oa Effects of Nitrogen Fertilization on Growth and Physiological Characteristics of Populus Sibirica Seedlings in a Desertification Area, Mongolia

Arid and semi-arid areas where desertification mainly occurs cover up to 40? of the world's land area. Mongolia is one of the arid and semi-arid areas, which 90? of the land is in effect of desertification. The government of Mongolia recognizes the need of afforestation for combating desertification, however, there are very limited practical afforestation techniques in Mongolia. Nitrogen fertilization, a major technique for afforestation, generally improves growth and physiological characteristics of plants. The optimal fertilizer application scheme may vary by locations due to the different responses of plants to nitrogen fertilization. Therefore, it is necessary to determine the optimal amount and the type of the nitrogen fertilizers for successful afforestation in the desertification area of Mongolia. The objective of this study was to investigate the effects of nitrogen fertilizer (three levels of amount and two types) on growth rate, photosynthesis and transpiration of Populus sibirica seedlings, a representative afforestation species, in Mongolia. In May, 2015, five plots (6 m ×  7 m) were installed at about 2 m distance apart and 2-year-old P. sibirica seedlings were planted in each plot; four plots for nitrogen fertilization and one plot for the control. Nitrogen fertilizers were applied to each seedling with 5 g (N1), 15 g (N2) and 30 g (N3) of urea and 33 g of ammonium sulfate (NS; same amount of nitrogen with N2). The number of seedlings in each plot was 22 for the control, 21 for N1, 29 for N2, 30 for N3 and 24 for NS plots, respectively. Each seedling was drip irrigated with 3 L per day for the first month and with 9 L at 3 day-interval for the rest of the period. Soil inorganic (NH4 ⁺ and NO3 ⁻ ) nitrogen concentration (mg kg^− 1), when measured 3 weeks after the fertilization, increased with the increasing amounts of nitrogen applied (Control: 2.16, N1: 4.33, N2: 4.84, N3: 5.97, NS: 5.53). Root collar diameter (RCD) and height of seedlings were measured in May and August, 2015. Growth rate of RCD and height were calculated as the increase of RCD or height from May to August divided by the initially measured value in May. Net photosynthetic rate and transpiration rate were measured by handheld photosynthesis system (CI-340, CID Bio-Science, USA) at 8:00?10:00 in June and at 16:00?18:00 in July (n?3). The differences in growth rate of RCD and height following nitrogen fertilization were analyzed using analysis of covariance and the differences in soil nitrogen concentration, net photosynthetic rate and transpiration rate following nitrogen fertilization were analyzed using one-way analysis of variance (SAS 9.3 software). Growth rate of RCD (?) was significantly higher only in the NS plot (14.99) than in the control plot (10.42). Growth rate of RCD of urea-fertilized plots did not significantly increase compared to the control plot and showed a tendency to decrease as the amount of urea increased (N1; 13.01, N2: 12.97, N3: 11.17). The decrease in growth rate of RCD with the increasing amounts of urea might be influenced by ammonia toxicity. When ammonium ion from urea is converted to ammonia in alkaline soils, ammonia toxicity which restricts growth of plants occurs. Although 33 g of ammonium sulfate (NS) has the same content of ammonium ion as 15 g of urea (N2), the growth rate of RCD was significantly increased in the NS plot. It can be explained that ammonium sulfate decreased soil pH which resulted in the decline of ammonia toxicity and improved uptake of nitrogen by roots. Growth rate of height (?) was 8.47 for the control, 8.42 for N1, 10.30 for N2, 9.29 for N3 and 8.74 for NS plots, respectively. There were no significant differences in growth rate of height among plots. It was related to the fact that trees concentrate more on diameter growth than height growth in arid environments. In June, net photosynthetic rate (μmol m^− 2 s^− 1) was significantly higher in the N2 (10.79) and NS (11.15) plots than in the control plot (4.05). There were no significant changes in net photosynthetic rate among plots in July, however, net photosynthetic rate showed relatively high values in the N2 (14.26) and NS plots (15.20), similar to the result of June. It seemed that nitrogen fertilization increased net photosynthetic rate. However, net photosynthetic rate was lower in the N3 plot, which had the highest amount of nitrogen, than in the N2 and NS plots. The reason for the lower net photosynthetic rate of the N3 plot can be related to the fact that excessive nitrogen decreases photosynthesis. In June, nitrogen fertilization significantly increased transpiration rate (mmol m^− 2 s^− 1) compared to the control plot (0.68). Transpiration rate of fertilized plots was highest in the N2 plot (2.79), followed by N1 (2.46), N3 (2.03) and NS plots (1.89). Transpiration rate in July revealed no significant differences among plots. Generally, transpiration rate might be increased by nitrogen fertilization with the increase of photosynthesis. However, the change of transpiration by nitrogen fertilization was significant in June, but not in July. It can be explained by the fact that the increase of transpiration occurs at the early stage after nitrogen fertilization. Growth rate of RCD was only increased in the NS plot, although net photosynthetic rate of the N2 and NS plots were higher than that of the control plot. This result might be related to the high transpiration rate of the N2 plot. It was reported that transpiration was decreased in order to reduce the water loss in dry environments. Although our study site is also a dry environment, transpiration was increased by nitrogen fertilization. We speculated that water stress caused by the increase of transpiration limits growth of seedlings. In conclusion, the response of growth and physiological characteristics of P. sibirica seedlings differed with the amount and the type of nitrogen fertilizer. 15 g of urea increased net photosynthetic rate and transpiration compared to other amounts of urea, but growth rate of seedlings did not differed with the amount of urea. Thus, the optimal amount of urea cannot be determined with the three amounts (5 g, 15 g, 30 g) used in this study. Ammonium sulfate and urea which have the same amount of nitrogen increased net photosynthetic rate. However, growth rate of seedlings was increased by ammonium sulfate. Ammonium sulfate seems to be more suitable fertilizer than urea for the early growth of seedlings in the desertification area of Mongolia. However, the effects of urea and ammonium sulfate on growth and physiological characteristics of P. sibirica seedlings were different. Therefore, further studies would be necessary to determine the optimal amount of ammonium sulfate.