Slash and burn effect on soil quality of an Alfisol: Soil physical properties

Kayode S. Are *, Gabriel A. Oluwatosin, Olateju D. Adeyolanu, Adebayo O. Oke

Institute of Agricultural Research and Training, Obafemi Awolowo University, Moor Plantation, Ibadan, Nigeria

Soil & Tillage Research 103 (2009) 4–10

A R T I C L E I N F O

Article history:

Received 23 November 2007

Received in revised form 27 August 2008

Accepted 29 August 2008

Keywords:

Slash and burn

Trash

Physical properties

Soil quality

Alfisol

A B S T R A C T

Slash and burn method of land clearing, an integral part of the traditional farming system, is widely

practiced by over 90% of farmers in Southwestern Nigeria. It is often used as means of land clearing to pave

way to minimum or zero tillage. Two experimental sites located on Alfisol were selected to evaluate changes

in soil physical properties following burning. Soils from 0 to 0.05 and 0.05 to 0.10 m depths and worm casts

on the surface were collected before and after burning. Soil strength, saturated hydraulic conductivity and

infiltration tests of pre- and post-burn soils were taken in-situ. In response to burning, pore volumes

reduced and concomitantly reduced infiltration rates, sorptivity and hydraulic conductivity. Infiltration

rates, sorptivity levels and saturated hydraulic conductivity decreased significantly after burning by 64.3,

58.9 and 64.3%, respectively, in Site 1 and 47.9, 58.9 and 47.8%, respectively, in Site 2. Mean weight-diameter

(MWD) decreased significantly following burning by 30.8 and 43.5% in Site 1 at 0–0.05 and 0.05–0.10 m

depths, respectively, and 46.2 and 44.7% in Site 2 at 0–0.05 and 0.05–0.10 m depths, respectively.

Appreciable but not significant reduction was recorded in water stable aggregates (WSA) for soils in both

sites. However, WSA of worm casts in Site 1 increased significantly from 0.765 to 0.873 kg kg�1. Despite

significant decrease of 19.7% in organic matter (OM), the stability of worm cast improved after burning.

Slash and burn did not affect soil class but there were appreciable changes in particle size distribution of

post-burn soils. Ash deposits in the pore spaces likely accounted for 0.8 and 3% reduction in available water

capacity (AWC) at 0–0.05 and 0.05–0.10 m depths, respectively, in Site 1; and 15.1 and 6.6% reduction at 0–

0.05 and 0.05–0.10 m depths, respectively, in Site 2 following burning. Soil strength increased after burning

but not significant at any of the soil layers while slight increase of 0.8 and 4% in bulk density was recorded at

the respective depths. Site 2 followed similar trends in soil strength and bulk density except that the

penetrometer cone could not be forced to 0.10 m depth. This study showed that slash and burn may have

immediate and direct impact on soil physical properties of an Alfisol; while the soil and worm casts on the

soil surface responded differently to burning.

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1. Introduction

Over 70% of Nigerian population, whose livelihood depends onfarming, lives in rural area. The method of land clearing is thetraditional slash and burn. This method, according to Babalola(2000), has been an integral part of shifting cultivation and widelypracticed by over 90% of farmers in south western Nigeria. As thetraditional shifting cultivation pave way to continuous cultivation,necessitated by demographic pressure, coupled with burning, thedetrimental effects of soil degradation became prominent.

Burning had been identified as one of the soil degrading practicesthat result in soil structural degradation (Giovannini et al., 1988;Valzano et al., 1997; Hubbert et al., 2006). Even then, it was reportedby Obale-Ebanga et al. (2003) in Cameroon that burning increases

* Corresponding author. Tel.: +234 803 5721035.

E-mail address: kayodeare@gmail.com (K.S. Are).

0167-1987/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2008.08.011

the percentage macro-aggregate stability of surface soil of Vertisol.However, the dependence on slash and burn as means of landclearing by our farmers is explicable. Its beneficial effects in clearingbush debris and reduction of weed infestation that would have beencompeting with crops for sunlight, water and soil nutrients havebeen reported by Babalola, 2000. The ash deposits after burning,helps to fertilize the soil. This is done by immediate release of theoccluded mineral nutrients—Mg, Ca, available P, for crop use(Scheuner et al., 2004; Niemeyer et al., 2005). Ojima et al. (1994) andBrye (2006) also submitted that increased soil temperatures afterburning, stimulate biological activity; increases organic mattermineralization to enhance nutrient availability.

