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Le projet majeur africain de la Grande Muraille Verte

 | 
Abdoulaye Dia
, 
Robin Duponnois

Partie I. Espèces végétales de la Grande Muraille Verte

The Native Shrubs Philiostigma reticulatum and Guiera senegalensis: The unrecognized Potential to Remediate Degraded Soils and Optimize Productivity of Sahelian Agroecosystems

R.P. Dick, M. Sene, M. Diack, M. Khouma, A. Badiane, S. A.N.A Samba, I. Diedhiou, A. Lufafa, E. Dossa, F. Kizito, S. Diedhiou, J. Noller y M. Dragila

Resumen

The global objective of this study was to determine the unrecognized role of shrubs as key determinants in sequestration of C, water relations, and soil degradation mitigation in semiarid climatic regimes of Senegal that are representative of much of Sub-Sahelian Africa. The results showed that shrubs are the dominant controllers of hydrology, C biomass on the landscape, microbiology, and crop productivity in agroecosystem of Senegal. The major findings were:
Shrubs residues decompose rapidly enough to allow non-thermal management.
Shrub residues promote crop growth but it takes 2 years of incorporation before beneficial impacts on crops were measured.
Both shrubs are doing hydraulic lifting of water from wet subsoils to dry surface soils
Shrubs are non-competitive with crops for water and increase water and nutrient efficiency.
During periods of excess rainfall shrubs promote groundwater recharge and therefore reduce surface runoff losses.

G. senegalensis had the most profound impact on yields which after fours cropping the declining yields in the absence of crops resulted in a 242% difference in yield between plots with and with out this shrub. These positive impacts occurred even in the absence of fertilizer applications

Texto completo

Introduction

1The Sahel is experiencing landscape and soil degradation that reduces food and economic security of rural, underprivileged communities that depend on ecosystem services. In cropped fields, the Parkland system of randomly distributed trees is an approach to address these challenges. However, trees are slow growing and can compete with crops for water and nutrients. Conversely, two native shrubs, Piliostigma reticulatum and Guiera senegalensis, coexist in farmers’ fields throughout the Sahel and until recently have are largely been overlooked components of parkland systems in semi-arid Sub-Sahelian Africa (Lufafa et al, 2005).

2Organic matter input to the soil has been shown to be critical for improving soil quality and optimizing nutrient and water efficiencies, and ultimately crop productivity in these degraded agroecosystems (Woomer et al., 1994; Sanchez et al., 1997; Badiane et al., 2000; Sinaj et al., 2001; Tschakert et al., 2004). Various non-indigenous vegetative systems to increase organic matter input to soils of the Sahel have been proposed, but received limited rates of adoption due to socio-economic constraints (Rhoades, 1997; Buresh and Tian, 1998; Bationo and Buerkert, 2001). Consequently, technologies that build upon farmers’ indigenous practices are most likely to have greater impact at the landscape level which is the case for these shrubs being locally available. Unfortunately, the current management, to coppice and burn the shrub biomass each spring to prepare for the cropping season, is not utilizing this organic matter effectively.

3Our team hypothesized that these two shrubs would be much more important than trees or other organic inputs such as animal manure at the landscape level in regulating C inputs/sequestration and hydrologic processes. Therefore the global objective was to determine the unrecognized role of shrubs as key determinants in sequestration of C, water relations, and soil degradation mitigation in semiarid climatic regimes of Senegal that are representative of much of Sub-Sahelian Africa.

Results & Discussion

Carbon Stocks and Plant Biomass of Native Shrubs as Landscape Levels

LANDSCAPE LEVEL CARBON AND BIOMASS

4Accurate and reliable estimates of carbon (C) storage in landscapes are critical to the development of effective policies and strategies to mitigate atmospheric and climate change. Carbon stocks of two native woody shrubs (Guiera senegalensis J.F. Gmel and Piliostigma reticulatum (DC.) Hochst) communities and associated soils within Senegal’s Peanut Basin were determined and the spatial structure of soil C quantified.

