Soil Microorganisms Can Reduce P Loss from Cropping Systems

Abstract

Declining supplies of quality phosphate ore and rising costs for P fertilizer, tripled in last 10 years, are changing the economics of food production and consumption. Simultaneously, P applied in crop production continues to be released as a non-point source pollutant that causes widespread degradation of water resources. Strong motivation thus exists to become more efficient in the use of P for crop production. Applications of P fertilizer to crops are commonly guided by single-point-in-time measurements of inorganic P residing in forms that are relatively easy to chemically extract from the soil. While these easily extractable inorganic P forms often represent P that is immediately available for plant uptake, these same P forms also represent P that is most subject to fixation in the soil (decreasing plant availability) or loss from the system via leaching, surface flow, and erosion. Point-in-time measurements ignore the dynamic nature of soil P, since soil biological activities continually transform P into forms with varying mobility and plant-availability and therefore introduce a kinetic aspect to P retention and plant availability. There is an urgent need for an alternative model for managing soil P fertility that accounts for the activities of soil microorganisms in controlling pools and fluxes of soil P and moderating plant P uptake. In this review, we synthesize the emerging literature from different scientific disciplines to provide a foundational understanding of the principles by which native soil microorganisms can improve the efficiency of P use in crop production systems. Following an introduction, we summarize the dual roles of P as a critical resource for food production and an environmental pollutant responsible for economic damages in billions of dollars. In Sect. 2.3 we outline the use of P in fertilization of cropping systems and its possible fates following application. We point out that commonly used indices of fertilization use efficiency fail to account for loss from the system and there is an alternative method that does account for losses. Sect 2.4 summarizes standard schemes for analytical soil P fractionation, their relationship with conventional soil fertility tests, and the limitations of using conventional soil tests to direct P applications. Section 2.5 describes the mechanisms by which soil microorganisms modify P mobility and bioavailability and surveys the emerging literature that demonstrates that microbial biomass turnover and biologically-catalyzed reactions can create an annual flux of 10-40 kg ha(-1) plant available P. Section 2.6 documents the role of an obligate plant symbiont, arbuscular mycorrhizal fungi, in providing P to plant hosts and increasing retention of P in the system. Section 2.7 discusses the relationships between plant and microbial diversity and their influence on soil nutrient dynamics. We conclude with a synthesis of agricultural practices documented to promote soil biomass, diversity, and activities that can improve P retention in crop production systems.