Nitrogen in physiology. An agronomic perspective and implications for the use of different nitrogen forms

Nitrogen (N) is an essential plant nutrient at the centre of plant metabolism being present in numerous vital organic compounds including amino acids, proteins, nucleic acids and phytohormones. In highly fertile arable soils supplied with N fertilisers, NO3 - and NH4 + ions are usually considered to be the main N forms in soil solution taken up by crop plants, the ratio of NO3-N to NH4-N often being in the order of 10-20:1. Soluble organic N is receiving increasing attention. The form of N uptake modifies rhizosphere pH which affects uptake of other soil nutrients. Acquisition of the various N forms is regulated not only by their chemical and spatial availability in the soil, but also by transport systems in the plasma membrane of root cells, root system architecture and mechanisms that regulate the activity of N transport systems and root growth, depending on plant requirements. In most fertile soils well supplied with mineral nutrients, neither N transport from the bulk soil to crop roots nor the efficient processes of NO3 - and NH4 + ion uptake by roots limit N acquisition. Uptake of N is regulated by N demand in which shoot-root interactions play an important role. Nitrate acts as a signal for metabolism and plant development which is associated with cytokinin transport. The biochemistry of N assimilation is well understood but control at molecular level much less so. Nitrogen status of plants can be characterised by the concept of critical N concentration (Nc) in relation to the long-term fall in N concentration in the dry matter as plants age. Nc represents the optimum for growth for a given crop and can be related to actual crop N concentration. Fertilisation is needed when [N] / [Nc], i.e. NNI (nitrogen nutritional index) < 1. By determining N status during crop growth, fertilisation can be carried out more efficiently as exemplified in melon cultivation. Examples are presented to demonstrate practical effects of different N forms including NO3-, NH4- and urea-N. These examples include the value of placed stabilised NH4- N in depressing rhizosphere pH on high-pH soils to increase the availability and uptake of sparingly soluble phosphate and micronutrients iron, manganese, copper, zinc and boron. These latter nutrients play a vital role, protecting plants against both biotic and abiotic stresses. The acidifying effect in the rhizosphere of symbiotically grown legumes can be of similar benefit on high pH soils. The use of NH4-N fertilisers to depress rhizosphere pH which is also associated with increased Si and Mn uptake is regularly practiced to control some plant diseases such as mildew and take-all in wheat. Water use efficiency (WUE) also relates to the form of N supply. Foliar application of urea is discussed. Signaling effects of stabilised N forms can be used to influence tiller number and yield components in wheat and barley crops. The use of NH4-N in restricting lateral shoot development in tomato production is discussed