5Peak-season shrub biomass C was measured in forty-five fields in 0.81 ha sampling plots at 8 villages across the Peanut Basin of Senegal. Direct density counts were supplemented with allometric calculations to determine biomass per hectare. Soil samples to a depth of 20 cm were taken and analyzed for total C using a Leco C analyzer at 4 sites.

6Table 1 shows that organic C and N as well as total P is higher beneath the canopy than outside the influence of the canopy. This follows the concept of islands of fertility or spatial patchiness of nutrients due to woody vegetation in unmanaged arid and semiarid environments (Schlesinger et al. 1996). The increased soil organic matter beneath the shrubs are likely due to the greater litter input and the root turnover. This show the ability of shrubs to remediate degraded soils.

7Estimates of peak-season aboveground biomass C ranged from 0.9 Mg C ha-1 to 1.4 Mg C ha-1 yr-1 with an overall mean of 1.12 Mg C ha-1 yr-1 in the G. senegalensis sites and from 1.3 to 2.0 Mg C ha-1 yr-1 in the P. reticulatum communities. The dominant crop residues are peanut for which the aboveground biomass is entirely harvested for livestock forage and pearl millet residue of which 50-100% is removed or consumed by livestock (0 to ~0.5 Mg ha-1 yr-1) (Badiane et al., 2000). Thus this leaves only animal manure as a source of organic inputs which based on Badiane et al.’s (2000) field surveys showed a typical amount of 0.048 Mg manure-C ha-1 yr-1 (based on being evenly spread on all millet fields of a village or 50% in a given year). Although trees practically contain more biomass C than do the shrubs (Tschakert et al., 2004; Woomer et al., 2004), the overall fraction of tree C applied to soils is only the leaf material whereas in theory, the entire shrub aboveground biomass could be returned to soils upon pruning. Furthermore, these current measurements (0.9 to 2.0 Mg C ha-1 yr-1 ) are based on current shrub densities of (200-600/ha) which is relatively low (we have done studies as reported below that have 1200 plants/ha). Consequently, our results show that not only are these shrubs the dominant C source to soils in the Peanut Basin of Senegal, but with optimized densities have great potential to more than double the C inputs.

8Lufafa et al. (2005), showed the potential distribution of the shrubs across the Peanut Basin to have approximate areal coverages of 2.34 x 106 and 9.14 x 105 ha, respectively, for G. senegalensis and P. reticulatum. Tottrup and Rasmussen (2004) reported an average cultivation density of 49% in sections of the Peanut Basin in 1999. Using this rather conservative cultivation density estimate and areal coverage of the shrubs reveals that these systems annually lose about 3.51 x 105 Mg of biomass C, an equivalent of 2.11x 105 Mg of CO2 per year (Brady and Weil, 1999).

LANDSCAPE BIOMASS ESTIMATING MODELS

9Allometric equations and community biomass stock estimating models are needed as a means to quickly assess the biomass of Guiera senegalensis J.F. Gmel and Piliostigma reticulatum (DC.) Hochst (Pr). This is important not only for research but ultimately to develop optimized management systems and to be able to verify C levels for Carbon Trading Schemes. In Year 1, best predictors of aboveground biomass were height and number of stems (G. senegalensis) and crown diameter (Pr); and for belowground biomass were height and basal diameter (Gs) and basal diameter (Pr). In Year 2, height and crown diameter were the best predictors of aboveground biomass (R2 1/4 0.90 for G. senegalensis and 0.87 for Pr), whereas basal diameter and number of stems (Gs) and basal diameter (Pr) were best predictors of belowground biomass. Peak-season biomass estimates ranged from 0.44 to 4.58 ton ha-1 (mean -1/4 2.38 Mg ha-1) in the G. senegalensis sites and from 0.33 to 7.38 ton -1 (mean 1/4 3.71 Mg ha-1) in the P. reticulatum communities. Both species exhibited unusually large root: shoot ratios (4.5:1 for G. senegalensis and 10.2:1 for Pr). Although models differ between years, allometric relationships provide reasonable biomass estimates for G. senegalensis and P. reticulatum.

