The Importance of the Microbial N Cycle in Soil for Crop Plant Nutrition

Nitrogen is crucial for living cells, and prior to the introduction of mineral N fertilizer, fixation of atmospheric N2 by diverse prokaryotes was the primary source of N in all ecosystems. Microorganisms drive the N cycle starting with N2 fixation to ammonia, through nitrification in which ammonia is oxidized to nitrate and denitrification where nitrate is reduced to N2 to complete the cycle, or partially reduced to generate the greenhouse gas nitrous oxide. Traditionally, agriculture has relied on rotations that exploited N fixed by symbiotic rhizobia in leguminous plants, and recycled wastes and manures that microbial activity mineralized to release ammonia or nitrate. Mineral N fertilizer provided by the Haber–Bosch process has become essential for modern agriculture to increase crop yields and replace N removed from the system at harvest. However, with the increasing global population and problems caused by unintended N wastage and pollution, more sustainable ways of managing the N cycle in soil and utilizing biological N2 fixation have become imperative. This review describes the biological N cycle and details the steps and organisms involved. The effects of various agricultural practices that exploit fixation, retard nitrification, and reduce denitrification are presented, together with strategies that minimize inorganic fertilizer applications and curtail losses. The development and implementation of new technologies together with rediscovering traditional practices are discussed to speculate how the grand challenge of feeding the world sustainably can be met.