Abstract: A field experiment was conducted during 2005-2006 and 2006-2007 growing seasons to determine, the effects of Rhizobium inoculation and different levels of nitrogen on the yield and growth of field pea (Pisum sativum sp. Arvense L.). Nitrogen application had significant effect on the plant height, number of branches, root and shoot dry weight, number of nodules, seed yield, biomass yield, harvest index, number of pods, as well as crude protein rate of seeds in both years. Plant height, number of pods, harvest index, number of nodules, crude protein rate and root dry weight were higher with application of 20 kg N ha^-1 while seed yield, shoot dry weight and number of branches were higher with application of 60 kg N ha^-1, in the 2 seasons. The lowest values related to these parameters were obtained from the control treatment. Inoculation treatment had also significant effect on the all parameters investigated in both years. The highest values regarding these parameters were obtained from inoculated plants, whereas the lowest values were obtained from the uninoculated plants. Interactions of nitrogen and inoculation had significant effects on the all parameters investigated, except for number of nodules in 2005-2006 and crude protein rate of seeds in both years. The highest seed yield was obtained under treatment 20 kg N ha^-1 + inoculation with 1654.3 and 1625.0 kg ha^-1 in 2005-06 and 2006-07, respectively.

INTRODUCTION

Nitrogen is major nutrient element for grain legumes. Although, nitrogen is abundant element in the atmosphere in gaseous form, plants cannot directly utilize it. Availability of nitrogen amount in soil is getting decreased by plant cultivation, increased oxidation in soil or it can also be easily washed away. Therefore, nitrogen becomes a limited factor in the plant cultivation. The main ways of the nitrogen application into soil are the mineral nitrogen fertilizer or Rhizobium inoculation of seeds or soil in order to obtain biological fixation of nitrogen. High cost and its dependence to moisture is the main disadvantageous of mineral fertilizer. Biological nitrogen fixation is an important nitrogen source due to the fact that it requires less energy and causes less environmental pollution.

Dry pea is a pulse crop and a member of the family Leguminacea. This crop like many other legumes is capable of fixing and utilizing atmospheric nitrogen through symbiotic relationship with Rhizobium. The crop thus, improves soil, economizes crop production reducing the requirement of added synthetic nitrogenous fertilizers. When properly inoculated with an appropriate Rhizobium inoculant, pea can derive up to 80% or more of its nitrogen through nitrogen fixation. The rest of the N must be provided from the soil or from fertilizer applications (Ali-Khan and Zimmer, 1989; Bowren et al., 1986). Seed inoculation is the most widely used methods of inoculant application. Many researchers (Rahman et al., 1994; Solaiman and Rabbani, 2005) have reported the beneficial effects of inoculation of grain legumes.

Nodule formation and subsequent nitrogen fixation are very sensitive to external nitrogen sources, including fertilizer and available soil nitrogen. Nitrogen fertilizer applications generally inhibit biological N-fixation by R. leguminosarum. The inhibitory effect of N fertilizer on nodule formation results from the fertilizer’s contribution to the soil N pool. Bowren et al. (1986) concluded that nodule formation became inhibited as soil N levels approached 40 kg ha^-1 and were progressively inhibited as levels exceeded this amount. This suggests that N fertilizer applications will reduce biological N-fixation by R. leguminosarum, except where the amount of N applied as fertilizer plus that contained in the soil is <40 kg ha^-1. Small N fertilizer applications have stimulated nodule formation on pea roots in some low N environments and other pulse crops (Kauskik et al., 1995). It can take 3-4 weeks after planting before nodules are fully functioning. Early plant growth may be poor in soils with low nitrogen levels and plants may appear yellow prior to the beginning of effective nitrogen fixation due to a nitrogen deficiency. This early deficiency can be overcome by adding low levels of starter nitrogen at seeding. Although, high levels of starter nitrogen may appear to help the crop overcome a nitrogen deficiency during early crop growth stages, final seed yields may not increase. Bowren et al. (1986) suggested that applications of 9-18 kg N ha^-1 enhanced pea seedling growth prior to nodule development in low N soils. The soil of the region where the study was conducted has low nitrogen and organic matter content and their availabilities are low due to the various soil and other environmental factors. Therefore, it was aimed to determine the effects of nitrogen fertilizer and Rhizobium inoculation on the yield and growth parameters, as well as crude protein rate of seeds in field pea.

MATERIALS AND METHODS

The study was carried out at experimental field of the Yüzüncü Yil University, Van, Turkey, where legumes have not be grown for at least 10 years, in 2005-06 and 2006-07 growing seasons using field pea genotype with number 110121 adapted to the region. Seeds were inoculated using peat inoculant included a mixture of nodule-forming strains of Rhizobium leguminosarum bv viciae specific to pea. Peat inoculant was prepared as commercial peat cultures by Soil and Fertilizer Research Institute, Ankara, Turkey, according to the method of Somasegaran and Hoben (1994). Peat inoculant was kept in refrigerator at +4°C until use. It was separately provided from Soil and Fertilizer Research Institute in 2005-06 and 2006-07 growing seasons. Content and activity of peat inoculant was checked before trials of 2005-06 and 2006-07. Peat inoculant was used after cells count were adjusted to 1x10⁸ Rhizobium cells g^-1 so content of peat culture was standardized by diluting peat inoculant. The Most Probable Number (MPN) method was used for estimating viable cells of Rhizobium (Somesagaran and Hoben, 1994).

The soil of the experimental field was clay-loamy in texture, slightly alkaline (pH 7.8), poor in nitrogen (0.065%) and organic matter (1.43%) and had an average level of phosphorus (539.1 ppm). The soil received rainfall 391.8 mm and 363.1 mm in 2005-06 and 2006-07, respectively. Annual average temperature was 10.5°C in 2005-06 and 9.4°C in 2006-07.

