/kg (range 4.1 – 10.6mg/kg), resulting in an 89% increase over 1997-99 levels (Table 2). Unlike the Brigalow, pH decreased (-0.4) on 2 Open Downs soils (Table 2).

Discussion

Monitoring of on-farm trials measured changes in surface (0-10cm) soil fertility over time, particularly decreases in organic carbon across both Brigalow and Open Downs soil types. In results comparable to other studies in central Queensland (Radford et al 2007; Millar and Armstrong unpublished data), it is apparent that the cropping practices implemented on these sites have had reduced organic carbon, and zero tillage in particular has not increased organic carbon levels as it has in southern Queensland (Dalal 1989). In semi-arid environments such as central Queensland carbon inputs (via plant material) are limited by rainfall, hence carbon outputs (organic matter decomposition by soil biota) are greater than the inputs (Dalal et. al. 2004).

All soils recorded a decrease in phosphorus levels, but significant changes were evident at only five of these sites. Where significant changes occurred, the change was large (generally more than 20mg/kg) indicating removal rates are much higher than replacement via fertilisers. This has important implications regarding the assessment of how much to apply. However, increased awareness of phosphorus status is needed as a matter of urgency. In general, trace elements changed very little between sample times, most likely due to low removal rates. However this doesn’t mean these nutrients should be ignored; on some sites, zinc levels are at or near deficient levels.

Changes in sulphate-sulfur levels were either static (open downs) or generally small (brigalow), hence are of little practical importance. However two brigalow soils (sites 6 & 7) recorded significant reductions in sulphate-sulfur, most likely due to crop removal and/or leaching into to deeper soil layers. Likewise, small but significant decreases have been recorded in soil salinity (chloride) levels in both soil types; however changes are of little practical importance. Of particular interest was the decrease in pH on 2 open downs soils. As these soils are strongly alkaline (8-9) a decrease in pH would improve the plant availability of a number of nutrients, leading to a positive impact.

Conclusion

Between the sampling periods changes in soil fertility parameters (organic carbon, phosphorus, sulphate-sulfur, trace elements) have been measured in the surface soil layer (0-10 cm) across many sites. During this period, rainfall and hence cropping opportunities have varied, which has affected nutrient inputs (via fertiliser, stubble, organic matter mineralisation) and removal (grain harvested). Some of the parameters measured in this study have been used as sustainability indicators, however the small number of samples collected, the variability measured and short time frame of this study preclude any assessment of the sustainability of the farming systems employed at these sites. However under the current dryland cropping system of zero till, controlled traffic and nutrient management the issue of diminishing organic carbon and other nutrients are not sustainable in the long term, and need to be addressed as a matter of urgency.

References

Dalal RC (1989). Long term effects of no tillage, crop residue, and nitrogen application on properties of a vertisol. Soil Science Society of America Journal 53, 1511 – 1515.

Dalal RC, Wang W, Mann S and Henry B. (2004). Soil organic matter decline and restoration: principles and practices. Natural Resource Management (Vol. 7)(No. 2) 2-15.

Dalal R C, Walker J, Shaw R J, Lawrence G, Yule D, Lawrence P, Doughton JA, Bourne A, Duivenvoorden L, Choy S, Moloney D, Turner L, King C, Dale A, Srivastrava K, Noble R, Ross A, Clarke P, Cummins V, Johnston D, Hunter H, Carroll C, Lockie S and Mullins J (1998). Monitoring sustainability in the grains industry: A Central Queensland pilot study. Department of Natural Resources, Queensland, DNRQ980073, 63 p.

Isbell RF (1996). The Australian Soil Classification. CSIRO.

Radford BJ, Thornton CM, Cowie BA and Stephens ML (2007). The Brigalow Catchment Study: III. Productivity changes on brigalow land cleared for long-term cropping and for grazing. Australian Journal of Soil Research 45, 512-523.

Rayment GE and Higginson FR. (1992). Australian Laboratory Handbook of Soil and Water Chemical Methods, Inkata Press, Australia.

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