Comparison of Fixed-Wall and Pressurized-Wall Minirhizotrons for Fine Root Growth Measurements in Eight Crop Species

will be greatly enhanced by effective information about the dynamics of fine root growth and dependent Studyofroot growthisimportantforunderstandingC flowthrough rhizosphere biology. Another area of application for plants to the soil and for modeling plant-soil interactions. Fine root growthcanbeobservedthroughminirhizotrons(MRs),tubesinstalled fine root growth information is in soil and crop manage- in the field. We compared the use of fixed-wall (F-wall) MRs, 5.6 cm ment. For example, Merrill et al. (1996) have linked the in diam., and pressurized-wall (P-wall) MRs, 9.6 cm in diam., with effects of conservation tillage management on wheat an inner cylinder and an outer wall of plastic sheeting kept under growth through MR measurement of fine root growth. constant air pressure. Root growth was measured with a micro-video A general model of plant-soil interactions for under- camera for3 yr ineight crop speciesgrowing on PachicHaplustoll soil: standing the effects of crop-soil management on the canola,Brassica rapaL.; crambe,Crambe abysinnicaHochst. ex R.E. environment, the Root Zone Water Quality Model Fries; dry bean, Phaseolus vulgaris L.; dry pea, Pisum sativum L.; saf- (RZWQM; Hanson et al., 1999), critically depends on flower, Carthamus tinctoris L.; spring wheat, Triticum aestivum L.; information aboutthe soilprofile distributionand depth soybean, Glycine max (L.) Merr.; and sunflower, Helianthus anuus L. of effective (meaning fine) root growth. Midpoint depths of root length density profiles measured with P-wall MRs were on average 20% greater than those from F-wall MRs, and The needs for nondestructive imaging and measure- maximum rooting depths observed were 15% greater with P-wall than ment of active, fine root growth, which have been out- with F-wall MRs. Fixed-wall MRs were forced into tight access holes lined here, are being met with the MR system. The to establish soil contact while the expectedly more uniform interfaces development and use of MRs have been reviewed by ofP-wall MRsapparently allowedmore rootpenetration. Averagesof Taylor (1987) and Johnson et al. (2001) among others. total root lengths (TRL) over rootzones and of root length per MR A current MR system typically consists of clear, plastic area (for upper halves of rootzones) measured by F-wall MRs were tubes installed in field or greenhouse soil, a color micro- 47 and 32% greater, respectively, than those measured by P-wall video camera, a video tape recorder with a microphone MRs. This is attributed to greater clarity and visibility in F-wall MRs for audio annotation, and a video monitor (Johnson et compared with P-wall MRs. Significantly more effort is required to al., 2001). buildand maintainP-wallMRscompared withF-wallMRs, butP-wall MRs could be considered for use in problem soils with swelling clay Root systems are hierarchical structures, and a large content, stoniness, or gravel. In soils with fine-textured, difficult sub- fraction of the length of a plant's root system is com- soils, use of P-wall MRs to supplement F-wall MRs could increase prisedofhigher-orderrootbrancheswithdiametersthat the accuracy of root growth depth measures.