The benefits of burning in improving soil by immediate releaseof occluded mineral nutrients for crop use seem to be short-liveddue to its degenerative effects on soil physical properties (Edenet al., 1991; Ketterings et al., 2000; Tucker, 2003; Wuest et al.,2005). Suffice it to say that burning, among other soil degradingfactors results in environmental damage in intensive arable lands.

K.S. Are et al. / Soil & Tillage Research 103 (2009) 4–10 5

Even worse, most lands that are left unused in a cropping year arebeing set on fire by our farmers. This is common with the livestockfarmers, for their animals to browse on young plants that growafter burning. Before the plants come up to cover the groundsurface, the soil is exposed to climatic element of rainfall.Subsequently, soil aggregates are dispersed; pores are cloggedwith particles and further result in much higher rates of surfacerunoff (Neary et al., 1999). The level of soil alteration may even beenormous if the quantity of trash is large and the residence timeof burning is long, or a thin, dry litter is completely incinerated(Wells et al., 1979). More severe burns may alter such fundamentalcharacteristics as texture, mineralogy and cation-exchange-capacity (Ketterings et al., 2000).

In the previous research, there have been conflicting results onthe response of soil physical properties to burning. Even then,most reported research focused on the effects of burning on soilphysical properties with very little or no attention paid to theresponse of worm casts on the surface to burning. Besides, fewstudies have compared the change accompanying soil physicalproperties at 0–0.05 and 0.05–0.10 m depths following slash andburn land clearing. Therefore, this study was set out to evaluateand compare the direct and immediate effects of slash and burnon soil physical properties at 0–0.05 and 0.05–0.10 m depthswith worm casts collected on the surface in an Alfisol (TypicKanhaplustalf) under regrowth forest and natural fallow in southwestern Nigeria.

2. Materials and methods

2.1. Sites description

Two sites located within the tropical rainforest of south westernNigeria were selected for the study. Site 1 was located within theexperimental station of the Institute of Agricultural Research andTraining (I.A.R.&T.), Ibadan (78230N; 38510E) while Site 2 waslocated on farmer’s field in Akure (78170N; 58140E), Nigeria. Ibadanand Akure have a tropical humid climate with mean annual rainfallof 1289.2 and 1454.0 mm, respectively, which were recorded forover a period of 16 years (Alabi and Ibiyemi, 2000). There are twogrowing seasons: early season (March/April to August) and lateseason (Mid-August to October/November). Annual temperatureranges from a high of 31.2 to 21.3 8C. The percentage sunshineranges between 16% in August and 59% in February and Decemberwith an average of 44%.

The soil of the study sites belong to Typic Kanhaplustalf (SSS,2003) and classified locally as Iwo series (Smyth and Montgomery,1962). Ibadan site (Site 1) covered an area of 432 m2, which hadbeen under fallow (Chromoleana odoratum L.) for 4 years prior tosampling. Akure site (Site 2) covered an area of 550 m2, which hadbeen under fallow (bush regrowth) for about 35 years, opened upfor the study.

2.2. Treatments

Land preparation began in the two sites in February 2007 withthe slash of regrowth bush using cutlass and axe (the traditionalland preparation technique). The trash was left on the two sites andallowed to dry for three weeks after which the biomass weight wasestimated with the aid quadrant (1 m � 1 m size). In each site, theslashed area was divided into five replicated plots of 79.3 and102 m2 in Sites 1 and 2, respectively. All the plots were demarcatedfrom one another by 1 m wide fire line around the perimeter ofeach plot.

Pre-burn soil and worm cast samples and field measurementswere taken before burning the trash at the end of 3 weeks after

slashing. Post-burn samples were taken and field measurementsmade within 2-week period after burning.

