Pierre H.Y. Hiernaux

Decomposition of and nutrient release from ruminant manure on acid sandy soils in the Sahelian zone of Niger, West Africa

In ago-pastoral systems of the semi-arid West African Sahel, targeted applications of ruminant manure to the cropland is a widespread practice to maintain soil productivity. However, studies exploring the decomposition and mineralisation processes of manure under farmers’ conditions are scarce. The present research in south-west Niger was undertaken to examine the role of micro-organisms and meso-fauna on in situ release rates of nitrogen (N), phosphorus (P) and potassium (K) from cattle and sheep‐goat manure collected from village corrals during the rainy season. The results show that (1) macro-organisms played a dominant role in the initial phase of manure decomposition; (2) manure decomposition was faster on crusted than on sandy soils; (3) throughout the study N and P release rates closely followed the dry matter decomposition; (4) during the first 6 weeks after application the K concentration in the manure declined much faster than N or P. At the applied dry matter rate of 18.8 Mg ha 1 , the quantities of N, P and K released from the manure during the rainy season were up to 10-fold larger than the annual nutrient uptake of pearl millet (Pennisetum glaucumL.), the dominant crop in the traditional agro-pastoral systems. The results indicate considerable nutrient losses with the scarce but heavy rainfalls which could be alleviated by smaller rates of manure application. Those, however, would require a more labour intensive system of corralling or manure distribution.

Effects of grazing on plant species composition and spatial distribution in rangelands of the Sahel

The effects of grazing by livestock on plant species composition and spatial distribution have been studied at Sadoré, Niger. Herbaceous species were recorded in plots of increasing size from 1/64 to 1024 m2 in ten fallow plots subjected to five different grazing treatments over the previous three years. Treatments consisted of three intensities of grazing, and of protection from grazing for either 3 or 14 years. For all treatments, the number of species fitted a normal distribution with the log (ln) of the area inventoried. However, the fit improved slightly when the model included two successive log-normal distributions respectively considered as species distribution within a patch and at the patch mosaic scale. Across treatments, the optimal sampling areas averaged 3.8 ± 1.1 m2 for the within-patch and 725 ± 113 m2 for the mosaic scale distributions. It is argued that similarity between treatments in the overall log-normal distribution resulted from compensations between the divergent trends that affected species distribution within and across patches depending on the grazing status. Long-term protection resulted in a regular spatial arrangement of highly contrasted, but internally homogeneous patches. Heavy grazing ensued the dominance of a few species in contagious patches but also left niches for scattered individuals of other species. Tests of the relative frequency of each plant species, together with the average area needed to record that species, were used to characterize specific response to grazing. A majority of species encountered in old Sahelian fallows were either fostered by grazing, indifferent or tolerant to grazing. However, more than a third of the species appeared sensitive to heavy grazing, and no relationships were found between species response to grazing and palatability.

The use of herders’ accounts to map livestock activities across agropastoral landscapes in Semi-Arid Africa

Improved understandings of the agricultural and range ecologies ofsemi-arid Africa require better information on the spatiotemporal distributionof domestic livestock across agropastoral landscapes. An empirical GIS-basedapproach was developed for estimating distributions of herded livestock acrossthree agropastoral territories (around 100 km2 each)over a two-year period. Algorithms developed from regression analyses of herdtracking data (with R2s ≥ 0.67) are used to transform a morecomprehensive but incomplete set of data generated from herders’ accounts oftheir herds’ grazing itineraries (400 herds following 6500 itineraries). Theresulting characterization registers 40 000 days of livestock activitiesacross694 land units (averaging 70 ha) over the study period. This studydemonstrates that rural producers’ knowledge of their daily extractionpracticescan be translated to fine-grained characterizations of extraction densitiesacross mixed landscapes. The spatiotemporal distribution of livestock that isrevealed by this approach diverges strongly from that predicted bycommonly-usedpoint-diffusion estimation procedures. Instead, the distribution reflects localpatterns of land use, topography, vegetation, settlements, and water points.Grazing and nongrazing times spent in land units are not spatially correlatedand the seasonality of grazing pressure is spatially variable. Therefore, theecological impacts of livestock grazing are spatially variable at fine scalesand there is a significant potential for livestock-mediated nutrient transfersacross agropastoral landscapes. The georeferenced data produced by thisapproachnot only will help evaluate the impact and sustainability of differentmanagement practices but also provides a strong empirical base for improvedspatial modeling of herded livestock.