Water Balance and Hydrology of Native Shrubs

WATER RELATIONS

10Soil water is an important resource that imposes limitations on optimal plant performance in semiarid regions. In the Sahel, shrubs are a component of farmers’ fields and would be expected to affect water relations with the adjacent crops. Consequently, a 2-year study on soil water dynamics and shrub rooting patterns was conducted during the dry season and transition into the wet season with fields having Pearl millet intercropped with shrubs.

11Millet roots predominantly exploited the 0.2–0.5m depth with 95% of shrub roots in the upper 0.5 m. Soil volumetric water content (soil water content) decreased with greater radial distance from shrubs up to 2m but progressively increased with soil depth. During the dry season, soil surrounding shrub roots was consistently wetter than adjacent bare soil. Soil moisture content declined steadily in the 0.9–1.2m depth range due to depletion by shrub roots. On the contrary, the 0.2 and 0.4m zones had slight increases in soil moisture which could be attributed to soil water redistribution by shrub roots (Kizito et al., 2009).

12During the rainy season, shrub presence had a considerable impact on the fate of the field soil moisture regime with shrub roots serving as pathways for deep profile recharge. Shrubs exploited the deeper profile (0.9–1.2 m) as opposed to the Pearl millet (0.2–0.5 m) suggesting that intercropping of annual crops with shrub stands could serve as an innovative and viable agronomic option in these vulnerable Sahel agro-ecosystems.

HYDRAULIC LIFT

13Hydraulic redistribution (HR) is the process of passive water movement from deeper relatively moist soil to shallower dry soil layers using plant root systems as a conduit. We hypothesized that one of the mechanisms implicated in the facilitation observed between two shrub species (Guiera senegalensis; Piliostigma reticulatum) and annual food crops in Sahelian systems is HR. Although soil water potential (Ψ s) at the 20 n Ψ s of approximately 0.6 to 1.1 MPa, resulting in rewetting of the drier upper soil layers overnight. Shrubs exhibited significant leaf water stress with eventual stomatal closure after midday with rehydration ceasing before midnight. This suggests that these shrubs have evolved to take advantage of the redistributed water from HR during dry periods. Neighboring annual crops exhibited stomatal closure before shrubs and recuperated after their shrub counterparts. Sap flow measurements on both tap and lateral shrub roots indicated daily reversals in the direction of flow. During the peak of the dry season, both positive (toward shrub) and negative (toward soil) flows were observed in lateral shrub roots with sap flow in the lateral roots frequently negative at night and rapidly becoming positive soon after sunrise. Flow in the larger descending tap root remained positive at this time. The negative sap flow at night in the superficial lateral root and the periodic positive flow in the descending taproot are also indicative of soil water redistribution. HR thus appears to be an important mechanism for drought stress avoidance and a mechanism to maintain plant physiological functions. This unique phenomenon has major implications for the understanding how Sahelian Agroecosystems function. First it seems reasonable that in the shrub rhizosphere the year around release of water at night from HR supports microbial populations and drives biogeochemical processes even in the dry season. This represents a major paradigm shift as conventional theories would predict all biological processes would shut down in the dry season. Secondly, the superior performance of crops in the presence of shrubs (as presented below) may in part be related to HR and better water relations of crops (HR may assist crops through drought periods). Lastly enhance shrubs enhance ground water recharge. Our conclusion is that the net effect of these shrubs is very positive in terms of water availability/efficiency for crops and for conserving water, precious commodity in this semiarid environment.

Shrub Litter Decomposition and Microbial Diversity

DECOMPOSITION AND MICROBIAL ACTIVITY OF SHRUB RHIZOSPHERES

14The parkland systems of Sahelian regions have woody shrubs that randomly grow in farmers’ fields and provide litter inputs and likely promote microbial communities beneath their canopies. However, farmers currently burn shrub residues.

