Influence of Re-Greening on the Infiltrability of Soils in South-Central Niger

In the Sahel, the climate and the agricultural practices are the main factors used to observe regreening process. But the extension of this re-greening is not uniform in the fields, so we can assume that there are other factors contributing to the greening of some areas and not others. The infiltrability of soils is considered as an important factor for these changes in the last decades. The influence of re-greening on the infiltrability of soils in the southern strip of Niger was studied through a comparison of zones according to the type of sandy soils (Jigawa/Rerey), silty-clay (Guiéza), silty-sandy (Hako) and clay-silty-sandy (Laka). The evaluated parameters are the texture, the infiltrability and the ecological characteristics (specific index of regeneration “SIR” and the Importance Value Index “IVI”). The main conclusions were: the soils of Niger South-Central are characterized by a sandy texture for more than 80%. The soils which are very sandy (“Jigawa”, “Rerey” and “Guieza”) are more permeable and have more trees contrary to the soil Hako and Laka. Their encrusting has the tendency to reduce the vegetation cover on the soil. The infiltration measurements indicated that the rate of initial and stationary infiltration diminishes while going down towards the South. In fact, it has been recorded a rate of 123 mm/min of initial infiltration and 87 mm/min stationary in Dan Saga on the sandy soils against 76 mm/min and 65 mm/min in Daré respectively for the initial and stationary rate. Therefore, the infiltration seems to influence the density, the SIR and the IVI of the trees from the North towards the South. Some additional work is necessary in order to determine the contribution of Farmer Managed Natural Regeneration (FMNR) in the improvement of the quality of soils and the movement of water in the superficial zones of the soil.


Introduction
The arid and semi-arid areas are characterized by a precariousness of their environmental conditions, thus making the balance of the ecosystems fragile [1].But in the last three decades, the zone of the Sahel is presented as a rare case of success of Man's action in matters of adaptation to environmental changes [2].In fact, the satellite images of 2003 show some important re-greening areas in the Sahel since the 1980s [3] [4].Eklundh and Olsson [5] underlined a great increase in the normalized difference vegetation index (NDVI) on large surfaces of the Sahel and Sudan during the period from 1982 to 1999.This positive tendency was confirmed by Anyamba and Tucker [6].
The re-greening identified as an increase in the normalized difference of the index of vegetation (NDVI) [5]- [7] is explained by the changes in the use of lands, some practices of the management of natural resources and some agricultural practices, notably some techniques of protecting trees by Farmer Managed Natural Regeneration [8] [9].
In Niger, various studies conducted in the Southern strip of Niger (regions of Maradi and Zinder) showed that the re-greening occurred thanks to the efforts of reclaiming lands carried out by the different actors, notably the farmers thanks to the FMNR [8] [10].Thus, the practice of the FMNR enabled to increase the number of trees on about five million hectares between 1983 and 2005 in that Southern strip [10].
Ali and Lebel [11] and Nicholson [12] showed that the dynamics of vegetation is closely linked to that of rainfalls.Thus, the rainfall stimulated the re-greening despite the serious droughts of the years 1970 and 1980.However, the extension of re-greening not being uniform [13], some researchers such as Nemani et al. [14]; Reij and Smaling [15] assert that the strong tendency of the ongoing re-greening cannot be explained by only one factor as the climate.In fact, the density of trees is not uniform in the fields of farmers [16].So, can the strong tendency of the ongoing re-greening be explained by the physical characteristics of the soils?
This article studies the texture and the infiltration of soils in order to understand the link between the physical characteristics of soil and re-greening for a durable use of these agricultural ecosystems.

Study Sites
The study took place in four (4) village areas of two regions located in the South-Central of Niger: Dan Saga and El Guiéza in the region of Maradi and Daré and AraSofoua in the region of Zinder (Figure 1).The area of study is relatively populated (about 100 inhabitants Km −2 ).The climate is semi-arid, characterized by a single modal model of precipitation, with a rainy season (from June to September) and a dry season (October to May).The pluvial agriculture constitutes almost 90% of the population's activities.The crops such as millet and sorghum are mainly grown for subsistence on the sandy and poor soils, entirely depending on rainfall.

