Short communication
Effects of different land management techniques on selected
topsoil properties of a forest Ferralsol
R. Ambassa-Kiki
a,*, D. Nill
baInstitute of Agricultural Research for Development (IRAD), PO Box 2067, Messa, YaoundeÂ, Cameroon bBerliner Str. 2-64342 Seeheim, Germany
Received 16 December 1997; received in revised form 27 April 1999; accepted 22 July 1999
Abstract
Soil erosion is a major threat for Ferralsols in Cameroon. The in¯uence of traditional intercropping (TI), disk-harrow ploughing (DH), no-tillage (NT), and Wischmeier bare fallow (BF) on runoff coef®cient, soil loss, organic carbon (OC) content and bulk density was evaluated in topsoils of a forest Ferralsol in Yaounde region, Central Cameroon, using erosion plots. This was to ensure the best conditions for the determination of the soil properties to be assessed. After two years of cropping, the mean runoff coef®cient remained very low for TI (<2% of the rain) as compared with NT (14%) and DH (15%). The same held true for soil loss which was in the order of 2, 68 and 109 Mg haÿ1for TI, NT, and DH respectively, and bulk density which was 1.06, 1.18 and 1.21 Mg mÿ3respectively. Comparing the latter with the measurements obtained from BF (1.23 Mg mÿ3) and the adjacent secondary forest (SF) (1.04 Mg mÿ3) showed that the disk-harrow treatment was the most degraded among the three. The same comparison was made for the OC content. It was found that while in SF, OC was as high as 30 g kgÿ1, it was only 11, 13, 15 and 18 g kgÿ1in BF, DH, TI, and NT respectively. On the average and for the time frame considered, TI adversely affected topsoil properties less than NT, DH and BF in this order. Based on the above, it can be concluded that TI is more conservative than the three other land management techniques investigated.# 1999 Elsevier Science B.V. All rights reserved.
Keywords:Land management; Topsoil properties; Ferralsol; Central Cameroon
1. Introduction
Ferralsols are upland soils covering 60% of the Cameroonian territory (Muller and Gavaud, 1979) including the Central Province where the YaoundeÂ
region is located. Although these soils have good physical properties Ð good structural stability through microaggregation (Ahn, 1979; Bernard et al., 1989) and little susceptibility to erosion (Bernard et al., 1989) Ð soil erosion is the major threat to their productivity due to their topographic situation in the landscape and the high rainfall intensity (Petri, 1992; Nill, 1993). Land management practices like e.g. traditional farming systems with long fallow periods, and no-tillage system of cultivation have shown
tre-*Corresponding author. Present address: Institute of Agricultural Research for Development, Nkolbisson, PO Box 2067, Messa, YaoundeÂ, Cameroon. Tel.:237-233105; fax:237-237440
E-mail address: iita-humid@iccnet.cm (R. Ambassa-Kiki)
mendous potential for preserving soil environment. Moreover, mixed-cropping systems are reported to favour fast soil cover which minimizes runoff, soil loss and nutrient leaching (Nye and Greenland, 1960; Norman, 1973; Suryatna and Harwood, 1976; Petri, 1992). Other reports suggest that no-tillage can increase soil water content, enhance in®ltration and lower soil temperatures (Allmaras et al., 1973; Stand-ford et al., 1973; Lal, 1982; Mengel et al., 1982). The objective of this study was to evaluate the effects of some land management techniques on runoff, soil erosion, bulk density and OC content in a Ferralsol in Central Cameroon.
2. Materials and methods
2.1. Experimental site
A 2-year ®eld experiment was conducted at two sites located in the forest zone of Central Cameroon (Yaounde region). The sites whose major character-istics are presented in Table 1 were Minkoameyos, the IRAD experimental farm, and Elig-Essombala, a farmer-managed site in a nearby village. The topography of the overall region is rolling with dominant slopes between 10% and 18%. Steeper slopes (25±35%) occur locally whereas ¯atter tracts of land are found on small plateaus. The soils are classi®ed according to FAO (1998) as Rhodic Ferral-sol. The average annual rainfall amounts to 1600 mm and occurs in a bimodal con®guration (Ambassa-Kiki, 1988) such that the major and minor cropping seasons, separated by a 4-month dry season, last from mid-March to early July and from late-August through mid-November respectively (Ambassa-Kiki, 1990).
2.2. Land management treatments
Four different types of land management techniques on soil conservation were evaluated on non-replicated observation plots. They included NT farming, TI system, DH and BF treatments.