15We hypothesized that thermal management could be replaced by microbial degradation of these residues. To test this hypothesis several studies were done to assess the influence of shrub canopies/rhizospheres and residue quality on microbial communities and rates of decomposition. This information will be important for practical management considerations to develop non-thermal management of residues.

16Diack et al., (2000) compared decomposition under field and lab conditions for P. reticulatum. Residue placed in the soil at the beginning of the rainy season (June) resulted in 80% losses after 8 months compared to a lab incubation that only had a 50% loss for the same period. Specific surface area of the shrub residue in relation to mass loss was 15 X 10-5 to 45 X 10-5 which was similar to crop residues. The elevated decomposition rate under field conditions was attributed to the presence of termites that were not present in the lab incubation.

17A second study was done to develop fundamental information about non-thermal residue decomposition of shrub residues in relation to microbial dynamics. The experiment was a 2 X 3 X 2 factorial design with two soil treatments (0-5 cm depth beneath and outside the influence of the shrub), three residue amendments (leaf, leaf+stem and stem), and 2 litterbag mesh size treatments (<0.7 mm to exclude macrofauna and >2 mm to permit entry by macrofauna). Litterbags were destructively sampled at 15, 30, 60, 120 and 210 days after the first rain of June 30, 2005. At each sampling, litter mass loss was measured and soil was assessed for microbial carbon biomass (MBC), inorganic N and enzyme activities. When macrofauna were not restricted in decomposing litter, there were greater rates of mass loss, MBC, enzyme activities, and mineral N. Rates of decomposition and microbial response were higher with soils beneath canopy than outside the canopy influence. ‚-glucosidase had a high correlation with mass loss and MBC. Chitinase showed a strong correlation with mass loss. The results provide evidence that non-thermal management has potential for practical use of coppiced shrub residues.

18The results from two decomposition studies indicates that under field conditions, about 80% of the shrub residues, when placed in the soil at the beginning of the rainy season, are decomposed within 9 months. This suggests non-thermal management of these residues should be possible and allow farmers to prepare seed beds to enable successful crop establishment.

SHRUBS AND MICROBIAL DIVERSITY

19The Sahel is dominated by the presence of permanent shrubs, regardless that they may be coppiced many times during the year. The hydraulic lifting of water reported above for Guiera senegalensis and Piliostigma reticulatum -where water from deep soil is lifted to the surface which benefits other plants should also affect microbial communities. However, the influence of these shrubs through root exudates and hydraulic redistribution on the soil microbial communities remains unknown. This study was carried out to determine the impact of the shrubs on the activity, structure and composition of the microbial communities and how those communities are modified during the dry season as well as the wet season.

20There were two experimental site locations in an area with 300 mm of rainfall a year and an area with 700 mm a year. The experimental design was a 2 x 2 factorial design with three soil treatments (rhizospheric soil, bulk soil and non-rhizospheric soil sampled two meters away from the shrub) and two seasons (wet season and dry season); the wet season lasts only 3 months. The rhizospheric soil was separated depending on the age of the roots for the denaturing gradient gel electrophoresis (DGGE) analysis of 16S rDNA and 28S rDNA. Additionally, the soil was subject to phospholipids fatty acid (PLFA) analysis, enzyme activities, microbial biomass carbon (MBC) and nitrogen (N) mineralization. For the DGGE profiling, the bacterial community responded more to the rhizospheric effect whereas the fungal community was more sensitive to the season. There were strong rhizospheric and seasonal effects as a result of analysis from PLFA, MBC, enzyme activities and N mineralization. Furthermore, the rhizospheric communities during the dry season were quite similar to the one during the wet season. This study showed the profound modification of the microbial communities due to the presence of shrubs through root exudate input and hydraulic redistribution.

