Groundnut (Arachis hypogeaa L.) productivity in Nigeria has remained low over the years due to the inherent low fertility status of the soils of the savanna, a region where it is mostly grown by small holder farmers with limited imputs. Groundnut can fix atmospheric nitrogen through symbiotic association with native rhizobia but unfortunately, the amount of N2 fixed is usually not enough due to the presence of ineffective or low numbers of native rhizobia. This study was designed to investigate the comparative response of groundnut to rhizobium inoculation and nitrogen fertilizer with or without phosphorus fertilizer on an Alfisol in the Northern Guinea Savannah of Nigeria. Soil samples were collected from a P-deficient plot on the Institute for Agricultural research/ Faculty of Agriculture (IAR/FOA) experimental farms located in Samaru Zaria and used to conduct two screen house trials using the groundnut genotype “SAMNUT 24” as the test crop. The first experiment consisted of six (6) inoculants (four indigenous rhizobial strains namely: SNN 343, KBU 26, SBG 234, SAMFIX 703 one commercial inoculant (HISTICK) and a reference strain (NC 92) alone, the inoculants each combined with 20 kg N ha-1mineral nitrogen as starter dose, and mineral nitrogen alone at 0, 20, or 40 kg N ha-1, adding up to fifteen treatments all termed as nitrogen (N) sources. The second factor was phosphorus at three rates (0, 30 or 60 kg 2O5 ha-1) applied as single super phosphate. The N sources (SNN 343, KBU 26, SBG 234, SAMFIX 703, NC 92, HISTICK, SNN 343 +20N, KBU 26 +20N, SBG 234 +20N, SAMFIX 703 +20N, NC 92 +20N, HISTICK +20 N, 0 kg N ha-1, 20 kg N ha-1and 40 kg N ha-1) and P rates (0 kg P2O5 ha-1, 30 kg P2O5 ha-1 and 60 kg P2O5 ha-1) were factorially combined (fifteen x three) to give a total of forty five (45) treatments and were laid down in a Randomized Complete Block Design replicated three times. The second trial was set up to assess the residual benefit of rhizobium inoculation on following crops. Soils previously inoculated with SNN 343, KBU 26, SBG 234, SAMFIX 703, NC 92 and HISTICK and a control, were combined factorially with three levels of P (0, 30 or 60 kg P2O5 ha-1) to give a total of twenty one (21) treatments. The RCBD was also used with three replications. A non- nodulating groundnut variety (ICGL 5) was included as a reference crop to estimate the amount of biological nitrogen fixed. In the first trial, the use of mineral nitrogen either alone or in combination with rhizobium inoculants suppressed nodulation. Addition of 20 kg mineral nitrogen per hectare to rhizobium inoculation as starter dose reduced nodule number by an average of 12.2 % but increased groundnut shoot dry weight by an average of 9.5 % and 9.8 % compared to inoculation alone and control respectively. The use of this starter dose of mineral nitrogen also increased the amount N2 fixed by 58.4 % compared to inoculation alone and by 51.7 % compared to the control. A similar trend was also observed for % Ndfa. The application of P at 60 kg P2O5 ha-1 gave rise to significant increases in all the parameters measured. However, P uptake efficiency and P agronomic efficiency decreased when P rate increased from 30 kg P2O5/ha to 60 kg P2O5 ha-1. Inoculation alone increased phosphorus uptake efficiency by 29.3 % while addition of starter nitrogen at 20 kg per hectare gave an increase of 35.4 % compared to the control. In the second trial, the residual effects of inoculation failed to produce a significant increase in most of the parameters measured. On average, shoot dry matter declined by 46.3 %, nodule number by 45 %, nodule dry weight by 53 % across all treatments when plants depended on the residual effect of the previous inoculation. Similarly the amount of nitrogen fixed decreased by an average of 34 % across all treatments except KBU 26 which indicates lack of persistence of the rhizobium strains and the need for inoculation with each sowing. KBU 26 amongst all the inoculants increased the amount of nitrogen fixed and % Ndfa by 18.1 % and 52.8 % respectively compared to the first trial indicating that the strains persisted in the soil. The result of this study shows the importance of the use of rhizobium inoculants coupled with a starter dose of mineral nitrogen at 20 kg N/ha to enhance BNF. It also shows that the application of P at 60 kg P2O5 ha-1greatly enhanced yield components and BNF indicating the beneficial role of phosphorus fertilizer on the growth and productivity of groundnut even though P supplied at 30 kg P2O5 ha-1was more efficiently used by the crop than at 60 kg P2O5 ha-1. The residual effect of KBU 26 amongst all the inoculants increased the amount of nitrogen fixed and % Ndfa indicating that the strains persisted in the soil. This potential when fully assessed and harnessed may constitute an evident advantage over the use of inorganic nitrogen fertilizer which has to be applied frequently for consistent high yields.?

