IF THE PLANT GOT THE CHOICE,AMMONIUM FORM IS PREFERRED BECAUSE IT  IS EASILY METABOLIZED AND COST THE PLANT LESS ENERGY.

An excellent answer from Konstantinos; however I have a little extra to add. This question is very specific to plant species, Blueberries for example have very low nitrate reductase activity and therefore are almost totally dependent on ammonium as the nitrogen source. Conversely the monocots especially grasses  appear to be comfortable in utilizing nitrate.

Apple prefer to pick N in Nitrate form, all the above answers are comprehensive. Species act according to their molecular code under certain conditions 

Nitrate for plants can be compared to sugar for Humans, in that it is not always good for us to have to much, even though we have the biochemical capabilities of using it.

See interesting "Chlorosis Paradox" article Romheld V, Journal of Plant Nutrition, Vol 23, 2000 pp 1629-1643 

Also in same Journal volume Lucena J, pp 1591 - 1606

Simply, Nitrate is the best source for N but in balance. the use of Nitrate or Ammonium depend on the environmental conditions. During prepairing the soil before cultivation, it's better to use ammonium fertilizers especially in sandy soils. while during the growth plant stages, Nitrate fertilizers are prefering. the amounts and dose during the different groth stages of the plant should be match the plant requirments and take in consider the environmental conditions beside the irrigation methods. Remember that for the most vegetable crops,  Ammonium as a source of N should not be over 15 % of the total N requirments.

A non-mentioned issue.  During production of forest seedlings in nursery, use of high amounts of ammonium promotes high abovegroung growth rates.  This may decrease seedling quality as the rate aboveground/belowground (transpining/wáter absorbing tissue) may be too high thus limiting field performance of seedlings. 

In aerobic soils, NO3 is the dominated form of N and preferred by most of the plants growing under aerobic conditions. However, NH3 is the form dominated under anaerobic soils and preferred by plants growing under such conditions e.g. rice. Most of the N is taken up by majority of plants as NO3, though it is converted into NH4 within the plant. 

Nitrate-N is mobile in the soil system because of  negative charge similar to most soils charges, it is preferred for uptake by most plants, however, it is reported that nitrates are not found in cell vacuoles of sugarcane

NO3 is relatively stable form and more available than the NH3 ammonical form. The losses of Ammonical form are more in hot climates or summer crops while nitrate form is leached down. Overall upto 39 % average loses are reported in nitrogenous fertilizers and then come s the efficiency of Nutrient use (NUE). In mix cropping system e.g for fodders ammonium nitrate fertilizer can be a suitable choice to satisfy all sorts of plants preferences for N. Soil 's overall charge also better helps to understand and decide about plants preference as well as from availability point of view.

Constantinos has given a good answer. That helps understand the importance of NH4+ and NO3- for plants. However, availability of NH4+ and NO3- depends on the type of soil, in particular availability of O2 and moisture. In aerated soils NH4+ is oxidized to NO3-, where as with reduced conditions NH4+ form is dominant. Both forms are preferentially absorbed by plants. As mentioned, NH4+ is directly integrated into glutamate in the root, where as NO3- has to be reduced for that. In reduced soils, NH4+ remains abundant, while NO3- will leach down and hence vulnerable for denitrification losses. Therefore depending upon the type of soil and level of moisture, the availability of N to the plant roots varies. In soils with organic matter, mineralization adds NH4+ form faster, but there O2 too needed for microbes to function optimum rates.  The same reaction can proceed to form NO3-, therefore addition of  organic material will be a ways to provide both forms.

I also agree with Constantinos that application of a mixture of both NH4+ and NO3- would promote growth faster than that with a single form of N.

Its totally depends on crop species. 

Its totally depends on crop species. 

I mostly agree with the answers thus far given. It is however a well known fact that most tropical plant species has a greater affinity for ammonium N while the temperate species is more relating on nitrate N for their needs. It is also true that access ammonium applications in cold climatic conditions may result in toxicity symptoms in crops like tomatoes.

Depends on oxygen bioavailability. In hypoxia, more NO3 is absorbed by snap bean.

As Philip stated above, it is a matter of plant species.  A brief experience! I tested various ammonium:nitrate ratios in nutrient solutións for Pinus patula.  Ammonium promoted aboveground biomass, especially foliage, while nitrate promoted a better balance between above and belowgrond growth, which is important for seedling survival in the field.

