Understanding and managing the rhizosphere in agroecosystems
Drinkwater, Laurie; Snapp, Sieglinde
Agricultural systems represent the major form of land management, covering 5 billion hectares of the global terrestrial land area. The unintended consequences of agriculture extend well beyond agricultural landscapes and include environmental degradation and social displacement (Hambridge 1938; Vitousek et al. 1997; Friedland et al. 1991). Many have advocated the adoption of an ecosystem-based approach that would incorporate multifunctionality as an agricultural goal and entail broad application of fundamental ecological principles to food production (Dale et al. 2000; Drinkwater and Snapp 2005). This approach would aim to reduce external inputs and environmental degradation by increasing the capacity for internal, ecological processes to support crop production while contributing to other ecosystem services (Dale et al 2000). Most efforts devoted to managing the rhizosphere in agricultural systems have emphasized processes that contribute directly to maximizing yield within the context of resource-intensive cropping systems. Several excellent reviews are available covering the role of rhizosphere biology in promoting crop growth under the nutrient rich conditions of high input agriculture (cf. Pinton et al. 2001; Lynch 1990). In particular, the biology of important root pathogens and plant-microbial N-fixing symbioses have been extensively studied within this context (Spaink et al. 1998; Whipps 2001). A smaller amount of rhizosphere research has focused on achieving modest improvements in yields under severe nutrient or water limitations that are commonly found in low-input, subsistence agroecosystems of the developing countries where farmers do not have access to purchased fertilizers and pesticides (Lynch 1990). In this chapter we will assess the current ecological understanding of the rhizosphere in agroecosystems and broaden the scope of rhizosphere contributions to encompass a variety of ecosystem functions beyond those directly related to maximizing crop growth and yields. Our aim is to examine the potential for rhizosphere processes and plant-microbial interactions to restore agroecosystem functions to reduce input dependancy and environmental degradation. We begin with an inventory of how conventional, high input management has altered the soil environment and biota in agroecosystems with particular emphasis on the consequences for the rhizosphere habitat. We then survey a range of rhizosphere processes and examine how current management practices enhance or hinder the process and evaluate the potential for improved functionality. Finally, we look ahead and discuss how management of the rhizosphere and plant-microbial interactions could be approached within multifunctional, ecologically-sound agricultural systems of the future.
ecology; plant-microbial interactions; agriculture; agroecology