Crop Productivity

EFFECT OF SHRUB RESIDUES

21Plots were established at the ISRA Bambey Research station on a field that had no shrubs grown on it for > 50 years with various crops grown over this period. A 3 X 3 X 2 factorial with: 3 residue rates (0, 1.5 or 3.0 Mg biomass ha-1); 3 residue treatments (shrub leaves, shrub stem, or leaf/stem mix); and 2 plant residue types (G. senegalensis or P. reticulatum) was started in 2004. All residues (whole leaf material and stems chopped to ~10 cm lengths) were incorporated with light tillage similar to tillage practices of farmers and crops received a standard fertilizer application (peanut -9, 30 and 15 kg N, P, and K ha-1; millet – 14, 7 and 7 kg N, P, and K/ha plus 45 kg urea-N ha-1).

22The results from this experiment are profound. The first 2 years of the experiment there was no effect of residues on crop productivity. However, by the fourth year on millet the there was as much as a 32% increase in yields at the high rate of G. senegalensis over the control (Table 3). These results are very important for several reasons. First it reinforces the previous findings that regular inputs of organic matter are critical for crop productivity in the Sahel. But secondly and most importantly this is one of the few if only studies to show that low quality residue (i.e. limited nutrient value) can be effective in stimulating this crop response in the Sahel. The other important consideration from a management perspective is that the addition of residues must take place for several years before the benefits can be seen.

PRESENCE OR ABSENCE OF SHRUBS

23Two native perennial shrubs (P. reticulatum and G. senegalensis) form an important vegetation component of parkland systems in Senegal. However, their biophysical interactions with soil and crop are largely uninvestigated. Hence, a four-year study was started in 2004 to investigate effects of the two shrub species at separate sites on crop productivity and extractable N and P dynamics. The design at both sites was a spilt plot factorial experiment with presence or absence of shrub as the main plot and fertilizer rate as the sub plot (0, 0.5, and 1.0 of the recommended N P K fertilizer where 1.0 is millet at a rate of 68.5 kg N, 15 kg P and 15 kg K ha-1 and for peanut it is 9 kg N, 30 kg P and 15 kg K ha-1).

24Crop biomass and N and P uptakes in biomass significantly increased with increasing fertilizer rate and were always higher in presence of shrubs, even in the absence of fertilizer. Contrary to P, mineral N in soils had a very rapid change, reaching very low levels by the end of growing season. Nutrient use efficiency was low in general but was improved in presence of shrubs.

25For peanut in 2004 and pearl millet in 2005 yields in the presence of P. reticulatum, yields were significantly higher in plots with shrubs than in sole crop plots across all fertilizer rates (data not shown). In 2006 (peanut) and 2007 (millet) yields were not significantly different or slightly higher in the absence of P. reticulatum.

26The most dramatic effect on yields occurred in the presence of G. senegalensis (Table 2). Here there was a consistent and significant effect across all fertilizer treatments for 4 years – even in the absence of any fertilizer applications. In the absence of shrubs yields steadily decreased G. senegalensis site. We believe this is due to the fact that the year before the experiment was started the no shrub plots had shrubs removed – thus in the first years there was residual effects of shrubs that had been in the plots previously. However, by the fourth year the soils were so degraded in the non-shrub plots that it was very difficult to establish a stand. So by Year 4 there was on average across fertilizer treatments a 242% yield increase in shrub plots over the no shrub plots.

27G. senegalensis dominates sandy soils and is in a drier region (~400 mm yr-1) compared to the P. reticulatum site that has soils with higher clay content and greater rainfall (~800 mm yr-1).

28We conclude that there is no competition of shrubs with crops for water and for G. senegalensis there is very dramatic and positive impacts on crop yields Indeed it seems that in the northern Senegal with it dry environment and sandy soils – this shrub appears to be essential for maintaining crop productivity. These profound findings, particularly for G. senegalensis indicate that these shrubs are a key vegetative component for stopping and reversing desertification and sustaining crops.

Conclusions

29Our global conclusion is that shrubs are the dominant controllers of hydrology, C biomass on the landscape, microbiology, and crop productivity in agroecosystem of Senegal. The major findings were:

  • Shrubs residues decompose rapidly enough to allow non-thermal management.

