Effect of Nitrohumic Acid Application on Some Morphological and Physiological Characteristics of Savory Plant (Satureja hortensis L.)

Savory is considered one of the most important medicinal plants, which is used in various food and medical industries. Nitrogen (N) plays a major role on the growth and yield of medicinal plants. Therefore, an adequate supply of N is required for successful production of savory. However, the application of chemical N fertilizers is associated with many obstacles such as groundwater ‎pollution, N enrichment of surface waters, and drop in the quality of plants. ‎Accordingly, nowadays, great attention has been paid to organic fertilizers. In this regard, humic acid-based fertilizers have shown promising results. Humic acids (HAs) could be converted into nitrohumic acids (NHAs) through the nitration process, in which nitro groups (NO2) are located on the aromatic rings. This process increases the N content of the HA. Thus, ‎‎NHAs can be used as organic N fertilizers in the cultivation of medicinal plants whose organic production is a priority. However, the ‎effects of these types of fertilizers on plant growth and physiological characteristics have not been well ‎understood. Accordingly, the present study for the first time investigates the effectiveness of NHA on the ‎morphological and physiological characteristics of savory, as well as N loss through leaching.‎

In the current study, HA was initially extracted from leonardite (purchased from Yazd Golsang Kavir Company) as a rich source of HA. Then, NHA was prepared through the nitration process using nitric acid (50% by volume). After that, using FT-IR (Fourier transform infrared spectroscopy) and CHNS analysis the extracted HA and NHA were characterized, and their N content was determined. Afterward a ‎greenhouse experiment in a completely randomized design (CRD) with three replications was conducted ‎to determine the effects of 16 treatments, including control (without urea, HA and NHA), urea (U1, U2 and U3), humic acid ‎‎(HA1, HA2 and HA3), nitrohumic acid (NHA1, NHA2 and NHA3), urea-humic acid (U1HA1, U2HA2‎ and U3HA3), and urea-nitrohumic acid (U1NHA1, U2NHA2 and U3NHA3) on the morphological ‎and physiological characteristics of savory plant. The treatment levels were ‎determined as 40, 80, and 120 mg N kg-1 for the first, second and third level of the treatments, respectively. In the combined treatments of urea and HA or NHA, an equal fraction of the total nitrogen (N) was applied. At the end of the experiment, standard methods were used to assess various 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, nitrate, and nitrate reductase in both the shoot and root. Additionally, leaching was conducted on specific days during the experiment, and the leachate was collected for nitrate measurement.

The results showed that using the nitration process, some characteristics of the NHA such as total acidity, the content of carboxylic and phenolic groups as well as N content improved as compared to the initial HA. Moreover, the results indicated that most of the morphological and physiological ‎traits of savory plants, including leaf area, plant height, root length, fresh and dry weights of root and shoot as well as chlorophyll index, and the concentration of nitrogen, phosphorous, potassium, nitrate and nitrate reductase enzyme were significantly higher in the NHA treatments than those of HA. In addition, the highest shoot dry weight was obtained in the combined treatments of U3NHA3 and U3HA3 as well as in the U3 treatment alone. The average rate of nitrate concentration increase in the U treatments was 1.77 times higher than the UNHA treatments. According to the results, U3 treatment indicated the highest nitrate loss which by using the U3NHA3 treatment, the mean concentration of nitrate ‎in the leachate decreased by about 40.5% as compared to the U3 treatment.‎

The findings of this research revealed that most of the morphological and physiological ‎traits of savory plant showed better responses to the combined treatments of U3NHA3 and U3HA3 as well as to the U3 treatment alone. However, with regard to the lower accumulation of nitrate in the shoot of savory as well as to the lower nitrate leaching, the combined treatments were preferred. Accordingly, NHA can be ‎a alternative nitrogen source in increasing the yield and growth indicators of savory. However, the reasons behind the fact of the better performance of combined nitrogen treatments than the individual ones require more research in the future.