Data Collection
With the purpose of evaluating the contribution of texture and that of infiltration of soils in the process of regreening in these village areas, a participative approach including the farmers through the holding of meetings in the villages was adopted.In fact, a classification of soils was established by the farmers.One participant in the meeting was invited to draw on a Kraft paper a map for the repartition of the types of soils, thus giving a view on their spatial repartition.This framework presents an interest for the perception of the quality of soils [17], through which color and texture were used as main characteristics.Then, a visit to the fields was organized with four farmers chosen by the participants in the meeting in order to confirm the classification.

Dendrometric Parameters
The data on flora and vegetation were collected between June and July 2012 on some small squares of inventories and the plotting of the vegetation of 2000 m 2 (50 m * 40 m) on the various types of soil.In each plotting, an exhaustive census of ligneous was taken.Some dendrometric measurements were taken in order to evaluate the height of trees, the circumference of the basis of the trunk (CBT) at 1.30 meters.For the multicaule individuals, the clump is considered as an individual and the measurements concerned only the dominant stalk.All the individuals whose diameter is inferior to 5 centimeters are considered as juvenile zone and systematically considered as part of the regeneration [18].The height of individuals above 2 meters was estimated by using a graduated pole and the other individuals were measured by using a tape measure.The measurements of the circumference were taken by using a soft tape measure.Because of the irregularity of the form of certain species due to anthropic pressure, the soft tape measure was used [19].In total, the sampling was carried out on 60 small squares; that is 4 small squares per type of soil and per village area.

Infiltration Test
The measurements of infiltration were carried out on each square during the dry season (February) in order to neglect the impact of the tenure of the soil in initial humidity [20].The infiltration was measured by the technic of infiltrometer with dual rings following the method described by Bouwer [21].With a height of 20 centimeters, the rings differ by their diameter, which is 11 centimeters for the internal ring against 34 centimeters for the external one.Four measurements were taken in each type of soil and by village.The samples of soil were constituted according to the types of soil at 0 -20 cm of depth.

Analysis of the Soils
The composite samples of soil coming from each site were well dried at ambient temperature, put in plastic bag, labeled and analyzed.The texture of soils was determined by the method of granulometry with five fractions according to the standard procedures of the pipette of Robinson.

Data Analysis
The data obtained from the plotting of infiltration, of vegetation and the analysis of soils were treated and analyzed using Microsoft Office (Excel, 2007) spreadsheet and the software XLStat 7.2.
The observed density or real density was obtained by the following formula: where: N is the total number of individuals in the sample and S is the sampled surface.The specific importance of regeneration was obtained from the ratio in percentage between the number of young plants of a species and the total number of counted young plants [22]: All the subjects whose diameter is inferior or equal to 5 cm were considered to belong to the regeneration [18].The ecological importance of species was appreciated from the Importance Value Index (IVI) that enables to appreciate better their importance in the plant community [23].It is a synthetic and quantified expression of the importance of a species in a plant population.This index, for a species, is defined as follows: ( ) ( ) ( ) where: Domr is the relative dominance, Fr is the relative frequency and Dr represents the relative density.The values of Domr, Fr and Dr vary between 0% and 100% whereas those of the IVI of the species vary from 0 to 300%.The species with IVI ≥ 20% are those ecologically important [24].

Classification of the Types of Soils Encountered from the Zone of Study
The meetings at the level of each selected village point out two main groups of soil: the crusted soils or Hako and Laka; and the sandy soils locally called Jigawa and Guéza (Table 1).In fact, the division is mainly based on the texture and has two exclusive aspects (rough and fine).There is a second repartition based on the difference of color.At this level, there are four exclusive aspects (red, white, brown, black).The sandy soils have a low rate of fine elements such as silt and clay whereas the crusted soils have a high rate of more fine elements (<0.25 mm).

Physical Characteristics of the Soils of Main Study Sites
According to the granulometric analyses (Figure 2), the soils Jigawa or Rerey and Guéza present in general, a low proportion in fine particles, compared to 95% of sand.There is no significant difference between the two types of soils.However, the rate of fine elements at the level of Hako in Dan Saga (Northern Maradi) is significantly inferior to that of the three other areas (El Guiéza, Daré and AraSofoua) (Zinder) with respectively 2% against 10%.

