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In the nitrogen cycle (ecology), it is usually described that plants can use nitrogen in the form of ammonium (NH4+) and nitrate (NO3-). Do plants prefer one form of nitrogen over the other?

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Quoting verbatim from this site. The reference is not really a scientific article but you can check the references it cites. Some were not in English so I did not check. However these points are fairly logical

Nitrates are the preferred nitrogen source:

  • Non-volatile: unlike ammonium, nitrate is non-volatile, so there is no need to incorporate it in the soil when applied by top- or side dressing, which makes it a convenient source for application.

  • Mobile in the soil - direct uptake by the plant, highest efficiency.

  • Nitrates synergistically promote the uptake of cations, such as K, Ca and Mg, while ammonium competes for the uptake with these cations.
  • Nitrates can be readily absorbed by the plant and do not need to undergo any further conversion, as is the case with urea and ammonium, before plant uptake.
  • No acidification of the soil if all the nitrogen is applied as nitrate-nitrogen.
  • Nitrates limit the uptake of harmful elements, such as chloride, into large quantities.
  • The conversion of nitrates to amino acids occurs in the leaf. This process is fuelled by solar energy, which makes it an energy-efficient process. Ammonium has to be converted into organic N compounds in the roots. This process is fuelled by carbohydrates, which are at the expense of other plant life processes, such as plant growth and fruit fill.

Many plants can carry out nitrification but this is not universal. From this article:

Nitrification is sometimes considered so universal and rapid that applications of NH4-N are considered equivalent to NO3-N. This is not true in many forest, orchard, and grassland soils.

However, nitrate being more soluble is susceptible to leaching and wash off, care has to be taken when using nitrate based fertilizers. From the previously mentioned paper:

Unlike the positively charged ammonium ion, which is relatively stationary because of its adsorption to organic matter or clay particles, the negatively charged nitrate ion is freely mobile in the soil solution (166). Thus, leaching and denitrification primarily involve a loss of NO3-N (171). Inhibition or retardation of nitrification of applied NH4-N can reduce nitrogen losses, increase efficiency of applied N, and establish a predominantly ammoniacal form of nitrogen available for plant uptake (105, 171,222, 232).

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  • $\begingroup$ @quibble Ammonium can at high concentrations even be toxic to plant (roots). This is not uncommon in aquatic peat vegetation. $\endgroup$ – RHA Nov 12 '15 at 20:46

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