Besides ammonium and nitrate plant absorb nitrites for nitrogen.

From soil system, ammonium and nitrates are absorbed by the plants. However, within the plant tissues or compartments, these forms can also be converted into various forms.

plants can use all sorts of reaktive nitrogen compounds, but they usually are ambient only in small quantities and therefore neglectible for the entire N balance of a plant

There are evidences that plant can also assimilate dissolve organic N too. You can find many literature on this topic.

Short and best reply by Sajid and Ewald

the best practical example is foliar application of disolved urea! the effect ist immediately, some people coined it "green spraying"!

Hi Felipe,

Higher plants can uptake many forms of N depending on the circumstances (soil-microbe-root interactions).  The review paper below (link provided) changed most of the perceptions held before 2000s relating to the acquisition of N from the root system by higher plants.  If you cannot access this publication, I can forward a copy.

-SJ

Schimel, JP and Bennett, J (2004) Nitrogen mineralization: Challenges of a changing paradigm.  ECOLOGY. Volume: 85 (3) Pages: 591-602

DOI: 10.1890/03-8002

Abstract: Until recently, the common view of the terrestrial nitrogen cycle had been driven by two core assumptions-plants use only inorganic N and they compete poorly against soil microbes for N. Thus, plants were thought to use N that microbes "left over," allowing the N cycle to be divided cleanly into two pieces-the microbial decomposition side and the plant uptake and use side. These were linked by the process of net mineralization. Over the last decade, research has changed these views. N cycling is now seen as being driven by the depolymerization of N-containing polymers by microbial (including mycorrhizal) extracellular enzymes. This releases organic N-containing monomers that may be used by either plants or microbes. However, a complete new conceptual model of the soil N cycle needs to incorporate recent research on plant-microbe competition and microsite processes to explain the dynamics of N across the wide range of N availability found in terrestrial ecosystems. We discuss the evolution of thinking about the soil N cycle, propose a new integrated conceptual model that explains how N cycling changes as ecosystem N availability changes, and discuss methodological issues raised by the changing paradigm of terrestrial N cycling. 

Total Times Cited: 818

http://onlinelibrary.wiley.com/doi/10.1890/03-8002/abstract;jsessionid=3B77F462CDE77414876A0075F761BDFA.f04t01

The learned scientists have pointed out all possibilities. I think some good answers have been uploaded on the subject under discussion.

Plants also can use amino acids by foliar application of leaves.For example in dry conditions spraying leaves with proline or other form of amino acids can help the plants.

To our knowledge there do not exist a different form of nitrogen uptake by plants than ammonium and nitrate and nitirites.

Evidence for plant uptake of N rich organic molecules via root system:

Marsden et al. (2015) Plant acquisition and metabolism of the synthetic nitrification inhibitor dicyandiamide and naturally-occurring guanidine from agricultural soils. Plant and Soil. Volume 395, Issue 1, pp 201–214

https://www.researchgate.net/publication/279231333_Plant_acquisition_and_metabolism_of_the_synthetic_nitrification_inhibitor_dicyandiamide_and_naturally-occurring_guanidine_from_agricultural_soils

http://link.springer.com/article/10.1007/s11104-015-2549-7

Pathways of dicyandiamide uptake in pasture plants: a laboratory study. Biology and Fertility of Soils · (February 2016)

https://www.researchgate.net/publication/294138522_Pathways_of_dicyandiamide_uptake_in_pasture_plants_a_laboratory_study

Article Plant acquisition and metabolism of the synthetic nitrificat...

Article Pathways of dicyandiamide uptake in pasture plants: a laboratory study

Interesting references have been uploaded by Susthantha.

I think some plants prefer to get nitrogen in forum of ammonium and others prefer nitrate 

Some plants can take up nitrogen in amide form.

Yes, Tarafdar

Thanks to all the People that answer the question. It is a very interesting discussion. Regards.