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Effects of cassava-based cropping systems on physico-chemical properties of soil and earthworm casts in a tropical Alfisol
Agriculture, Ecosystems and Environment, 35 ( 1991 ) 55-63
55
Elsevier Science Publishers B.V., Amsterdam
Effects of cassava-based cropping systems on physico-chemical properties of soil and earthworm casts in a tropical Alfisol N.R. Hulugalle* and H.C. Ezumah International Institute of Tropical Agriculture, PMB 5320, Oyo Road, lbadan (Nigeria) (Accepted for publication 7 August 1990)
ABSTRACT Hulugalle, N.R. and Ezumah, H.C., 1991. Effects of cassava-based cropping systems on physicochemical properties of soil and earthworm casts in a tropical Alfisol. Agric. Ecosystems Environ., 35: 55-63. The effects of cassava-based intercropping systems and rotations on physical and chemical properties of earthworm casts and the adjacent soil were studied on an Oxic Paleustalf in south-western Nigeria. Earthworm activity was greater with intercropping although it was not significantly affected by the number of component crops in a mixture. The particle size distribution, bulk density, exchangeable cations, Bray-I-P, pH and effective cation exchange capacity (CEC) of soil and earthworm casts did not differ among the cassava-based cropping systems investigated. Greatest values of mean weight diameter, organic C and total N were observed in earthworm casts from three component crop mixtures, although the adjacent soil was not similarly affected. The results suggest that although the cropping system changed some physico-chemical properties of earthworm casts, similar changes did not occur in the adjacent soil. Water infiltration into the soil was, however, increased by intercropping and may be related to earthworm activity. Cropping system may, therefore, influence soil fertility indirectly by changing water infiltration characteristics and hence, nutrient losses in runoff and erosion. In relation to soil, earthworm casts had higher silt and clay contents, bulk density, mean weight diameter, pH, Bray-l-P, Organic C, total N, C : N ratio, exchangeable cations and effective CEC, and lower sand content.
INTRODUCTION
Cassava (Manihot esculenta (Crantz.) is one of the major food crops in the h u m i d and sub-humid zones ofsub-Saharan Africa (Terry et al., 1984, 1987 ). The total area planted to cassava in Africa is 7.48 million ha (52.9% of world area) (Terry et al., 1987). A major proportion of this cassava ( > 9 5 % ) is grown as an intercrop with crops such as maize (Zea mays), sorghum *Address correspondence to: IITA, c / o Ms. M. Larkin, L.W. Lambourne and Co., 26 Dingwall Road, Croydon CR9 3EE, Gt. Britain.
0167-8809/91/$03.50
© 1 9 9 1 - Elsevier Science Publishers B.V.
56
N.R. HULUGALLE AND H.C. EZUMAH
(Sorghum bicolor), yam (Dioscorea alata and D. rotundata), cocoyam ( Xanthosoma sagittifolium ) and melon ( Citrullus lanatus ) ( Okigbo, 1978; Terry et al., 1984, 1987 ). Research work in the past has concentrated primarily on yield and other crop growth aspects of such cassava-based cropping systems (Okigbo, 1978; Terry et al., 1984, 1987 ). The effects of cassava-based cropping systems on the soil environment have been little studied. The few studies conducted have shown that in relation to monocropping, intercropping cassava increases water infiltration and decreases water runoff and soil erosion (Aina et al., 1979). This has been ascribed to protection of the soil surface from the compactive effects of high intensity rainfall (Aina et al., 1979) and greater earthworm activity (Lal, 1983 ). Increased earthworm activity results in a high proportion of water transmission pores which in turn lead to greater water infiltration rates and hence, lower erosion (Lee, 1985). Increases in soil water content and decreases in soil