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

ROLE OF PHOSPHORUS AND ARBUSCULAR MYCORRHIZAL FUNGI ON
NODULATION AND SHOOT NITROGEN CONTENT IN
GROUNDNUT AND LABLAB BEAN
YLVER L. BESMER 1, R.T. KOIDE 1 and S.J. TWOMLOW 2
Abstract
1Pennsylvania State University, University Park, PA, USA
21CRISA T, Matopos Research Station, Bulawayo, Zimbabwe
Rotations with legumes have been suggested as a means to increase cereal production in low-input agriculture in Zimbabwe,
as cereal yields are currently limited by nitrogen (N). However, NJixation by legumes is often phosphorus (P)
limited. Soil availahle P explained 67% of the variation in nodule numbers when groundnut was grown on a wide
range of soils collected from subsistence farm er's fields in southern Zimbabwe. P applications on a luvisol and vertisol
in Tsholotsho, south -western Zimbabwe, can significantly increase nodule mass, aboveground biomass and total N in
residues of groundnut (Arachis hypogaea L), lablab bean (Lablab purpureus) and pigeonpea (Cajanus cajan (L)
Millsp.) . However, P fertilizers are often beyond the economic means of subsistence farmers. The success of the legumes
therefore, will strongly depend on their ability to utilize the P already l/1 the soil. Arbusclilar mycorrhizal fungi (AMF)
are components of most natural ecosystems and form a symbiosis, arbuscular mycorrhiza, with approximately 80 percent
of all terrestrial plants. The fungi can increase plant P uptake by increasing the surface uptake area. We have
shown in a pot trial that by enhancing the AM colonization through an inoculation with AMF in a luvisol from
Tsholotsho, nodule number and N content of the shoot significantly increased in groundnut and lablab bean. This study
indicates that by exploring the biology of the agro-ecosystem , beneficial effects could be obtained by optimizing the mlltualistic
interactions between the plant, bacteria and fungi. Ways to enhance AMF inoculum potential in the field s of
subsistence farmers are currently being tested and are disCllssed.
Key words: Arbuscular mycorrhiza, rhizobia, phosphorus, groundnut
Introduction
A majority of subsistence farms in Zimbabwe occur
on communal land. Maize (Zea mays) is grown as a
staple, often on nutrient depleted sandy soils iow in
organic matter (Grant 1967, 1981, 1985). Inorganic
fertilizers, once subsidized by the government, are
scarce in rural areas and often beyond the economic
means of subsistence farmers (Mapfumo and Giller,
2001). With declining maize yields due to nitrogen
(N) limitations (Snapp, 1998), there has been renewed
interest by researchers in using N2 fixing legumes
in intercropping and rotational cropping systems
to increase soil fertility (Snapp et al., 2002; Ma
et aI, 1998), a common practice in much of southern
Africa prior to the introduction of mineral fertilizers
(Howard reprinted in Small Farmer's Journal, 1999).
Groundnur (Arachis hypogaea L), cowpea (Vigna llndiculata
(L) Walp.) and bambara groundnut (Vigna
subterranean (L) Thou.) are currently grown for huan
consumption and animal feed in Zimbabwe. However,
their capacity to improve soil fertility might be
limited for at least two reasons. First, they are all
grain legumes where much of the N is translocated
to the seeds and removed from the field at harvest.
Second, optimal N2 fixation might be limited by
phosphorus (P), as soil P availabilities are generally
low in the old, highly weathered soils of sub­
Saharan Africa (Warren, 1992; Buresh et a L, 1997;
Giller, 2001).
In a previous experiment we have shown that when
groundnut was grown on a wide range of soils collected
from subsistence farmer's fields in southern
Zimbabwe, nodule numbers differed by an order of
magnitude. Further, soil available P explained 67%
of this variation (Besmer et al., unpublished), suggesting
the importance of this element for nodulation
and N 2 fi xa tion. Applications of P (40 kg P20 s/
ha) to two soils from Tsholotsho, a luvisol and vertisol
(Moyo, 2001), both low in P, significantly
(p