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Discoveries in Agriculture and Food Sciences - Vol. 11, No. 6
Publication Date: December 25, 2023
DOI:10.14738/dafs.116.16105.
Akuru, G., Tumuhairwe, J. B., Ebanyat, P., Tenywa, J. S., & Nabirye, L. D. (2023). Municipal Solid Waste Compost as Alternative
Carrier Material for Rhizobia Inoculants. Discoveries in Agriculture and Food Sciences, 11(6). 67-80.
Services for Science and Education – United Kingdom
Municipal Solid Waste Compost as Alternative Carrier Material
for Rhizobia Inoculants
G. Akuru
College of Agricultural and Environmental Sciences,
Department of Agricultural Production, P.O Box 7062,
Makerere University, Kampala, Uganda
J. B. Tumuhairwe
College of Agricultural and Environmental Sciences,
Department of Agricultural Production, P.O Box 7062,
Makerere University, Kampala, Uganda
P. Ebanyat
College of Agricultural and Environmental Sciences,
Department of Agricultural Production, P.O Box 7062,
Makerere University, Kampala, Uganda
J. S. Tenywa
College of Agricultural and Environmental Sciences,
Department of Agricultural Production, P.O Box 7062,
Makerere University, Kampala, Uganda
L. D. Nabirye
Bugema University, School of Agriculture and Applied Sciences,
Department of Agriculture and Environment, P.O Box 6529, Luwero
ABSTRACT
Biological nitrogen fixation (BNF), mediated by micro-symbionts (Rhizobia spp.),
remains the primary source of N in the arable soils of sub-Saharan Africa (SSA).
Rhizobia inoculants are delivered securely to legume seeds, through carrier
materials, in order to augment the BNF processes in legume systems. In SSA, peat is
the primary carrier material used for this purpose; whose natural reserves are
quickly dwindling. It is, therefore, imperative that alternative organic materials,
with comparative potential for providing similar services, are evaluated. The
objective of this study was to assess the potency of municipal solid waste compost
(MSW) as replacements in admixtures of peat material, as carriers of rhizobia
inoculants for administering to legume seeds prior to planting. A laboratory study
was conducted for duration of 270 days (2016 – 2017), at the College of Agricultural
and Environmental Sciences, Makerere University in Uganda. Treatments included
five carrier formulations with municipal solid waste compost (MSW) as the base
material (w/w); namely 100:0, 60:40, 40:60, 20:80 and 0:100 MSW to peat (Pt),
ratios with Bradyrhizobium diazoefficiens USDA strain of rhizobia. Result revealed
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Discoveries in Agriculture and Food Sciences (DAFS) Vol 11, Issue 6, December- 2023
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that all carrier formulations maintained a high B. diazoefficiens population >1 x 109
CFU/g for the first 120 days of storage. By the 270th day of storage, the best
performing carrier formulation was pure municipal solid waste compost (100:0)
with B. diazoefficiens population of 2.08 x 109 CFU/g. This was followed by the pure
peat carrier (0:100) with B. diazoefficiens population of 1.57 x 109 CFU/g, which
was not significantly different from municipal solid waste compost carrier. The
least performing carrier combination was 40:60 (MSW: Pt) with B. diazoefficiens
population of 3.08 x 108 CFU/g. From the results, the municipal solid waste compost
is an effective replacement for peat in rhizobia inoculant formulation; however, the
inherently high alkaline pH (8.96) of municipal solid waste compost needs
adjustment to the recommended optimal range (6.0 to 7.0). Overall, the 60:40
admixture is the nearest to the optimal alternative carrier formulation, to
substitute for peat as the conventional carrier in the production of rhizobia
inoculants in Uganda.
Keywords: BNF, Bradyrhizobium diazoefficiens
INTRODUCTION
Biological nitrogen fixation (BNF), mediated by Rhizobia spp., remains the primary source of N
in the arable soils of sub-Saharan Africa (SSA). Rhizobia inoculants are delivered securely to
legume seeds, through carrier materials, in order to augment the BNF processes in legume
cropping systems. Carrier materials are used to conserve the bacteria by prolonging their shelf
life under specified conditions (Kaljeet et al., 2011).