2.3. Field and laboratory measurements

The severity of burning in each site was measured qualitativelyfrom the degree of litter consumption (Wells et al., 1979). Toensure representative sampling, bulk soil and worm cast sampleswere composite of five random samples taken within a replicatedplot. An undisturbed soil cores were taken with a cylindrical coresampler (0.05 m – height and inner diameter) from 0 to 0.05 and0.05 to 0.10 m depths. The soil cores were saturated with waterovernight and thereafter weighed at saturation. Water retentioncharacteristics between saturation and 10 kPa matric potential(�100 cm water) were determined by tension plate apparatus;similar to that of Topp and Zebchuk (1979). Pressure was alsoimposed between 10 and 1500 kPa for the determination ofavailable water capacity (AWC). However, field experiment wasalso conducted in both sites to confirm laboratory procedure onavailable water at field capacity (FC) and permanent wilting point(PWP). Field measurements of FC and PWP were made by pondinga dyke of 9 m2 with water up to 0.20 m. Free drainage of water wasallowed while evaporation was prevented using polythene. Soilsamples were taken 2 days after saturation; when the draining ratebecame negligible for FC determination (SSSA, 1997). PWP wasdetermined after the moisture content is negligible and the soil canlonger transfer water towards the roots of the plant seedlings on it(Briggs and Shantz, 1912).

Bulk density was estimated by dividing the oven-dry mass ofthe soil by the volume of the soil as described by Grossman andReinsch (2002). This is computed by dividing the oven-dry mass ofthe soil by the volume of soil core. Gravimetric moisture contents(Lowery et al., 1996) at FC and PWP were calculated on dry massbasis. AWC on volume basis was calculated by multiplying thegravimetric moisture content between FC and PWP by thecorresponding bulk density (Eq. (1)):

AWC ¼ ð? FC � ? PWPÞrb (1)

where 1 is the gravimetric moisture content (%) and rb is the bulkdensity at the required depth in mg m�3.

Pore size distribution and total porosity (TP) were calculatedusing the water retention data and capillary rise equation asdescribed by Flint and Flint (2002). Macropores (pores >30 mm),taken as drain pores were estimated at 10 kPa matric potential.Total porosity was estimated as water content at saturation usingthe following relationship:

TP ¼ MSW �Mds

Vb(2)

where Msw is the mass of soil at saturation, Mds is the mass of drysoil and Vb is the volume of the soil.

Particle size distribution was carried out on the soils and wormcasts using hydrometer method as described by Gee and Or (2002).Undisturbed soil samples and worm casts were taken for thedetermination of organic matter (OM), aggregate stability andaggregate size distribution. Water stable aggregates (WSA) weredetermined on soil and worm casts by a modified Kemper andRosenau wet sieving method described by Nimmo and Perkins(2002). 50 g (oven-dry mass equivalent) of air dry soil at 0–0.05and 0.05–0.10 m depths and worm casts collected before and afterburning was placed on a set of sieves (5.00, 2.00, 1.00, 0.25 and0.045 mm) attached to a dipping machine. The set of sieves wascycled through a column of water for 10 min (30 cycles per min,4.0 cm stroke length). The percentage of WSA as fraction of the

K.S. Are et al. / Soil & Tillage Research 103 (2009) 4–106

total sample was calculated. Mean weight-diameter (MWD), astatistical index of aggregation, was calculated from aggregate sizedistribution data, after correction had been made for sand fractionsby dispersion with sodium hexametaphosphate (HMP).

OM was determined by loss-on-ignition (LOI) as described byCambardella et al. (2001). Air dry soil and worm cast samples wereoven dried at 105 8C to a constant weight. 5 g of oven-dry samplewas used for LOI by mass difference after 4 h in a muffle furnace at500 8C. Soil strengths at 0.05- and 0.10-m depths as described byBradford (1986) were measured before and after burning using agauge penetrometer with a 608 cone and base area of 104 mm2.

