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Life on Earth is sustained by a small volume of soil surrounding roots, called the rhizosphere. The soil is where most of the biodiversity on Earth exists, and the rhizosphere probably represents the most dynamic habitat on Earth; and certainly is the most important zone in terms of defining the quality and quantity of the Human terrestrial food resource. Despite its central importance to all life, we know very little about rhizosphere functioning, and have an extraordinary ignorance about how best we can manipulate it to our advantage. A major issue in research on rhizosphere processes is the intimate connection between the biology, physics and chemistry of the system which exhibits astonishing spatial and temporal heterogeneities. This review considers the unique biophysical and biogeochemical properties of the rhizosphere and draws some connections between them. Particular emphasis is put on how underlying processes affect rhizosphere ecology, to generate highly heterogeneous microenvironments. Rhizosphere ecology is driven by a combination of the physical architecture of the soil matrix, coupled with the spatial and temporal distribution of rhizodeposits, protons, gases, and the role of roots as sinks for water and nutrients. Consequences for plant growth and whole-system ecology are considered. The first sections address the physical architecture and soil strength of the rhizosphere, drawing their relationship with key functions such as the movement and storage of elements and water as well as the ability of roots to explore the soil and the definition of diverse habitats for soil microorganisms. The distribution of water and its accessibility in the rhizosphere is considered in detail, with a special emphasis on spatial and temporal dynamics and heterogeneities. The physical architecture and water content play a key role in determining the biogeochemical ambience of the rhizosphere, via their effect on partial pressures of O 2 and CO 2 , and thereby on redox potential and pH of the rhizosphere, respectively. We address the Plant Soil

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Soils are the product of the activities of plants, which supply organic matter and play a pivotal role in weathering rocks and minerals. Many plant species have a distinct ecological amplitude that shows restriction to specific soil types. In the numerous interactions between plants and soil, microorganisms also play a key role. Here we review the existing literature on interactions between plants, microorganisms and soils, and include considerations of evolutionary time scales, where possible. Some of these interactions involve intricate systems of communication, which in the case of symbioses such as the arbuscular mycorrhizal symbiosis are several hundreds of millions years old; others involve the release of exudates from roots, and other products of rhizodeposition that are used as substrates for soil microorganisms. The possible reasons for the survival value of this loss of carbon over tens or hundreds of millions of years of evolution of higher plants are discussed, taking a cost-benefit approach. Coevolution of plants and rhizosphere microorganisms is discussed, in the light of known ecological interactions between various partners in terrestrial ecosystems. Finally, the role of higher plants, especially deep-rooted plants and associated microorganisms in the weathering of rocks and minerals, ultimately contributing to pedogenesis, is addressed. We show that rhizosphere processes in the long run are central to biogeochemical cycles, soil formation and Earth history. Major anticipated discoveries will enhance our basic understanding and allow applications of new knowledge to deal with nutrient deficiencies, pests and diseases, and the challenges of increasing global food production and agroecosystem productivity in an environmentally responsible manner.

Over the last 50 years, the use of nitrogen (N) and phosphorus (P) fertilizers has increased at a faster rate than global food production, resulting in substantial decreases of N and P efficiency in agriculture (Fig. 1). Thus, considerable amounts of N and P fertilizers have been wasted in agroecosystems, and the alteration of N and P biogeochemical cycles is among the most visible impacts of human activities on ecosystem services, far exceeding the hypothesized "planetary boundaries" for N and approaching those for P (Rockström et al., 2009). Projections for the future suggest that a substantial further increase in N and P fertilizer use will occur to cope with increasing food demand (Tilman et al., 2001;Vance et al., 2003). Tilman et al. (2001) predicted that global consumption of fertilizer P will increase from 34.

SummaryThe rhizosphere differs from the bulk soil in a range of biochemical, chemical and physical processes that occur as a consequence of root growth, water and nutrient uptake, respiration and rhizodeposition. These processes also affect microbial ecology and plant physiology to a considerable extent. This review concentrates on two features of this unique environment: rhizosphere geometry and heterogeneity in both space and time. Although it is often depicted as a soil cylinder of a given radius around the root, drawing a boundary between the rhizosphere and bulk soil is an impossible task because rhizosphere processes result in gradients of different sizes. For instance, because of diffusional constraints, root uptake can result in a depletion zone extending <1 mm for phosphate to several centimetres for nitrate, while respiration may affect the bulk of the soil. Rhizosphere processes are responsible for spatial and temporal heterogeneities in the soil, although these are sometimes difficult to distinguish from intrinsic soil heterogeneity. A further complexity is that these processes are regulated by plants, microbial communities and soil constituents, and their many interactions. Novel in situ techniques and modelling will help in providing a holistic view of rhizosphere functioning, which is a prerequisite for its management and manipulation.New Phytologist (2005) 168 : 293-303 © New Phytologist (2005