The optimum quality of a good carrier has been prescribed by Ben Rebah et al. (2007) to be
high in organic C with optimal N contents, near neutral pH and of high moisture holding
capacity. Additionally, the material should be non-toxic, inexpensive and accessible all year- round (Ben Rebah et al., 2007). Also, it should be easy to process, free from lump forming, and
easy to sterilize by autoclaving or gamma-irradiation (Somasegaran and Hoben, 1994).
No single inoculant is known to possess all these quality characteristics; however, a good
inoculant should have as many as possible of these features (Bashan et al., 2014). Peat-based
carriers are the most commonly used materials in rhizobia inoculant industry (Bashan et al.,
2014) because they maintain high levels of viable rhizobia populations; attributed to high
carbon content, high water-holding capacity and large surface area (Kaljeet et al., 2011).
Unfortunately, despite of its endowment with such positive carrier attributes, peat is getting
depleted in many countries in sub-Saharan Africa. It is, therefore, imperative to explore other
alternative competitive materials that are locally accessible, cost effective and environmentally
safe, for commercial inoculant production.
Various materials have been tested as alternative rhizobia inoculant carriers to peat, based on
maintenance of high populations of viable bacteria. Examples of these include sawdust (Singh
et al., 2014), rice husks + kaolin (Kaljeet et al., 2011), water hyacinth compost (Mohamed and
Abdel-Moniem, 2010), biochar (Hale et al., 2015; Hardy and Knigh, 2021) and crop residue
composts (Phiromtan et al., 2013). Municipal Solid Waste (MSW), which is in abundant supply
in SSA, but cumbersome to dispose of, particularly in urban and peri-urban environments of
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Akuru, G., Tumuhairwe, J. B., Ebanyat, P., Tenywa, J. S., & Nabirye, L. D. (2023). Municipal Solid Waste Compost as Alternative Carrier Material for
Rhizobia Inoculants. Discoveries in Agriculture and Food Sciences, 11(6). 67-80.
URL: http://dx.doi.org/10.14738/dafs.116.16105
SSA, still remains unexplored for this purpose. A number of studies have been conducted to
determine the quality of MSW compost for use in agricultural production in Uganda (Amoding
et al., 2011; Komakech et al., 2014; Kabasiita et al., 2022). Kabasiita et al. (2022) characterised
MSW compost in 12 municipal composting sites in Uganda and noted that nutrient endowments
were generally low. In addition, MSW compost has been reported to have alkaline pH, to levels
that may be non-conducive to rhizobia inoculant production (Kabasiita et al., 2022). Hence,
considering that peat is highly acidic and requires liming to raise the pH to favourable levels for
rhizobia, MSW compost could be mixed with reduced quantities of peat carrier to obviate costly
lime requirements. The objective of this study was to assess the potency of municipal solid
waste (MSW) compost as replacements in admixtures of peat materials as carriers of rhizobia
inoculant production.
MATERIALS AND METHODS
A laboratory study was conducted at Makerere University in Uganda, for a duration of 270 days
during 2016 to 2017. Treatments included sterilised carrier formulations of MSW compost,
admixed with peat (Pt) in ratios (w/w) in the order of 100:0, 60:40, 40:60, 20:80 and 0:100.
These formulations were selected after adjusting a range of (MSW: Pt) carrier admixtures to
rhizobia conducive pH values (see below). The treatments were laid out in a completely
randomised design (CRD) and replicated three times.
Carrier Preparation
Two types of carrier materials, namely MSW compost and peat (Pt) were used in this study. The
MSW was obtained from the Mukono Municipal Composting site in Central Uganda (latitude: 0°
21' 11.99" N and longitude: 32° 45' 19.01" E). Mukono municipal composting site is one of the
12 municipal composting sites under the World Bank Clean Development Mechanism (CDM)
programme that finances low-income countries’ greenhouse gas (GHG) reduction mechanism,
including composting. Windrow composting is the technique practiced at Mukono municipal
composting site (Kabasiita et al., 2022).
Peat soil used in this study was obtained from Kabale district in South-western Uganda
(latitude: -1° 14' 54.85" S and longitude: 29° 59' 23.75" E). Separately with MSW, the materials
were air dried, screened through a 5 mm sieve; and later ground to a very fine powder texture
using a hammer mill.
