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Plant and Soil 238: 11–20, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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Microbiological and chemical properties of soil associated with macropores at different depths in a red-duplex soil in NSW Australia
C.E. Pankhurst1,4, A. Pierret2 , B.G. Hawke3 & J.M. Kirby2
1 CSIRO

Land & Water, PMB P.O. Aitkenvale, Townsville QLD 4814, Australia. 2 CSIRO Land & Water,GPO Box 1666, Canberra ACT 2601, Australia. 3 CSIRO Land & Water, PMB 2, Glen Osmond, Adelaide SA 5064, Australia. 4 Corresponding author∗
Received 17 January 2001. Accepted in revised form 30 July 2001

Key words: Biolog, PLFA, macropores, macropore sheath, microbial functional groups, roots

Abstract Some agricultural soils in South Eastern Australia with duplex profiles have subsoils withhigh bulk density, which may limit root penetration, water uptake and crop yield. In these soils, a large proportion (up to 80%) of plant roots maybe preferentially located within the macropores or in the soil within 1–10 mm of the macropores, a zone defined as the macropore sheath (MPS). The chemical and microbiological properties of MPS soil manually dissected from a 1–3 mm wide region surroundingthe macropores was compared with that of adjacent bulk soil (>10 mm from macropores) at 4 soil depths (0–20 cm, 20–40 cm, 40–60 cm and 60–80 cm). Compared to the bulk soil, the MPS soil had higher organic C, total N, bicarbonate-extractable P, Ca+ , Cu, Fe and Mn and supported higher populations of bacteria, fungi, actinomycetes, Pseudomonas spp., Bacillus spp., cellulolytic bacteria,cellulolytic fungi, nitrifying bacteria and the root pathogen Pythium. In addition, analysis of carbon substrate utilization patterns showed the microbial community associated with the MPS soil to have higher metabolic activity and greater functional diversity than the microbial community associated with the bulk soil at all soil depths. Phospholipid fatty acids associated with bacteria and fungi were alsoshown to be present in higher relative amounts in the MPS soil compared to the bulk soil. Whilst populations of microbial functional groups in the MPS and the bulk soil declined with increasing soil depth, the differentiation between the two soils in microbiological properties occurred at all soil depths. Soil aggregates (< 0.5 mm diameter) associated with plant roots located within macropores werefound to support a microbial community that was quantitatively and functionally different to that in the MPS soil and the bulk soil at all soil depths. The microbial community associated with these soil aggregates thus represented a third recognizable environment for plant roots and microorganisms in the subsoil. Introduction Many of the duplex soils under agricultural production in South EasternAustralia have subsoils with high bulk density (Turner et al., 1992). In these soils, crops appear unable to extract water that is potentially available in the subsoil because their roots are not evenly distributed throughout the soil matrix but are confined within or closely associated with macropores in the soil structure (Cresswell and Kirkegaard, 1995; Passioura, 1991; Pierret et al., 1999;Stewart et al., 1999).
∗ FAX No: +74-75-38600.

Up to 80% of the plant root system may be preferentially located within or closely associated with the macropores (within 1–10 mm), a zone defined as the macropore sheath (Stewart et al., 1999). This type of root distribution implies that nominally available water in the subsoil is poorly accessible and hence plants experience water stress earlierthan would be expected from measurement of bulk soil hydraulic properties (Pierret et al., 1999). Failure to access this water results in reduced crop production and the unused water may drain below the root zone and contribute to groundwater and hence exacerbate salinity problems in these soils.

E-mail: Clive.Pankhurst@tvl.clw.csiro.au

12 Subsoil macropores are formed by plant roots,...