Sampling fine roots from mature trees for experimental analyses of physiological and morphological traits (e.g., Brundrett et al., 1990; Comas et al., 2002; Guo et al., 2004; King et al., 2005; Kosola et al., 2001, 2006; Pregitzer et al., 1997) or monitoring root health (e.g., Rizzo et al., 1998) is time-consuming. The most commonly characterized methods for collecting fine roots from the field involve taking soil cores or monoliths and subsequently washing the soil away from the roots. The method of washing influences the effectiveness of root recovery from soil cores; small-diameter roots are particularly prone to loss when roots are washed from soil cores onto a screen (Livesley et al., 1998; Pallant et al., 1993). Hydropneumatic root elutriation was developed to provide recovery of up to 95% of roots of all size classes from soil cores with minimal damage and good repeatability (Smucker et al., 1982). In this method, air is bubbled up through a flowing water column to carry roots and organic matter up to a capture screen.
High-velocity air excavation of roots is an alternative method for root sampling that allows relatively rapid isolation of roots from an individual tree; it is usually not feasible to identify the origin of roots taken from a soil core. The use of air excavation to expose structural roots is a well-established method in arboriculture to expose girdling roots and remove excess soil from root collars of trees planted too deeply (Smiley, 1999). This method has also been used in analyses of root-disease distribution in orchards (Rizzo et al., 1998) and for excavation of root systems to apply instruments to measure sap flow (Nadezhdina and Cermak, 2003). In air excavation, a stream of pressurized, high-speed air is applied to penetrate soil pores, leaving roots largely intact. Current tools provide about mach 3 air, if supplied with high-pressure air (6.21 × 105 Pa, 7.1 × 10−2 m3·s−1) that penetrates soil pores and shatters soil aggregates, while leaving nonporous objects, like roots, intact.
Fine-root recovery by air excavation of pear trees apparently caused the loss of roots < 0.5 mm diameter, but detailed descriptions of root damage were lacking (Rizzo and Gross, 2000). Does air excavation cause more damage than the standard method of hydropneumatic elutriation? In this paper, we examine fine-root samples from tree species with a range of root diameters sampled by either air excavation or hydropneumatic elutriation of soil cores. We compared root damage and root-diameter class distribution in fine-root samples collected by these two methods.
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