Response of Faba Bean to Starter Nitrogen Dose Application and Rhizobial Inoculation in the Major Growing Areas of Arsi Zone

Application of starter Nitrogenous fertilizer with rhizobial strains have been hypothesized to increase yield and yield parameters. Hence, to exploit productivity potential of Faba bean(Vicia faba L) a field study was conducted in Arsi zone 2015/16 and 2016/17.The test crop was uninoculated, inoculated with strain FB-1017,and supplied with six rates of Nitrogen (T1 = -Ve control ( No input),T2 = 9 N kg/ha,T3 = 18 N (kg /ha),T4 = 27 N (kg /ha),T5 = 36 N (kg /ha),T6 = 54 N (kg /ha)).The treatments were laid out in randomized complete block design with split plot arrangements with three replications. Application of inoculants and starter nitrogen at different levels indicated that plant height (at three locations), grain yield (at one location) and number of seeds per pod were improved significantly (p<0.05) otherwise did not improve none of yield and yield parameters in both years. In that case, though inconsistent a grain yield of more than five quintals per hectare than the negative control (uninoculated, unfertilized) was found by the highest fertilized treatment (54 kg ha-1). The results suggest that application of starter nitrogen to small holder farmers is not economical and inoculation with trust worthy and viable strains would suffice in compromise to environmental and profit margins and more studies in different locations ought to be executed for a few locations as such might not give required result.


Introduction
Enhancing agricultural productivity is one of the central challenges to achieving food security and poverty reduction in Ethiopia. Considering the fact that soil fertility is one of the biggest challenges, an obvious strategy is to increase fertilizer application and promote good agronomic practices to enhance productivity (Birhan Abdulkadir et al., 2017). Among which Nitrogen is one of the most abundant elements on earth. However, it is one of the most limiting factors of growth and production of crops. Which its requirements exceed any other and rarely do soils in the tropics have enough of this nutrient to produce high sustainable yields (Otieno et al., 2007).Currently inorganic fertilizer is an immediate supply but by far the most important source of fixed nitrogen derives from the activity of certain soil bacteria that absorb atmospheric N2 gas and convert it into ammonium which according to Zahran (1999) and Simon et al. (2014) approximately reduce 20 million tons of atmospheric nitrogen to ammonia which is 50% -70% of the world Biological Nitrogen Fixation.
Integration of multipurpose, N-fixing legumes into farming systems commonly improves soil fertility and agricultural productivity through symbiotic associations between leguminous crops and Rhizobium. However, the contribution of N fixation to soil fertility varies with the types of legumes grown, the characteristics of the soils, and the availability of key micronutrients in the soil to facilitate fixation, and the frequency of growing legumes in the cropping system (Birhan Abdulkadir et al., 2017).
It is widely acknowledged that inoculation of legumes with effective rhizobia can improve yields and provide a substitute to inorganic fertilizers and different research works made in recent years revealed that inoculation of Faba Bean with R.leguminosarum increase yield by 10-50%(Asfaw Hailemariam and Angaw Tsgie, 2003;Chemining'wa et al., 2007). The technology, therefore, is good for Ethiopian soils where 85% are reported to have low levels of Nitrogen(EIAR, 2014) and several field demonstrations have confirmed that leguminous crops show remarkable growth and yield response to rhizobia inoculations in different agroecologies in Ethiopia. As a result, the use of rhizobia inoculants has shown spectacular growth in Ethiopia (EIAR, 2014).
Faba Bean is a crop of high economic value with its edible seed serving as an important protein complement in the cereal based Ethiopian diet (Tamene Temesgen et al., 2015) and also contribute to smallholder income (FAO, 2014). Furthermore, it supplies an important added value to the crop by fixing atmospheric nitrogen in symbiosis, with root nodule bacteria known as Rhizobium leguminosarum bv. Viciae (Mutch and Young, 2004).An estimate of 240-235 amounts of N2 fixed (kilograms of N2 fixed per hectare) by Faba Bean with Faba Bean-rhizobial symbioses (Somasegaran and Hoben, 1994) thus, reducing costs by less fertilizer use and minimizing impact on the environment by natural soil maintenance (IFPRI, 2010;Alghamdi et al., 2012).
Currently rhizobial inoculants are widely used in various parts of the world. They are the solution to dwindling soil fertility, inexpensive, environment friendly, and easy to use with no side effects in most cases(Wondwosen Tena et al., 2016).In response to this, promising Faba Bean Rhizobia screening activities were carried out during the past ten years in the country and revealed that there is diversity in different agroecologies (Abere Mnalku et al., 2009;Alemayehu Workalemahu, 2009;Zerihun Belay and Fassil Assefa, 2011;Anteneh Argaw, 2012;Solomon Legesse and Fassil Assefa, 2014;Dereje Tsegaye et al., 2015;Getahun Negash 2015;Wendesen Melak et al., 2018). Indeed,he symbiotic interaction is initiated after an initial exchange of signals: when nitrogen present in the soil is scarce, legumes exude a series of phenolic compounds into the rhizosphere, mainly flavonoids and isoflavonoid (Clúa et al., 2018),however, Crop yield increases of 51-158% were reported in nitisols at Holleta due to the combined application of 20 kg ha-1P with strain over non-inoculated ones ,none of the above studies reported on the agronomic efficiency, profitability and the level of risk associated with applying starter N fertilizer on Faba bean.
However crucial is the use of organic inputs as external nutrient sources and has been advocated as a logical alternative to expensive fertilizers in Africa, application of starter Nitrogen with biological nitrogen fixing bacteria has been hypothesized to ameliorate yield and yield related parameters, but not explored.This paper reports results from a study on the effects of starter N fertilizer application and to evaluate the symbiotic potential of popular faba bean against nitrogen fixing rhizobial isolates, the essence of starter N application and to determine the starter N dose that the farmer should use and its effects on yield, agronomic efficiency, profitability, the risk associated with faba bean, in selected districts of Arsi Zone.