Infiltration rates were measured before and after burning usinga double ring infiltrometer (Reynolds et al., 2002). The inner ring(measuring cylinder) is 30 cm long with a diameter of 30 cm theouter ring (buffer cylinder) has the same length as the inner ringwith a diameter of 50 cm. A constant head of 0.10 m water wasmaintained in the measuring cylinder in the course of measure-ments. Grass residues were put on the soil in the inner surfacesof the rings prior to water application to minimize surfacedisturbance when applying water. The amount of water infiltratedwas recorded at 1 min for the first 10 min and then every 5 min for1 h. In each case, steady-state infiltration was attained within themeasurement period. Sorptivity and infiltration data wereanalyzed according to Philip’s equation (Phillips, 1957). Sorptivityis a measure of the rate at which water is absorbed into the soil, andit is the slope of the straight-line portion found in the first fewminutes of a curve of accumulated infiltration against time1/2 (t1/2).Initial soil moisture content in the surface (0–0.15 m) wasdetermined at the time of each infiltration measurement.

Saturated hydraulic conductivity (Ks) was estimated using therelationship described by Reynolds and Elrick (1990):

Ks ¼qs

½H=ðC1dþ C2rÞ� þ f1=½aðC1dþ C2rÞ�g þ 1(3)

where Ks is the field saturated hydraulic conductivity (cm s�1), qs isthe steady-state infiltration (cm s�1), H represents water pondingdepth (cm), d is the ring insertion depth (cm), r is the inner ringradius (cm), a is the microscopic capillary length put at 0.12 cm�1,

Table 1Effects of burning on particle-size distribution, mean weight-diameter (MWD), water s

Property (depth) Soil treatment

Site 1

Before burning

Worm casts

Texture (g kg�1) 20–2000 mm 529

2–20 mm 286

<2 mm 185

MWD (mm) 2.55aWSA (kg kg�1) 0.765

Organic matter content (g kg�1) 82.20

Soil samples

Texture (g kg�1) (0–0.15 m) 20–2000 mm 699

2–20 mm 113

<2 mm 188

MWD (mm) (0–0.05 m) 2.21

MWD (mm) (0–0.10 m) 0.85aWSA (kg kg�1) (0–0.05 m) 0.733aWSA (kg kg�1) (0.05–0.10 m) 0.436

Organic matter content (g kg�1) (0–0.15 m) 50.90

Within each row in each site, (*) indicates mean values are significantly different (/ �a Concentration of WSA > 0.25 mm.b NWC – no worm casting.

C1 and C2 are constants with the values of 0.316p and 0.184p,respectively.

All data obtained were fitted into general linear model, whiletreatment effects (pre- and post-burn data) were compared usinganalysis of variance and assuming a randomized complete blockdesign (SAS, 2002).

3. Results

The total biomass estimated before burning at Sites 1 and 2were 77 mg ha�1 and 212 mg ha�1, respectively. Although, thequantity of trash on Site 2 was higher than on Site 1 but the burnseverity was significant in Site 1. The particle size distribution ofsurface soil samples and worm casts were summarized in Table 1.The textural classes of surface soil samples in both experimentalsites before and after burning were sandy loam. On the other hand,the textural class of earthworm casts collected from Site 1 beforeand after burning was loam. Worm casting was not noticeable inSite 2 at sampling time, perhaps due to prolonged fallow period.However, regardless of burning, sand content in the casts wasconsistently lower than the soil samples (Lal and Akinremi, 1983).The mean sand contents before burning in Site 1 were 529 g kg�1

and 699 g kg�1 for worm casts and soil samples, respectively. Afterburning, sand content was 502 g kg�1 for cast and 702 g kg�1 forsoil samples. Meanwhile, there were decreases of 27 and 29% in theclay contents of the soil samples collected after burning from Sites1 and 2, respectively. The decrease was however compensated forby higher silt contents with corresponding increase of 35 and 26%.

OM contents in the soils and worm casts reduced after burning(Table 1). However, the reduction in soil OM in both sites were notstatistically significant (a � 0.05) albeit 27 and 22% decreases inOM values after burning in Sites 1 and 2, respectively. On the otherhand, OM reduced significantly from 82.2 to 66.0 g kg�1 in wormcast after burning.

