Little is known about the root distribution pattern of different root diameter classes as affected by crop sequence. Barley roots were sampled using the monolith method from 45 to 155 cm soil depth at the time of anthesis (BBCH = 61) after growing taprooted chicory and fibrous-rooted tall fescue precrops for two consecutive years. Image-based analysis of the root-length density (RLD, cm cm(-3)) of four root diameter classes, viz., very fine ( 0.1 mm), fine (0.1-0.2 mm), medium (0.2-0.5 mm), and coarse roots (> 0.5 mm), was done by using the WinRHIZO Pro software. Proportional distribution of very fine, fine, medium, and coarse roots across the soil depth was 4.4, 33.6, 50.4, and 11.6 , respectively. In case of very fine and fine roots, the RLD of barley was significantly higher when grown after tall fescue (0.35 cm cm(-3)) than chicory (0.53 cm cm(-3)). In contrast to these results, barley roots sampled after the taprooted chicory resulted in a higher RLD of two upper root diameter classes (medium and coarse roots; 0.38 cm cm(-3)) in comparison to tall fescue (0.23 cm cm(-3)). Our data suggest the root architecture of precrops resulted first in different patterns of soil biopores and second in a different morphology of the root system of the subsequent crop. More precise root sampling inside and outside the pore channels might be helpful for direct comparisons of root morphological traits. Additionally, functional-structural plant models allowing the modeling of root growth in three dimensions will be helpful tools for understanding the interrelations between root-designed soil structure, root development plasticity, and nutrient uptake processes.
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