ASSESSMENT OF BIOLOGICAL NITROGEN FIXING POTENTIALS OF PIGEONPEA GENOTYPES INTERCROPPED WITH SORGHUM FOR SOIL FERTILITY IMPROVEMENT IN SOUTHERN GUINEA SAVANNA OF NIGERIA

INTRODUCTION In the tropics, nitrogen is the most important nutrient required by plants and it is often the most limiting (Kaleem,2000). Biological nitrogen fixation (BNF) is important in legumebased cropping systems when fertilizer nitrogen is limiting. A combination of legumes and cereals is popular, mainly due to the economic value of the legumes and the expectation of sustaining the soil fertility (especially nitrogen) over years (Ofori and Stern, 1987; Kumar Rao et al.,1996). The inclusion of pigeonpea in intercropping systems helps to minimize competition for nitrogen with the cereal component (Adu-Gyamfi et al., 1997) because pigeonpea is able to meet a proportion of its own N requirement through BNF (Kumar Rao et al. 1987) and also improve the soil organic matter status of the systems. Kumar Rao et al. (1987) estimated that between 68-88kg of symbiotically fixed N was derived from pigeonpea in India depending on the season. Using sorghum as the non-fixing control and N-difference method, estimates of fixed N in pigeonpea genotypes of different maturity ranged between 6-69kg N per ha (Kumar Rao and Dart, 1987). AduGyamfi et al. (1997) reported that the amount of N fixed by pigeonpea estimated by the 15 N naturalabundance method, during the entire growth period of 210 days, was 120-170kg N per ha The amount was higher in intercrop than Agro-Science Journal of Agriculture, Food, Environment and Extension Volume 6, Number 1 January 2007


INTRODUCTION
In the tropics, nitrogen is the most important nutrient required by plants and it is often the most limiting (Kaleem,2000).
Biological nitrogen fixation (BNF) is important in legumebased cropping systems when fertilizer nitrogen is limiting.A combination of legumes and cereals is popular, mainly due to the economic value of the legumes and the expectation of sustaining the soil fertility (especially nitrogen) over years (Ofori and Stern, 1987;Kumar Rao et al.,1996).The inclusion of pigeonpea in intercropping systems helps to minimize competition for nitrogen with the cereal component (Adu-Gyamfi et al., 1997) because pigeonpea is able to meet a proportion of its own N requirement through BNF (Kumar Rao et al. 1987) and also improve the soil organic matter status of the systems.
Kumar Rao et al. (1987) estimated that between 68-88kg of symbiotically fixed N was derived from pigeonpea in India depending on the season.Using sorghum as the non-fixing control and N-difference method, estimates of fixed N in pigeonpea genotypes of different maturity ranged between 6-69kg N per ha (Kumar Rao and Dart, 1987).Adu-Gyamfi et al. (1997) reported that the amount of N fixed by pigeonpea estimated by the 15 N natural-abundance method, during the entire growth period of 210 days, was 120-170kg N per ha The amount was higher in intercrop than .sole-croppedpigeon pea (Katayama et al., 1995;Tobita et al., 1994).Quantification of the amount of nitrogen fixed by the newly introduced pigeon pea genotypes under intercropping with sorghum in Southern Guinea Savannah of Nigeria was undertaken to assess their N contributions to the system and their role in the improvement of farm productivity.Quantitative data on such are meagre or, in most cases, lacking in literature.

