For nearly 200 years, crop nitrogen (N) nutrition has focused on inorganic forms of N, assuming that plants primarily take up nitrate and ammonium. In this excellent review, the authors summarize what is known about organic forms of N that plants may also take up. They suggest an emerging model of plant N nutrition in which, as an intermediate step before mineralization, soil organic matter compounds get transformed into organic N forms that roots may absorb.
In agricultural soils, manure, crop residues, roots, and root exudates as well as dead organisms are the main sources of organic N, while the bulk of it seems to be of microbial origin. The authors discuss several hypothetical mechanisms by which organic N uptake and assimilation may increase crop N use efficiency. Although the quantitative role of organic N uptake remains largely unknown, this is an area of research that should receive more attention in our quest for further fine-tuning nitrogen management in agricultural systems. This paper provides plenty of food for thought for that.
