Saturday, 15 July 2006

Hydrological Consequences of Native Shrubs in Semi-Arid Sahel: Implications on Sahelian Soils and Agroecosystems.

F. Kizito1, Maria Dragila1, M. Sène2, J. R. Brooks3, Abel Lufafa1, and Richard Dick4. (1) Oregon State Univ, Dept of Crop and Soil Science, ALS Building 3017, Corvallis, OR 97331, (2) Institut Sénégalais de Recherches Agricoles (ISRA)/CERAAS, B.P 3320 Thiès-Escale, Thiès, 3320, Senegal, (3) Western Ecology Division, US EPA/NHEERL, 200 SW 35th St, Corvallis, OR 97333, (4) Ohio State Univ, School of Natural Resources, 2021 Coffey Road, Columbus, OH 43210-1085

Water availability is a critical limiting element in semi-arid agriculture which presents particular challenges in Sahelian countries such as Senegal. The landscapes are characterized by widely occurring yet unrecognized shrubs (Guiera senegalensis and Piliostigma reticulatum), both of which are important hydraulic regulators in these water-limited agro-ecosystems. However, these shrubs fall prey to continuous destruction, especially for fuel and increment in agricultural acreage. They possess deep taproots that permit all year round survival in farmers' fields, even at periods when soil water levels are barely sufficient for crop survival. In the dry seasons, preliminary results revealed substantial moisture levels in soil surrounding the shallower roots of these shrubs. We therefore hypothesized the presence of a natural irrigation phenomenon termed as “hydraulic-lift”. In this regard, this dissertation reports three studies. In the first study, seasonal changes in soil water variation as well as shrub root dynamics were investigated. In the dry season, soil moisture content declined steadily in the 0.9 - 1.2 m depth range due to depletion by shrub roots. On the contrary, both the 0.2 and 0.4 m zones depicted slight increments in soil moisture which we attributed to soil water redistribution by shrub roots. During the rainy season, shrubs where intercropped with Pearl millet and depicted considerable impact on the field moisture regime with their roots serving as pathways for deep profile recharge. Shrubs also exploited deeper soil horizons as opposed to Pearl millet hence making them suitable candidates for intercropping in these fragile agro-ecosystems. The second study built on findings of the former one and further assessed water balance dynamics at two sites at the plot scale. Notably, shrub water uptake from the water table served as a crucial contribution of moisture to the water balance of the system. Shrub mediated effects were more pronounced when the profile was dry (~0.03 m3m-3) than when soil was not water deficient (~0.15 m3m-3). Consequently, in a third study, dry-season shrub mediated water provision as they tap ground water resources was investigated. Crop-shrub associations were evaluated using novel combinations of soil water potential, sap flow and canopy-level physiological measurements. This study reported both existence and magnitude (~0.1 mm d-1) of hydraulic redistribution (HR) in these agro-ecosystems. The shrub root zone at the 0.20 m depth exhibited diel changes in soil water potential (-6.2 to -3.7 MPa), representing a rewetting of the upper profile due to passive water loss from the root system into the drier upper soil layers. Reverse sapflow was detected in instrumented shrub roots with both positive and negative flows reported on tap and lateral shrub roots respectively. Sap flow reversals at night in the lateral roots and the periodic positive flow in the descending taproots are attributed to hydraulic lifting of water from lower moister layers. The HR mechanism and shrub root patterns serve for drought avoidance and maintanance of plant physiological functions which could promote greater plant growth with practical implications on nutrient cycling and water balance in these fragile ecosystems.

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