The Rhizosphere - Roots, Soil and Everything In Between
Written by David H. McNear Jr. (Assistant Professor of Rhizosphere Science)
What is the Rhizosphere and how can understanding rhizosphere processes help feed the world and save the environment? This article will review the critical biogeochemical processes occurring at the plant root-soil interface.
Roots serve many functions for a plant including anchorage and acquisition of vital nutrients and water necessary for growth. The plant root-soil interface is a dynamic region in which numerous biogeochemical processes take place driven by the physical activity, and the diversity of chemicals released by the plant root and mediated by soil microorganisms. In turn the processes occurring in this region control a host of reactions regulating terrestrial carbon and other element cycling that sustain plant growth and which have an enormous influence on plant and microbial community function and structure which greatly influence a variety of ecosystem level processes (van der Heijden et. al., 2008; Wardle, 2004; Berg and Smalla, 2009). Understanding and harnessing these interactions for the sustainable production of food, fuel and fiber to support a growing world population on a dwindling supply of arable land will be the challenge of generations to come.
Meeting the Global Challenge of Sustainable Food, Fuel and Fiber Production
Soil is one of the last great scientific frontiers (Science, 11 June, 2004) and the rhizosphere is the most active portion of that frontier in which biogeochemical processes influence a host of landscape and global scale processes. A better understanding of these processes is critical for maintaining the health of the planet and feeding the organisms that live on it (Morrissey et al., 2004). There is a small but concerted effort under way to harness the root system of plants in an attempt to increase yield potentials of staple food crops in order to meet the projected doubling in global food demand in the next 50 years (Zhang, et al. 2010; Gewvin, 2010). These efforts are being done in the face of a changing global climate and increasing global population which will inevitably require more productively grown food, feed and fiber on less optimal (and often infertile) lands; a condition already encountered in many developing countries (Tilman, et al, 2002). Meeting the global challenges of climate change and population growth with a better understanding and control of rhizosphere processes will be one of the most important science frontiers of the next decade for which a diverse, interdisciplinary trained workforce will be required.
The Rhizosphere Defined
In 1904 the German agronomist and plant physiologist Lorenz Hiltner first coined the term "rhizosphere" to describe the plant-root interface, a word originating in part from the Greek word "rhiza", meaning root (Hiltner, 1904; Hartmann et al., 2008). Hiltner described the rhizosphere as the area around a plant root that is inhabited by a unique population of microorganisms influenced, he postulated, by the chemicals released from plant roots. In the years since, the rhizosphere definition has been refined to include three zones which are defined based on their relative proximity to, and thus influence from, the root (Figure 1). The endorhizosphere includes portions of the cortex and endodermis in which microbes and cations can occupy the "free space" between cells (apoplastic space). The rhizoplane is the medial zone directly adjacent to the root including the root epidermis and mucilage. The outermost zone is the ectorhizosphere which extends from the rhizoplane out into the bulk soil. As might be expected because of the inherent complexity and diversity of plant root systems (Figure 2), the rhizosphere is not a region of definable size or shape, but instead, consists of a gradient in chemical, biological and physical properties which change both radially and longitudinally along the root.