MATERIALS AND METHODS
Field experiments were conducted each year during the wet seasons of 2002 and 2003 at National Root Crops Research Institute sub-station, Otobi (07 0 10 N 08 0 39′ E elevation 105.1m) in Benue State located in the Southern Guinea Savanna ecological zone of Nigeria (Kowal and Knabe, 1972).Rainfall at the site was 1712.00mm between June and November in 2002 and 1665.60mmwithin the same period in 2003.A preplant soil sample was taken to the depth of 0-30cm and characterised as sandyloam with a pH of 7.60 in water.Organic carbon was 2.23%, total nitrogen, 0.71%, available phosphorus (Bray 1) 36ppm, and potassium 0.14%, CEC, 31.80meq/100g in 2002.In 2003, a pre-plant soil sample analysis also characterised the soil as sandy loam with pH in water, 7.20, organic carbon 2.3%, total nitrogen, 1.04%, available phosphorus (Bray1) 72ppm,and potassium, 0.17%, CEC, 40.40meq/ 100g of soil.The experiment was a 3 x 15 factorial set out in a split plot design with three replications.The main plot treatment consisted of two sole cropping systems (pigeon pea or sorghum) and one intercropping (pigeon pea + sorghum).The sub-plot treatments were 15 pigeon pea genotypes, including a local check, namely: -ICPL 85010, ICPL 84031, ICPL 87, ICPL 161 (short duration), ICPL 8863, ICPL 85063, ICPL 87119, ICPL 7120, ICEAP 00068 (medium duration), ICPL 7035, ICPL 8094, ICPL 87051, ICPL 9145, ICEAP 00040 (long duration) and Igbongbo.The sorghum variety was the tall (4m or more) photoperiod-sensitive red sorghum popularly used by the local farmers.The use of the sorghum as the non-fixing plant was borne out of its popularity among the farmers in this region.The gross plot size was 3.0m x 4.0m while the net plot measured 2m x 2m.The pigeon pea genotypes and sorghum were sown as sole or intercrops on ridges spaced 1m apart.The optimum plant population densities for the respective sole crops of pigeon pea and sorghum were 66,666 and 40,000 per ha.The replacement series method of mixture was adopted.The agronomic parameters measured on pigeonpea genotypes were: number of nodules per plant, nodule biomass per plant, leaf litter at harvest and shoot dry matter yield.Soil tests were done in each of the years of experimentation.Soil samples were taken at 0-30cm depths from each treatment plot, air-dried and ground to pass through a 0.3mm screen for chemical analysis.Nitrogen in soil was estimated by indophenols colour formation method (Chaykin, 1969) after micro-Kjeldhal digestion.At harvest, oven-dried plant parts (pigeon pealeaves, stems, roots, fallen parts, nodules and pod with seed) were ground to pass through a 0.3mm screen and used for chemical analysis.Nitrogen concentration in plant parts as estimated by the indophenol colour formation method (Chaykin, 1969) after micro-Kjeldhal digestion was determined.Similarly, oven-dried shoot samples of intercropped pigeonpea and sorghum were separately ground to pass through a 0.6mm screen for chemical analysis.Nitrogen yield in shoot samples was determined as outlined by Chaykin (1969)

Estimation of N -fixation:
Estimation of nitrogen fixation in the cropping systems was determined by the N-difference method using the formula outlined by Papastytianou (1999) for estimation of the apparent net amount of atmospheric N 2 fixed by legumes in short-and longterm cropping systems: N 2 = (L-M) + (fi -fm) Where N 2 = amount of nitrogen fixed by system; L = N harvested in a N 2fixing legume M = the amount of N harvested in a non-N 2 _ fixing crop grown under the same condition as the legume; fi = soil N after the legume fm = soil N under the non-N 2 _ fixing crop.This equation assumes that the legume and the non-legume crop absorbed the same amount of soil N. The soil N value (fi-fm) in the equation could be positive, zero or negative, depending on whether the legume system removed less, equal or more soil N than the nonlegume grown in monoculture.The amount of total N fixed per ha for each pigeon pea genotype was obtained by multiplying the proportion of N derived from N fixation (P) by the dry shoot weight of each genotype per ha in the intercrop system.All data generated were analysed using GENSTAT Release 3.2 (Copyright 1995, Lawes Agricultural Trust Rothamsted Experimental Station).Statistical tests or mean differences and treatment effects followed standard analysis of variance procedures for a split plot `design (Gomez and Gomez, 1984).Wherever differences between treatment means were significant, mean separation was by F-LSD at 5% probability (Obi, 1986).Similarly, intercropping decreased nodule biomass production in such genotypes as ICPL 87 and Igbongbo and