Variation of the Rates of Infiltration According to the Types of Soils
At Dan Saga (Northern Maradi), the rate of initial infiltration is 123; 126 and 72 mm/min respectively on the Jigawa, Guéza and Hakosoils against a rate of infiltration of the stationary state of 87, 61 and 54 mm/min respectively (Figure 3(a)).On the contrary, at El Guiéza (Southern Maradi), the rate of initial infiltration is 81; 73 and 50 mm/min respectively on the Jigawa, Guéza and Hako soils against 64, 59 and 28 mm/min, as a rate of infiltration of the stationary state (Figure 3(a)).
The village areas of the region of Zinder are characterized by dune grounds.At Daré, the initial rate of infiltration is 76, 65 and 75 mm/min respectively on the Rerey, Laka and Hako soils against 65, 45 and 48 mm/min, as a rate of infiltration of the stationary state (Figure 3(b)).On the contrary, at AraSofoua, the rate of initial infiltration varies 92, 81 and 2 mm/min respectively on the Rerey, Laka and Hako soils against 74, 21 and 2 mm/ min, respectively as a rate of infiltration of the stationary state.

Ecological Importance of Ligneous Species According to the Types of Soils
In all the sites of Rerey and Jigawa soils, the density of trees is significantly more important in relation with the Guiéza, Hako and Laka soils (Table 2).At Dan Saga, the density of trees is significantly different between the Guiéza, Hako and Laka soils whereas this difference is not significant in the areas of El Guiéza, AraSofoua and Daré.Hako/Laka 82c 73b 61b 36b Rerey/Jigawa 206a 151a 180a 84a Guiéza 177b 99b 98b 55b The averages followed by the same letter are not significantly different at the threshold of 5%.
On the sites of the region of Maradi, it was taken a census of 10 ligneous species on the Hako soils against 16 on the Guiéza and Jigawa soils.60% of the species on the Hako soils of Dan Saga have an IVI ≥ 20.On the contrary, on the Guiéza and Jigawa soils, only 25% of the species have an IVI ≥ 20 (Table 3(a)).Globally, the prevailing species, that represent the highest values of ecological importance are Faidherbia albida (89.24%) at Dan Saga on the Hako, Piliostigma reticulatum (82.65%),El Guiéza on the Hako, Combretum glutinosum (113.32%) in the North (Dan Saga) on the soil Guiéza, Piliostigma reticulatum (88.49%) in the South (El Guiéza) on the Guiéza, Annona senegalensis (33.21%), and Azadirachta indica (164.09%) in the North on the Jigawa, Hyphaene thebaica (124.02%) in the South on Jigawa.
The importance of regeneration according to the various species was apprehended by the calculation of the specific index of regeneration (SIR).At Dan Saga, the Pilostigma reticulatum and Annona senegalensis with 20.63% represent the best SIR on the Hako (Table 3(b)), Combretum glutinosum (30.50%) on the Guiéza and Azadirachta indica soils (31.25%) on the Jigawa.At El Guiéza, it is Hyphaene thebaica that has the best potential of regeneration on the Hako, Guiéza and Jigawa with respectively a SIR of 55.22%, 50.00% and 81.16%.
At Daré and AraSofoua, less than 10 ligneous species on the Hako, Rerey and Laka soils were counted.Globally, the prevailing species that represent the highest values of average ecological importance is Faidherbia albida on all the three types of soil.
The best potential of regeneration was recorded at the level of ligneous species such as Annona senegalensis, Faidherbia albida, Azadirachta indica and Piliostigma reticulatum.

Influence of the Texture of Soils
In general, the soils of the area of study are characterized by a sandy texture for more than 80%.Clay and fine silts, which ensure the role of cohesion of the different constituents of the soils, the basis of a good structure, are in low proportion.The "Jigawa" or "Rerey" soils are very sandy and very poor in fine elements and more particularly in clay.Thus, several authors [25] [26] have shown that on the sandy soils of West Africa, the putting under cultivation of lands favor the elimination of the finest elements through eluviation and water erosion and/ or wind erosion and therefore, the increase in rough sand on the surface.For the Guéza soil on the contrary, Ambouta and Amadou [25] have pointed out its extreme sensitivity to the degradation of its physicochemical properties, thus transforming it into Jigawa soil.