Study sites
The study was undertaken for two years in seven districts of two potential Faba bean production weredas namely Tiyo &Limu-Bilbilo( Figure) The soils are classified as follows: at Bekoji a haplic Nitisol, and at Kulumsa an intergrade between a haplic Nitisol and a luvic Phaeozem(Amanuel Gorfu et al., 2000) where Wheat is dominantly produced with mean annual rain fall 823 mm and 1020mm respectively of Kulumsa and Bekoji.

Planting and Agronomic Practices
Field experiments were carried out in the two successive years of growing seasons, 2015/2016 and 2016/2017at seven locations in Arsi zones where Faba bean production is at potential and monoculture production system is dominant. Faba bean seeds were sown in the rate of 100 kg seeds ha−1 and were cultivated in strips. Each block (4m× 37.2 m) consisted of twelve plots. Each plot area was 10.4m 2 and consisted of 10 rows, spaced 0.4m apart. An additional eleventh row was placed in each plot and served as a border, and was not involved in calculations. Each strip was spaced apart by 1m apart to prevent bacterial migrations. Weeds, insects, and fungal pathogens were controlled by chemical spray applications, as required, at rates according to manufacturers' recommendations. At harvest, yield was determined by the manual mechanical harvesting of the entire plot.

Statistical Analysis
The ANOVA model was used according to Jones and Nachtssheim (2009)

Results and Discussions
Rhizobial inoculation with starter-N treatments had no significant effect on all of parameters studied (Table and  Table ) the two years growing seasons but plant height at Bekoji where unfertilized plots performed significantly (p<0.05) less than other treatments during the first year. This result did not agree to previous studies, that goes in line with the general notion that biological nitrogen fixation is affected by inorganic nitrogen. Starter nitrogen caused decline in grain yield that was explained could suppress nodulation and hence yield related parameters, but it was also noted that a moderate dose of starter-N demonstrated to stimulate seedling growth and subsequent nitrogen fixation .Inorganic Nitrogen is required by legume plants during the 'nitrogen hunger period' for their nodule development, shoot and root growth before the onset of N2-fixation. The success of legume grain crops is dependent on their capacity to form effective nitrogen-fixing symbioses with root-nodule bacteria (Youseif et al., 2017). However, many soils may do not have adequate amounts of native rhizobia in terms of number, quality, or effectiveness to enhance biological nitrogen fixation.
The increments in seed yields in most of the N-fertilized plots and/or inoculated plots, in relation to the uninoculated non-N fertilized plots controls indicate that, in these soils, nitrogen is not a limiting factor, and that crop yields could be strongly improved by means of competitive and viable strains inoculation but fertilization might not help. However, we found that response to inoculation with the best rhizobia strains was greater comparably better than the full N fertilization even though statistically not different. Our results showed that faba bean inoculation could effectively reduce the need of applied inorganic N-fertilizers while achieving higher grain yield, but in this particular case even the main plot factor ,that inoculating did not have improved difference than un inoculated control while a study reported in Youseif et al. (2017) from which effective inoculation improved grain yield by 35%-48% and faba bean yield and yield components could be significantly improved through the combined use of Rhizobium/Agrobacterium inoculations and starter N application (48kg N·ha−1) under low fertility sandy soil conditions but according to Chemining'wa et al. (2007) even Rhizobia inoculation failed to improve yield .The mean grain yield in the two years varied 3.69 to 4.34 ton ha -1 in 2015/16 and 2.80 to 5.40 ton ha -1 in 2016/17 which actually is far better than the national average 1.91 and 2.05 ton ha -1 (CSA, 2016;CSA, 2017),that can be ascribed to the relatively low precipitation and higher temperature (Figure ) during floral initiation when nodule initiation Is maximum Means followed by the same letter within a column are not significantly different at the P=0.05 level using LSD test

Conclusions and recommendations
Application of starter nitrogen with faba bean had not had any apparent effect on yield and yield components. The reason could be attributed to poor inoculant viability, adequate soil mineral, or highly competitive indigenous strains, besides to that Legumes have a high internal phosphorous requirement for their symbiotic nitrogen fixation, in addition to the nodule formation, deficiency of phosphorous in legumes also markedly affects the development of effective nodules and the nodule leghaemoglobin content which directly affect productivity. It is therefore suggested that the status of available phosphorous in soils ought to be taken through as in the soils of the experimental field may be beneficial to nodule nitrogen fixation through the prevention of the decrease of the phosphorous concentration in the plants at the later growth stage. Hence For better verification of this investigation it is also suggested to repeat the study in other carefully evaluated in prevalent N depleted in small holder farmers with other cultivars and bio-fertilizers and even the inoculants of different strains of rhizobia that was formed from indigenous soils of Arsi.