Rhizobia populations were lowest in soils amended with DAP. This is in agreement with
previous studies that have shown that nitrogen fertilizers have a negative effect on
nodulation of common bean (Diouf et al., 2008; Bekunda et al., 2010). Mehrpouyan
(2011) reported reduced nodulation and nitrogen fixation in three common bean cultivars
planted in plots treated with commercial nitrogen fertilizers. Higher concentrations of
DAP have also been reported to lower the growth rate of rhizobia and lead to cell lysis
and death under laboratory conditions (Maheshwari et al., 2010). Reduction of microbial
activity in the soil can also occur due to toxicity of metal contaminants present in mineral
fertilizers especially P fertilizers that contain significant levels of mercury, cadmium and
lead (Bunemann et al., 2006). Mineral nitrogen inhibitory effects on nodulation and
nitrogen fixation depends on the concentration as it is the case with soybean plants in
which inhibitory effects of mineral nitrogen on nodulation and nitrogen fixation occur at
high concentrations (>5 mM), but it is very low at lower concentrations (Weisany et al.,
2013). Nitrogen fertilization affects nodulation of bean plants and the recommended rates
There were infective and effective rhizobia in soils treated with DAP. Plants inoculated
with soil that had been treated with DAP nodulated especially in the first soil serial
dilution of 10-1 and their leaves were deep green in color as compared to non nodulated
units at serial dilution 10-6 (Plate 4.11). It is therefore possible to get rhizobia isolates
tolerant to DAP fertilizer application in Lake Victoria Basin. Under laboratory conditions
Maheshwari et al. (2010) also reported finding rhizobia strains that maintains their
growth in high concentration of DAP and urea fertilizers. In his experiments 0.35M
concentrations of DAP were found lethal to most of the rhizobia isolates, tested but some
were tolerant.
There were significantly (p = 0.009) higher rhizobia populations in soils from plots
inoculated with commercial rhizobia strain (Rhizobium leguminosarum (strain 446)) in
comparison with soil from the uninoculated control plots. Similarly, Raposeiras et al.
(2006) reported that inoculation and continuous cultivation help build-up of rhizobia in
soil which can lead to enhanced nodulation and yield. Increased nitrogen fixation in
inoculated treatments as opposed to non inoculated treatments has been reported on
studies carried out on nitrogen fixation efficiency of indigenous and non native strains of
rhizobia (Mehrpouyan, 2011).
The rhizobia populations in soils from plots treated with water hyacinth compost were
significanly (p = 0.009) higher compared to those in the control experiment, soils
amended with DAP and commercial rhizobia inoculum. According to Reid et al. (2011)
levels which enhance microbial growth in the rhizosphere which may have lead to the
significantly (p=0.009) higher rhizobia populations in soils treated water hyacinth
compost as compared to those treated with DAP, rhizobia inoculum and the untreated
control.
Application of organic matter into the soil has been reported to fix the soil pH in acidic
soils for example, Suryantini (2014) reported change of acidic soil to almost neutral in
one cropping season after adding organic matter into soil which may also have
contributed to the higher rhizobia populations in the water hyacinth compost treated plots
as compared to the other treatments for near neutral pH in the soil enhances rhizobia
populations. Field studies carried out in Kakamega, Kenya in which soil was amended
with water hyacinth compost prepared using manure as starter culture and water hyacinth
compost prepared using EM, also demonstrated enhanced common bean nodulation as
compared to DAP treatment and untreated (control) soil (Naluyange et al., 2014).
Soils treated with water hyacinth compost prepared using EM had a significantly (p =
0.009) higher populations of rhizobia as campared to soils treated with water hyacinth
compost prepared using manure as starter culture. This is possibly because, EM is
reported to enhance nitrogen availability of compost by reducing C:N ratio more than
manure (Sangakkara et al., 2008). Organic matter with low C:N ratio, is reported to
provide rhizobia with nutrients over a long duration, which may enhance their population
in the soil (Sangakkara and Higa, 1994). Effective microorganisms (EM) is a mixed
lactic acid bacteria, yeast, ray fungi and molds (Hussain et al., 2007). Effective
microorganisms have the ability to breakdown organic matter releasing plant nutrients
and improving the physical and chemical properties of compost and soil (Hussain et al.,
2007).
Presence of common bean nodulating rhizobia in all soil treatments including the
untreated control soil, indicated the avalailabilty of indigenous common bean nodulating
bacteria in all the farms. Similar findings have been reported by Otieno et al. (2009), who
established that, nodulation of common beans and other legumes occurred even in soils
that were not innoculated with a rhizobia inoculum.
The rhizobia populations’ variation within the farms especially the control soils could be
due to the differences in soil characteristics. Various researchers such as Chemining’wa
et al. (2011); Mwenda et al. (2011) and Werner and Newton (2005), when they
researched using different legumes and sites attributed the differences in rhizobia
population to variations in soils characteristics including, soil type, soil moisture, levels
of soil pH, nutrient availabilty, temperature, crop/soil management and osmotic stress
among other factors.
Rhizobia populations significantly (p = 0.046, R2 = 0.203) increased with increase in soil
pH towards the neutral pH 7. Zengeni and Mpereki (2003) also reported significant
increase (p < 001) of rhizobia population in the soil with increase in soil pH. Acidic pH
and formation of nodules (Zengeni and Mpereki, 2003; Kawaka et al., 2014).
Molybdenum which is an important component of the nitrogenase enzyme complex is
usually low in acidic soils which may affect nitrogen fixation in such soils (Ferguson et
al., 2013). Acidic pH has been reported to limit the chances of rhizobia survival and
persistence in the soil, however some fast growing rhizobia like R. tropici and R.loti are
moderately acid tolerant and can infect their host at a pH of 4.5 ( Zahran, 1999).
Rhizobia populations in soils with higher available P concentration were significantly (R2
= 0.40, p = 0.003) higher than those with lower Av. P concentration. Available
phosphorus is essential for microbial population growth due its importance in energy
transfer process (Kisinyo et al., 2012). Phosphorus is also essential for the plant
metabolic processes that are important for growth and directly affects nodule structure
(Gentilli et al., 2006).
Indigenous rhizobia populations significantly (R2 = 0.327, p = 0.008) increased with
increase in SOM (soil organic matter). According to Das et al. (2010) soil organic matter
stimulates biological activity and improves microbial populations in the soil which may
have contributed to the high populations of rhizobia in soils with high SOM as compared
to those with low organic matter.
Rhizobia populations significantly (R2 = 0.243, p = 0.027)increased with increase in N
concentration in the soil however, presence of nitrogen in the soil at high concentration of
al., 2013). According to Oliveira et al. (1998), mineral N has been observed to stimulate
nodulation of soybean in soils with high Av. P. In this study the soils with high nitrogen
also had high Av. P concentration and therefore the high rhizobia populations in soils
with high nitrogen could have been contributed by the mineral nutrient balance between
Av. P and N.
Potassium (K) plays a role in enzyme activation and enhancement of legume nodulation
and nitrogen fixation (Mmbaga et al., 2014). In this study, indigenous rhizobia population
were observed to increase with increase in K concentration, however the relationship was
not significant (R2 = 0.413, p = 0.280).
Low calcium concentration in the soil is reported to inhibit formation of nodules
(Oliveira et al., 1998). In this study regression analyses showed an increase in rhizobia
populations with increase in calcium concentration in the soil, however, the relationship
was not significant (R2 = 0.03, p = 0.811).