Authors: Gwenzi, W; Veneklaas, EJ; Phillips, I; Bleby, TM; Hinz, C


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Gwenzi, W, Veneklaas, EJ, Phillips, I, Bleby, TM & Hinz, C 2009, 'Spatial distribution of fine roots on a rehabilitated bauxite residue disposal area in Western Australia', in AB Fourie & M Tibbett (eds), Mine Closure 2009: Proceedings of the Fourth International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 317-327,

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Root spatial distribution controls water and nutrient uptake, and is a key input in ecohydrological and biogeochemical models. In particular, fine roots are responsible for resource acquisition and represent the most dynamic component of root biomass. While root distribution on natural and agro-ecosystems is relatively well documented, few studies have investigated root spatial distribution on rehabilitated mined ecosystems, where adverse physical and chemical conditions may limit root growth. The authors investigated the spatial distribution of fine roots (< 4.5 mm diameter) on a rehabilitated bauxite residue disposal area under Mediterranean conditions. The objectives of this study were: to determine the vertical and horizontal spatial variability of fine roots at plot scale; to develop a spatial model for root distribution for use in ecohydrological modelling and other numerical applications; and to compare observed results to those reported in literature for natural ecosystems under similar climatic conditions. A 20 × 20 cm grid sampling scheme was used to collect 226 core samples (10 cm diameter and 10 cm height) from a 700 cm long and 150 cm deep trench. Samples were analysed for root length (L), root diameter (D), root length density (RLD) and root biomass density (RBD). Soil dry bulk density, pH and electrical conductivity (EC) were used as indicators of soil physical and chemical constraints to root growth. Root characteristics showed high spatial variability with coefficient of variation (CV) ranging from 51–200%. The top 20 cm had the highest mean RLD (8.4 cm cm-3) and root mass density (RMD) (1 g cm-3) which decreased with depth according to a power function (RLD = 2474 × (SD)-1.8, r2 = 0.92). About 80% of the total root length and biomass were in the top 40 cm, while the remainder was in the deeper layers (40– 140 cm). At all depths, very fine roots (≤ 1.5 mm in diameter) constituted about 95% of the total root length, suggesting a root system adapted for water uptake in the dry season when soil moisture is limited. Soil EC values were generally low (mean 1.1 dS m-1), but showed high spatial variability (CV = 91%) probably due to non-uniform incorporation of chemical amendments. For 19 out of the 24 samples, EC values were below 2.5 dS m-1, considered the upper limit for normal plant growth. Soil pH was slightly alkaline (mean of 8.2), and showed low spatial variability (CV = 4%). In all cases, bulk densities (mean = 1.3 g cm-3) were below the critical value for restricted root growth (1.6 g cm-3). Correlation analysis suggested that root distribution was not limited by soil dry bulk density, pH and EC. Accordingly, the depth distribution of cumulative RLD and RMD closely agreed (r2 = 0.93) with the general root depth distribution models for natural vegetation ecosystems in Mediterranean climates. The results are discussed in the context of vegetation water uptake on rehabilitated mined ecosystems.

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