NITROGEN IS the most important element in plant nutrition beyond any doubt. Its multifunctional nature places it to the most distinguished rank among nutrients. It plays a vital role in plant metabolism, physiological activity, defense mechanism, growth, development and efficiency by influencing a number of key enzymes, precursors and metabolites. Nitrogen is most abundant gas in air (about 79 per cent) but unfortunately plants are unable to use this atomic N and thus uptake from root zone fixed in the form of nitrates, ammonium and organic acids. Major portion utilized by the plants is uptaken in the form of nitrates but numerous losses are associated with this form. That’s why scientists are seriously concerned about nitrification and its regulation in ecosystems to sustain the nitrogen use and to enhance its use efficiency.
Nitrogen is converted into its reactive form by the process of nitrification. It is the Oxidation of ammonia into nitrite and then into nitrate with the help of ammonia oxidizing bacteria (Nirtrosomonas and nitrobacter spp.). Nitrification provides available form of nitrogen to plants via nitrogen cycle. Although nitrification is enhanced by fertilizers as well as biologically so as to increase fertility of soil but nitrate is lost through leaching or runoff because it is less stable and is loosely bound with soil particles. This loss of highly active form of nitrogen to environment is causing an alarming situation because of participation in global warming and environmental pollution via agricultural systems. Also the assimilation of nitrates in plants requires more metabolic energy as compared to the assimilation energy for ammonia. Thus unchecked and uncontrolled nitrification is becoming an evil, so we can say it “a necessary evil”.
Ammonia has capacity to bind with soil particles but nitrate does not have this capacity so nitrate can easily leach down through soil during nitrogen cycle as compared to ammonia leading to the nitrogen pollution. Nitrous oxide (N2O) is potentially more harmful greenhouse gas than carbon dioxide. So, present trend in agriculture system is to reduce nitrification for increasing the nitrogen use efficiency of plants. Nitrification regulation may be of two types:
• Agronomic strategies for nitrification inhibition
• Biological nitrification inhibition
Various agronomic strategies are there for regulation of nitrification in soil. These strategies are specifically focused on the utilization rate and timing of fertilizers application. According to these measures, nitrogen fertilizers are applied in synchronization with the nitrogen requirements of the crops to facilitate the rapid uptake. In this way residence time of nitrogen in soil becomes less and there is minimum chance of nitrogen leaching. Hence nitrogen use efficiency is increased. These strategies are:
1. Synthetic nitrification inhibitors (NIs)
2. Slow and controlled release fertilizers (SCRFs)
Nitrification inhibitors are in the synthetic chemical form. These inhibitors depress the nitrification activity of soil nitrifying bacteria by delaying the oxidation if ammonia. This reduction of nitrification until the maturity stage of crop provides more time for crops to utilize nitrogen in an efficient way and the risk of nitrogen leaching becomes also very low. Nitrapyrin, Dicyanndiamide (DCD), 3,4-dimethyl pyrazole phosphate (DMPP) have been successfully used for nitrification inhibition. But the problem is their high prices and localized efficiency. Such serious limitations do not allow suitable adaptation especially in developing countries.
A better technique to reduce the nutrients loss is the use of slow and controlled release fertilizers. These are quite effective in their specific mode involving the release of nutrient element in required amounts at regular but well spaced intervals. Nitrogen is major element applied by this particular mechanism. These increase time of release and availability of nitrogen. Normally these are made by applying special type of impermeable or semi-permeable protective coatings. Due to SCR the availability of NH4+ to nitrifiers is reduced and thus the loss through nitrification is reduced. Polyolefin coated urea (POCU) reduces N losses up to 40 per cent but the problem is high cost. So, these are effective at varying degrees but major concern is their high cost and unavailability.
Biological nitrification inhibition (BNI) is a plant mediated phenomenon in which chemicals released by plants inhibit its conversion from ammonium form to nitrate form and thus reduce its fixation deriving it to other forms. Actually these plant exudates which are released in rhizosphere are secondary metabolites i.e. allelochemicals which act in two ways to inhibit nitrification. Major mechanism is the suppression of nitrifying bacteria by inhibiting the enzymes necessary for their functioning and they may also directly retard the growth and proliferation of such microbial colonies. Major enzymatic pathways inhibited are ammonium oxidase (AMO) and hydroxylamine oxidoreductase (HOA) pathways which are of key importance for ammonium assimilation. Certain biomolecules have varying degree of BNI potential depending upon the rate of exudation, local edaphic factors, soil microbial status, temperature and many other environmental conditions. Plants are known to release a wide range of substances with biological activity. These include molecules that belong to phenolic, alkaloid, fatty acid, isothiocyanate, and terpene groups. Scientists are working on such molecules that can be added to agro-ecosystems for this function.
Allelochemicals are major BNI agents. The research on their screening and potential measurement is still at very preliminary stages. Different crops have varying degree of BNI potential. Some wild grasses have shown superb results and similarly cultivated cereals also have potential. Sorghum is of key importance in this regard having maximum detectable BNI capacity. However, legumes have least or nearly non-detectable potential as it is a regulatory phenomenon which prevents present ammonium form to convert in nitrate form but we know that biological nitrification is maximum in legumes through nodule formation facilitating nitrification process. BNI act as adaptive mechanism and improves NUE in systems where it is already in fewer amounts. It helps to increase intrinsic as well as agronomic N use efficiency. A specific bioassay has been developed known as bioluminescence bioassay to evaluate and measure BNI potential of molecules in plant soil systems. It detects the inhibition and then measures its capacity as well on the basis of bioluminescence emission. It measures BNI in allylthiourea (ATU) units.
These advanced tools help to frame and determine the capacity. It will significantly improve the knowledge and research into subject and will ensure the use of biomolecules as N regulating agents in agricultural regimes.
In modern agriculture intensive and indiscriminate use of fertilizers is causing global pollution. Major portion of these fertilizers are N fertilizers and after application their dominated form is nitrate form which is more prawn to losses and fixation. Due to this fact NUE is decreasing increasing cost of production also. BNI is a novel approach to address this problem. Future research endeavours must be direct in a way to establish more pragmatic tools and protocols for such experimentation. Screening, detection, potential capacity and stability of a wide range of phytochemicals should be focused regarding biological nitrification inhibition. It would be a great breakthrough to improve crop nutrition and ultimately agricultural economics as well as global food security.
The writers are associated with the University of Agriculture, Faisalabad, Pakistan.
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