CHAPTER ONE: INTRODUCTION

1.1 Background of the Study
Groundnuts (Arachis hypogaea L) also known as peanuts, is a leguminous crop, a member of the genus Arachis in the family Leguminosae. It is widely grown in the tropics and subtropics due to its nutritional and economic importance. Groundnut is the most widely grown major legume worldwide cultivated in 118 countries and occupies more than 22.6 million ha that produce about 36.4 million MT, with average yield of about 1600 kg ha-1(Abate et al., 2012).
Groundnut seeds (kernels) contain 40 -50 % fat, 20 - 50 % protein and 10 - 20 % carbohydrates (ICRISAT, 2003). Groundnut seeds are nutritional source of vitamin E, niacin, calcium, phosphorus, magnesium, zinc, iron, ribloflavin, thiamine and potassium (Kumar and Shankar, 2013). In most of the developing countries it provides high-quality cooking oil and is an important source of protein for both human and animal diet and also provides much needed foreign exchange by exporting the kernels and cake. The uses of groundnut plant therefore, make it an excellent cash crop for domestic markets as well as for foreign trade in several developing and developed countries (FAO, 2002).
In Africa, groundnut is grown mainly in Nigeria, Sudan, Senegal, Chad, Ghana, Congo, and Niger. Average productivity is 1720 kg ha-1 in Nigeria which is poor when compared to the USA and other developed countries where it is close to 3500 kg ha -1 (Vara-Prasad et al., 2009). The crop is grown mostly by smallholder farmers under rainfed conditions with limited inputs (Samson, 2012).
Majority of the soils of the Guinea savanna of Nigeria are inherently low in fertility especially organic matter, phosphorus and nitrogen (Odunze and Kureh, 2009; Oluwasemire and Alabi, 2004). To address this challenge, farmers in the northern Guinea savanna use strategies such as application of organic and inorganic fertilizers (Samson, 2012). The use of inorganic fertilizers is effective but costly (Obisesan et al., 2013); this therefore, leads to application at sub- optimal level that is, below the recommended rate (Olawale et al., 2009). On the other hand, organic fertilizers, such as crop residues and animal manures are bulky.They contain relatively low concentration of nutrients and handling them is laborious. ICRISAT (1995) recommended an application of 10 -12 t ha-1of chicken manure or 20 t ha-1of well decomposed farm yard- manure for groundnut production. Wamba et al. (2012) gave the nutrient content of poultry manure as 21.76 N g kg-1, 8.74 P g kg-1 and 11.22 K g kg-1.
