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.