In Site 1, MWD consistently reduced after burning by 31% for 0–0.05 m and 44% for 0.05–0.10 m depths. Also, the percentagedecrease in WSA concentration after burning was 20% for 0–0.05 mand 12% for 0.05–0.10 m depths (Table 1). MWD and WSA in Site 2showed trends similar to those obtained in Site 1 with MWD at

table aggregates (WSA) and organic matter contents for soils and worm casts

Site 2

After burning LSD0.05 Before burning After burning LSD0.05

502 ns NWCb NWC –

326 ns NWC NWC –

172 ns NWC NWC –

2.95 ns NWC NWC –

0.873 0.05* NWC NWC –

66.00 13.93* NWC NWC –

702 ns 704 727 ns

153 ns 113 143 ns

138 ns 183 130 ns

1.53 ns 1.43 0.77 0.37*

0.48 0.35* 0.94 0.52 0.37*

0.586 ns 0.533 0.443 0.106*

0.384 ns 0.449 0.392 ns

37.12 ns 40.10 31.20 ns

0.05). ns – no significant difference between the treatment means (/ � 0.05).

Table 2Summary of the response of soil strength, bulk density, moisture content, total porosity, macro- and micro-pores, available water holding capacity and field saturated

hydraulic conductivity to burning

Soil property (depth) Soil treatment

Site 1 Site 2

Before burning After burning LSD0.05 Before burning After burning LSD0.05

Soil strength (kPa) (0–0.05 m) 97.7 106.7 ns 112.6 126.6 ns

Soil strength (kPa) (0.05–0.10 m) 352.4 370.9 ns NPb NPb ns

Bulk density (mg m�3) (0–0.05 m) 1.23 1.24 ns 1.13 1.18 ns

Bulk density (mg m�3) (0.05–0.10 m) 1.45 1.51 ns 1.31 1.38 ns

Moisture contenta (cm3 m�3) (0–0.15 m) 0.107 0.087 ns 0.106 0.091 ns

Total porosity (m3 m�3) (0–0.05 m) 0.542 0.535 ns 0.565 0.511 ns

Total porosity (m3 m�3) (0.05–0.10 m) 0.455 0.434 ns 0.545 0.480 ns

Macropores (>30 mm) (m3 m�3) (0–0.05 m) 0.187 0.172 ns 0.172 0.140 ns

Macropores (>30 mm) (m3 m�3) (0.05–0.10 m) 0.125 0.104 ns 0.143 0.112 ns

Micropores (<30 mm) (m3 m�3) (0–0.05 m) 0.355 0.363 ns 0.393 0.370 ns

Micropores (<30 mm) (m3 m�3) (0.05–0.10 m) 0.329 0.329 ns 0.402 0.368 ns

Available water (m3 m�3) (0–0.05 m) 0.344 0.341 ns 0.378 0.321 ns

Available water (m3 m�3) (0.05–0.10 m) 0.294 0.285 ns 0.332 0.310 nszHydraulic conductivity (10�3 cms�1) 10.50 3.75 1.12* 8.99 4.69 2.16*

ns – no significant difference between the treatments (/ � 0.05). Within each row, (*) indicates that mean values are significantly different (/ � 0.05).a Moisture content was measured at the time of soil strength measurements.b NP indicates that mean could not be calculated as the penetrometer could not be pushed to 0.10 m depth on Site 2.z Geometric mean value.

K.S. Are et al. / Soil & Tillage Research 103 (2009) 4–10 7

0–0.05 and 0.05–0.10 m and WSA at 0–0.05 m depths decreasesignificantly after burning (Table 1). Similar trends in WSA andaggregate sizes for mineral soils were reported by Eden et al.(1991), Tucker (2003), Wuest et al. (2005) and Hubbert et al.(2006). Conversely, the concentration of WSA in worm castscollected in Site 1 increased significantly from 0.765 to0.873 kg kg�1 after burning (Table 1). Despite not significantlydifferent, an appreciable increase of 16% in the MWD of worm castaggregates was also recorded after burning.

Soil strength and bulk density responded to burning by increasein their values after burning, even though there were no significantdifferences between pre- and post-burn soils (Table 2). However,penetrometer cone could not be forced to 0.10 m depth for anysampling points in Site 2 at sampling time. Therefore, we couldonly take the soil strength of 0.05 m depth in Site 2. The variabilityin soil strength is unlikely to be related to soil moisture contentbecause we found no significant difference in the mean moisturecontent before and after burning in both sites (Table 2).