This however depends on the handling conditions. Much of the N in manure may be lost to the air if they are allowed to dry out or stored for a long time. Unlike cereals, legumes are able to supplement their nitrogen (N) demand and contribute to soil N through various processes. Legumes fix atmospheric nitrogen (N2) in the soil through its symbiotic relationship with N- fixing bacteria. Legumes also add nitrogen to the soil through falling leaf litter and, to a lesser extent by decaying roots and root nodules below- ground, and thus they have great potential for restoring degraded soils. The organic matter produced by legumes is generally rich in nitrogen and of good quality, meaning that it decomposes quickly and is a good source of nitrogen for other plants (Giller, 2010). The ability of legumes to fix N2 allows farmers to grow them with minimal inputs of N fertilizer (Lupwayi et al., 2011). Therefore, farmers usually intercrop non-legumes with leguminous crops including groundnut having considered legumes to stabilize crop yields and also serves as a source of income and protein for their families. In the northern savanna of Nigeria, the bulk of groundnut is produced by small scale farmers using traditional system of mixed cropping with maize, millet and sorghum. (Samson, 2012).
Nitrogen is considered as the most limiting plant nutrient for crop production in West Africa (Sangakkara et al., 2003). Despite its abundance in the atmosphere, plants are unable to use it directly because it is present in an inert form (N2) and the nitrogen in the soil is lost through microbial dinitrification, soil erosion, leaching, chemical volatilization, removal of nitrogen containing crop residues from the field. As earlier mentioned, groundnuts like other legumes play a primary role of fixing atmospheric N through their symbiotic relationship with Rhizobium spp., usually associated with the host's root system. This contributes nitrogenous compounds to the soil, either directly, by nodule excretion, or indirectly, by decomposition of root nodules and tissues (Giller, 2003). Biological Nitrogen Fixation (BNF) is an inexpensive, renewable resource option for smallholder farmers, permitting them to redirect limited farm investment toward other pressing household needs.
One of the ways to increase biological nitrogen fixation is by inoculating legume seeds with rhizobium inoculants. Research has also shown that legumes grow best if there is some mineral N available as nodules form and a small amount of starter nitrogen (10 to 30 kg ha-1) at planting may increase total BNF over the crop?s lifetime (Woomer, 2010). This starter dose of nitrogen is necessary due to the lag period between rhizobium colonization and the onset of nodule functioning. The starter dose however, increases yield only on soils that are extremely deficient in nitrogen and where crop yield potential is high ( Kucey, 1989;Woomer, 2010).
Phosphorus (P), in addition to its role in crop nutrition is an essential element for rhizobium bacteria to convert atmospheric N into ammonium (NH4+) and ammonia (NH3) which is usable by plants. Phosphorus in?uences nodule development through its basic functions in plants as an energy source. Furthermore, phosphorus plays a major role in many plant processes such as storage and transfer of energy, stimulation of root growth, flowering, fruiting and seed formation, nodule development and N2 fixation (McLaren and Cameron,1996).?
One of the frequently asked questions with respect to rhizobial inoculation is the frequency at which legume crops should be inoculated (Drew et al., 2014). The answer to this question is connected to the ability of the rhizobial strain to persist in the soil. Rhizobia, once introduced to the soil, will be affected by the same factors that affect native rhizobia: vegetation, soil moisture, pH, and temperature. Where soil conditions are favourable, rhizobia are able to survive in the soil for many years, even in the absence of their legume host (Drew et al.,2014). Therefore the inoculation of legume seeds with reasonably large population of effective and persistent rhizobial strains constitutes an evident advantage over inorganic nitrogen fertilizer which has to be applied frequently for consistent high yields (Ojo and Fagede, 2002).