temperature also occur concurrently in cassava-based intercropping systems (Ikeorgu et al., 1983). In addition, soil fertility decreases are reported to occur at a lower rate, particularly in crop mixtures of three or more species (Okigbo, 1978). Lower soil erosion and water runoff, and hence, lower nutrient losses are likely to be the primary cause for the lower rate of fertility decrease in such cropping systems. A secondary cause may be the accumulation of higher levels of nutrients in earthworm casts voided in intercropping systems of three or more species. The breakdown of these casts over time could result in the release of nutrients to the adjacent soil (Lee, 1985 ). Hence, long-term management of soil fertility in cassavabased cropping systems would appear to require detailed information on the physical and chemical characteristics of earthworm casts voided therein. The objective of the present study, therefore, was to quantify the effect of cassavabased cropping systems on physico-chemical properties of earthworm casts and adjacent soil in a tropical Alfisol. MATERIALS AND METHODS
The experiment was conducted at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria ( 7 ° 30' N, 3 ° 54' E). The IITA is located in the forest-savannah mosaic zone of south-western Nigeria. Mean annual rainfall is 1250 m m . Rains commence in late March and continue until late July. Following a short dry spell of approximately one m o n t h the rains recommence in late August and continue until early November. The dry season lasts from November to March. The soil at the experimental site is a kaolinitic, skeletal, isohyperthermic, Oxic Paleustalf (Ferric Luvisol) belonging to the Ibadan series with a sandy loam topsoil and a sandy clay subsoil ( M o o r m a n et al., 1975). Particle size analysis showed that the surface 0.15 m consisted of 71% sand, 12% silt and
CASSAVA-BASEDCROPPING SYSTEMS
57
17% clay. Effective cation exchange capacity (CEC) ( 1N ammonium acetate extractable) was 108.4 mmol ( + ) kg -1, with 69.2, 22.1, 11.1, 4.8 and 1.2 mmol ( + ) kg-1 being contributed by Ca, Mg, K, Na and total acidity ( A I + H ) , respectively. Soil pH ( H 2 0 ) , Bray-l-P and total N were 6.7, 73.7 mg kg- 1 and 0.22%, respectively. Three cassava-based rotation systems in two phases were used in the trial, which had been established since 1985. Table 1 summarizes the crop combinations, rotations, their respective chronologies and populations. The experimental design was a randomized complete block design with four replications. Plot size was 15 m × 5 . 5 m. Prior to commencement of the trial in 1985 the site was disc-harrowed to a depth of 0.20 m. Thereafter no-tillage (Lal, 1983 ) was used in all treatments except when yam was planted, when ridges 0.30 m high and 0.75 m apart were constructed. Weeds were controlled by spraying 'Galex' (metabromouran + metolachlor) at a rate of 3 kg a.i. ha- ~ before planting. Fertilizer was applied at planting to all plots at rates of 45 kg N, 60 kg P and 60 kg K hain the forms of calcium ammonium nitrate, single superphosphate and muriate of potash. Soil bulk density was determined on undisturbed cores, 50 mm high and 51 TABLE 1
Crop combinations, rotations, and their respective chronologies and populations during the study ( + = intercrop; - = rotation )
Rotation systems
Phases Years 1985
Plant population ( m -2) 1986
Cycle 1 A
A~
Cassava
1987
Cycle 2 Maize-
Cassava
cowpea A2
Maize-
Cassava
cowpea B
B~ B2
C
C~
C2
1988
1989
Cycle 3 Maize-
Cassava
cowpea Maize-
Cassava
cowpea
Maize-
cowpea
Cassava = Cowpea = Cassava = Cowpea =
1; Maize = 5.3 6.2 1; Maize = 5.3 6.2
Maize+ cassava Yam+
Yam+ cowpea Maize+
Maize+ cassava Yam+
Yam+ cowpea Maize+
cowpea
cassava
cowpea
cassava
Maize+ M a i z e = 4; C a s s a v a = 1 cassava Yam = 1; Cowpea = 6.2 C a s s a v a + M a i z e = 4 ; C a s s a v a = 1; cowpea Yam = 1; Cowpea = 6.2
