Use of calliandra–Napier grass contour hedges to control erosion in central Kenya
Introduction
Soil erosion by water is a global problem and more so in the tropical regions due to the torrential nature of rainfall and highly erodible soils. While several methods exist for control of water erosion, the use of tree hedges (hedgerows) on contours of steep slopes has become increasingly important (Young, 1989, Young, 1997, Angima et al., 2000). Success in the use of hedgerows has been observed in Nigeria, Columbia, and Kenya where 48–85% reduction in soil loss has been observed (Young, 1989, Kiepe and Young, 1992, 1997; Angima et al., 2000). Trees in hedgerow systems can serve as soil erosion barriers and nutrient retention enhancers through their influence on the supply and availability of nutrients in the soil through biological N2 fixation, retrieval of nutrients from below the rooting zone of crops, and reduction in nutrient losses from leaching and erosion such as P and N. The ability of trees to enhance nutrient availability is greater on soils with high base saturation than those with low base saturation (Szott et al., 1991).
One tree species used in agroforestry systems that has had remarkable success in conserving soil, nutrient cycling, and nutrient retention is calliandra (Calliandra calothyrsus). Calliandra, indigenous to central America, is a small tree that reaches about 10 m in height and grows naturally in moist, tropical regions up to an altitude of 1500 m (Paterson, 1994). Calliandra can improve soil quality and increase yields of associated crops and grass species such as Napier grass (Pennisetum purpureum) (National Research Council, 1983, Nitrogen Fixing Trees Association, 1988, Goudreddy, 1992). Napier grass is a tall perennial grass reaching over 3 m high, resistant to drought, and grows at altitudes up to 2400 m with a minimum rainfall of 900 mm (Henderson and Preston, 1959). Biomass yields from Napier grass range between 12–150 Mg ha−1 per year depending on fertility, management, and the variety of Napier grass used (Henderson and Preston, 1959, Purseglove, 1985, Orodho et al., 1992). The effectiveness of combinations of calliandra with Napier grass used in hedges for erosion control is thought to be due to the stem strength of the calliandra and the massive near-surface lateral root system of the Napier grass.
Data from soil erosion studies can be used in soil erosion prediction models including the Revised Universal Soil Loss Equation (RUSLE) developed and used for conservation planning in the USA (Renard et al., 1997) and that has been used in many countries. The use of RUSLE, however, requires site-specific parameters that adequately address the erosion hazard specific to the locality. RUSLE computes the average annual erosion expected on field slopes by multiplying the rainfall and runoff erosivity R-factor, soil erodibility K-factor, slope length and steepness LS-factor, cover and management practices C-factor, and support practice P-factor (Foster et al., 1977, Renard et al., 1997). On croplands, support practices include contouring (tillage and planting on or near the contour), strip cropping, terracing, and subsurface drainage (Renard et al., 1997). The support practice P-factor is affected most by management practices carried out by landowners. Thus, it is important that local management practices be considered in the development of the sub-factor, so that conservation planning will reflect local conditions.
This research was conducted in the Kianjuki catchment area located in the Embu District of central Kenya, which is within the research mandate region of the Kenya Agricultural Research Institute (KARI) working collaboratively with the International Center for Research in Agroforestry (ICRAF). Objectives for this study were to: (1) determine erosion rates from on-farm plots with and without contour calliandra–Napier grass hedges; (2) use the soil loss data to develop a support practice P-subfactor for use with the RUSLE soil erosion prediction computer model; (3) determine biomass production from the hedges; and (4) determine N and P losses in eroded sediments from the runoff plots to gauge the effectiveness of the combination hedge system in retaining nutrients. The study had the following hypotheses: (1) the calliandra–Napier grass hedges, when used as contour hedgerows, will significantly reduce soil loss; (2) the support practice P-subfactor for calliandra–Napier grass hedges will be less than the support P-subfactor for terracing found in the RUSLE database; (3) the combined biomass yields from the calliandra–Napier grass hedges will be lower on steeper slopes as a result of soil and nutrient losses; and (4) losses of N and P with the eroded sediments from plots with calliandra–Napier grass hedges will be significantly less than the control.
Section snippets
Study site
The study site was located in the Kianjuki catchment area in the Embu District of central Kenya. The climate of this area is representative of the east African highlands. The catchment lies on latitude, 00°30′S, longitude, 37°27′E and an altitude of 1480 m above sea level (Angima et al., 2000, O’Neill et al., 1993). Average annual rainfall is 1500 mm, which comes in two seasons referred to as the long rains (March–September) and the short rains (October–February). This catchment, or watershed, is
Soils, runoff and erosion
Soil properties for the Kianjuki catchment are presented in Table 1. The amounts of clay, silt, and sand on the two slopes varied significantly under each treatment at the conclusion of the study. However, organic-clay complexes, organic carbon, and infiltration rates were not significantly different between treatments (Table 1).
Total annual rainfall for the study years 1997, 1998, and 1999 were 1898, 1296, and 590 mm, respectively. There was a drought in 1999, accompanied by crop failures.
Conclusions
The calliandra–Napier grass hedge significantly reduced both runoff and soil loss. A support practice P-subfactor of 0.7 was calculated for this hedge system for use with the RUSLE erosion prediction model. This P-subfactor value is less than the default RUSLE subfactor value for terracing but comparable to the value for meadow buffer strips. The P-subfactor for this hedge system can be used in RUSLE for conservation planning in the humid and sub-humid tropics.
The added benefit of biomass
Acknowledgements
This work was supported by funds to the National Agroforestry Research Project from the International Center for Research in Agroforestry (ICRAF), the Kenya Agricultural Research Institute (KARI), and the Swedish International Development Agency (SIDA). Dr. Angima’s travel and research were supported by the Rockefeller Foundation (Grant No. RF95022 #758). The authors thank Mr. S.P. Gachanja, Center Director for the KARI Regional Research Centre, Embu, which hosted this project, the two farmers,
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Present address: University of Missouri, 100 W. Franklin, Courthouse, Room 16, Clinton, MO 64735, USA.
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Present address: Agricultural Science Center, New Mexico State University, P.O. Box 1018, Farmington, NM 87499, USA.