A higher volume of pores >30 mm (macropores) was observedin Site 1 than in Site 2 with in 0–0.05 m depth. Even though poresizes decreased after burning, slash and burn did not havesignificant effect on the response of total porosity, macro- andmicro-pores to burning in both sites (Table 2). However, in Site 1,

Fig. 1. Soil moisture retention characteristics before and

while volume of micropores increased from 0.355 to 0.363 m3 m�3

at 0–0.05 m depth, no percentage increase recorded in microporevolumes at 0.05–0.10 m depth (Table 2).

Soil water characteristic curves (Figs. 1 and 2) showed that theobserved differences in moisture retention of pre- and post-burnsoils in both sites were more at low than at high suctions. Althoughthe changes were not significant, AWC dropped by 0.9% and 3% forsoils at 0–0.05 m and 0.05–0.10 m depths, respectively, in Site 1(Table 2). In Site 2, AWC showed trends similar to those obtained inSite 1 with 15 and 7% reduction in available water after burning at0–0.05 and 0.05–0.10 m depths, respectively. However, thedifference between laboratory and field AWC (data not shown)was not significant (/ � 0.01) but strongly correlated (r = 0.95).

Field saturated hydraulic conductivity (Ks) significantlyreduced after burning in the soils of both sites (Table 2). Themean saturated hydraulic conductivity reduced significantly afterburning by 64 and 47% in Sites 1 and 2, respectively.

Table 3 summarizes the mean value of infiltration rates,sorptivity and initial surface moisture contents (0–0.15 m) beforeand after burning. In both sites, soil before burning maintainedtheir initial Infiltration rates vis-a-vis a significant decline in thesoil infiltrability after burning (Figs. 3 and 4). The steady-stateinfiltration into the soils before burning was significantly higher

after burning of an Alfisol in Ibadan (Site 1), Nigeria.

Fig. 2. Soil moisture retention characteristics before and after burning of an Alfisol in Akure (Site 2), Nigeria.

Table 3Effects slash and burn on initial moisture contents, sorptivity and steady-state infiltration rates

Property Soil treatment

Site 1 Site 2

Before burning After burning LSD0.05 Before burning After burning LSD0.05

Moisture content (m3 m�3) 0.103 0.168 ns 0.112 0.173 ns

Sorptivity (cm h�1/2) 83.4 34.3 2.87* 125.0 51.4 8.70*

Infiltration (cm h�1) 67.2 24.0 7.65* 57.6 30.0 13.80*

Within a row in each, (*) indicates that mean values are significantly different at / � 0.05.

K.S. Are et al. / Soil & Tillage Research 103 (2009) 4–108

than after burning. Here, infiltration rates after burning reduced by180% in Ibadan site and 92% in Akure site. Initial moisture contentsin both sites may unlikely has influence on the steady-stateinfiltration because there was no significant difference in theirvalues between pre- and post-burn soils (Table 3). Sorptivityshowed trends similar to infiltration rates with a significantreduction in sorptivity levels after burning (Table 3).

4. Discussion

4.1. Texture and structural stability

The textural classes of both soils and worm casts were notaffected by slash and burn system. Soil textural class is apermanent and natural attribute of the soil (Hillel, 1982). This

Fig. 3. Influence of burning on infiltration of water into an Alfisol in Ibadan (Site 1),

Nigeria. Vertical bars represent standard error bars.

suggests that the textural class of a particular soil may notnecessarily change even if the soil management changed. None-theless, there were slight changes, albeit not significant, in theparticle size distribution of both the soils and worm casts followingburning. The reduction in the clay fractions of soils and worm casts(Table 1) suggests that there was some aggregation of finerparticles (clay) into larger silt-size particles, which perhapsincreased the silt fractions after burning (Neary et al., 1999;Ketterings et al., 2000; Hubbert et al., 2006).

