Charles Bielders

Transport of soil and nutrients by wind in bush fallow land and traditionally managed cultivated fields in the Sahel

In the West African Sahel, few direct measurements are currently available for the major land-use types on the extent of soil losses by wind erosion. A measurement campaign was therefore carried out in 1997 to monitor windblown sediment fluxes using Big Spring Number Eight (BSNE) sand-traps in a conventionally managed cultivated field and bush fallow in western Niger. Sediment balances were derived from the measured windblown sediment mass fluxes. Results indicate that sediment fluxes in a cultivated field increased linearly over distances up to 76 m irrespective of wind speed and duration. Sediment deposition over distances up to 47 m in an adjacent bush fallow was well described by an exponential decay function with a near constant trapping efficiency coefficient of 0.11 m−1 for incoming sediment mass fluxes between 10 and 45 kg m−1. Soil mass balances up to −17.5 and +10.5 Mg ha−1 were measured in a single storm in the field and fallow, respectively. However, 89% of the sediment deposition observed in the fallow occurred within the first 20 m. The nutrient content of windblown sediment generally declined with distance into the field and increased with distance into the bush fallow. Because of the low nutrient content of the native soil, total nutrient losses remained very low (<163 mg m−2 for any given nutrient). However, such losses were by no means negligible compared to the average nutrient uptake by a millet crop. The present measurements confirm that wind erosion can result in substantial soil losses in traditionally managed fields on the sandy soil of the Sahel. The bulk of sediment transport is, however, predominantly short range as the saltating material is efficiently trapped by the natural vegetation of fallow land.

Farmer perception of runoff and erosion and extent of flooding in the silt-loam belt of the Belgian Walloon Region

Flooding, and in particular muddy floods resulting from erosion of agricultural land, is of growing concern to decision makers and the public in western Europe and is frequently blamed on agricultural land use. Two surveys were therefore under-taken in the Walloon Region of Belgium, one among all municipalities to assess the extent of flooding, and one among 1500 farmers in the silt-loam and sandy-loam agricultural regions to evaluate the extent of runoff and erosion problems on-farm and what, if any, control measures are currently implemented by farmers. Twenty-seven percent of the municipalities in Wallonia have been confronted with floods at least once every 2 years over the last decade. For the silt-loam and sandy-loam regions specifically, 68% of the municipalities have had to deal with at least one flooding event directly attributed to agricultural runoff during the last 10 years. Runoff or erosion was observed at least once every other year on average by 26% of the farmers during the last decade. The occurrence of erosion was positively correlated to the area of row crops and negatively correlated to winter cereals. On average, 74% of the fanners are taking active measures to control erosion but farmers with erosion problems are more likely to take measures. Farmers with a higher level of education are also more likely to implement erosion control measures (ECMs) than less educated farmers. It appears therefore that greater efforts need to be expended on awareness raising among less educated farmers and towards the development, testing and demonstration of additional ECMs if flooding problems are to be reduced in the future

Integrated nutrient management of pearl millet in the Sahel combining cattle manure, crop residue and mineral fertilizer

In the Sahelian zone of Niger, there is a need to develop guidelines fir integrated nutrient management, which relies on the potential nutrient sources of manure, pearl millet residue and mineral fertilizers. A fully factorial on-station experiment was conducted during the 1994 and 1995 rainy seasons at Sadore, Niger, combining application of: (i) broadcast millet residue (300, 900 and 2700 kg ha(-1)), (ii) broadcast cattle manure (300, 900 and 2700 kg ha-(1)) and (iii) mineral fertilizer (unfertilized control, 15 kg N ha(-1) + 4.4 kg P ha(-1) and 45 kg N ha(-1) + 13. l kg P ha(-1)). Manure and fertilizer increased millet yields in both years whereas residue was effective in 1995 only: The effect of manure and residue were additive, as was the effect of manure and fertilizer but only up to 50 kg N ha(-1). However in 1995, the response to fertilizer was approximately doubled in the presence of 900 or 2700 kg residue ha(-1) compared to fertilizer with 300 kg ha(-1) residue, indicating a strong synergistic effect. This synergistic effect was reflected in the partial factor productivity of nitrogen and phosphorous in both years. Two treatment combinations stand out as particularly relevant based on yield, partial factor productivity and nutrient balance criteria: 2700 kg manure ha(-1) combined with (i) 300 kg residue and no fertilizer (95% grain yield increase); (ii) 900 kg residue ha(-1) and 15 kg N + 4 kg P ha(-1) (132 % grain yield increase). There is a need for similar, long-term experiments to confirm the present results.

