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ARS Home » Plains Area » Mandan, North Dakota » Northern Great Plains Research Laboratory » Research » Publications at this Location » Publication #199147
Title: COMPARISON OF FIXED-WALL AND PRESSURIZED-WALL MINIRHIZOTRONS FOR FINE ROOT GROWTH MEASUREMENT IN EIGHT CROP SPECIES

Author
item MERRILL, STEPHEN - RETIRED - USDA-ARS
item Tanaka, Donald
item Hanson, Jonathan

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2005
Publication Date: 11/1/2005
Citation: Merrill, S.D., Tanaka, D.L., Hanson, J.D. 2005. Comparison of fixed-wall and pressurized-wall minirhizotrons for fine root growth measurement in eight crop species. Agronomy Journal 97:1367-1372.

Interpretive Summary: Understanding how carbon flows from the air through plants and through their roots and into the soil is a key part of the science of understanding the science of carbon sequestration. A scientific understanding of how crop root systems grow and where in the soil profile they are located is very important for integrated agricultural modeling. One of the currently best technologies for observing and quantifying root growth is the field minirhizotron (MR) system. Clear plastic observation tubes are placed in the soil and roots growing up against the walls are observed with miniature video cameras that magnify images of roots. MR’s are typically forced into holes in the soil sized to establish a tight fit between wall and soil. There are often visibility problems with soil separating from the wall, with soil smearing of the wall in high clay soils, and with rocks and pebbles. The pressurized-wall MR’s (P-wall MR) was designed to overcome these problems by having a rigid-wall inner cylinder and an outer wall of plastic sheeting kept under constant air pressure after installation. We compared the performance of P-wall MR’s with standard, rigid-wall minirhizotrons (Stand MR’s) in an alternative crops study with 8 species (canola, crambe, dry bean, dry pea, safflower, spring wheat, soybean, and sunflower). Fine roots (often 1/100 inch or less in diameter) were often more visible in Stand MR’s as was apparent from the greater total root lengths measured in Stand MR’s compared to P-wall MR’s. However, P-wall MR’s did a better job than Stand MR’s for accurately observing the soil depths of root growth. We have recommended that researchers use P-wall MR’s in situations involving problem soils (high clay, stony, etc.) or where accurately observing rooting depth is a priority. Stand MR’s are less time- and money-expensive and can be used in other situations.

Technical Abstract: Study of root growth dynamics is important for understanding carbon flow through plants to the soil and for the modeling of plant-soil interactions. The dynamics of fine root growth can be observed in tubes (minirhizotrons, MR), which are installed in the field and are read with a miniature video camera. We compared the use of two types of MR devices, standard MR’s (Stand MR), 5.6 cm in diameter with rigid plastic walls, and pressurized-wall MR’s (P-wall MR), 9.6 cm in diameter, with a rigid-wall inner cylinder and an outer wall of plastic sheeting kept under constant air pressure after installation. Root growth measurements were conducted on eight crop species (canola, Brassica rapa; 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.) over a period of three years on Pachic Haplustoll soils. Median depths (half of root length above) of root growth observed with P-wall MR’s were on average 26% greater than those from Stand MR’s, and maximum rooting depths from P-wall MR’s were 19% greater than from Stand MR’s. Stand MR’s must be forced into the soil to establish soil-wall contact, and the more uniform soil-wall interface established by P-wall MR’s appear to underlie these results. Total root lengths (TRL) measured by Stand MR’s were greater than those measured by P-wall MR’s in over 70% of cases. This is attributed to greater clarity and visibility afforded by Stand MR’s compared to P-wall MR’s. Stand MR’s are recommended for use where soil properties are not Aa limitation and observation of the dynamics of overall root growth and structures is a priority. P-wall MR’s require significantly more effort to build and maintain than Stand MR’s, but should be considered for use in situations of problem soils (higher clay content, stones, shrink-swell potential) and where the accuracy of observing root growth depths is a priority.