Indeed several points have been raised already. However, allow me to stress that Getachew Agegnehu raised two questions which are to large extent not related. And the way the question was put suggests that we talk about well-aerated soils used to grow agricultural and horticultural crops (but not ornamentals), not forests or forest species, peat soils, or paddy fields. In this connection some generalization can be made:

1. Which form of N is most readily available for plant uptake? Is NO3-N or NH4-N?

As already mentioned by Salah Mukhtar and others nitrate is by far more mobile and fairly stable in aerated soil.

2. Is there preferential difference in crops for these nutrients?

This addresses a completely different issue, and as mentioned above plants are not intelligent. Annual crops grown in well-aerated soils can be considered early-successional species (compare Konstantinos Simoglou) and tend to prefer nitrate for uptake (got nothing to do with the N form fertilised!). Several additional aspects why this might be the case can be found in a closely related thread I also contributed to (https://www.researchgate.net/post/Why_nitrate_is_considered_to_be_the_best_form_of_nirogen_for_plant)

And yes, I fully support the statement that mixed forms are usually best.

https://www.researchgate.net/post/Why_nitrate_is_considered_to_be_the_best_form_of_nirogen_for_plant

Thanks Joska Gerendas for your answer.

Plants prefer NH4+ rather than NO3-. Another thing is that in the field most of N source is nitrate.

Yes, I agree with Marcos

Why the plants prefer NH4 not NO3- and what the references for this information.

Because nature tend to keep energy as much as it can. Amonia is reduced, so it is cheaper energically, although it is toxic. Every book of metabolism of N talk about this. For example, this article too: Britto and Kronzucker. 2002. J. Plant Physiol. 159. 567 –584.

As both sources are available form of N in the soil, hence both are equally good for plant growth. However, selection of source either from ammonical or nitrate form of N should be based on a number of factors e.g. crop species (already a large number of people said), growth stage (even), soil texture (nitrate form should not be used in sandy soils or should be used very carefully in split forms, because of its mobility), irrigation water availability and type of irrigation (surface flooding, drip irrigation, sprinker, fertigation), other fertilizer sources (P and K).

Plants generally prefer ammonical form, because of low energy requirement.

Very interesting article on ammonia vs nitrate, a function for ecological impact.

https://www.jstor.org/stable/10.1086/665997?seq=1#page_scan_tab_contents

Yes, I agree with the Professor Dr. Tariq Aziz. However, if you talk about preferential nutrient (ammonium) uptake. In energy efficiency case, plants prefer ammonium uptake as plants spends 5 moles of ATP for ammonium uptake and 20 moles of ATP for nitrate assimilation. If plants assimilate nitrate, finally they have to convert into ammonium and again in organic forms.

• In most of systems, plants preferred inorganic form of N over organic form of N. However, preference for different inorganic forms of N depends on plant adaptation to soil. Plants adapted to low pH, anaerobic soil tend to take up NH4-N while plants adapted to high pH and aerobic soil (well aerated agriculture soil) take up NO3-N. Plants preferred NO3 - in the drier environments and NH4 + in the wetter environments. NO3- is main form of nitrogen taken up by upland crops while NH4+ is prevalent form taken up by plant grown under submerged condition. Wheat, maize, canola, beans, sugar beet, Arabidopsis and tobacco grow preferentially on NO3− nutrition; whereas, rice pine and grow preferentially on NH4+ nutrition as described by many researchers.
• In energy expenditure point of view, uptake and assimilation of NH4+ is less costly compared to NO3- uptake because NH4 + directly incorporated into glutamate during N assimilation while NO3- first reduced to NO2- and then converted to NH4+ before assimilation. Uptake of NO3- is more costly.
• However, NO3- is more available for uptake because it is more mobile. Higher mobility leads to more rapid diffusion to root and easier access to plant as mass flow and diffusion is main pathway for N uptake. Due to electronic charge, ammonium is less mobile, often adsorbed to organomineral complexes, retained in soil and has lower rate for diffusion and mass flow.
• Some studies also revealed that only NH4+ can cause severe toxic symptom. The NH4+ toxicity could counterbalance the energetic advantage of NH4+. According many researchers the NH4+ toxicity may be due to several speculation such to proton extrusion with NH4+ uptake, cytosolic pH disturbances, decreased water use efficiency, shifts in plant carbohydrate status, the uncoupling of photophosphorylation, high energy costs of membrane transport, and displacement of crucial cations (e.g. K)

Plants can utilize both nitrate and ammonium forms of nitrogen with equal ease. It all depends upon which form is more available in a given environment. For example, in submerged rice it is ammonium which is more available in the soil. Whether nitrates are not formed or lost under anaerobic conditions is a different issue. In upland crops like wheat, nitrate-N are not preferred because it is nitrate-N, but because ammonium-N is readily converted to nitrate-N. I would like to repeat - - plants can use both nitrate- and ammonium-N with equal ease.

