Soil organic nitrogen: an overlooked but potentially significant contribution to crop nutrition. Plant Soil 462: 7–23
Farzadfar, S., Knight, J.D., Congreves, K.A. 2021.
Summary by Dobermann, A.
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 may also be taken up by plants. 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 may also be absorbed up by roots. In agricultural soils, manure, crop residues, roots and root exudates as well as dead organisms are the main sources of such organic N, but the bulk of it seems to be of microbial origin. The authors discuss several hypothetical mechanisms by which such 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
Classic Plant Nutrition Paper
The direct assimilation of inorganic and organic forms of nitrogen by higher plants. J. Agric. Sci. 4: 282–302.
Hutchinson, H.B., Miller, N.H.J. 1912.
Summary by Dobermann, A.
The idea that plants may take up organic N compounds such as amino acids dates back to the 19th century. Working at the Rothamsted Experimental Station, Hutchinson and Miller in this paper first provide an extensive review of the literature on uptake of inorganic and organic N forms by plants. They show that for the majority of organic compounds no evidence of uptake had been found, but they also list 15 compounds for which an N uptake of at least 1 mg had been shown, including urea. They go on to study many of those in a series of solution culture experiments with peas, which also illustrate some of the difficulties associated with such research, such as keeping things sterile enough to avoid contamination with inorganic N. Based on the results, they provide a provisional grouping of compounds as (i) readily assimilated, (ii) assimilated, (iii) doubtful and (iv) toxic. Within the first group, the best results were obtained with urea and barbituric acid, the former assimilating rather more nitrogen than the latter whilst barbituric acid gave a greater amount of dry matter. Of course, none of this challenges the fundamental quantitative importance of mineral forms of N for crop nutrition.