Long-term effects of conservation soil management in Saria, Burkina Faso, West Africa

The negative degradation spiral that currently leads to deteriorating soil properties in African drylands is a serious problem that limits food production and threatensthe livelihoods of the people. Nutrient depletion and water and wind erosion are the main factors in soil degradation in Africa. This thesis describes field research conducted from 2006 through 2008 to assess how changes in physical and hydrological soil properties, induced by differences in land management and macro-faunal biodiversity determine water and nitrogen use efficiencies in Burkina Faso. The methodology involved systematic soil sampling of selected treatments (including a fallow control) coupled with macro-fauna identification. Measurements were used to generate information on the effect of the long-term land management practices on soil properties and the different terms of the field water balance. Crop sampling (leaves, stem and grains) allowed determination of plant nutrient uptake and calculation of water and nitrogen use efficiency. Differences in soil properties between treatments were smaller than expected after so many years of applying the same soil management practice. Results indicate that long-term permanent cultivation result in a decrease in the quality of most soil properties when compared with the fallow. We also found that there are clear benefits from inclusion of cowpea in a rotation system due to its N fixation and deeper root system. Regarding soil fauna, long-term application of the same soil management practices resulted in specialization of the food type for the macro-fauna leading to less fauna diversity. Also, more diverse and abundant macro-fauna was discovered under superficially tilled plots compared to tractor plowed plots. The contribution of the soil fauna to aggregate building depends on the amount and type of organic material available to the fauna as well as the soil management regime. In spite of the amount of applied organic amendments used in our trials, the C-stock in the soil has decreased at a rate of 0.25 % per year, perhaps limiting macro-fauna activity. The 3-year average of the green water use efficiency (ratio T/P) was only 14% and the crop yield was also low due to less than optimal crop management. Results further suggest that systematic, rather than strategically timed, N applications (organic and/or mineral) are likely to lead to N losses. Synchronizing N fertilizer application with crop-N demand and accounting for residual Nitrogen will lead to higher N fertilizer use efficiency. Soil management practices, crop selection and fertilizer regime can have positive or negative impacts on water and nutrient use efficiency. Practices with positive impact should be encouraged in order to increase crop productivity and improve food security in Burkina Faso.