Biological nitrification inhibition-an efficient approach
Nitrogen (N) is an essential element, needed by plants in highest quantity followed by carbon. It is crucial for the synthesis of proteins, DNA, RNA, chlorophyll, vitamins, plant hormones and many other biomolecules. Almost 20-50 g of nitrogen is taken up by the plant to synthesize 1 kg of dry matter thus crop yield is highly dependent on nitrogen supply. Annually 88 Mt of nitrogen fertilizer is utilized by agriculture sector. Agriculture sector is the main contributor towards the global warming due to emissions of nitric oxide (NO) and Nitrous oxide (N2O) from cultivated soils. Every year about 17000 kg of applied N fertilizers becomes the atmospheric part as N2O which is 300 times more harmful to ozone layer than carbon dioxide. According to recent estimates, the global N2O emissions will surpass four folds due to high nitrogenous fertilizer inputs in the coming future.
The biological oxidation of NH4+1 to NO3-1 is known as nitrification that converts relatively immobile form of nitrogen (NH4+) to highly mobile form (NO3-1) by ammonia oxidizing bacteria (AOB) especially Nitrosomonas species in the soil. In most of the plants, some of NO3-1-N is directly assimilated into the root cells and remaining NO3-1-N is transported to the upper parts of the plant like shoot via xylem. In shoot cells nitrate is reduced to nitrite by the enzyme nitrate reductase, meanwhile, the enzyme nitrite reductase converts some nitrate molecules into ammonium. This NH4+1-N directly taken up by the roots from soil solution and transported to shoots via ammonium transportrs. Then it is assimilated by the plants into amino acids through GOGAT pathway. It has been reported that out of total added ammonical N fertilizer, 90 per cent is nitrified into nitrate within four weeks. Nitrate thus formed is highly vulnerable to leaching. This huge and rapid conversion of ammonium into nitrates limits the effectiveness of applied nitrogen by high nitrogen losses from the soil. So, by controlling high rates of soil nitrification process nitrogen use efficiency (NUE) can be improved.
Many fertilizer management strategies like band, basal, deep as well as foliar application of nitrogen fertilizer have been practiced to improve nitrogen use efficiency but have limitations as they are laborious and expensive. Many synthetic compounds have been commercialized for nitrification inhibition but few of them have been considered and patented as nitrification inhibitors (NIs). Among these dicyandiamide, nitrapyrin and 3, 4-dimethyl pyrazole phosphate perform better under field conditions. But Synthetic nitrification inhibitors are not being applied widely in agriculture sector due to their high cost, less efficacy and inconsistency under different soil and climatic conditions.
Now, Nitrogenous fertilizers are being coated with some synthetic organic compounds which decrease the time of fertilizer release and hence enhance opportunity for the uptake of nitrogen by the plant. Polyolefin-coated urea (POCU) is very effective and makes the nitrogen available for plant at right time and substantially reduces the loss of nitrogen due to nitrification because most of the fertilizer is absorbed by the plant due to slow release. But POCU use is limited to high value horticultural crops because of its high cost about four to eight times more than normal urea and hence not cost-effective.
Allelopathy is a phenomenon in which plants and microorganisms like fungi and viruses influence biological and agricultural systems via secondary metabolites, this effect can be either stimulatory or inhibitory. The chemical compounds which are released by the plants and other micro organisms are termed as allelochemicals which are synthesized in all parts of plant like roots, shoots, leaves, seeds, and flowers and released into the surrounding environment by volatilization from leaves, decomposition of plant residues, and exudation from roots.
Allelopathy provides a natural and better option to check nitrification process in the soil whereby allelochemicals from plants suppress the activity of nitro bacteria. Allelochemicals which are released by plants in the rhizosphere to control the soil nitrification process are termed as biological nitrification inhibitors (BNIs). Many tropical forage grasses, cereals and legume crops have been evaluated for BNI capacity and includes creeping paspalum, sorghum, soybean and mammoth wild rye and sunflower etc. A cyclic diterpene (brachialactone), two unsaturated free fatty acids (linoleic acid and α-linolenic acid) and two phenyl propanoids (Methyl p-coumarate and Methyl ferulate) from Brachiaria humidicola, a phenyl-propanoid (Methyl 3-(4-hydroxyphenyl) propionate) from sorghum, Karanjin (3-methoxy-2-phenylfuro-[2, 3-h] chromen-4-one) from Pongamia glabra have been isolated and identified as BNIs because they block ammonium mono-oxygenase (amo) and hydroxylamine oxidoreductase (hao) enzymes responsible for nitrification reaction.
Most of the methods to control the nitrification process are expensive and laborious, while the allelopathy, being a novel approach, is not only cheaper, safer, environment friendly but also a comprehensive solution to improve the nitrogen use efficiency as well as healthy approach to reduce the pollution threats to our environment.
The authors are from the University of Agriculture, Faisalabad, Pakistan.