Effect of intercropping period management on runoff and erosion in a maize cropping system

The management of winter cover crops is likely to influence their performance in reducing runoff and erosion during the intercropping period that precedes spring crops but also during the subsequent spring crop. This study investigated the impact of two dates of destruction and burial of a rye (Secale cereale L.) and ryegrass (Lolium multiflorum Lam.) cover crop on runoff and erosion, focusing on a continuous silage maize (Zea mays L.) cropping system. Thirty erosion plots with various intercrop management options were monitored for 3 yr at two sites. During the intercropping period, cover crops reduced runoff and erosion by more than 94% compared with untilled, post-maize harvest plots. Rough tillage after maize harvest proved equally effective as a late sown cover crop. There was no effect of cover crop destruction and burial dates on runoff and erosion during the intercropping period, probably because rough tillage for cover crop burial compensates for the lack of soil cover. During two of the monitored maize seasons, it was observed that plots that had been covered during the previous intercropping period lost 40 to 90% less soil compared with maize plots that had been left bare during the intercropping period. The burial of an aboveground cover crop biomass in excess of 1.5 t ha(-1) was a necessary, yet not always sufficient, condition to induce a residual effect. Because of the possible beneficial residual effect of cover crop burial on erosion reduction, the sowing of a cover crop should be preferred over rough tillage after maize harvest.

Vertical particle segregation in structural crusts: experimental observations and the role of shear strain

Abstract There is ample evidence that coarse-textured soils can be highly sensitive to crust formation under rainfall. One of the most controversial aspects of crust formation on such soils is the frequent occurrence of a thin clay band buried below a washed-out layer. In order to better understand the mechanisms that may lead to this vertical sorting in structural crusts, mixtures of 92.5% sand and 7.5% clay were exposed to simulated rainfall for 1 h at 63 mm h−1. The sand fraction of the samples consisted of a binary mixture of contrasting particle-size classes between 106 and 1000 μm, mixed in varying proportions. Micromorphological changes in surface structure were observed on thin sections made from undisturbed samples. Before rainfall, the clay was present as coatings around the sand grains. In all samples subjected to rainfall, a washed-out layer 2 to 2.5 mm thick formed at the soil surface. Except for the samples containing 50% or more 106–150 μm sand, the lower boundary of the washed-out layer was marked by an accumulation of clay material, generally in the form of a band. This clay material was present as microaggregates that were probably derived from the initial grain coatings. Within the washed-out layer a vertical sorting of the sand grains was sometimes observed, which resulted in a relative concentration of the coarsest particles at the surface. The sorting was more pronounced as the ratio between the diameter of the large and small sand grains increased, and as the relative percentage of finer sand decreased. By analogy with a physical model derived for particle segregation in granular media, it is proposed that the vertical sorting of particles according to their size was caused by the shear strain created in the first few millimeters of the samples by drop impact. The strain is believed to have induced temporary changes in pore size which allowed the preferential downward movement of finer grains that would otherwise riot percolate freely. The present observations suggest that strain-induced segregation may be one of the main mechanisms leading to clay band formation in structural crusts formed on coarse-textured soils. The implications of the model are discussed in terms of the range of conditions under which segregation is expected to occur. The experimental results also point to the distinctive role of the particle-size distribution of the sand fraction on the morphology of structural crusts.

Soil water crop modeling for decision support in millet-based systems in the Sahel: a challenge

Food insecurity in the Sahelian environment was extensively shown to be a result of low soil fertility and high climate risks. But decisions and recommendations made from the great wealth of research have little been adopted by farmers. Soil water crop models (SWCM) can assist researchers and development actors in this environment if they can appropriately deal with the constraints and mainly farmers agricultural development goals.

Effect of Zai Soil and Water Conservation Technique on Water Balance and the Fate of Nitrate from Organic Amendments Applied: A Case of Degraded-Crusted Soils in Niger

Experiments were conducted on degraded crusted soils to study water status and nitrogen release in the soil during the dry seasons of 1999 at ICRISAT research station and on-farm during the rainy seasons of 1999 and 2000 in Niger. Zai is a technology applied on degraded crusted soil, which creates conditions for runoff water harvesting in small pits. The harvested water accumulates in the soil and constitutes a reservoir for plants. The organic amendment applied in the Zai pits releases nutrients for the plants. Soil water status was monitored through weekly measurement with neutron probe; access tubes were installed for the purpose. Nutrient leaching was measured as soil samples were collected three times throughout the cropping season. A rapid progress of the wetting front during the cropping period was observed. It was below 125 cm in the Zai-treated plots 26 days after the rain started versus 60 cm in the non-treated plots. Applying cattle manure leads to shallower water profile due to increased biomass production. Total nitrate content increased throughout the profile compared to the initial status, suggesting possible loss below the plant rooting system due to drainage, which was less pronounced when cattle manure was applied. This study shows that the system improves soil water status allowing plants to escape from dry spells. However, at the same time it can lead to loss of nutrients, particularly nitrogen.