In NT, the seeds were placed in a slit 5 cm wide and deep, opened with a garden hoe and covered with soil thereafter. In DH, the plot was prepared twice a year by means of a tractor-pulled offset DH prior to plant-ing. NT and DH were grown each year to a rotation of groundnut (Arachis hypogaeaL.) in the major crop-ping season followed by maize (Zea maysL.) during the minor cropping season. Organic residues were removed to mimic farmer's practice. Fertilizer was applied in a band and incorporated into the soil 2 weeks after planting. Weeds were controlled by her-bicides (glyphosate, paraquat, atrazine) and manual weeding as necessary. Both treatments were estab-lished only at Minkoameyos like BF. Prior to this, the land was subjected to clearing using a tree-pusher (bulldozer).
The TI was implanted at Elig-Essombala alone and managed by farmers for two consecutive years without interference. After clearing the bush fallow [avocado trees (Persea Americana Mill) and other useful per-ennials like oilpalm (Elaeis guineensisJacq.) were left standing] and burning the biomass, the plot was planted to groundnut as dominant crop (150 000 plants/ha) mixed with maize, cassava (Manihot escu-lentaCrantz), plantain (Musa sp.AAB Colla)/banana (Musa sp. AAAColla), sugarcane (Saccharum of®ci-narum L.), and a wide range of vegetables in low density. This was followed by fallow of mainly chro-molaena (Chromolaena odorata L.) (R.M. King and Robinson) so that 3 years later (i.e., 1 year after the
Table 1
Characterization of the two selected topsoils and the nearby secondary forest from the forest zone of Central Cameroona
Site No. Locationb Prior land useb
Destination (mean slope)
Physical and organic properties
Clay g kgÿ1 Silt Sand BD Mg mÿ3 OC g kgÿ1
1 MM Bulldozer-cleared SF RO plots (18%) 310 260 460 1.10 20 2 EE Hand-cleared bush fallow RO plots (30%) 200 290 510 1.05 21
3 MM Non-cleared SF None (25%) 130 220 650 1.02 32
aAll the soils were classified as Rhodic Ferralsols (FAO).
farming phase), a strati®ed fallow vegetation of up to 5 m high had completely covered the plot. Palm tree and avocado constituted the highest storey (4±5 m high) followed by banana/plantain and sugarcane (3± 4 m high). The understorey was composed of cassava and chromolaena (1±2 m high) underlain by a variety of smaller Herbs and grasses which completed the dense vegetation. Cassava, banana/plantain, oilpalm and sugarcane were still selectively harvested during the fallow. After 2 years of fallow, the vegetation was slashed and burned as at the study initiation, and the same cropping pattern as described above was pursued. The BF treatment was set up solely at Minkoa-meyos and consisted of maintaining a plot free of any vegetation and/or cropping. To this end, the plot was tilled to 15 cm depth using an offset disk-harrow, then hand-raked and levelled to seed bed conditions. Hand-raking was regularly repeated after heavy storms in order to destroy surface seals.
2.3. Soil measurements
In view of providing the best conditions for the determination of the soil properties to be assessed, mostly runoff and soil loss, all plots Ð 50 m long and 10 m wide each Ð were delimited by earthen ridges on three sides and equipped with cemented gutter at the fourth and lower side for the collection of runoff water and sediments. The gutter collected and directed runoff to a measurement system consisting of a ¯ume, a water-level recorder and a Coshocton sampler (Fig. 1). The water ¯owing out of the ¯ume turned the Coshocton wheel of 60 cm in diameter. By passing underneath the water-jet, a slot in the wheel, 2.5 cm wide, sampled an aliquot of suspension corresponding to 1.33% of the runoff. The aliquot was conducted into a 33 l bucket placed in a 200 l drum (Fig. 1). Thus, small runoff volumes would be measured in the bucket. Bigger runoff volumes caused over¯ow which was stored and measured in the drum. The soil in the gutter and the bucket was weighed in the ®eld and samples for moisture determination were taken. If the bucket had over¯own, the sediment in the drum was stirred and a 1.5 l sample of the suspension was taken. All samples were dried and weighed in the laboratory (Nill, 1993). Runoff (RO [l/plot]) was calculated by:
ROf VbVdÿ SbSs=D;
withDbeing the sediment density (D2.55) [kg/l],f the factor for the aliquot [ÿ],Sbthe dry-weight of soil
in the bucket [kg],Ssthe dry-weight of suspended soil
[kg],Vbthe suspension volume in the bucket [l], and
Vdthe suspension volume in the drum [l].