increased that of ICPL 8863, ICPL 85063, and ICPL 7120 in the two years of experimentation (Table 2).The effect of intercropping on nodule biomass yield of the remaining genotypes tested seemed inconsistent as reflected by the cropping system means.In 2002, monocropping had significantly higher nodule biomass (x=2.02g)than intercropping (x = 0.85g), while in 2003, intercropping produced higher nodule biomass (x =2.82g) than monocropping (x = 2.05g) (Table 2).6 and 7. Nodules had the highest concentration of total nitrogen among the plant parts of the pigeon pea genotypes intercropped with sorghum, followed by pod with seed, leaf, root, stem and fallen parts in that order.The total nitrogen concentration was higher in sole cropped pigeon pea plant parts than in intercropped pigeon pea, particularly, in pod with seed and nodules.The concentration of total N in nodules and fallen parts were nearly the same, irrespective of the cropping system adopted.The results of total N concentration in pigeon pea plant parts in intercrop or monocrop for both years of experimentation showed a similar trend.The interaction between cropping systems and genotypes on the nitrogen yield of shoot of pigeon pea genotypes intercropped with sorghum was not significant in both 2002 and 2003 (Table 8).The total nitrogen yield of shoot of pigeon pea varied significantly with pigeon pea genotypes and ICPL 87 had the highest total nitrogen yield of shoot in both years of experimentation under intercropping.The influence of cropping systems on nitrogen yield of pigeon pea genotypes intercropped with sorghum was inconsistent in both years of the study.The total N yield of shoot of sole sorghum was superior to that of intercropped sorghum combined with all pigeon pea genotypes in 2002and 2003, except ICPL 7120 in both years, and ICPL 87, ICPL 87051 and Igbongbo in 2003 (Table 9).Mean total nitrogen concentration of shoot of intercropped pigeon pea genotypes at harvest in both years (2.38%) was higher than that of intercropped sorghum (1.83%) in both years combined (Tables 8 and 9).This result was consistent with the findings of earlier workers (Nambiar et al;1983;Wahua and Miller, 1978) who observed reduced nodulation and nitrogen fixation by groundnut cultivars mixed with sorghum and attributed the response to the effects of shading.The exceptional response of ICPL 7120 in 2002 and the early-maturing genotypes (ICPL 85010,ICPL 84031,ICPL 87,ICPL 161) under intercropping in 2003, suggested that shading from the sorghum might not have been intense enough to influence nodulation in these genotypes, considering the time (56 days after planting) at which nodulation score was done.The decreased nodule biomass of ICPL 87 and Igbongbo under intercropping with sorghum was in agreement with the findings of Nambiar et al. (1983) who reported decreased nodule dry weight of groundnut intercropped with sorghum, millet and maize.Kaleem (2000) similarly reported decreased nodule biomass in soybean intercropped with sorghum.These workers attributed this response to adverse shading effects due to the tall cereal canopy.Reduced light in intercropping situations could affect nodule biomass by restricting photosynthesis of the pigeon pea shoots and consequently the energy supply to the nodules.But other workers (Searle et al., 1981) have also found that this negative response observed with ICPL 87 and Igbongbo under intercropping is not invariably the case as found with ICPL 8863, ICPL 85063 and ICPL 7120, which had increased nodule biomass.The erratic response of the other intercropped pigeon pea genotypes with sorghum in nodule biomass yield might have resulted from the different distribution of rainfall received in the two experimental years.Some earlier workers (Kumar Rao and Dart, 1979;Thompson et al., 1981) reported that nodule formation and development are affected by soil type, the season and the duration of the cultivars of pigeon pea.The reduction in leaf litter of pigeon pea intercropped with sorghum as compared to monocrop situation might be due mainly to the fewer number of plants in intercropped plots as compared to the plants in sole crop environment.The means of leaf litter produced by pigeon pea under intercropping in 2002 (0.76 t/ha) and in 2003 (0.79 t/ha) are small compared to the 3.0 t/ha of leaf fall from intercropping of pigeon pea with maize in Malawi (Sakala, 1994).Sakala (1994) further stated that if seeds of intercropped plants are harvested for food, the leaf fall is sufficient to make a significant contribution to N accumulation.Kumar Rao et al. (1983) also reported less leaf fall under intercropping than in pure stands.
