Perspectives from the Fritz‐Scheffer Awardee 2020—The mutual interactions between roots and soil structure and how these affect rhizosphere processes^#

Abstract

Roots growing into soil interact with soil structure in numerous ways. They can grow into the soil matrix and leave elongated macropores after decomposition, that is, biopores. Conversely, the soil may already have a large and connected macropore system through which the roots can expand, and thus reach deeper soil layers. Both, the formation of new biopores or the reuse and occupation of existing macropore systems are expected to affect major soil processes like water and gas flow through the soil profile, as well as water flow toward the root. Despite the increasing research interest in the limitation of root growth by soil structure, as well as the modification of soil structure by roots, the mutual interactions between the two are largely overlooked. This study highlights new methodological developments which enable describing interactions between roots and soil structure with X‐ray computed microtomography. It further shows how roots affect the pore system and can create a massive biopore system in less than a decade. After this, it is evaluated how the mutual interaction of roots and structure determines the physical properties of the rhizosphere. It is outlined that this has implications for major rhizosphere processes. Thus, it is emphasized, that the role of structure needs to be considered in future experiments with plants, in particular if extrapolation of results from laboratory experiment with sieved, homogenized substrates to field conditions with well‐established soil structure is intended. Lastly, in this study, research gaps are outlined remaining in respect to the dynamics of biopore creation and destruction and their consequences for processes in rhizospheres like carbon storage. These reveal the need for novel research approaches that consider the mutual interactions of root growth and soil structure.

Cover crop influence on pore size distribution and biopore dynamics: Enumerating root and soil faunal effects

“…After roots exploring the soil matrix die and decay, they leave behind the biopores of comparable sizes ( Lucas, 2022 ). Indeed, we found differences among the studied plant species in terms of <0.2 mm and 0.2–0.5 mm Ø pores and biopores, the sizes that match well the prevailing root sizes of the investigated cover crops ( Figures 1 – 3 ).…”

“…For example, after the first root growth period, the OA soil samples contained the largest pore volumes of <0.5 mm Ø pores, while the corresponding root and biopore volumes were the lowest among the five cover crops observed. In addition, similar to earthworms creating macropores resulting in the loss of smaller pores, roots also compact their surrounding and therefore potentially reduce the volumes of pores of certain size classes ( Bruand et al, 1996 ; Lucas et al, 2019a ; Phalempin et al, 2021b ; Lucas, 2022 ). Indeed, roots <0.5 mm in diameter and total root volume and biopore volumes of all size classes correlated negatively with <0.2 mm Ø pores ( Figure 4 ).…”

“…However, roots only create new biopores and potentially compact their surroundings, when they grow into the dense soil matrix ( Jin et al, 2017 ; Lucas, 2022 ). Indeed, the volume of roots growing in the dense matrix was positively correlated with pores between 0.5–1 mm, while negatively with pores <0.2 mm Ø.…”

“…Plant roots are the main modifiers of the pore structure, affecting it through a variety of mechanisms, including direct creation/modification of soil pores, increases in soil organic matter (SOM), exudation of mucilage, and water uptake ( Gregory, 2022 ). A growing root interacts with pore structure in several ways: (1) roots can elongate into the soil matrix containing only the pores smaller than the root diameter (2) they can grow along an existing pores including biopores and (3) they can negotiate an existing pore at some angle before penetrating the soil ( Jin et al, 2017 ; Lucas, 2022 ). During their growth, roots overcome soils penetration resistance and compress exiting pores creating new biopores ( Dexter, 1987 ; Lucas et al, 2019a ).…”

Root and root‐derived biopore interactions in soils: A review

“…This indicates that biopores formed by bio‐tillage play a significant role in promoting plant growth in compacted soil. Thus, the utilization of plant roots to improve soil pore structure receives much attention globally (Lucas, 2022).…”