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

Study of root growth is important for understanding C flow through plants to the soil and for modeling plant–soil interactions. Fine root growth can be observed through minirhizotrons (MRs), tubes installed in the field. We compared the use of fixed-wall (F-wall) MRs, 5.6 cm in diam., and pressurized-wall (P-wall) MRs, 9.6 cm in diam., with an inner cylinder and an outer wall of plastic sheeting kept under constant air pressure. Root growth was measured with a micro-video camera for 3 yr in eight crop species growing on Pachic Haplustoll soil: canola, Brassica rapa L.; crambe, Crambe abysinnica Hochst. ex R.E. Fries; dry bean, Phaseolus vulgaris L.; dry pea, Pisum sativum L.; safflower, Carthamus tinctoris L.; spring wheat, Triticum aestivum L.; soybean, Glycine max (L.) Merr.; and sunflower, Helianthus anuus L. Midpoint depths of root length density profiles measured with P-wall MRs were on average 20% greater than those from F-wall MRs, and maximum rooting depths observed were 15% greater with P-wall than with F-wall MRs. Fixed-wall MRs were forced into tight access holes to establish soil contact while the expectedly more uniform interfaces of P-wall MRs apparently allowed more root penetration. Averages of total root lengths (TRL) over rootzones and of root length per MR area (for upper halves of rootzones) measured by F-wall MRs were 47 and 32% greater, respectively, than those measured by P-wall MRs. This is attributed to greater clarity and visibility in F-wall MRs compared with P-wall MRs. Significantly more effort is required to build and maintain P-wall MRs compared with F-wall MRs, but P-wall MRs could be considered for use in problem soils with swelling clay content, stoniness, or gravel. In soils with fine-textured, difficult subsoils, use of P-wall MRs to supplement F-wall MRs could increase the accuracy of root growth depth measures.