Kenta Ikazaki

“Fallow Band System,” a land management practice for controlling desertification and improving crop production in the Sahel, West Africa. 1. Effectiveness in desertification control and soil fertility improvement

Wind erosion is a major contributor to desertification in the Sahel. Although three effective countermeasures for wind erosion (i.e. ridging, mulching with post-harvest crop residue, and windbreaks) have been proposed, they are not practical for Sahelian farmers. Therefore, we designed a new land management practice, termed the “Fallow Band System,” which can be used for both controlling wind erosion and improving soil fertility and crop production. This method does not impose additional expense and labor requirements on Sahelian farmers who are economically challenged and have limited manpower. The objective of this study was to evaluate the effects of this system on wind-erosion control and soil-fertility improvement. We conducted field experiments at the International Crops Research Institute for the Semi-Arid Tropics West and Central Africa and showed that (i) a fallow band can capture 74% of wind-blown soil particles and 58% of wind-blown coarse organic matter, which suggests that it can effectively control wind erosion, (ii) the amount of soil nutrients available for crops in a former fallow band was increased by the decomposition of trapped soil materials containing considerable amounts of nutrients, and (iii) the amount of soil water available for crops in a former fallow band was increased by the trapped wind-blown soil materials through improvement of rainwater infiltration into surface soil. These results lead to the conclusion that the “Fallow Band System” can be useful for preventing desertification and improving soil fertility in the Sahel, West Africa.

Sediment catcher to trap coarse organic matter and soil particles transported by wind.

Wind can erode fertile topsoil and reduce soil fertility. Evaluating the effect of wind erosion on soil fertility is crucial to achieve sustainable agriculture in areas suffering from desertification caused by wind erosion. To estimate soil loss and associated soil nutrient loss by wind erosion, flux of coarse organic matter (COM) (defined here as free organic debris larger than 200 µm) and soil particles (defined as the other soil components) must be measured separately. This is because their modes of transport are different, and COM plays a prominent role in soil nutrient dynamics in some semiarid zones where COM accounts for a large percentage of the total soil carbon. Because the Big Spring Number Eight (BSNE) sampler can trap both COM and soil particles 0.05 m above the surface, we designed a sediment catcher, the Aeolian Materials Sampler (AMS), to trap these components below 0.05 m. This device can be manufactured easily at low cost. AMS performance was tested by wind tunnel experiments over a range of wind velocities typically observed in erosive storms and with six incident wind angles because the AMS is a buried-type sampler that is unable to rotate toward the wind. The trapping efficiency of the AMS for COM and soil particles was not 100%, but it can be calibrated easily using wind data. Therefore, we can estimate the mass flux of COM and soil particles and evaluate the effect of wind erosion on soil fertility using the AMS with the BSNE sampler.

Technical Note: Aeolian Materials Sampler for Measuring Surface Flux of Soil Nitrogen and Carbon During Wind Erosion Events in the Sahel, West Africa

In the Sahel, determining the effects of wind erosion on soil fertility and soil carbon balance is crucial for achieving sustainable agriculture and for carbon sequestration, respectively. We designed the Aeolian Materials Sampler (AMS) to measure the surface flux of soil nitrogen and carbon, which limit crop production as well as water in the Sahel. The AMS should not be used alone, but with the Big Spring Number Eight (BSNE) sampler, which is a quasi-isokinetic sampler. We examined the performance of the AMS in estimating the surface flux of soil nitrogen and carbon by conducting wind-tunnel and field experiments. In the former experiment, we measured the trapping efficiency of the AMS for nitrogen and carbon content in coarse organic matter (COM) and soil particles. We observed that the AMS allows estimation of the surface flux of soil nitrogen and carbon associated with the movement of COM and soil particles; wind data and empirical equations were used for the estimation. In the field experiment, we assessed the contribution of the inherent error of the AMS to the total measurement error and found that the inherent error was negligible and did not increase the total measurement error in the estimation of the surface flux of soil nitrogen and carbon. Therefore, we concluded that the combination of the AMS and the BSNE sampler could be effectively used to evaluate the effects of wind erosion on soil fertility and soil carbon balance in the Sahel.

Desertification and a new countermeasure in the Sahel, West Africa

Abstract In this paper, I firstly describe wind erosion, the primary cause of the desertification in the Sahel region, from three different perspectives – namely, on microscopic (a few centimeters), mesoscopic (a few tens of meters to a few hundred meters) and macroscopic (a few kilometers to a few tens of kilometers) scales to offer a multilateral description of the realities of desertification. The desertification in the Sahel region can be understood on a microscopic scale as a deterioration of soil properties, caused by the loose sand layer being blown away by the wind, and by the associated exposure of the crust (layer); on a mesoscopic scale as a phenomenon that involves the topsoil and its component nutrients being spatially localized to an area not available for crop production; and on a macroscopic scale as a phenomenon in which the land becomes patchy with decreasing vegetation and eventually turns into bare land. Then, I introduce a new countermeasure against desertification, called the fallow band system. It is a technique on a microscopic scale that can be used to cover the crust (layer) by placing a loose sand layer on top of it without investing in capital or labor, and on a mesoscopic scale that can be used to connect the spatially localized topsoil and its nutrients with crop production. Finally, I mention future research needs.