Observations and interpretation of seasonal ERS-1 wind scatterometer data over Northern Sahel (Mali)

Abstract This article presents an analysis of ERS-1 wind-scatterometer (WSC) data acquired over a region located in the northern Sahel. The considered period extends from January 1992 to December 1995, that is, covering four vegetation cycles. Experimental observations show that WSC responses at 45° of incidence angle display a marked seasonality associated with the development and senescence of annual grasses during the successive rainy seasons. The interpretation of the σ° temporal plots is performed with the assistance of a semiempirical backscattering model combined with an ecosystem grassland model. Contributions of the various components of the Sahelian landscape to the total backscattering are identified. Overall, the soil contribution is always large but the σ°(45) temporal plots reflect well the vegetation development. The analysis of the different contributions leads to a simplification of the backscattering model. Finally, this latter is parameterized as a function of two surface parameters, namely, the soil volumetric water content and the vegetation biomass. This simplified model allows the vegetation biomass to be estimated with a 33% error.

Livestock related nutrient budgets within village territories in western Niger

Poor cropland fertility and a rapidly rising demand for food force Sahelian farmers to cultivate more land and shorten fallow periods. This mostly leads to a gradual decline in crop yields per hectare, which can be counterbalanced by the systematic use of livestock manure on cropland. To assess the potential and limits of manuring practices, annual nutrient budgets were established for different land-use types, based on forage and crop yields and livestock and cropland management in five village territories in western Niger, which were selected along the Sahelian climatic gradient. Stocking rates per km2 of pasturing area range from 8–22 tropical livestock units (TLU, animal of 250 kg live weight). Faecal excretion during grazing directly returns 18–25% of the consumed forage dry matter, 21–29% of the ingested nitrogen and 44–56% of ingested phosphorus to the grazed land. Corralling animals on fields at night leads to a spatial concentration of nutrients, benefiting at most 9% of the arable village land. Where livestock consume only 15–20% of the total amount of forage produced, there is some scope for increasing village livestock numbers in order to increase the area manured, but eventually manuring must be complemented by additional measures such as the application of inorganic fertilizers to sustain overall productivity of the farming systems.

A regional Sahelian grassland model to be coupled with multispectral satellite data. II: Toward the control of its simulations by remotely sensed indices

Abstract An approach for combining remote sensing spectral measurements with an ecosystem model was presented in an accompanying article (Mougin et al., 1995). The sahelian grassland ecosystem STEP model developed for that purpose was also described and validated. In order to fulfill a prerequisite for using coarse resolution optical satellite data with the STEP model, the present paper presents i) a modeling of the reflectance which is adapted to the sahelian landscape and ii) a study based on the coupled ecosystem-reflectance modeling to assess the potential of vegetation indices for inferring vegetation parameters. The modeling of the landscape reflectance is based on existing soil and canopy reflectance models, and considers area-weighted contributions from green and dry vegetation, and bare soil components. The ecosystem model provides the landscape reflectance models with inputs like vegetation cover fraction ( f v ) and leaf area index (LAI) to characterize the vegetation present. Atmospheric effects are also accounted for using an existing simplified radiative transfer model. Simulated top of the atmosphere reflectances confronted to real satellite data during a growing season indicate that the modeling is adequate to reproduce temporal profiles of vegetation indices when atmospheric conditions are not prohibitive. Simulated vegetation indices (NDVI, SAVI, GEMI, SR) compared to vegetation characteristics show that a good tracking of the evolution of LAI and v during the growing season is possible before maturation. A sensitivity study of the four VIs to green biomass, soil brightness, and atmospheric water vapor is carried out for the specific case of the Sahel. The SAW and NDVI are both found to be adequate if atmospheric effects are minimized. NDVI integrated over the growing season is compared to net primary productivity (NPP) for different sites, regions, and growing seasons. A near-linear relationship is found, but the same relationship may not be applicable to different regions or growing seasons. On the whole, the results suggest that vegetation indices contain information which are useful for the ecosystem model, despite the fact that perturbating factors make the retrieval of these informations difficult. The possibility of using satellite data to drive the STEP model, or control its simulations, will be assessed in a forthcoming article.

The effect of clipping on growth and nutrient uptake of Sahelian annual rangelands

Growth and nutrient-uptake responses of annual rangeland to defoliation were studied at 13 sandy range sites located across the Sahelian zone of Mali between 1977 and 1989. 34 clipping experiments (site-years) were conducted using identical treatment designs with respect to the timing and frequency of clipping. The effect of highly variable growing conditions (rainfall and nutrient availability) on the response to clipping was analysed through a series of regression analyses. The growth response of vegetation to clipping is more related to variables associated with rainfall and growing condition than to clipping frequency. Total yields were lowered most during periods of rapid growth. Nitrogen (N) and phosphorus (P) yields were not affected by growing condition while clipping consistently increased N and P yields. Greater sink strength in clipped plants better explains the observed stimulation of N and P yields than does the increased nutrient availability that could result from modified soil water status after clipping. The significance of these results for the ecological management of Sahelian rangelands is discussed.