  • Shrub residues promote crop growth but it takes 2 years of incorporation before beneficial impacts on crops were measured.

  • Both shrubs are doing hydraulic lifting of water from wet subsoils to dry surface soils

  • Shrubs are non-competitive with crops for water and increase water and nutrient efficiency.

  • During periods of excess rainfall shrubs promote groundwater recharge and therefore reduce surface runoff losses.

  • G. senegalensis had the most profound impact on yields which after fours cropping the declining yields in the absence of crops resulted in a 242% difference in yield between plots with and with out this shrub. These positive impacts occurred even in the absence of fertilizer applications

30The increase of rural populations and associated harvesting of these perennial woody components may be decreasing the density of these woody species in the semi-arid Sahel Loss of these shrubs on the landscape could potentially reduce profile recharge while accelerating runoff, increasing surface soil temperature and soil evaporation, resulting in long-term detrimental impact to agricultural productivity and greater land degradation. We believe it is imperative that these shrubs be actively managed for optimal and sustainable crop production in the Sahel.

Future directions

31These results are very promising but considerable more work is needed before full scale management systems can be recommended. First these experiments need to be repeated in other regions of the Sahel to verify these findings. Studies are needed to determine the optimal density of shrubs per hectare. The experiments we conducted were at about 1200 plants per hectare which is well above what we found in farmers’ fields where they could be near zero or more typically in the 200 to 500 plants ha-1 More closely controlled studies are needed to determine the amount of water uptake there is from hydraulic lifted water to surrounding crops (using deuterium tracing studies). Shrub rhizospheres need to be characterized for beneficial microorganisms that may assist crops for drought tolerance, stimulate plant growth by releasing hormones, fixing atmospheric N2,and suppressing diseases (even when nutrient limitations are removed and there is high rainfall there is an unexplained yield response with shrubs which could be due to beneficial microbes ). Another question is whether P. reticulatum, a non-nodulating legume could be converted to a nodulating legume to increase N fixation and provide more N for adjacent row crops.

32We have developed simple ways for propagating and establishing shrubs in farmers’ field. Hence, pilot studies should be initiated where farmers manage an optimized, non-thermal system in comparison to the traditional coppice and burn system. There should be close monitoring of the biophysical and socio-economic performance of these systems to develop practical shrub-crop systems that can be widely adopted.

Table 1

Table 1

Chemical properties of soils beneath and outside the influence of native shrubs (n=4) (adapted from Dossa et al., 2008)

Table 2

Table 2

Crop yield as affected by presence or absence of shrub (G. senegalensis), and fertilizer rate at Keur Mata.
Values within columns followed by the same superscript letter are not significantly different at P<0.05.

Table 3

Table 3

Crop yield as affected by shrub residue rates with and without fertilizer from 2004 to 2007(Bambey)

Figure 1

Figure 1

Variation of mean volumetric moisture content (a) vertically, below G. senegalensis root zone and an adjacent bare soil matrix in the 2003 season;
(b) vertically, below G. senegalensis root zone and an adjacent bare soil matrix in the 2004 season;
(c) vertically, below P. reticulatum root zone and an adjacent bare soil matrix in the 2003 season;
(d) vertically, below P. reticulatum root zone and an adjacent bare soil matrix in the 2004 season. Inset graphs are variations of soil temperature 10 cm below shrubs and bare soil at both sites and are labeled as
(e) G. senegalensis for the 2003 season;
(f) G. senegalensis for the 2004 season;
(g) P. reticulatum for the 2003 season;
(h) P. reticulatum for the 2004 season (adapted from Kizito et al., 2006)

Bibliografía

Badiane, A.N., Khouma, M., Sene, M., 2000. Région de Diourbel: Gestion des sols. Drylands Research Working Paper 15, Drylands Research, Somerset, England. 25 pp.

Bationo, A., Buerkert, A., 2001. Soil organic carbon management for sustainable land use in Sudano-Sahelian West Africa. Nutr. Cycl. Agroecosyst., 61: 131-142.