Soil toposequence, productivity, and a simple technique to detect petroplinthites using ground-penetrating radar in the Sudan Savanna

ABSTRACT In the Sudan Savanna of West Africa, Plinthosols with a petroplinthic or pisoplinthic horizon at ≤ 50 cm from the surface comprise the major soils. Because these horizons limit the rooting volume and water and nutrient storage capacities of the soils, they should be a major cause of decreased crop yield in the Sudan Savanna. However, the local distribution of Plinthosols is not precisely known, and the relationships between soil classes, effective soil depth, and crop yield, which are considered to be closely related to each other on the Plinthosol soils, are not fully understood. To clarify these relationships, we first reassessed the soil toposequence on a slope at the Institute of Environment and Agricultural Research Saria station in Burkina Faso using the current World Reference Base soil classification system. We then determined the relationships between soil classes and sorghum yield and between the effective soil depth and yield. We also assessed whether ground penetrating radar could predict the position of a petroplinthic horizon. We found (1) that Pisoplinthic Petric Plinthosols were found at the upper slope, Petric Plinthosols were found at the middle slope, and Ferric Lixisols were found at the lower to toe slope; (2) that sorghum yield was significantly larger at the Ferric Lixisols, then at the Petric Plinthosols, and lower at the Pisoplinthic Petric Plinthosols; (3) that sorghum yield was proportional to the effective soil depth at which upper boundary of petroplinthic horizon was found (n = 26, R2 = 0.78*** exclusion of waterlogged soil); and (4) that ground penetrating radar could predict the effective soil depth and the position of petroplinthic horizons (n = 4, R2 = 0.99**), suggesting that we could roughly but easily predict sorghum yield and local distribution of Plinthosols having a petroplinthic horizon using GPR. These results may enable us to take more account of the inherent soil conditions when studying soil and water conservation, fertilization methods, and crop breeding, all of which are crucial if sustainable agricultural methods are to be achieved in the Sudan Savanna.

Are all three components of conservation agriculture necessary for soil conservation in the Sudan Savanna

ABSTRACT Conservation agriculture (CA) as recommended by the Food and Agriculture Organization of the United Nations consists of three components: minimum soil disturbance, soil cover, and crop rotation/association. CA was expected to become an effective countermeasure against water erosion in the Sudan Savanna, but it has not been adopted by local smallholder farmers. As markets for grain legumes (including cowpea) have not been developed in the Sudan Savanna, crop rotation/association should be considered impractical for these farmers. Therefore, we examined whether legume intercropping as a crop rotation/association component is necessary for preventing soil erosion in the Sudan Savanna. Three-year field experiments were conducted in runoff plots at Institute of Environment and Agricultural Research Saria station. The four treatments were conventional practice (full tillage, no sorghum residue mulching, and no intercropping), two-component CA (minimum tillage (MT) and sorghum residue mulching without intercropping), and three-component CA with velvet bean (VB) or pigeon pea (PP) intercropping. It was revealed that: (1) MT and sorghum residue mulching (without intercropping) effectively reduced the annual soil loss by 54% mainly due to the improvement of soil permeability by the boring of termites and wolf spiders found under the sorghum stover mulch; (2) intercropping in combination with MT and crop residue mulching had no effect on soil erosion control mainly because: (a) PP did not survive the long dry season; (b) VB did not serve effectively as a cover crop since soil loss was concentrated at the beginning of the rainy season when VB was still too small; (c) unexpectedly, in combination with MT and crop residue mulching, intercropping with VB did not increase mulch biomass, especially sorghum biomass which prompts the boring of termites and wolf spiders. These results demonstrate that the third component of CA, namely legume intercropping, is not always necessary; rather, the two remaining components – minimum soil disturbance and soil cover – are sufficient for soil conservation in the Sudan Savanna. This finding lightens the burden of adopting CA and thus facilitates its future promotion to the smallholder farmers in the Sudan Savanna.

Grassland degradation caused by tourism activities in Hulunbuir, Inner Mongolia, China

The recent increase in the number of tourists has raised serious concerns about grassland degradation by tourism activities in Inner Mongolia. Thus, we evaluated the effects of tourism activities on the vegetation and soil in Hulunbuir grassland. We identified all the plant species, measured the number and height of plant and plant coverage rate, and calculated species diversity, estimated above-ground biomass in use plot and non-use plot. We also measured soil hardness, and collected soil samples for physical and chemical analysis in both plots. The obtained results were as follows: a) the height of the dominant plants, plant coverage rate, species diversity, and above-ground biomass were significantly lower in use plot than in non-use plot, b) Carex duriuscula C.A.Mey., indicator plant for soil degradation, was dominant in use plot, c) soil hardness was significantly higher in use plot than in non-use plot, and spatial dependence of soil hardness was only found in the use plot, d) CEC, TC, TN and pH in the topsoil were significantly lower in use plot than non-use plot. On the basis of the results, we concluded that the tourism activities can be another major cause of the grassland degradation in Inner Mongolia.