Abstract
Agricultural crop production depends upon judicious use of nitrogen (N) fertilizers to sustain yields. Globally, the N recovery rate by crops is about 60%, meaning that the rest of the N applied to agroecosystems is transformed to forms that are not available for crop uptake or are lost to the environment. Considering that part of the soil N supplied to crops comes from biological N2 fixation and mineralization of soil organic N, quantifying these contributions could reduce our reliance on exogenous N inputs. This review examined how the microbially mediated reactions of N mineralization and nitrification contribute to the soil N supply, and biotic controls on these reactions in the soil food web. Potential N mineralization by heterotrophic bacteria and fungi can exceed 10% of the total soil N per year, and ammonium released by mineralization is rapidly transformed to nitrate through the action of chemoautotrophic ammonia oxidizers (bacteria and archaea) followed by heterotrophic and chemoautotrophic nitriflers (bacteria and fungi). Predation of these micro-organisms, primarily by soil microfauna, accounts for additional release of ammonium, estimated at 32-38% of the annual N mineralization. Soil meso- and macro-fauna also contribute to N mineralization and nitrification by accelerating the decomposition of organic substrates and modifying the soil habitat in ways that favour microbial activity. Tillage, application of organic amendments and improving soil drainage in humid temperate regions should favour N mineralization and nitrification processes in soil food web, whereas agrochemical use is expected to have a negligible effect. In summary, the soil food web contribution to N mineralization needs to be included in the soil N supply concept, which requires the development of field-based measurements and models of the soil N supply.