Population dynamics and management response of Nitrogen-fixing indigenous legumes for soil fertility restoration in smallholder farming systems of Zimbabwe
Tauro, Tonny Phirilani
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The main problem undermining food security in most smallholder farming systems in Zimbabwe is the limited range of alternative nutrient resources for a wide range of farmers and soils. This study was initiated to investigate field establishment patterns, population dynamics and management dynamics of potential indigenous legume species found in smallholder farming systems of Zimbabwe. Germination patterns were determined under laboratory conditions following various scarification procedures using a germination test technique while field studies were conducted under low (450-650 mm yr-1) to high (>800 mm yr-1) rainfall conditions in Zimbabwe on nutrient depleted soils. Indigenous legume seeds were broadcast on disturbed soil in mixtures at 120 seeds m-2 species-1 over two growing seasons: 2004/05 and 2005/06 rainfall seasons. Low emergence rates among species were attributed to seed hardness and low seed viability, accounting for > 50 % and 10 - 30 % of germination failure, respectively. Acid scarification significantly (P<0.05) increased germination while hot water treatment had no effect on germination. Crotalaria pallida and C. ochroleuca were the first to emerge within 14 days from seeding while emergence for other species was spread over two months. Eriosema ellipticum had a highest population of 42 plants m-2 (8 %) while Crotalaria pallida, Crotalaria cylindrostachys, Crotalaria ochroleuca and Indigofera arrecta had populations of < 23 plants m-2 (5 %). Species biomass contribution into the system was spread across time and in space characterized by slow initial growth rate compared to that of non-legume species. Three months after sowing, annual herbs such as C. cylindrostachys and C. glauca attained peak biomass of 0.5 t ha-1 (dry weight) while other annuals, C. ochroleuca and C. pallida, attained maximum biomass (> 5 t ha-1) within a growth period of six months. Most biennials only attained maximum biomass in the second season with seasonal yields not exceeding 2 t ha-1. Application of single super phosphate (SSP) or Dorowa phosphate rock (DPR) had no significant (P>0.05) effect on legume species composition and establishment patterns but SSP significantly increased biomass productivity by 20-60 %. Non-legume species responded to SSP application and recovered more phosphorus than legumes. Following addition of SSP, the amount of N fixed by Crotalaria cylindrostachys, C. ochroleuca and C. pallida increased by 2, 27 and 51 kg N ha-1 respectively, resulting in increased total N fixed within the system. Over time in the first season, the abundance of predominant non-legume species such as Cynodon dactylon (a grass species) and Richardia scabra (a broadleaf species) was suppressed to 17 % and 7 % respectively, while the abundance of the same species in natural fallows increased. There was insignificant change in legume species diversity in the second season characterized by persistent growth of about 50 % of the sown species. However, non-legume species diversity decreased as C. dactylon dominated apparently associated with increased N availability. These findings suggest that the indigenous legumes could contribute in restoring productivity of soils continuously cultivated fields with little or no nutrient inputs in most parts of Zimbabwe and similar agro-ecologies in Sub-Saharan Africa. However, low germination of these indigenous legumes still remains a challenge.