The soil physical disruption following burning was reflected inthe reduced aggregate sizes and concentration of WSA in the soils(Table 1). Giovannini et al. (1988), Wuest et al. (2005) and Hubbertet al. (2006) found a reduction in aggregate stability followingburning. However, it was observed in our analyses that the lowervalues of MWD and WSA of soil aggregates after burning, were

Fig. 4. Influence of burning on infiltration of water into an Alfisol in Akure (Site 2),

Nigeria. Vertical bars represent standard error bars.

K.S. Are et al. / Soil & Tillage Research 103 (2009) 4–10 9

primarily due to considerable lower percentage of aggregatespresent in the 2.0–5.0 mm diameter size class. The reduction inclay fractions likely accounted for the reduction in soil WSA, due tothe cohesive influence of clays that may have reduced afterburning (Duriscoe and Wells, 1982). On the contrary, despite thereduction in clay fractions and significant loss of OM (Table 1),worm casts became more stable after burning. This is reflected inthe increased concentration of WSA and MWD following burning.Alegre and Cassel (1986) have however reported an increase inmacro-aggregate stability following burning on a fine loam soils inPeru. Similar observations were also made by Obale-Ebanga et al.(2003) on zero-tilled Vertisols in Cameroon. It therefore appearsthat soil with greater percentage of finer particles has its structuralquality improves after burning. Schrader and Zhang (1997)reported that high clay content leads to high dispersion and lowWSA. It follows that some types of soil or worm casts with high claycontent will have their WSA increases after burning once their clayfractions reduce after burning.

The role of SOM in improving aggregate stability has beenreported by Channey and Swift (1984) and others. Cook et al.(1992) for instance, suggested that it is soil carbon that reducesclay dispersion and therefore maintains soil structure. Even then,an inverse relationship between worm cast stability and OM wasobserved following burning. However, Shipitalo and Protz (1989)identified clay-polyvalent cation-organic matter linkages asresponsible for the unusual increase in aggregate stability ofworm cast. It therefore calls for further research to investigate thefractions of OM that contribute to the stabilization of worm castand some other soils after burning. Nevertheless, the increasedworm cast stability resulting from burning suggests that thevulnerability of soil surface to rainfall impact can be underminedsince worm casts provide enough roughness to the soil surface.However, if mechanical tillage is a must use after burning, thebeneficial effect of worm cast in providing surface roughness maybe shattered.

4.2. Soil strength and bulk density

The increase in soil strength was in response to an increase in soilbulk density following burning. In Site 1, 26% of the variation in soilstrength may be explained by bulk density using linear regression.Although penetrometer cone could not penetrate up to 0.10 m soildepth in Site 2 at sampling time, 30% of the variation in the soilstrength at 0–0.05 m depth may be explained by changes in bulkdensity. Similar trends in bulk density for mineral soil followingburning were reported by Giovannini et al. (1988) and Hubbert et al.(2006). Giovannini et al. (1988) suggested that the increase in bulkdensity can be ascribed to the disruption of soil aggregation and lossof OM following burning. However, the observed difference inmoisture contents at the time of soil strength measurement, may beattributed to burning (Tucker, 2003; Hubbert et al., 2006), and thevalues are inversely proportional to soil strengths.

4.3. Pore size distribution, soil water characteristics and

hydraulic conductivity

The effects of slash and burn on pore size distribution showedthat the volume of larger drain pores and total pore volumesdecreased after burning (Table 2). In Site 1, 1% reduction in totalporosity for 0–0.05 m soil layer, may be explained by 8% decreasein macropores and 2% increase in micropores following burning. Itwas also observed that at 0.05–0.10 m depth, 17% decrease inmacropores with no change in micropore volumes, accounted for5% reduction in total porosity after burning. However, in Site 2, asubstantial decrease of 19% and 22% in macropores for soil at 0–

0.05 and 0.05–0.10 m depths accounted for 10% and 12% decreasein total porosity, respectively. Reduction in larger pores and totalporosity following burning has also been reported by Mallik et al.(1984), Hubbert et al. (2006) and Zhang et al. (2007). It thereforeappears that the reduction in macropores and total pore volumeswas perhaps due to ash deposits in larger pores. The ash depositedmight have probably reduced the large pore density andconcomitantly increased the small density (Mallik et al., 1984;Tucker, 2003). The effect was however substantial at 0.05–0.10than 0–0.05 m depths (Table 2).