Buresh, R.J., Tian, G., 1998. Soil improvement by trees in sub-Saharan Africa. Agrofor. Syst., 38: 51-76.

Diedhiou, S., A.N. Badiane, I. Diedhiou, M. Khoum, A.N.S Samba, M. Sène and R.P. Dick. 2009. Succession of Soil Microbial Communities during Decomposition of Native Shrub Litter of Semi-Arid Senegal. Pedobiologia (in press).

Diack, M., M. Sene, A. N. Badiane, M. Diatta, and R. P. Dick. 2000. Decomposition of a native shrub (Piliostigma reticulatum) litter in soils of Semiarid Senegal. J. of Arid Soil Research and Rehabilitation 14(3):205-218.

Diangar, S., A. Fofana, M. Diagne, C.F. Yamoah, and R. P. Dick. 2004. Pearl millet-based intercropping systems in the semiarid areas of Senegal. African Crop

Science J. 12:133-139.

Dossa, E.L., J. Baham, M. Khouma, M. Sene, F. Kizito, R.P. Dick. 2008. Phosphorus sorption and desorption characteristics of soils incubated with native shrub residues in semiarid Senegal. Soil Science (in press).

Dossa, E.L, M. Khouma, I. Diedhiou, M. Sene, F. Kizito, A.N.. Badiane, S.A.N. Samba, R.P. Dick. 2008. Carbon, nitrogen and phosphorus mineralization potential of semiarid Sahelian soils amended with native shrub residues. Geoderma 148:251-260.

Iyamuremye, F., V. Gewin, R.P. Dick, M. Diack, M. Sene, A.N. Badiane, and M. Diatta. 2000. Carbon, nitrogen, and phosphorus mineralization of agroforestry plant residues in soils of Senegal. J. of Arid Soil Research and Rehabilitation 14:359-371.

Kizito, F., M. Senè, M. I. Dragila, A. Lufafa, I. Diedhiou, E. Dossa, R. Cuenca, J. Selker, R. P. Dick. 2006. Soil water balance of annual crop-native shrub systems in Senegal’s Peanut Basin. Ag. Water Management 90:137-148.

Kizito, F., M. Dragila, M. Sène, A. Lufafa, I. Diedhiou, E Dossa, R.P Dick, M Khouma, A. Badiane, and S. Ndiaye. 2006. Seasonal soil water variation and root dynamics among two semi-arid shrubs coexisting with Pearl millet in Senegal, West Africa. J. of Arid Environments 67:436.

Kizito, M. I. Dragila, R. Brooks, M. Senè, M. Diop, R. Meinzer, A. Lufafa, I. Diedhiou, R. P. Dick. 2009. Hydraulic redistribution by two semi-arid shrubs: Implications on agro-ecosystems. J. Arid Environments (in press).

Lufafa, A., I. Diédhiou, S. Ndiaye, M. Séné, M. Khouma, F. Kizito, R.P. Dick, and J.S. Noller. 2008. Carbon stocks and patterns in native shrub communities of Sénégal’s Peanut Basin. Geoderma 146: 75-82.

Lufafa, A., Wright, D., Bolte, J., Diédhiou, I., Khouma, M., Kizito, F., Dick, R.P., Noller, J.S., 2005. Regional carbon stocks and dynamics in native woody shrub communities of Senegal’s Peanut Basin. Agriculture, Ecosystems and Environment, 128:1-11.

Lufafa, A., I. Diedhiou, S.A.N. Samba, M. Sène, F. Kizito, Ph.D.; R. P Dick, and J. Stratton Noller. 2009. Allometric relationships and peak-season community biomass stocks of native shrubs in Senegal’s Peanut basin Journal of Arid Environments (in press).

Rhoades, C. C., 1997. Single-tree influences on soil properties in agroforestry: lessons from natural and savanna ecosystems. Agrofor. Syst., 35: 71-94.

