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Nitrogen is one of the major mineral nutrients that are required by plant for its growth and development. It is a major component of chlorophyll (a pigment responsible for food synthesis in plants) and amino acids which are the building blocks of proteins. Some proteins act as structural units in plant cells while others act as enzymes. Nitrogen deficiency in plants leads decreased growth and yellowing of leaves. Although nitrogen is present in all soils, addition of fertilizers improves the plants growth and yield. This is due to the fact that nitrogen is stored in the soil as organic matter and plants cannot take up this form. Soil N is easily soluble and is readily lost through drainage or volatilization (Corbeel, Hofman & Cleemput 1999; Hadasa et al. 1999; Inubushi, Barahona & Yamakawa 1999).
Nitrate (NO3-) and ammonium (NH4+) ions are two main forms of nitrogen easily available to the plants both of which are water-soluble (Wolt 1994). Plants absorb NO3- and NH4+ from the soils solution (Bloom 1988). Nitrate ions are absorbed quickly by plant roots, but leach easily. Ammonium ions are attracted to soil particles and move slowly through the soil to plant roots. Plant's preference to ammonium and nitrate varies depending on several environmental factors like soil pH, soil temperature etc and also the energetic cost involved. Even though plants prefer both NH4+ and NO3- many researches indicated that NH4+ has an inhibitory effect on the nitrate uptake of plants. Some scientists in the 70's said that there is a little or no effect of NH4+ on the NO3- uptake (Oaks, Stulen & Boesel 1979; Schrader et al. 1972a; Smith & Thompson 1971)). Early studies have showed that the inhibitory effect of NH4+ on NO3- uptake is because of the reversible inactivation of nitrate reductase in Chlorella fusca (Losada et al. 1970). But because of its inconsistency and rapidity with which it occurs, the explanation was difficult (Pistorius et al. 1976). The inhibition of NH4+ on NO3-uptake is a result of its effect on the net uptake of NO3- (Aslam, Travis & Huffaker 1994) or due to the effect on NO3- influx (Glass, Thompson & Bordeleau 1985) or by stimulation of efflux(Aslam, Travis & Rains 2001). In contrast to the above said inhibition, a number of studies demonstrated the preference of ammonium over nitrate when they are applied as mixed sources of N. While the concentration of NH4+ and NO3- were maintained at 10 mmol m-3, the absorption of NH4+ was more in barley at all temperatures as compared to NO3- (Macduff & Jackson 1991) but the preference varied with time. Plants grown in acidic soils favored NH4+ to NO3- where as plants cultivated in neutral soils prefer a mixture of both NH4+ and NO3- as their N source (Falkengrengrerup & Lakkenborgkristensen 1994). Past researches in corn demonstrated that a combination of NH4+ and NO3- as the nitrogen source increased the total nitrogen uptake of plants (Schrader et al. 1972b). In addition, Ammonium has a positive effect on the nitrate reduction in the corn seedlings as demonstrated by (Oaks, Aslam & Boesel 1977). However the interaction between ammonium and nitrate in the soil is unclear and not many studies are conducted on this.
Recent researches conducted by scientists in ACPFG found that addition of a small quantity of ammonium in the form of ferrous ammonium sulphate increased the plant growth compared to those plants where NO3- was the sole source of N. In the field conditions, ammonium is present in small quantities where addition of nitrate fertilizers increase plant growth and improve the yield. This research will focus on the contribution of this ammonium in the nitrogen uptake of corn plants when grown in hydroponics. The study will also answer the questions whether ammonium has any effect on the accumulation of Fe in the tissues. In addition experiments will be carried out to know the affect of ammonium on the total nutrition of maize plant. Ammonium and nitrate fluxes will be measured from the plants and the tissue analysis will be done to quantify the nitrate absorption and accumulation of other nutrients in the plants.
Sources of Nitrogen in Soils
Atmosphere contains about 78% of nitrogen in its gaseous form and it is the largest nitrogen reserve. This nitrogen must be converted before it becomes useful for plants in the soil. The presence of nitrogen in the soil is either through the deposition of atmospheric nitrogen by precipitation and lightning (Soderlund 1981), or from plant residues and animal manures. It is also available in the soil by nitrogen fixation or by the addition of commercial fertilizers. The availability of N not only affects the yield but also the growth, extent of penetration and morphological development of roots which in turn effects the tillering in cereals as demonstrated in wheat (Belford, Klepper & Rickman 1987).
About 90% of the totals N in the most soils are present in the organic matter produced by the microbial decomposition of plants and animal residues(Rosswall 1976). (Haynes 1986), in his studies pointed out that climate, vegetation, topography is some of the major factors that influence the availability of organic matter in the soil. The organic nitrogen present in the soil should be converted to either NH4+ or NO3-, so that it becomes available to the plants. As ammonium is nitrified to nitrate rapidly; nitrate is the most predominant form present in most of the soils (Haynes 1986; Tills & Alloway 1981). In tundra and boreal ecosystems, ammonium is the major form of nitrogen in soils because of low nitrification (Keeney 1980). Studies have also reported that surface soil constitutes about 5% and subsoil contains about 60% of fixed ammonium (Stevenson & Dhariwal 1959). But this fixed ammonium is not available to the plants. Under submerged conditions in wetland soils, where the main crop is rice, mineral N is available in the form of Ammonium because ammonium is stable under anaerobic conditions (Islam & Islam 1973).In most of the fertile agricultural soils the major form nitrogen is nitrate and plants prefer nitrate to ammonium because it can be mobilized easily to the different parts of the plants.
Plant preferences to Different Nitrogen Sources
One of the important factor in the discussion about plant absorption of ammonium and nitrates their preferences by different plant species. Both Ammonium and nitrate are easily absorbed by plants. Plants preferences to these forms vary. Plant uptake varies depending on the availability of the source of N. Many researches in their studies demonstrated that plants preferences for different forms vary at different stages of plant growth. Experiments on various plant species in arctic tundra region showed that most of them preferred Ammonium and glycine as their N source during their initial growth stages (McKane et al. 2002). He also pointed out that the most productive species Eriophorum preferred a combined N source glycine and ammonium which were the most available N form as compared to the nitrate preference by Carex, although nitrate was the least available form. Species specific preferences to either form of N have been established in some grassland (Weigelt et al. 2003) and in some alpine communities (Miller & Bowman 2003).
Experiments on Maize (Schrader et al. 1972b) and cranberry (Greidanu.T et al. 1972) showed more affinity towards ammonium nitrogen. Plants in cold climates prefer ammonium as it is the major form of nitrogen in these regions (Keeney 1980). There are exceptions like two species of Eriophorum from the arctic region used nitrate as their nitrogen source(Koch, Bloom & Chapin 1991) even under low ratio of nitrate nitrogen and ammonium nitrogen (Nadelhoffer et al. 1991). Thus plant preferences to different nitrogen forms vary depending on species variations, climate, Soil pH, root temperature and also based on the energetic cost involved in their assimilation.