Under oxidize zone, nitrate nitrogen is taken by plant. Under reduced condition, ammonical nitrogen prefer by plant. Rice, potato, beetle vine banana taken it both the form of nitrogen, depend upon soil refox potential. Regards

Dear Bijay,

Thank you all for your interesting answers. Would you please elaborate this sentence? "In upland crops like wheat, nitrate-N are not preferred because it is nitrate-N, but because ammonium-N is readily converted to nitrate-N." Do you mean 'ammonium-N is not preferred in upland crops ?

This is very interesting question, but with no black and white answer to it. Let me mention some of the thoughts I came across while seeking answer to a similar question over the last three years. Because nitrate is much more mobile than ammonium, it is thought to be more likely accessible by plant roots than ammonium. There is also a view that the mobility of nitrate with soil solution makes it more prone to loss compared to ammonium, making it less available for plant uptake. The question about plant nitrate vs ammonium preference is also highly debated these days. Results from tracer studies suggest plants prefer nitrate to ammonium even though nitrate uptake and assimilation is more costly to plants. However, this assertation has been recently challenged, raising question how tracer results are interpreted. Other researchers argue that plants preference is related to adaptation, e.g., some conifers and crop plants such as rice that are adapted to acidic soil prefer ammonium. Then, there is also emerging evidence that plants likely use either N forms depending on its availability, which, in turn, is controlled by many factors, climate being one. So, we really do not have a clear answer to it. The following paper has some good answers to the above question:

Chalk and Smith 2020 'On inorganic N uptake by vascular plants: Can 15N tracer techniques resolve the NH4+ versus NO3− “preference” conundrum? Article On inorganic N uptake by vascular plants: Can 15N tracer tec...

The absorption of any of these forms depend on plant and soil conditions, many of them already considered. I would said that under optimal soil conditions the absorption of NO3 will be favored in plants with high photosynthetic capacity and no light restrictions. Those plants with low photosynthetic rates prefer more NH4 than NO3.

It is already well documented that NH4-N forms are readily available as well as more quickly assimilated than any other forms of nitrogen

Since N comes as NH4+ from two main sources, concentrated forms as either Urea, Ammonia, or Ammonium sulfate , and mineralizing organic matter, this form is abundant. However., under aerated condition in highlands, transformation to NO3- happens often. Being NH4+ available in soil, it is the same form as the transformation to NH4+ requires the availability of NH4+. Depending on the vegetation and root distribution NH4+ is rapidly absorbed by plants leaving negligible level for transformation.

The NO3- ion is vulnerable to leaching due to its negativity but can even reach such leached ions to the root surface through mass flow depending upon soil moisture status, environmental condition (warm or cool), type of plants, and rates of ET, etc. On the other hand, NH4+ is a cation that is adsorbed onto colloids, and hence it may be available to plant roots longer duration than NO3- depending upon the type of soil. The way the question has been asked, obviously one would say NO3- that will be fast absorbed by plants compared to NH4+ if it remains available. NO3- is vulnerable to leaching depending upon the rate of downward water movement, rainfall, or irrigation. In oxidized soils (upland soils) NH4+ is oxidized to NO3- and hence the latter would be abundant. In reduced soils (poorly drained and rice soils), the former remains abundant. In that situation, NH4+ remaining adsorbed on cation exchange complex would be available for plant roots to exchange over a longer period compared to NO3- that may remain for a short period until the soil solution stays. Since the way, the root getting contact with ions determines the rate of absorption. It is governed by many factors. Isolated conditions could not be found in soil environments to point out which would be taken up faster. NO3- is in soil solution whereas NH4+ is adsorbed on cation exchange complex. One would be able to compare the rates of uptake of preference using control experiments using hydroponic systems in which the nutrients will remain only in the solution with no cation exchange complexes.

Both nitrate and ammonium are uptaken by forest trees. However, ammonium is preferable to nitrate by trees in forest ecosystems if the amounts of the two nutrients close each other. In case ammonium is high in the soil, nitrate uptake is inhibited. The amount of the nutrients depend on environmental conditions due to the effects of them on ecological processes such as nitrification and ammonification. The uptake of the nitrohen form also can change with tree species.