A Modified Soil Monolith Technique for Characterizing Root Systems

Abstract

A soil and root system sampling technique that accurately measures root distribution within the soil profile without causing excessive damage to experimental plots would improve the efficiency of root system research. A monolith mapping root sampling technique is described that combines the positive attributes of the soil monolith and profile wall methods with a less destructive hand tool sampling protocol. The objective of this field study was to compare monolith mapping with a standard monolith washing technique for the purpose of measuring root distribution in the upper 30 cm of the root zone. Root systems of maize (Zea mays L.) (V6 stage of development) from plots (Vienna loams oils; fine‐loamy, mixed Udic Haploborolls) treated with broadcast or banded P were sampled using the modified monolith method. The roots present in the soil monolith were mapped (monolith mapping method), and the maps used to represent two‐dimensional root distribution. The soil monolith was then grid‐sectioned into 5.1 by 5.1 by 7.6 cm rectangular blocks and washed to separate roots from the soil. Root length was then measured using a line‐intersect method (monolith washing method). Monolith mapping and monolith washing methods both detected differences in root system distribution in the soil profile. A significant linear relationship (r = 0.87) between root length density and root number as measured by the two methods allows conversion of the numerical data collected by the monolith mapping method into root length density values. We conclude that monolith mapping works well for determining the spatial differences in root distribution in the upper 30 cm of the root zone. The relative time requirements, accuracy, and less destructive nature of the technique (compared with the trench profile and framed monolith methods) result in more efficient collection of data on root system characteristics.

Spatial distribution of maize roots by complete 3D soil monolith sampling

“…Methods for characterizing the spatial distribution of root systems in soil (Böhm 1979) include root mapping (Pagès and Pellerin 1996), minirhizotron observations (Liedgens and Richner 2001), auger or core sampling (Chassot et al 2001;Sharratt and McWilliams 2005), and soil monolith excavation (Buman et al 1994;Gajri et al 1994;Li et al 2006). Most of the experimental studies considered 1D root distribution in vertical direction (i.e., soil depth) and 1D/2D in horizontal direction (i.e., perpendicular to plant rows), and soil water flow and crop growth models frequently assume simplified 1D vertical (e.g., Jones and Kiniry 1986;Jansson and Karlberg 2004) or 2D vertical RLD distributions (e.g., Heinen et al 2003;Simunek et al 1999).…”

“…Previous monolith-type field samplings were restricted mostly to 2D vertical soil slices normal to the direction of plant rows (Buman et al 1994;Gajri et al 1994;Li et al 2006). This restriction was mainly based on the prevailing simplifying assumption that variability parallel to the plant rows is negligible.…”

Root growth of <i>Lotus corniculatus</i> interacts with P distribution in young sandy soil

“…The climate chamber experiment, where all other heterogeneities had been evened out by soil homogenization, confirmed that preferential allocation of root growth is indeed a response of L. corniculatus that can be induced by heterogeneous P distribution. The ability to respond to locally increased P availability with enhanced root proliferation has been shown also for many other plant species (Ma and Rengel, 2008;Ma et al, 2007;Kume et al, 2006;Robinson, 1994;Weligama et al, 2007;Denton et al, 2006) in climate chamber experiments, but seldom in the field (Eissenstat and Caldwell, 1988;Buman et al, 1994).…”

“…Indeed, we found a increase in labile P in the rootdistant soil with distance from stems on the CCC. Most authors investigating herbaceous plants or grasses found that root length density decreased with distance from stem after one growing season (Buman et al, 1994;Majdi et al, 1992;Milchunas et al, 1992). But they did not study perennial growth.…”

“…Laboratory and greenhouse experiments with constructed heterogeneities and/or split root systems have shown that localized P supply can induce preferential root proliferation in many plant species (Kume et al, 2006;Ma and Rengel, 2008;Ma et al, 2007;Robinson, 1994;Weligama et al, 2007;Denton et al, 2006). Some authors also studied preferential root growth in response to localized P fertilization in the field (Eissenstat and Caldwell, 1988;Buman et al, 1994;Caldwell et al, 1996). In studies with artificially created heterogeneity, the contrast in P concentrations between fertilized and non-fertilized soil patches was usually high.…”