Increasing irrigation efficiency is an important component of optimizing water use in containerized nursery operations. Grouping plants by water use requirements and using appropriate irrigation systems can aid in reducing container leachate (Bilderback, 2002). However, containerized plants have a limited substrate volume, often times requiring several irrigation events per day. Moreover, variability in substrate moisture within a container can be substantial, making characterization of available moisture for root uptake a challenge (Daniels et al., 2012). Given that water uptake by plant roots is the primary driver of soil moisture depletion in irrigated crop production systems (Clothier and Green, 1994), surprisingly few studies report how root system development affects water extraction patterns (Andreu et al., 1997; Coelho and Or, 1999; Daniels et al., 2012). Consequently, the influence of root development on containerized substrate moisture variation, which is known to influence root water uptake, is insufficiently described to inform irrigation applications (Beeson, 2007; Cannavo and Michel, 2013).
Not all root systems have the same architecture or are equally distributed throughout the soil profile (Fitter, 1987; Hodge et al., 2009). For example, numerous studies have examined root distribution in response to irrigation in field-grown crops (Andreu et al., 1997; Coelho and Or, 1999; Yu et al., 2007), concluding that it is common for root density to decrease with soil depth (Passioura, 2002). Recently, total root distribution, expressed as root biomass, has been shown to affect substrate heterogeneity and the retention of water within soilless container substrates (Cannavo and Michel, 2013). However, measuring root biomass alone does not take into account the changes in root foraging ability (Hodge, 2004). Root exploration within the soil volume is largely dictated by root systems architecture, including variables such as root length, depth, lateral root expansion, and branching (Hodge et al., 2009). Nonetheless, the perennial growth habit and sampling difficulties have resulted in few studies that investigate root distribution changes in containerized woody plant species.
The substrate moisture and root system matrix within containers is a heterogeneous volume that provides a means to compare species differences in root morphology. The objective of this study was to examine the relationship between root occupation within the soil volume and soil moisture sensor variability. We hypothesized that high variability in fine root density will result in high sensor variability. Additionally, we predicted that species with a high proportion of course roots would result in high sensor variability due to displacement of soil and water by the disproportionate large root mass. Understanding the effects of root growth and developmental patterns will have clear implications on how we water, fertilize, and manage our trees.
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