Soil loss (SL [kg/plot]) was calculated by:
SLSg f SbSs;
withSgthe soil in the gutter [kg] andf,Sb,Ssas above.
Bulk density (BD) in 0±5 cm depth was measured bimonthly on all plots using 100 cm3cores. Up-, mid-and downslope segments were sampled individually. Four samples were taken for each slope segment resulting in 12 samples per plot and sampling date. Samples were then ovendried for 24 h at 1058C.
All the measurements were done over 2 years, except for the determination of the organic carbon by Walkley and Black method (Nelson and Sommers, 1982) which was accomplished by the end of the experiment through analyses of composite samples collected from individual up-, mid- and dowslope segments at 0± 5 cm depth. The same applied to the calculation of the run off coef®cient (RC [% of rain]) although the rainfall height was registered with a self-recording raingauge long before the beginning of the study.
Because of the continuity of the TI system, almost all the data were calculated for the bulk of 2 years of measurements. Thus, RC and BD were expressed as the average of 2 years calculations, except for the SF's BD. Runoff sediments were given as the total amount of SL over the same period of time.
3. Results and discussion
the soil through the breakdown of its structure was not seriously affected by the mechanical operations made, which agrees with earliest statements (Ahn, 1979; Bernard et al., 1989). However, there was signi®cantly more runoff from NT and DH compared with slight runoff from TI (RC14% and 15% vs. 1%). Similar results were reported in a watershed in western Nigeria (Lal, 1981).
Soil loss (soil erosion) was greatly affected by land management techniques. As expected, the highest soil loss was observed on bare-fallow treatment (Table 2).
This high amount of loss corresponded to about 4 cm of desurfaced soil. However, there was comparatively less erosion on DH (109 Mg haÿ1, i.e., approximately 1 cm of topsoil removed), and less so on TI (despite its 30% slope gradient), which agrees with earlier ®nd-ings (Lal, 1981). Apart from the mechanical manip-ulation undergone by DH twice a year, another reason for these differences is that mixed cropping tends to produce less erosion than mono-crops under same climatic and topographic circumstances, and so con-tains an element of resource maintenance (Norman,
1973). Where there is a vertical arrangement, the rain, for example, falls from the plantain onto the cassava and then onto the vegetables, and only then does it reach the soil (Ruthenberg, 1980). In fact, 43 days after planting, mix-crops attain 75% of soil coverage (Petri, 1992) and then they have a protective effect similar to that of the original bush vegetation.
The OC content was signi®cantly in¯uenced by land management techniques (S.D.7.5; C.V.
43%) (Table 2). The DH, TI and NT were rated ``medium'', whereas BF and SF were rated ``low'' and ``high'', respectively (Euroconsult, 1989). There-fore, any mechanical manipulation of the soil results in OC decline (Allison, 1973; Tate, 1987). However, the rate of decline was different depending on the degree of manipulation (Table 3). The high rate of OC decline for BF is thus easily explained by bulldozer clearing followed by soil desurfacing resulting in a very high amount of soil loss. The same applies to DH
except that the decline may have been also caused by soil cultivation (Allison, 1973). Whatever the reasons, the data in Table 2 show that OC was much higher on NT compared with DH, BF and TI. Thus, relative to OC content, NT proved less degradative than the three other treatments. The low OC level in TI could be explained, as for DH, by soil cultivation.
4. Conclusion
This study established that traditional intercropping preceded by manual clearing produces less runoff and soil loss, and conserves a low bulk density compared to the treatments that involve sole cropping preceded by the use of heavy machinery for clearing like NT and DH. Consequently, although it has a relatively lower OC content than NT, TI proved more conservative than the other treatments, and may be improved by a better organic matter management. On the other hand, NT which is being practiced in many parts of the forest zone, mainly in homegardens, performed better than DH. Consequently, given the present move in the country towards mechanized agriculture, it may be recom-mended as a way to reduce land degradation through erosion, and to relatively protect the environment.
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RC, SL, BD, total porosity (P) and OC as affected by four different types of land management techniques Land management
aAverage of two years of measurements. bTotal amount of two years of measurements.
cObtained from only one set of samples taken at the end of the experiment. dData not available, and consequently, SF not included in the calculation.
Table 3
Soil OC decline relative to secondary forest
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