Potential of phosphate solubilising bacteria for enhancing Minjingu rock-phosphate solubility and phosphorus-use efficiency for sustainable maize production in acid soils
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Date
2021
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Publisher
Sokoine University of Agriculture
Abstract
Maize is a primary staple crop for over 80% of Tanzanians, accounting for more than
70% of the country's starch requirements. Small-scale farmers produce the majority of the
crop, with production unpredictable and inconsistent with agricultural requirements.
Despite the fact that total maize production has increased over the last decade, owing
primarily to extensification, yields per unit area remain low, averaging around 1.6 Mg/ha
and translating to just 17.8 to 26.7 percent of the feasible yield. In order to fulfill the
demands of the country's fast rising population, maize production must be increased. Low
soil fertility, which has been related to inadequate fertilization and poor soil fertility
management practices, is one of the reasons for low maize production. Soil phosphorus
(P) insufficiency is one of the most critical soil fertility concerns in most Tanzanian soils,
owing to the preponderance of soils with high Phosphorus-fixing capacity. Water-soluble
P fertilizers (WSF) and local rock phosphates (RP) have been advised to ease the P deficit
problem. While WSF are agronomically very effective, their application is limited due to
their expensive cost, whereas directly applied RP are associated with delayed agronomic
effectiveness and low plant phosphorus consumption efficiency.
A potent solution to the low reactivity of RP is its co-application with phosphate-
solubilising microorganisms (PSMs) in an appropriate carrier or carbon source. PSMs are
soil microorganisms, capable of converting the insoluble P sources into its plant available
forms. When co-applied with RP, PSMs could help solubilize the insoluble P in RP.
However, there is limited knowledge on PSMs characterized and selected based on their
ability to solubilize local phosphate rocks while utilizing locally available, naturally
occurring organic carbon sources. To evaluate potentials of PSB-RP combination of maize productivity, the first specific
objective was set to characterize phosphate solubilising bacterial (PSB). Forty (40) PSBs
were previously isolated from various agricultural fields in eastern and southern highlands
of Tanzania. Eleven (11) Isolates were selected, molecularly identified, screened for
plant-growth promoting traits such as (1) ability to solubilize three P-sources namely
ferric-, tricalcium-, and hard Minjingu rock phosphate, (2) Indole Acetic Acid (IAA)
production, (3) Siderophore production, (4) Ammonia gas production, (5) Hydrogen
cyanide (HCN) gas production, and (6) Activity against phytopathogenic fungi. The
second objective was to examine the influence of organic carbon sources and fertilizer-
grade nitrogen sources on phosphate solubilising abilities of PSBs. Three organic carbon
sources namely Molasses (ML), sunflower seedcake (SC), and filter mud (FM) were
initially evaluated for their physicochemical properties according their comparative
potentials to enhance solubilisation of Tri-calcium phosphate (TCP) and Ferric phosphate
(FP) by ten PSB isolates. In place of glucose, 20 g per liter of either ML, SSC or FM were
added in Pikovskaya Agar (PVK) broth. Accordingly, a double-modified synthetic
minimal media (dSMM) was prepared which contained all components of the Pikovskaya
broth, except the carbon and Nitrogen source. The third objective focused on evaluating
the influence of organic carbon sources and fertilizer-grade nitrogen sources on antifungal
efficiencies of PSB isolates against phytopathogenic fungi. Each of the PSB isolate was
grown in the modified synthetic minimal agar medium supplemented separately with
either ML, SSC, FM, or glucose as the sole source of carbon. Accordingly, agar plates
were spot-inoculated with different PSB isolates on one end of the 9 cm diameter Petri
dish and test fungi on the other end. Antifungal efficiency (AE) of each PSB under each
treatment was determined. Lastly, influence of co-application of MPR and PSBs in a
molasses-based carrier was evaluated for enhancing maize yield and phosphorus use
efficiency. Field experiments were laid out in randomized complete block design in quadruplicates under two sites, Madaba and Magadu, respectively located in the southern
highlands and eastern zones of Tanzania. The soils of each study area were characterized
for soil fertility parameters. Experimental treatments were PSB inoculum co-applied with
varying P rates (0, 20, 40, 60, and 80 kg P /ha) from MPR. Two P rates without PSB
inoculum (0 and 40 kg P /ha) were set respectively as absolute and positive control. Soil
available P, maize grain yield, biomass yield, phosphorus uptake, and use efficiency for
each treatment were evaluate at harvest maturity.
The results indicated that six bacterial isolates had the highest homology with the known
strains of the genus Klebsiella, two isolates related to the strains of genus Burkholderia,
while one isolate is still unidentified. All PSB isolates were positive for ammonia
production test with highest amount being 168.3 ug/ mL produced by Klebsilla sp.-
NA19a while none of the isolates exhibited ability to produce hydrogen cyanide gas. The
highest amount of IAA produced was 24.32 ug/ mL by Klebsilla sp.- SI-SP1. Similarly,
Klebsilla sp-NA5 showed a significantly (P≤ 0.05) bigger solubilisation index on ferric
phosphate agar plate while K. variicola spp.-MdG1 showed highest solubilisation index
on ZnCO 3 agar plates. All the eleven isolates inhibited growth of F. proliferatum on agar
plates with the highest inhibition of 47.7% by Burkholderia sp.-MK10.
Among organic carbon sources tested, molasses significantly (P ≤ 0.05) enhanced
solubilisation of both tri-calcium and ferric-Phosphates compared to filter mud or
sunflower seedcake while urea-containing media resulted into a significantly (P ≤ 0.05)
lower amount of P-solubilized as compared to fertilizer-grade or laboratory-grade
ammonium sulfate (SA) as N source. The evaluation of antifungal activity showed that
the use of molasses as carbon source retains most of antifungal activity of isolates but
filter mud and sunflower seedcake do not. Fertilizer-grade SA enhanced the antifungal activity of most bacterial isolates than urea. Furthermore, the bacterial isolates under
study retained their antifungal efficiencies whether yeast extract is included in the growth
media.
A study on field evaluation of RP-PSB co-application revealed the following. The
characterized soils of both study sites were acidic and deficient of nitrogen, phosphorus,
and potassium. The data collected at harvest time indicated that soil available P of both
sites responded positively to increasing P rates from Minjingu rock phosphate +
phosphate solubilising bacteria (Bio-rock). Unlike Magadu soil, sole application of PSB
inoculum in soils of Madaba increased soil available P to values statistically comparable
to applying either sole MPR at 40 kg P /ha or 20 kg P /ha + PSB inoculum. Compared to
applying sole MPR at 40 kg P /ha, maize grain yield, P uptake and phosphorus use
efficiency (PUE) were improved by inoculation of PSB combined with either 20 kg P /ha
or 40 kg P /ha. It is recommended, for enhanced maize productivity and efficient use of
phosphorus, co-application of PSB inoculum and Minjingu RP at 20 kg P /ha for the soils
of Madaba and 40 kg P /ha for the soils of Magadu.
Description
Dissertation
Keywords
Phosphate solubilising bacteria, Minjingu rock-phosphate, Phosphorus-use efficiency, Maize production sustainable, Acid soils