How pixel size affects a sediment connectivity index in central Belgium

Connectivity has become an increasingly used concept in hydrological and sediment research. In order to quantify it, various indices have been proposed since the start of the 21st century including the index of connectivity. This index is based on a limited number of factors, the most important one being topography. Sediment connectivity indices values probably depend on the digital elevation model (DEM) resolution. The aim of this study was, first, to compare the effect of DEM pixel size (between 0.25 and 10m, using an UAV) in the Belgian loess belt, a lowland area. We show that the index values were lower when the pixel size decreased (a difference of about 20% in value between 0.25 and 10m). In addition, the impact of linear features in the watershed (e.g. grass strip, bank and road) was lower with the largest pixel sizes, and the connectivity pattern was affected with a pixel size of 5m or more. At lower pixel sizes (1m or below), some more disconnected regions appeared. These corresponded with zones where there had been water stagnation during and after rainfalls, and was corroborated by field observations. This confirmed the need for a proper resolution according to the objectives of the study. The second aim of this study was to deduce a minimum pixel size for connectivity study, helping local erosion or sedimentation location and consequent land management decisions. In our context, 1m stands as the optimum DEM resolution. This pixel size permitted location of all ‘key areas’ in terms of erosion. Very high resolutions (<0.5m) did not generate much more information, and their calculation time was far greater. Copyright

Soil piping: networks characterization using ground-penetrating radar

Soil piping remains a relatively unexplored phenomenon despite its substantial impacts on watershed-scale water and sediments transfer in numerous locations around the world. One of the main limits regarding the study of this singular process is characterization of the pipe networks (defining number, position, dimension and connectivity of pipes). In this context, non-invasive sub-surface imaging using ground-penetrating radar (GPR) seems a promising technique. An exploratory methodology was developed to assess the ability of GPR to characterize pipe networks in Loess-derived soil. This methodology relies on (1) high spatial resolution scanning and (2) detection of electromagnetic sub-surface indicators of soil pipe (reflection hyperbolas and strongest reflections). For a 50 m × 50 m scanned zone, results show that combining these indicators can provide interesting clues about a potential pipe network. Three probably interconnected pipes were revealed. However, results show that the proposed methodology needs specific improvements in signal processing, object detection and system configuration in order to enhance and facilitate subsurface networks characterization.

Efficient use of nutrients and water through hill-placed combination of manure and mineral fertilizer in smallhoder maize farming system in northern Benin

Maize, a major staple food in many farming systems in sub-Saharan Africa, is characterized by low productivity due to the scarce availability and use of external inputs and recurrent droughts exacerbated by climate variability. Within the integrated soil fertility management framework, there is thus a need for optimizing the use of fertilizers and manure for the efficient use of limited nutrient resources and rainfall, and to increase crop yield and farmer income. On-station experiments were conducted in Northern Benin over a 4-year period using a split-plot design with three replications to evaluate the effect of hill-placed mineral fertilizer and manure on yields and soil chemical properties. The treatments consisted of the combination of: (i) three rates of manure (main plot): 0, 3 (3M) and 6 (6M) t ha−1; and (ii) three levels of fertilizer (sub-plot): 0%(NF), 50% (50F) and 100% (100F) of the recommended rate (76 kg N + 13.1 kg P + 24.9 K ha−1). Hill-placement of manure and/or fertilizer significantly improved soil organic carbon content, available P and exchangeable K in the vicinity of the planting hills. As a result, yields increased steadily over time for all manure and fertilizer combinations, with yields up to 5 times higher than the control for the 6M-100F treatment. Value-cost ratios and benefit-cost ratios were >2 and generally as good or even better for treatments involving 50F compared to NF or 100F. Although applying half the recommended rate of fertilizer is performed by many farmers and appears to make economic sense, this practice is unlikely to be sustainable in the long run. Substituting 50F for 3M or complementing 50F with 3M are two possible strategies that are compatible with the precepts of ISFM and provide returns on investment at least as good as the current practice. However, this will require greater manure production, made possible by the increased stover yields, and access to means of transportation to deliver the manure to the fields.