Effect of Application of Nitrogen-enriched Humic Acid (NHA) on Morphological ‎and Physiological Characteristics of Maize (Single cross 704) ‎

Nitrogen (N) plays a major role in maize growth and yield. Therefore, adequate supply of N is required ‎for successful maize production. However, application of chemical nitrogen fertilizers is associated with ‎some problems such as groundwater ‎pollution, nitrogen enrichment of surface waters, and nitrate ‎accumulation in agricultural products. ‎Accordingly, nowadays a great attention has been paid to the ‎slow-release fertilizers. Nitrogen-enriched humic acids ‎‎(NHAs) are considered as promising slow-release ‎nitrogen fertilizers in agricultural systems. However, the ‎effects of these types of fertilizers on plant growth ‎and physiological characteristics have not been well ‎understood. For this purpose, the present study ‎investigates the effectiveness of NHAs on the ‎morphological and physiological characteristics of maize as ‎well as nitrogen loss through leaching. ‎

‎The Nitrogen-enriched humic acids (NHAs) were prepared through the simple process of nitration, and ‎from the reaction of nitric acid with humic acid (HA) ‎extracted from leonardite of Yazd Golsang Kavir ‎Company as an organic carbon source. Then, a ‎greenhouse experiment in a completely randomized ‎design (CRD) with three replications was conducted ‎to determine the effects of 16 treatments, including ‎control, urea (U1, U2 and U3), humic acid ‎‎(HA1, HA2 and HA3), nitrogen-enriched humic acid (NHA1, ‎NHA2 and NHA3), urea-humic acid (U1HA1, U2HA2 ‎and U3HA3), and urea-nitrogen-enriched humic acid ‎‎(U1NHA1, U2NHA2 and U3NHA3) on the morphological ‎and physiological characteristics of maize plant ‎‎(Single cross-704). The levels of treatments were ‎determined as the quarter (50 mg N kg-1), half (100 mg ‎N kg-1) and equal (200 mg N kg-1) to the maize ‎fertilizer requirement. In the combined treatments of urea ‎and HA or NHA, an equal fraction of the total ‎nitrogen was considered. After the end of the experiment, ‎using the standard methods, some ‎characteristics including root length, leaf area, plant height, root ‎volume, wet and dry weights of shoot ‎and root, leaf chlorophyll index, concentrations of phosphorus, ‎potassium, nitrogen and nitrate, and ‎nitrate reductase activity in both shoot and root were determined. ‎Moreover, during the experiment and ‎on given days, the maize pots were leached and the obtained ‎leachate was collected for the nitrate ‎measurement.

‎Findings

According to the results, the nitrogen content of the produced NHA (3.3%) was about two times‏ ‏higher ‎than the HA (1.6%). In addition, the NHA had higher‎‏ ‏carboxyl and phenolic hydroxyl content than the ‎HA. The FT-IR analysis showed the characteristic peaks of nitro (NO2) groups at wavenumbers of 1541 ‎and 1336 cm-1 in the spectrum of NHA. Germination test indicated that the NHA was not toxic to the maize ‎seeds. The results showed that the NHA treatments had a much better influence on the plant ‎morphological characteristics than ‎the ‏HA treatments. This observation may be due to the negative effects ‎of HA application at high dosages. In comparision, the UNHA treatments were only slightly more efficient ‎than the urea treatments. Combining‏ ‏NHA with urea diminishes the adverse impacts of separate ‎application of these two fertilizers. On average, leaf chlorophyll index and concentrations of total ‎nitrogen, nitrate and nitrate reductase enzyme in shoot part of plants in the NHA treatments were 11.5, ‎‎17.0, 35.2 and 29.4% higher than the HA tratments. The nitrate reductase concentration in the roots was ‎‎40.4% lower than the shoots. However, the UNHA and urea treatments showed almost similar efficiency ‎in improving physiological characteristics. The U3NHA3 or U3 treatments, i.e. the highest level of ‎nitrogen, showed the highest efficiency which means the high nitrogen requirements of maize in pot ‎experiments. Based on the results, the nitrogen supply to the maize plant increased the shoot ‎concentration of potassium higher than that of phosphorus. Although the U3 treatment indicated ‎the ‎highest nitrogen and nitrate concentrations in both root and shoot, the highest nitrate leaching was ‎also ‎observed for this treatment. However, by using the U3NHA3 treatment, the mean concentration of ‎nitrate ‎in the leachate decreased by about 48.7% as compared to the U3 treatment. ‎

‎Findings of this research revealed that the combined fertilizer of UNHA can be ‎a good alternative for ‎urea. It could not only supply nitrogen for plants, but could improve plant vegetative ‎growth, and in turn ‎considerably reduce nitrate leaching, which has highly beneficial effects on nitrogen use ‎efficiency as well ‎as environmental issues.‎