Saturday, 15 July 2006

Effect of Traditional Range Burning on Short-Term Soil Chemical Changes and Perfomance of Introduced Forage Legumes in Semi-Arid Rangelands of Kenya.

P.N. Macharia, Kenya Agricultural Research Institute-Kenya Soil Survey, P.O. Box 14733, Nairobi, 00800, Kenya, C.K.K. Gachene, Univ of Nairobi, Kabete Campus, P.O. Box 30197, Nairobi, 00100, Kenya, and J.G. Mureithi, Kenya Agricultural Research Institute-Legume Research Network Project, P.O. Box 14733, Nairobi, 00800, Kenya.

Communities living in Kenya's extensive arid and semi-arid regions basically depend upon livestock and livestock products for subsistence. Thus livestock production is the main farming system in these regions whereby the people practice nomadic pastoralism as a strategy for their livestock to exploit the available graze and browse resources. To enhance pasture regeneration, the pastoralist's use fire as a tool to control accumulation of old herbage, thereby stimulating new grass growth which is also of higher quality. However, fire has profound effects on the soil's nutrient dynamics, composition and dry matter (DM) yield of the vegetation regrowth in these grazing ecosystems. Thus a study was conducted in Mashuru Division of Kajiado District to investigate the effect of range vegetation burning on soil chemical changes immediately after burning and DM yield of introduced Neonotonia wightiii (Arn.) Lackey (Glycine) and Macroptilium atropurpureum (DC) Urb (Siratro) which are forage legumes for pasture improvement. The study was conducted in two adjacent sites, one where herbage was subjected to a back fire while herbage in the other was unburned but slashed to ground level. The experimental design was an RCBD split plot in a 2 x 5 factorial with three replications, where burning/slashing were main plots and five treatments as sub-plots. Before planting the legumes, soil samples were collected using a soil auger at 0-15 cm depth and analyzed for pH (H2O), organic carbon (C), nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca). Statistical analysis was conducted at P0.05 level of significance. The results of soil analysis showed that the soil pH in unburned site (5.25) was significantly higher than in burned site (5.06). Similarly, organic C was significantly higher in unburned (1.34 %) than in burned site (1.29 %). However, the soil's N, P, K and Ca levels increased after burning with N and P achieving a significant increase while the increase in K and Ca was not significant. Specifically, the soil N content was significantly higher in burned (0.16 %) than in unburned site (0.15 %). Similarly, the soil P was significantly higher in burned (177.9 ppm) than in unburned site (166.8 ppm). Potassium levels in burned site (1.25 %) was higher (though not significantly) than in unburned site (1.20 %). Calcium levels were also higher (though not significantly) in burned (5.34 %) than in unburned site (5.12 %). The increased nutrient availability in the burned site may have been utilized by the treatments which manifested in a 54 % more DM yield than in unburned site. In addition, Glycine grown as monocultures yielded more DM in burned site (6.87 t ha-1) than in unburned site (4.81 t ha-1). But on the other hand, monocultures of Siratro yielded more DM in unburned site (2.84 t ha-1) than in burned site (2.73 t ha-1). When the two legumes were grown as mixtures with natural pasture, the pattern was the same as in monocultures. These results showed that fire can act as a mineralizing agent of nutrients mobilized in foliage to enhance short-term availability of soil nutrients for plant growth, while others are likely lost through volatilization, runoff and leaching to deeper soil layers. In addition, the results imply that soil chemical changes as a result of burning affected the growth of Glycine and Siratro in different ways, an aspect that needs to be further investigated.

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