Grain Legumes and Green Manures for Soil Fertility in ... - cimmyt

Arbuscular mycorrhizal fungi (AMF) colonize roots
of about 80% of terrestrial plant species and can increase
plant P uptake by increasing the surface uptake
area (Koide 1991). Synergistic effects on legumes
are frequently seen when both symbionts (the
rhizobia and the fungi) are present (Goss and de
Varennes, 2002; Sanginga et ai., 1999; Fitter and Garbaye,
1994). Both nodule number and dry weight
usually increase after mycorrhizal colonization
(Reddy and Bagyraj, 1991), which is often explained'
by increased P uptake by the fungi. While mycorrhizal
fungi are components of most natural ecosystems,
their abundance and efficacy can be severely
retarded by common agricultural practices such as
fallowing, soil disturbance through tilling and weed
management, and prolonged cultivation of non-host
plants (Boswell et al. 1998; Kabir et al., 1997; Douds
et al. 1995; Harinikumar and Bagyraj, 1989).
The objective of our work was to understand the
role of AMF for legume performance in subsistence
farmers' fields, and to determine if an enhanced
AMF abundance can promote nodulation and N2
fixation. In this paper we discuss the results of two
pot experiments. In Experiment 1, the effect of an
altered AMF abundance on nodulation and shoot N
content on groundnut was determined by enhancing
the AMF abundance through an inoculation
with a common AMF, or by reducing the indigenous
AMF abundance through a fungicide application.
In Experiment 2, the abundance of indigenous
fungi was enhanced and the effects on nodule number,
nodule mass and shoot N content were determined
on lab lab bean. Groundnut was chosen since
it is a common legume grown by the subsistence
farmer in Zimbabwe, and lablab bean because it is a
green manure crop and therefore has a higher potential
to improve soil fertility.
Material and methods
General
The soil used in both Experiment 1 and Experiment
2 was a Tsholotsho luvisol (from Simeon Moyo's
farm) where legume P limitations had been demonstrated
previously. The pH of the soil was 6.2 (1:2 V
soil: V water), and available P was 1.2 ppm (Olsen).
Soil was collected to a depth of 15 cm in December
1999, for Experiment 1, in April 2001 for the inoculum
production part of Experiment 2, and in November
2001, for the inoculation part of Experiment
2. For Experiment 1 the soil was collected randomly
from the field where plants were currently grown
and no fertilizers had been added, and for Experiment
2 from areas where maize had been grown the
previous year without fertilizer additions.
Experiment 1
Groundnut (var. Falcon) was planted on December
28 1999, in 1.6 L pots and grown for 6 weeks in soil
amended with P [2 g single superphosphate pot- l
(19% P20S)], AMF (2000 spores pot l of Glomus intraradices,
Schenk and Smith), a fungicide Benomyl
(200 mL of 0.1% solution added one day prior to
planting), or control consisting of non-sterile soil.
No additional fertilizers were added and the plants
were watered as needed. At harvest, nodule numbers
were determined along with shoot Nand P
concentrations and AM colonization using standard
procedures (Brundrett et ai., 1996; Watanabe and
Olsen, 1965; Jensen 1962).
Data were analyzed using a one-way ANOV A and
transformed where appropriate. When transformation
failed to generate data that fulfilled the underlying
assumptions, data were analyzed using nonparametric
tests.
Experiment 2
Production ofAMF and control inoculum
Maize was planted in the soil in July 2001, and
grown for three months to enhance the abundance
of indigenous AMF. Control pots consisted of
maize grown in sterile soil that had been given
spore washings containing soil bacteria and fungi
but lacking AMF. The plants were fertilized six
times with Peters fertilizer 15-0-15NK plus micronutrients
(at a N concentration of 100 ppm) and
amended with 0.15 giL MgS04 and 5 mg PIL as
KH2P04. After three months the maize plants were
allowed to wilt, and dry soil and cut root pieces
served as a source of inoculum, which consisted of
AMF spores, external hyphae and colonized root
pieces. The control roots were non-mycorrhizal.
Inoculation experiment
Lablab bean (var. Rongai) was planted on 3 November
2002, in pots amended with either 200 mL control
inoculum or 200 mL of AMF inoculum, and
grown for 7 weeks and watered as needed. At harvest,
nodule number and weight were recorded,
AM colonization determined and shoot Nand P
concentration measured (Brundrett et ai., 1996; Watanabe
and Olsen, 1965; Jensen 1962). Data were
analyzed using a one tailed paired t-test where
+AMF and control pairs shared the site origin from
the field.
Results
Experiment 1
Enhancing AMF in the test luvisol significantly increased
AM colonization compared to the control
(Table 1). This resulted in a doubling in nodule
numbers and a significantly higher N content in the
shoot. There was a strong beneficial effect of P on
the number of nodules, which resulted in almost a
doubling in N content in the shoot. However, inter-
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Grain Legumes and Green Manures for Soil Fertility in Southern Africa