Sanchez, P.A., Shepherd, K.D., Soule, M.J., Place, F.M., Buresh, R.J., Izac, A-M.N., Mokwunye, A.U., Kwesiga, F.R., Ndiritu, D.G., Woomer, P.L., 1997. Soil fertility replenishment in Africa: An investment in natural resource capitol. In: Buresh, R.J., Sanchez, P.A., Calhoun, F. (Eds), Replenishing Soil Fertility in Africa. Soil Science Society of America Special Publication 51, pp. 1-46.

Schlesinger WH, Raikes JA, Hartley AE, Cross AF (1996). On the spatial pattern of soil nutrients in desert ecosystems. Ecology 77:364-374.

Sinaj, S., Buerkert, A., El-Hajj, G., Bationo, A., Traoré, H., Frossard, E., 2001.

Effect of fertility management strategies on phosphorus bioavailability in four West African soils. Plant Soil, 233: 71-83.

Tschakert, P., Khouma M, Sene, M., 2004. Biophysical potential for soil carbon sequestration in agricultural systems of the Old Peanut Basin of Senegal. J. Arid Environ., 59: 511-533.

Woomer, P. L., Touré A. and Sall, M., 2004. Carbon stocks in Senegal’s Sahel Transition. Journal of Arid Environments 59, 499-510.

Índice de ilustraciones

Título Table 1
Leyenda Chemical properties of soils beneath and outside the influence of native shrubs (n=4) (adapted from Dossa et al., 2008)
URL http://books.openedition.org/irdeditions/docannexe/image/2126/img-1.jpg
Archivo image/jpeg, 68k
Título Table 2
Leyenda Crop yield as affected by presence or absence of shrub (G. senegalensis), and fertilizer rate at Keur Mata.Values within columns followed by the same superscript letter are not significantly different at P<0.05.
URL http://books.openedition.org/irdeditions/docannexe/image/2126/img-2.jpg
Archivo image/jpeg, 148k
Título Table 3
Leyenda Crop yield as affected by shrub residue rates with and without fertilizer from 2004 to 2007(Bambey)
URL http://books.openedition.org/irdeditions/docannexe/image/2126/img-3.jpg
Archivo image/jpeg, 148k
Título Figure 1
Leyenda Variation of mean volumetric moisture content (a) vertically, below G. senegalensis root zone and an adjacent bare soil matrix in the 2003 season;(b) vertically, below G. senegalensis root zone and an adjacent bare soil matrix in the 2004 season;(c) vertically, below P. reticulatum root zone and an adjacent bare soil matrix in the 2003 season;(d) vertically, below P. reticulatum root zone and an adjacent bare soil matrix in the 2004 season. Inset graphs are variations of soil temperature 10 cm below shrubs and bare soil at both sites and are labeled as(e) G. senegalensis for the 2003 season;(f) G. senegalensis for the 2004 season;(g) P. reticulatum for the 2003 season;(h) P. reticulatum for the 2004 season (adapted from Kizito et al., 2006)
URL http://books.openedition.org/irdeditions/docannexe/image/2126/img-4.jpg
Archivo image/jpeg, 313k

Autores

School of Environment and Natural Resource, Ohio State University, Columbus, OH, USA

Sciences du Sol, Institut Sénégalais de Recherches Agricoles (ISRA)/ CERAAS, BP 3320 Thiès: Escale, Thiès, Sénégal

University of Gaston Berger, St. Louis, Sénégal

UNOPS Dakar, Sénégal

USAID/ Sénégal, Dakar, Sénégal

University of Thiès, Thiès, Sénégal

University of Thiès, Thiès, Sénégal

World Bank, Washington, D.C. USA

International Fertilizer Development Center, Accra, Ghana

California Water Board, Davis California, USA

Oregon State University, Department of Crop and Soil Sciences, Corvallis, OR USA

Oregon State University, Department of Crop and Soil Sciences, Corvallis, OR USA

Oregon State University, Department of Crop and Soil Sciences, Corvallis, OR USA

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