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Eight years of Distichlis spicata growth improves soil properties in a saline discharge zone

Mark Sargeant, Caixian Tang and Peter Sale

Department of Agricultural Sciences, La Trobe University, Bundoora Vic 3086, www.latrobe.edu.au/agriculture. Email mrsargeant@students.latrobe.edu.au

Abstract

Comprehensive soil sampling was conducted at Wickepin, Western Australia to determine if Distichlis spicata improves the soil chemical and physical properties while growing in a saline discharge zone. Preliminary results for the saturated hydraulic conductivity and water stable aggregates are presented in this paper. Results obtained so far indicate that significant improvements in saturated hydraulic conductivity and aggregate stability are achieved with the growth of D. spicata.

Key Words

Saturated hydraulic conductivity, Water stable aggregates, Soil fertility, Saltland pasture, Distichlis spicata, NyPa Forage, Halophyte.

Introduction

Saline discharge sites are generally seen as degraded environments, both in a chemical and physical sense. These sites contain high concentrations of salts within the soil, and are commonly waterlogged, which creates a very hostile environment for plant growth. Distichlis spicata (NyPa Forage) is a C4 halophytic pasture species that is able to grow productively in these environments and has been used on a trial basis for approximately 10 years at Wickepin, Western Australia. Over this time, the land manager has observed improvements in the soil appearance where D. spicata was planted. To test the hypothesis that D. spicata improves the soil chemical and physical properties, comprehensive soil sampling was undertaken at the site and the samples were subject to analysis. This paper reports preliminary results of this work.

Methods

Soil samples were collected from Wickepin, Western Australia in July 2005. The site had been established to D. spicata 8 years prior to sampling, with an adjacent area that was not planted for comparison. Soil samples were also taken from the spreading margin of the stand of D. spicata, where it was estimated that the D. spicata had been growing for 2 years. Intact soil cores were sampled at 0-6, 10-16 and 20-26 cm depth with 6 replicates to determine differences in saturated hydraulic conductivity and root density. Destructive soil samples were taken from 0-10, 10-20, 20-30 and 30-50 cm depth with 6 replicates to determine differences in water stable aggregates and soil chemical properties. This paper will focus on the results of saturated hydraulic conductivity and water stable aggregates.

The intact cores were stored in a cool room prior to saturated hydraulic conductivity measurements. The destructive soil samples were used to source aggregates of approximately 10 mm in diameter for water stable aggregate determination. Ten aggregates from four replicates of each treatment were randomly selected and wet sieved in distilled water at 34 rpm for a period of 5 minutes through 2, 1, 0.5 and 0.25 mm sieves. These sieves were then dried in an oven at 120 C for 1 hour and weighed to determine the mass of aggregates of different size. Soil texture was determined mechanically for each soil depth, and the results used to correct for the sand particles and gravel component. The texture of soil at the 0-10 and 10-20 cm depths was sandy loam, and sandy clay loam at 20-30 and 30-50 cm depths (Figure 1A).

Results

Eight years of D. spicata growth significantly increased the saturated hydraulic conductivity of the topsoil (Figure 1B). The greatest increase can be found after 8 years of growth, with an increase from approximately 0.5 cm/hr to 7.5 cm/hr in the top 0-6 cm. There were also increases with 8 years of D. spicata growth compared to the control soil at all depths, with the size of the increase decreasing with depth. Two years of growth of D. spicata resulted in a small increase in conductivity at the 0-6 cm depth compared to 8 years of D. spicata growth.

Figure 1. A) Contents of clay and silt in the soil at varying depths. B) The effect of growth duration of D. spicata growth on the saturated hydraulic conductivity of the soil at varying depths. Error bars are SE of the mean.

Eight years of D. spicata growth significantly improved the stability of the soil aggregates in the top 10 cm of the soil (Figure 2). Approximately 75 % of the aggregates remained larger than 2 mm in diameter after 5 minutes of wet sieving. This improvement in stability did not occur where D. spicata had been growing for 2 years. This improvement in aggregate stability was limited to the top 0-10 cm of soil, with no significant differences between D. spicata treatments for aggregate stability at the 10-20, 20-30 and 30-50 cm depths.

Figure 2. Percentage of water stable aggregates of the soil at varying depths after growing D. spicata for 0, 2 and 8 years. Error bars are the SE of the mean.

Conclusion

The results indicate that improvements in the soil physical properties occur where D. spicata is grown in saline discharge zones. Further analysis will be conducted to determine if this also extends to soil chemical properties.

Acknowledgements

This work was supported by the Australian Research Council, NyPa Australia Ltd, Department of Primary Industries Victoria, Buloke Park Pty Ltd and Elders Ltd.

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