Significant reductions in shoot dry weight of intercropped pigeon pea as compared to sole pigeon pea might derive from reductions of dry stem weight, leaf weight, dry pod weight and dry grain yield (data not provided).Earlier studies in pigeon pea +sorghum intercropping (Kumar Rao et al.,1983;Natarajan and Willey, 1980a;Tobita et al.,1996;Katayama et al.,1995) reported severe reductions in shoot growth and consequently dry matter accumulation in the shoot of intercropped pigeon pea as compared to Monocropping.These reports further explained that the canopy of the cereal develops more rapidly and is relatively unaffected by the intercrop, whereas the pigeon pea canopy is shaded and its growth is substantially reduced and consequently accumulation of dry matter is drastically reduced in the shoot.Soils under both mono-and inter-crop pigeon pea had slightly higher N than soils under mono-and inter-crop sorghum in either of the years, although this was not statistically significant.Soils under Môn cropped pigeon pea gave total N of soils ranging from 0.73-0.96% in 2002 and 0.79-1.03% in 2003 while that of intercropping varied from 0.73-0.93 in 2002 and 0.76-1.03 in 2003.Wani et al. (1996) reported higher mineral N content in soils under legume as compared to non-legume at ICRISAT Asia Centre (IAC), India.
Intercropped pigeon pea plant parts accumulated less N compared to the Môn cropped pigeon pea plant parts.This is in agreement with the findings of Tobita et al.(1996) that less N was accumulated in the shoot of intercropped pigeon pea than of sole pigeon pea.This might be due to shading and competition for N between the intercrop components.The higher concentration of N in pods with seeds as compared to leaves and stem were similar to the observations of Devries (1986) and Lawson and Troedson (1990) that between 28 and 56% of the N accumulated by the pigeon pea shoots was recovered in the seeds.Several reports (Kumar Rao et al., 1983;Giller, 2001) have shown that N concentration is highest in nodules of all plant parts in pigeon pea, consistent with the observations in the current study.This suggested that inter-species competition and shading might have exerted more effect on N-accumulation under intercropping environment.This is consistent with the findings of Kumar Rao et al. (1983) andTobita et al. (1996).The interaction between cropping systems and genotypes on the nitrogen yield of shoot of pigeon pea genotypes intercropped with sorghum was not significant in both 2002 and 2003.The total nitrogen yield of shoot of pigeon pea varied significantly with pigeon pea genotype.The high dependence of shoot N on pigeon pea genotype rather than on the pigeon pea/sorghum intercropping system agreed with the findings of Kaleem (2000) which revealed variability in N yield among soybean cultivars intercropped with sorghum.The total nitrogen yield of shoot in intercropped pigeon pea genotype at harvest in both years (2.57%) was higher than that of intercropped sorghum (1.83%) in both years combined.The values of total N yield of shoots of intercrop pigeon pea and sorghum obtained in this work were higher than the 1.49% and 0.52% for pigeon pea and sorghum, respectively obtained by Ito et al. (1997) indicating higher N values for shoot of pigeon pea than shoot of sorghum.
The total nitrogen fixed by pigeon pea under intercropping with sorghum varied with the genotypes while ICPL 87 had the highest value of the nitrogen fixed in both years of experimentation.
The significant variability in biological nitrogen fixation among pigeon pea genotypes intercropped with sorghum supported earlier findings of Kumar Rao et al. (1983) andKumar Rao et al. (1996) that variability existed among legumes and genotypes of legumes in the amount of N fixed and the proportion of plant -N Egbe, O.M. derived from biological nitrogen fixation.ICPL 87 fixed the highest amount of N (164.82kg/ha),followed by Igbongbo (99.74kg/ha).The good performance of Igbongbo in nitrogen fixation in the pigeonpea/sorghum intercrop might be ascribed to its long association with the indigenous symbiotic bradyrhizobia.The values of the atmospheric nitrogen fixed by pigeon pea under intercropping with sorghum were comparable to values reported by Peoples and Craswell (1992).Results showed clearly that pigeon pea under intercropping with sorghum can lead to a reduction of nitrogenous fertilizer use of up to 37.52 -164.82kg N/ha depending on the pigeon pea genotype.

Table 3
shows the leaf litter produced by pigeon pea genotypes intercropped with sorghum.Intercropping reduced leaf litter produced by pigeon pea in all genotypes for both years of the study.The reductions were to about 50% level