Pre-study pH tests were taken on MSW compost : peat ratios, 100:0, 90:10, 80:20, 70:30, 60:40,
50:50, 40:60, 30:70, 20:80, 10:90, 0:100 based on the pH suitable for rhizobia inoculant
production (Somasegaran and Hoben, 1994). From these tested formulations, five MSW: Pt
carrier formulations namely; 100:0, 60:40, 40:60, 20:80 and 0:100 was selected as treatment
formulations for the study.
Preliminary analytical test results (Table 1) showed that sole peat (Pt) (0:100) was strongly
acidic (pH = 3.91) and contained high organic C (C = 16.59%); while MSW compost was fairly
alkaline (pH = 8.96) with organic C content of 4.63%. Therefore, peat soil was limed by
incubating with different quantities of limestone (CaCO3), namely 4, 5, 6, 7 and 8 grammes each
in different 250 ml beakers containing 10 g of peat, with 90 ml of distilled water. The quantities
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Discoveries in Agriculture and Food Sciences (DAFS) Vol 11, Issue 6, December- 2023
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of limestone were based on the recommended procedures by (Somasegaran and Hoben, 1994).
The setups were managed in triplicates.
The mixture in each beaker was stirred using a magnetic stirrer, and the pH monitored using a
pH electrode, for three days until stable results were obtained (Somasegaran and Hoben, 1994).
The peat (Pt) carrier formulation, in which 6 g of CaCO3 was added, achieved a pH of 6.62 and
was used for to raise the pH of peat. Several researchers have observed calcium or magnesium
carbonate to be the most effective reagent to raise the pH of peat to the desired level, for
rhizobia survival (Somasegaran and Hoben, 1994; Tittabutr et al., 2012; Thirumal et al., 2017).
In order to determine how much Al2 (SO4)3 that would be sufficient to neutralise the alkalinity
of MSW to the pH suitable for the rhizobia inoculants, the pH of the MSW compost was lowered
by adding different quantities of Al2 (SO4)3, namely 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2
and 3 g, each in a different 250 ml beaker containing 10 g of MSW compost with 90 ml of
distilled water, replicated 3 times.
The mixtures were stirred using a magnetic stirrer and the pH monitored using a pH glass
electrode for three days, until stable pH values were obtained. The MSW formulation to which
0.3 g of Al2 (SO4)3 was added achieved a pH of 6.77 and was selected for use in lowering the pH
of MSW. Therefore, the final pH used in the study was 6.77 and 6.62 for pure MSW (100:0) and
pure peat (0:100), respectively.
Carrier Characterisation
The carrier formulations slated for the study, were analysed for total organic carbon (OC), total
N, Bray 1 P, lead (Pb), Cadmium (Cd), Nickel (Ni) and Iron (Fe). Total organic carbon was
analysed by the Walkley and Black method; and total N was analysed by the Kjeldhal technique,
as described in by Okalebo et al. (2002).
For the heavy metals, 5 g of the air-dried samples were extracted in 50 ml of 1%
ethylenediaminetetraacetic acid (EDTA). The filtrate was then aspirated into an air-acetylene
flame in the atomic absorption spectrophotometer (AAS), and Pb, Cd, Ni and Fe, were read off
at their specific wavelengths. The carrier formulations’ analytical results are presented in Table
1. The results revealed low concentrations of heavy metals (Fe, Pb and Cd) in the materials used
in the different carrier formulations (Westfall et al., 2005).
Table 1: Chemical characteristics of the carrier materials (w/w) used in the study
MSW: Peat pH (H2O) Total N Organic carbon C/N ratio Fe Pb Cd Ni
w/w % μg g-1
100:0 8.96 0.59 4.63 7.85 148.00 0.02 0.01 nd
60:40 7.44 1.01 10.84 10.75 52.30 nd nd nd
40:60 6.99 1.24 12.28 9.90 42.30 nd nd nd
20:80 6.38 1.27 11.89 9.59 66.20 nd nd nd
0:100 3.91 1.55 16.59 10.70 38.20 nd nd nd
nd = not detected