Burning likely accounted for the reduction in AWC for bothsites. Besides, in response to the reduction in macropores and totalporosity following burning, moisture retention characteristics ofthe soils dropped at a layer of 0–0.10 m depth in both sites (Figs. 1and 2). However, a noticeable decrease in water retained by post-burn soil was observed mostly at low than at higher suctions(Figs. 1 and 2). Hubbert et al. (2006) reported similar reduction inmoisture retention from 0.13 to 0.03 m3 m�3 at a depth of 0–0.05 m in a steep chaparral watershed, southern California,following burning. The results contradict those of Mallik et al.(1984) who reported an increase in water retained after burning.However, at higher suction, a slight increase in moisture retentionwas observed after burning in 0–0.10 m for soils in both sites(Figs. 1 and 2). The reduction in soil moisture retention, apart fromdecrease in pore sizes, can be ascribed to decrease in clay fractionsor loss in SOM (Alegre and Cassel, 1986).

In response to the reduction in pore volumes following burning,saturated hydraulic conductivity reduced significantly in Ibadanand Akure sites. The ash deposits, which perhaps reduced thedensity of large pores to a preponderance of small pores, likelyimpeded the rate of water inflow through the soil profile, andconsequently reduced saturated hydraulic conductivity (Table 2).

4.4. Infiltration rates and sorptivity

The decrease in infiltration rates and sorptivity was in responseto the reduction in large pores (macropores) caused by ashdeposits. It was however expected that initial infiltration will beaffected by initial moisture content of the soil at the time ofmeasurement (Lowery et al., 1996); we made a comparisonbetween steady-state infiltration before and after burning. Also,post-burn sorptivity levels of the two soils were significantly lowerat the soil layers examined compared to the sorptivity levels of pre-burn soils. It follows that infiltration rates, sorptivity, saturatedhydraulic conductivity and pore size distribution are subject to soilpore geometry. Nevertheless, the reduction in these soil hydraulicproperties may be ascribed to ash deposits that clogged themacropores after burning (Mallik et al., 1984). It should be notedthat the much reduced infiltration rates and sorptivity couldgenerate excess runoff water from rainfall (Babalola et al., 2007);which may increase the risk of soil erosion while nutrients appliedto crops in form of fertilizers may be a waste.

5. Conclusions

Slash and burn systems has immediate and direct effects onphysical and hydrological properties of the soils studied. In thisstudy burning has no impact on soil class; even then, there wereslight changes in the particle size distribution of post-burn soils.No significant changes were observed in bulk density, soil strength,moisture contents, pore size distribution and AWC. However, werecorded a significant reduction in the values of structural stability,saturated hydraulic conductivity, sorptivity and infiltration rateafter burning. On the contrary, the structural stability in terms ofmacro-aggregate stability (MAS) and MWD of worm casts

K.S. Are et al. / Soil & Tillage Research 103 (2009) 4–1010

improved and are more stable after burning. Consequently, it isimportant to know that tropical soils generally have an extremelydelicate nature and lack resilience once degraded. With the currentincrease in demographic pressure on land in southwestern Nigeria,which has led to a continuous and intensive cultivation of land(including very steep and marginal lands), slash and burn mayhinder a sustainable cropping system. However, over 90% offarmers in south-western Nigeria are small farm holders; theycultivate less than 2 ha of land with zero or minimum tillage. Withtheir traditional farming system, slash and burn may have little orno effect on sustainable cropping system, since occluded mineralsare quickly mineralized while increased worm cast stability afterburning will stabilized the soil surface before vegetation cover. Onthe contrary, slash and burn may affect sustainable agriculture ifpracticed under continuous and intensive cultivation usingmechanical tillage operation. Here, the collective degenerativeeffects of tillage and burning on soil properties may be enormous ina way that the fragile tropical soil may not be rejuvenated oncedegraded.

Acknowledgements

The authors are grateful for the assistance rendered by the field,technical and laboratory staff of Land and Water ResourcesManagement programme in the Institute of Agricultural Research& Training, Ibadan, Nigeria.

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