1.2 Statement of Research Problem
The soils of the dry savannas of Nigeria where groundnut cultivation is most popular are generally sandy, poor in terms of nutrient content arising from low organic carbon, low nitrogen and phosphorus content (Ajeigbe et al., 2015). Farmers use strategies such as application of organic and inorganic fertilizers to address this challenge. These strategies have their own limitations which include the costly nature of inorganic fertilizers leading to application at sub-optimal levels (Obisesan et al., 2013; Olawale et al., 2009), while the organic fertilizers are not available in the right quantity and quality to meet the crop?s requirement. Rhizobial inoculants have been used to address the problems of soil fertility and inadequate fertilizer application in grain legume production and have been found to be a cheaper and usually more e?ective agronomic practice for ensuring adequate N nutrition of legumes, compared with the application of N fertilizer (Paynel et al., 2008). However, som results obtained from inoculation studies in comparison with the use of nitrogen fertilizer reveal that biomass production and yields were increased significantly by mineral N application over inoculation (Ahmed et al., 2014; Abubakar, 2015). The successful use of rhizobial inoculants as a substitute for nitrogen fertilizers for groundnut production depends on the ability to screen and select rhizobia strains that will produce higher yields than the use of nitrogen fertilizers.
Differences have been observed in the response of groundnut to rhizobial inoculation and Phosphorus fertilizers and this may be linked to the findings of Cassman et al. (1981) who reported that rhizobium strains differ markedly in external P requirements for growth which seem to affect their effectiveness. Assessing the performance of different strains of rhizobia under different phosphorus levels is important in determining the strains of P- efficient rhizobia best suited for the P-deficient Alfisols of the northern Guinea savanna of Nigeria.

1.3 Justification of the Study
Woomer (2010) reported that nitrogen fixation in groundnut is about 150 kg N ha-1 which offers strong residual benefits to following crops. However results obtained in Nigeria with respect to the amount of nitrogen fixed by groundnut gave much lower range of values. Yakubu et al. (2010) reported that groundnut inoculated with rhizobia fixed 27.19 kg N ha-1.
There is a need for research to screen bacterial strains and traits that are useful and necessary for different environmental conditions and plants so that optimal bacterial strains that will improve nitrogen fixation and yields of groundnuts can be selected.Biodiversity and economic potential of African rhizobia is largely unexplored, yet potential exists for native rhizobia to outperform exotic commercial strains. A study conducted by Yusuf et al. (2011) in the northern Guinea savanna of Nigeria showed that the commercial rhizobial inoculants used gave no increase in groundnut yield. The rhizobium strains in the commercial inoculants used were not only ineffective but were inferior to the indigenous soil population. Therefore, it is vital to assess the effectiveness and competitiveness of indigenous strains against commercial strains in order to identify potentially useful inoculants strains.
There is also a need for further assessment of existing commercial inoculants to assist in the selection of rhizobia with specific symbiotic and competitive attributes suited to a range of soil environments.
Addition of starter dose of N at planting according to Woomer (2010) has the potential to increase total BNF over the crop?s lifetime and this may be useful especially on the soils of the northern Guinea savannah of Nigeria (Alfisols) which are highly deficient in Nitrogen. Several researches carried out confirmed the importance of phosphorus for groundnut production (Kamara et al., 2011, Abdul-latif, 2013, Nwokwu, 2011, Amba et al., 2013). However, the recommended rates vary between 20 - 45 Kg P ha-1depending on locations and soil types, hence the need to determine the adequate rate of phosphorus that will improve groundnut yields and in the case of inoculation, to determine the effectiveness of different rhizobial strains under different P levels in the northern Guinea savanna of Nigeria.

1.4 Aim and Objectives of the Study
The aim of this study is to improve symbiotic biological nitrogen fixation in groundnut on an Alfisol in the northern Guinea savanna of Nigeria through the use of rhizobial inoculants, nitrogen and phosphorus fertilizers. The specific objectives are:
1. To determine the effectiveness of groundnut rhizobia vis-a-vis nitrogen fertilizer and the effect of phosphorus fertilizer on the yield components and biological nitrogen fixation in groundnut.
2. To evaluate the effect of rhizobial inoculation vis-à-vis nitrogen fertilizer and the effect of phosphorus on phosphorus use efficiency in groundnut.
3. To evaluate the residual benefit of introduced rhizobia and phosphorus fertilizer on biological nitrogen fixation and yield components of groundnut.?