Rain-Use-Efficiency: What it Tells us about the Conflicting Sahel Greening and Sahelian Paradox

Abstract: Rain Use Efficiency (RUE), defined as Aboveground Net Primary Production (ANPP) divided by rainfall, is increasingly used to diagnose land degradation. Yet, the outcome of RUE monitoring has been much debated since opposite results were found about land degradation in the Sahel region. The debate is fueled by methodological issues, especially when using satellite remote sensing data to estimate ANPP, and by differences in the ecological interpretation. An alternative method which solves part of these issues relies on the residuals of ANPP regressed against rainfall (―ANPP residuals‖). In this paper, we use long-term field observations of herbaceous vegetation mass collected in the Gourma region in Mali together with remote sensing data (GIMMS-3g Normalized Difference Vegetation Index) to estimate ANPP, RUE, and the ANPP residuals, over the period 1984–2010. The residuals as well as RUE do not reveal any trend over time over the Gourma region, implying that vegetation is resilient over that period, when data are aggregated at the Gourma scale. We find no conflict between field-derived and satellite-derived results in terms of trends. The nature (linearity) of the ANPP/rainfall relationship is investigated and is found to have no impact on the RUE and residuals interpretation. However, at odds with a stable RUE, an increased run-off coefficient has

Beyond adding up inputs and outputs: process assessment and upscaling in modelling nutrient flows

Departing from the historical background of scientific interest in soil fertility and sustainable agricultural production in sub-Saharan Africa, a review is conducted of nutrient budget studies carried out in semi-arid West Africa at scales ranging from individual fields to the sub-continent. For both, nitrogen and phosphorus, the comparison discloses largely diverging balances calculated for similar agro-ecosystems. In a first step, the modes of calculation of the nutrient budgets are examined. It is demonstrated that the calculations used in the different studies differ by the variables and biophysical processes taken into account, and by the choice of spatial scales as well as related time scales. One important discrepancy between approaches is whether and to which extent nutrient flows are internalized when upscaling. The extent to which the impact of individual and communal management, especially of pastoral and forestry resources, on nutrient flows is accounted for is a second cause of divergence. Moreover, it was observed that nutrient budgets tend to be increasingly negative as the spatial scale of the study increases from farm to sub-continent. This unexpected trend is traced back to the lack of internalization of nutrient flows when upscaling. The complexity of the scale patterns of nutrient flows and that of the interactions and the tradeoffs in the effects of management calls for the use of models to calculate nutrient budgets. Therefore, in a second step, examples of a static model, a multiple-goal linear programming model and a decision rules model were reviewed, all of which include the calculation of nutrient flows and balances and which were applied to West-African farming systems. The models are analyzed for their way of dealing with the critical issues of spatial and temporal scales and the impact of resource management on nutrient flows, taking into account that they have different objectives and were designed for different spatio-temporal scales. To conclude, suggestions were made for strengthening the use of models as tools enabling ex-ante testing of alternative agricultural technologies and policies that could improve soil nutrient balances in semi-arid sub-Saharan Africa.

Analysis of ERS wind scatterometer time series over Sahel (Mali)

Abstract ERS wind scatterometer (WSC) time series are analyzed over different ecoclimatic regions of the African Sahel during the period 1991–1995. At 45° incidence angle, the strong seasonality of σ o time series can be directly linked to the successive wet and dry seasons. Moreover, the annual σ o dynamic range was found to be strongly correlated to total rainfall. The interpretation of the σ o temporal plots is carried out by combining a backscattering model to a grassland growth model. Results highlight the decreasing contribution of the herbaceous component with latitude. However, its contribution is far from negligible and can reach 60% in the Soudano-Sahelian subzone at peak herbaceous mass. Additionally, the tree layer has a negligible effect on the radar signal at the scale of a resolution cell. Finally, a simple parametric backscattering model is calibrated and used in an inversion process. The resolution of the inverse problem is based on a ‘brute-force’ method that consists of exploring all the combinations of parameters of interest. Despite a poor estimation of the temporal variation of the herbaceous mass B t , the retrieved maximum mass compares well with ground estimates.