Effects of Plant Density and Nitrogen Fertilizer on Nitrogen Uptake from Soil and Nitrate Pollution in Potato Tuber

MATERIALS AND METHODS

In order to evaluation of plant density and nitrogen fertilizer on nitrogen uptake from soil and nitrate pollution of potato tuber Agria cultivar, a factorial experiment based on randomized complete block design was carried out with three replications in Ardabil, Iran, in 2006. First factor was nitrogen level (0, 80, 160 and 200 kg ha^-1 net nitrogen) and second was plant density (5.5, 7.5 and 11 plant m^-2). Nitrogen was of urea source and applied in two stages, planting date and earthling up stage. Based on soil test pH depth of 0-30 cm, Total Saturated Electrical Conductivity (TSEC) was 3.68 mmhos cm^-1, soil pH was 8.09, total nitrogen was 0.56 % and soil texture was loamy sand. Rows were spaced 60 cm. Plots were included six rows each three meters. In order to preventing nitrogen effects in adjacent plots, they were placed 1.5 meters distance. Tubers of 60-70 g were sown in 13 May 2006. Sowing depth was 12-13 cm. Last harvest was assigned for yield. Promoting storage capability, 10 days before harvest, aerial parts were removed (Khajehpour, 2004). Sampling was done from 2 m² plot area, then, tubers were transferred to the laboratory.

Before measurements, tubers were washed along with roots and stolons. Different plant tissues were dried separately for 48 h in 75°C and weighed. Tuber nitrate pollution was calculated by sulfosalicylic acid method using spectrophotometer device (Cecile, France). Calculation of nitrogen uptake rate was made according to the Hashemidezfooli et al. (1998):

Where:

Results were analyzed by SAS software, mean comparisons were done via Duncan`s multiple range test and graphs were drawn by Excel software.

RESULTS AND DISCUSSION

Nitrogen Uptake
Results showed that simple effects of plant density and nitrogen level on nitrogen uptake by aerial parts and tubers (p<0.01) and interaction effects of plant densityxHnitrogen level (p<0.05) only for nitrogen uptake by aerial parts, were significant. Since, increasing nitrogen application led to over-growth of aerial parts and consequently, increase of leaves and stems dry weight, so, it increased nitrogen uptake. The most nitrogen was uptaken at 200 kg ha^-1 nitrogen and the less at control level, for all aerial parts. But in tuber, it was increased up to 160 kg ha^-1 and then, decreased (Table 1). Increment of density increased dry matter of aerial parts per unit area. This led to more nitrogen uptake in aerial parts and tubers so, the most and the less uptake was observed in 11 and 5.5 plant m^-2 (Table 1). With increasing plant density and constant rate of available nitrogen, competition for nitrogen, increased. In 200 kg ha^-1 nitrogen and 11 plant m^-2 treatments, the most uptake and in 80 kg ha^-1 nitrogen and 5.5 plant m^-2, the less uptake was observed (Table 2). Haase et al. (2007) reported that with increasing N application, nitrogen uptake in tuber was increased and it is in accordance with present study. Also, they revealed more nitrogen uptake by tuber in case of increased nitrogen. Since, nitrogen uptake in tuber per unit area increased as a result of plant density and nitrogen level increment, so this increase, has affected positively tuber yield and yield components and probably, the best reason to yield increment.

Nitrate Pollution
Simple effect of plant density and nitrogen and their interaction effect on tuber nitrate pollution based on dry weight (p<0.01) and fresh weight (p<0.05) was significant. With increasing nitrogen level, nitrate content in tuber dry and fresh weight significantly increased. More nitrate content in tuber, as a result of increase nitrogen application, has been reported by Wadas et al. (2005). Increase of density, reduced tuber nitrate pollution, as well (Table 1). Perhaps, this is because of low fertilizer distribution between the large number of plants and consequently, the tubers. In 200 kg ha^-1 nitrogen and 5.5 plant m^-2, the most nitrate pollution in fresh and dry weight, was observed (Table 2). In all nitrogen levels, Agria Cu. has accumulated the less nitrate rate in both fresh and dry tuber weight. Also, it could be found that nitrogen usage over the favorite range either caused to yield reduction or, increased nitrate accumulation in tuber.

Yield and Yield Components
Effects of plant density, nitrogen level (p<0.01) and their interaction effect (p<0.05) on tuber yield, were significant. Also, effects of plant density and nitrogen level (p<0.01) on tuber dry weight and tuber yield per unit area, were significant. Effects of plant density and nitrogen level (p<0.05) on mean tuber weight and number of tuber per unit area, were significant, as well. With increasing plant density, tuber yield was decreased per plant and increased per unit area (Table 1). This result has been reported by many of other researchers (Osaki et al., 1995; Georgakis et al., 1997; Karafyllidis et al., 1997). In 160 and 80 kg ha^-1 nitrogen and density of 5.5 plant m^-2, the most tuber yield per plant was observed (Table 2). Increment of nitrogen application up to 160 kg ha^-1, led to increase of number, dry weight, tuber yield per unit area and mean tuber weight and then decreased. This is because of high growth of aerial parts as a result of over-usage of nitrogen (more than 160 kg ha^-1) and consequently, increases of intra competition to reach water, mineral elements and light that significantly reduced yield and yield components at 200 kg ha^-1 nitrogen. In density of 5.5 plant m^-2, the most mean tuber weight was gained. Increase of plant density, decreased mean tuber size probably as a result of lack of available nutrient elements, intra competition or great number of produced tubers per plant (Beraga and Caeser, 1990).

Table 1:	Effect of plant density and nitrogen levels on measured traits
Numbers with same words in each column, have no significant differences to each other

Table 2:	Effect of plant density and nitrogen levels on measured traits
Numbers with same words in each column, have no significant differences to each other

CONCLUSIONS

In general, the most amount of nitrate in dry and fresh weight of tuber was observed in 200 kg ha^-1 nitrogen, 5.5 plant m^-2 treatments and the most tuber yield per plant was gained in jointly 80 and 160 kg ha^-1 nitrogen, 5.5 plant m^-2 treatments. Nitrate pollution at 80 and 160 kg ha^-1 net nitrogen was 170.52 and 214.47 mg kg^-1 tuber dry weight and 38.88 and 50.82 mg kg^-1 tuber fresh weight, respectively. At these nitrogen levels especially 80 kg ha^-1, nitrate accumulated was lower than critical range so, application of 80 kg ha^-1 nitrogen to gain most tuber yield with less nitrate pollution in tuber, is recommended for Agria Cu in Ardabil region. Noticing mean tuber yield in Ardabil region of 28.7 t ha^-1 and its comparison with yield of 80 and 160 kg ha^-1 nitrogen treatment (29.44 and 31.74 t ha^-1, respectively), it seems that can be recommended for this region. Also, density of 11 plant m^-2 is suitable to obtain planting seed (according to reduction of tuber weight and size). But, density of 7.5 plant m^-2 is recommended for eating usages.

ACKNOWLEDGMENTS

This study was supported by the Central Laboratory of Agricultural Faculty, University of Mohaghegh Ardabili. Valuable experimental support by Aziz Jamaati-e-Somarin and Rogayyeh Zabihi-e-Mahmoodabad is greatly appreciated. This work was extracted from M.S. thesis of Shahzad Jamaati-e-Somarin.