The study was conducted in split-plot design with 3 replications. In order to eliminate the contamination, inoculation treatments (with and without) were randomly applied to main plots and nitrogen doses (0, 20, 40 and 60 kg N ha^-1) as ammonium sulphate were randomly applied to the subplots before sowing. Plot size was 5x1.2 m. The seed was sown by hand with 25 cm row spacing in late October in both years (18 October 2005 and 20 October 2006). The seeding rate was 45 seeds m^-2. A basal dose of 60 kg ha^-1 triple super phosphate was given to each plot at the time of sowing. Seeds were inoculated with R. leguminosarum bv viciae bacteria at the recommended rate before sowing in all plots, except for uninoculated plots (Vincent, 1982). Application of the peat inoculant on the seeds was carried out by water, which contains 2% sugar. The experiment was carried out as rainfed. Plots were hand-weeded 2 times when needed in each season. Plants were harvested in late June in both years (24 June 2006 and 26 June 2007). At flowering, 10 random plants were removed from each plot. Numbers of nodules as well as dry weight of root and shoot/plant were recorded. Similarly, 10 plants from each plot were removed at the harvest to determine pods number per plant as well as plant height and branches number per plant. At harvest, 2 outer rows for each plot and 50 cm from each end of the plots were left as borders and the middle 4 m of the central rows were harvested. Biomass yield and seed yield were determined after harvest. In addition to these criteria, crude protein rate of seeds were determined using the methods proposed by Kacar and Inal (2008).

The influence of treatments on the field pea and differences among treatments were analysed using analysis of variance procedures for split-plot design in randomised blocks. Means of measured parameters were compared using Least Significance Difference (LSD) at p<0.05.

RESULTS AND DISCUSSION

The results indicated that the effects of nitrogen application and inoculation on the investigated parameters differed significantly between years. In general, compared to those in 2006-07, occurred significant increases in the values of parameters in 2005-06 when the soil received higher amount of rainfall (Table 1 and 2). Nitrogen application had significant effect on the plant height, number of branches, root and shoot dry weight, number of nodules, seed yield, biomass yield, harvest index, number of pods, as well as crude protein rate of seeds in both years. The lowest values related to these parameters were obtained from the control treatment. Plant height, number of pods, harvest index, number of nodules, crude protein rate and root dry weight were higher with application of 20 kg N ha^-1 while seed yield, shoot dry weight and number of branches were higher with application of 60 kg N ha^-1, in the 2 seasons.

Table 1:	Effect of inoculation and nitrogen on the seed yield, biomass yield, harvest index, number of pods and crude protein rate

Table 2:	Effect of inoculation and nitrogen on the root and shoot dry weight, number of nodules, plant height and number of branches
ns: Means in the same column are not significantly different; Means in the same column followed by the same letter are not significantly different at p<0.05

Table 3:	Interaction effects of inoculation and nitrogen on the seed yield, biomass yield, harvest index and number of pods (2005-06)
Means in the same parameter followed by the same letter are not significantly different at p<0.05

However, differences between 20 and 60 kg N ha^-1 were not significant in terms of plant height, number of pods, harvest index and number of branches (Table 1 and 2). The number of nodules differed significantly among nitrogen treatments. However, it was observed that nodules formed also in the uninoculated plots. Availability of the nodules in the uninoculated treatment indicates that experimental soils contain native rhizobial population. Increases in these parameters by the application of 20 kg N ha^-1 indicates the importance of starter nitrogen dose in the legumes for improving the efficiency of rhizobia for biological nitrogen fixation responsible for higher growth. These findings are close conformity to the findings of Igbasan et al. (1996) and Azad et al. (1992).

Inoculation treatment had also significant effect on the plant height, number of branches, number of nodules, root and shoot dry weight, number of pods, seed yield, biomass yield, harvest index and crude protein rate of seeds in both years (Table 1 and 2). The highest values regarding these parameters were obtained from inoculated treatment, whereas the lowest values were obtained from the uninoculated treatment. However, differences between inoculated and uninoculated treatments were not significant in terms of harvest index in both years and root dry weight in 2006-07.

Table 4:	Interaction effects of inoculation and nitrogen on plant height, number of branches, root and shoot dry weight (2005-06)

Table 5:	Interaction effects of inoculation and nitrogen on the seed yield, biomass yield, harvest index and number of pods (2006-07)

Table 6:	Interaction effects of inoculation and nitrogen on plant height, number of branches, root and shoot dry weight and nodules number (2006-07)
Means in the same parameter followed by the same letter are not significantly different at p<0.05

This positive relationship of yield and growth parameters with inoculation could be related to the nitrogen fixation ability of nodules, which consequently results in increased growth and yield. Similar results are found by Jha and Singh (1996) and Idris and Sandhu (1981).

Interactions of nitrogen and inoculation had significant effects on the seed yield, biomass yield and root and shoot dry weight, number of pods, plant height, number of branches and harvest index in both years, except for crude protein rate in both years and number of nodules in 2005-06 (Table 3-6).

While the application of 20 kg N ha^-1 + inoculation gave the highest values related to all parameters, the lowest values were obtained from control without inoculation treatment. The highest seed yield was recorded in 20 kg N ha^-1 + inoculation treatment with 1654.3 kg ha^-1 in the 1st year and 1625.0 kg ha^-1 in the 2nd year. These findings are in agreement with the results of Semu et al. (1982) and Kaya (2000).

CONCLUSION

It may be concluded that 20 kg N ha^-1 + inoculation gave highest values related to yield and yield components of field pea. Thus, 20 kg N ha^-1 + inoculation treatment could be preferred to grow in areas where amount of nitrogen are limiting biological nitrogen fixation in field pea production.