Influence of Soil Infiltration
Infiltration is the process through which surface water penetrates in the soil.It is related to the surface outflow and that of ground water, determines the fraction of irrigation water or rain water that penetrates in the soil and therefore, affects the runoff responsible of soil erosion [27].
The results of the analysis of the infiltrability enables to notice that the soils which are very sandy ("Jigawa" or "Rerey" and "Guiéza") are more permeable than the Hako and laka soils.This established fact is explained by the rough and porous texture that favors rapid infiltration.The same tendency was observed by Makungo and Odiyo [28] who determined the rates of infiltration of the silty-sandy soils, sandy-clayish soils and silty-clayish soils which have some rates of initial infiltration that vary from 133.5 to 325.[29] found very significant positive correlation between the tenure in sand and the final rate of infiltration.Some similar results were also obtained by several authors [30] [31], who noticed that the speed of infiltration of the rough soil is higher than that of the soils with fine and average textures.They obtained some infiltration rates which are superior to 100 mm/hour and inferior to 10 mm/hour for the average and fine soils.
The Hako and laka soils are less permeable because of their encrusting.Hillel [32] and Talaat [33] reported that the presence of layers of clay hinders the infiltration of water because of the weakness of the macro-pores.

Relation between Infiltration and Vegetation
The rate of infiltration is more important on the very sandy soils ("Jigawa" or "Rerey" and "Guiéza") and they also have an important density of trees.This tendency was observed by Rao et al., [34] in the Kenyan Savanna.Lili et al. [20] showed that the presence of vegetation is positively correlated with infiltration.The positive effect of trees on infiltration is due to the formation of root channels, macro pores and micro pores.Also, the litter increases the activity of the soil's faun [35] [36].The vegetation delays the transfer of surface water, breaks the kinetic energy of rain drops, which leads to the reduction of the effect of slaking.Also, the extension of roots, the expansion of the crown can contribute to the accumulation of organic matter and nutritive elements in the arable layer of the soil leading to a physical, chemical and biological improvement in the rhizosphere [37].A diminution of the tenure in the organic matter in the soil diminishes the macro-porosity, which reduces the infiltration [38].The activity of the fauna was found to be an important agent for the control of the quality of soils.The macro fauna of soil (mostly termites and ants) improve the physical properties of soil and considerably contributes to increase the infiltration of the dry season ecosystems [39]- [41].
Faidherbia albida is the species that has the best SIR and IVI in the sites of the region of Zinder.Hansson [42] has noticed the capacity of infiltration of soil under Faidherbia albida was 69 mm/hour in the fields in the West of Burkina Faso.
But, the success of restoration is hindered by the excessive tree felling, generally carried out by the transhumant stock breeders, debarking of trees done by certain categories of people and the overexploitation make certain species vulnerable [16] [43].The system of leading the herd into the farming fields in Niger contributes to the diminution of the vegetable cover and the biomass and increases the runoff because of crust colonization [44].In fact, Hiernaux et al. [45] showed that stepping and compacting are important reducing agents of soil hydrology.This combination of factors leads to the reduction of the vegetable cover, which has as consequence the development of the crusts of the soil surface [44] thus, favoring the runoff to the detriment of infiltration [44].

Conclusion
The measurement of infiltration of water in the soil is an important indication of efficiency of irrigation and drainage, the optimization of the availability of water for plants, the improvement of the yields of crops and the minimization of erosion.When the soils are crusted (Laka and Hako), the infiltration of water is slow or quasiinexistent.Also, the vegetable cover is low.As the precipitations are the main sources of water supply, ensuring the growth and production, rain water is less beneficial for the development of vegetation.But if the structure of the soil surface is improved by the work on the soil, there will be a bigger capacity of infiltration of water and the efficiency of use is improved in the long term.Other additional work is necessary in order to determine the contribution of the Farmer Managed Natural Regeneration (FMNR) in the improvement of the quality of soils and the movement of water in the superficial zones of the soil.

Figure 1 .
Figure 1.Location of the study sites.

Figure 2 .Figure 3 .
Figure 2. Average tenures in rough sands, fine sands, rough silt, fine silts and clay according to the types of soils.A: Clay; LF: Fine Silt; LG: Rough Silt; SF: Fine Sand; SG: Rough Sand.

Table 1 .
Classification of the soils at the study sites.

Table 3 .
(a) List of species according to the specific index of regeneration (SIR) and the Importance Value Index (IVI) following the types of soils in the sites of Maradi; (b) The list of species according to the specific index of regeneration (SIR) and the Importance Value Index (IVI) following the types of soils in the sites of Zinder.
5; 71.6 to 124.8 and 120.4 to 160.8 mm/hour respectively whereas the rates of infiltration of the stationary state vary from 50 to 110; 19 to 22 and 30 to 40 mm/hour.Agrawal et al.