How to study deep roots-and why it matters

Abstract

The drivers underlying the development of deep root systems, whether genetic or environmental, are poorly understood but evidence has accumulated that deep rooting could be a more widespread and important trait among plants than commonly anticipated from their share of root biomass. Even though a distinct classification of "deep roots" is missing to date, deep roots provide important functions for individual plants such as nutrient and water uptake but can also shape plant communities by hydraulic lift (HL). Subterranean fauna and microbial communities are highly influenced by resources provided in the deep rhizosphere and deep roots can influence soil pedogenesis and carbon storage.Despite recent technological advances, the study of deep roots and their rhizosphere remains inherently time-consuming, technically demanding and costly, which explains why deep roots have yet to be given the attention they deserve. While state-of-the-art technologies are promising for laboratory studies involving relatively small soil volumes, they remain of limited use for the in situ observation of deep roots. Thus, basic techniques such as destructive sampling or observations at transparent interfaces with the soil (e.g., root windows) which have been known and used for decades to observe roots near the soil surface, must be adapted to the specific requirements of deep root observation. In this review, we successively address major physical, biogeochemical and ecological functions of deep roots to emphasize the significance of deep roots and to illustrate the yet limited knowledge. In the second part we describe the main methodological options to observe and measure deep roots, providing researchers interested in the field of deep root/rhizosphere studies with a comprehensive overview. Addressed methodologies are: excavations, trenches and soil coring approaches, minirhizotrons (MR), access shafts, caves and mines, and indirect approaches such as tracer-based techniques.

Keywords: biogeochemical and ecological functions; deep roots; root measure.

Figure 1

Summary of major impacts of deep roots on the subsoil and deep roots' functions, i.e., water uptake and hydraulic redistribution, nutrient uptake, physical–chemical weathering and C sequestration, and deep root-fauna and -microbial interactions. See text for further information.

Figure 2

(A–C) Illustrations of some direct field methods to access (deep) root systems. (A) Excavation, soil coring and soil trenching techniques. (B) Minirhizotron (MR) techniques with image acquisition devices (i.e., Digital Camera or Scanner MR) and different options to install the MR tubes, i.e., angled or vertical from the soil surface or horizontally from trenches. (C) Schematic view of the access shafts technique. Left: Location of the well in relation to a tree row (vertical projection). Right: Side view of the soil volume excavated for angled root window installation. See text for further information.

Figure 3

Root mapping and collection in a trench (4 m deep) in Thailand (Maeght, 2009).

Figure 4

Root sampling from an excavation (7 m deep) in Lao PDR (Maeght, 2009).

Figure 5

Root scanning in access shaft (5 m deep) in Lao PDR (Maeght, 2012).

Figure 6

Cave prospection (12m deep) for root studies in Lao PDR (Pierret, 2010).