Maize+ cassava+
Maize+ cassava+
Maize+ cassava+
Maize+ cassava+
Maize+ cassava+
melon
okra
melon
okra
melon
Maize + cassava+
Maize + cassava+
Maize + cassava+
Maize + cassava+
Maize + cassava+
okra
melon
okra
melon
cowpea
M a i z e = 4; C a s s a v a = 1; M e l o n = 2; O k r a = 3.0 Maize = 4; Cassava = 1; Melon =2; O k r a = 3.0 Cowpea = 6.2
58
N.R. HULUGALLE AND H.C. EZUMAH
m m in diameter, taken at r a n d o m from five locations in the 0-0.05 m and 0.05-0.10 m depth of each plot. Composite soil samples collected at r a n d o m in the 0-0.10 m depth from 10 locations in each plot were air-dried, ground and analyzed for particle size distribution (hydrometer m e t h o d ) (Klute, 1986), organic carbon (dichromate oxidation), pH ( 1 : 1 soil:water suspension), total N (Kjeldahl analysis), Bray-l-P and 1N a m m o n i u m acetate-extractable Ca, Mg, Mn, K and Na, and 1N KCl-extractable total acidity (A1 + H ) (Page et al., 1982 ). Aggregate stability was determined by wet sieving air-dried, unground samples and expressed as the mean weight diameter of the aggregates (Klute, 1986 ). Soil sampling for physical and chemical analyses was carried out during September 1989. Earthworm activity was measured as the mass of earthworm casts per unit area of a 10 m × 3 m area in each plot during November 1988 and September 1989. The worm casts collected were the columnar casts of Hyperiodrilus africanus (Madge, 1969), which was the predominant earthworm species at the experimental site. Except for bulk density, the previously described physical and chemical analyses were also performed on earthworm casts collected during September 1989. Bulk density of earthworm casts collected during the same period was determined by the water displacement m e t h o d (Lal and Akinremi, 1983 ). Water infiltration was measured during September with a double-ring infiltrometer at one location in each plot. The results of the infiltration tests were analyzed according to Philip's equation (1957) to compute water sorptivity and transmissivity. RESULTS AND DISCUSSION
Earthworm activity Earthworm activity measured as the mass of earthworm casts per unit area did not differ significantly between years, but was greater ( P < 0.01 ) with intercropping (Table 2). There were no significant differences between twoand three-component crop systems. Phase 2, especially for the two and three crop mixtures tended to amass more earthworm casts than the monocrop sequence, though the difference was not significant. Increase in casting activity arises from cooler and wetter soil in intercropped systems (Ikeorgu et al., 1983). Supra-optimal soil temperatures for earthworm activity occur frequently in monocropped cassava systems (Lavelle, 1975; Ikeorgu et al., 1983 ).
Physical and chemical properties In relation to soil in the 0-0.10 m depth, silt and clay contents, and chemical properties except total acidity were significantly greater ( P < 0.001 ) and sand content lower ( P < 0.001 ) in earthworm casts (Table 3 ). Cropping sys-
59
CASSAVA-BASEDCROPPING SYSTEMS TABLE 2 Effect of cropping system on mass of earthworm casts per unit area Sampling time
November 1988 September 1989 Phase means 2 Rotation means
Mean
Mass of earthworm casts (t ha-~ )
Aj
A2
Bi
B2
CI
C2
0.10 0.16 0.25
0.44 0.41 0.30
0.75 1.14 0.78
1.46 0.81 1.30
0.37 1.24 0.76
0.94 1.15 1.09
+SE Between years Between rotation means Between phases
0.92
1.04
0.28
0.68 0.82
0.094 0.125 0.217
2AI in 1988 carries the same crop as A2 in 1989. The phase mean is therefore the average of the 2 years. Phase means for systems B and C were calculated following the same procedure. TABLE3 Comparative sand, silt and clay contents, and chemical properties of earthworm casts and soil in the 0-0.10 m depth Soil property
Soil
Earthworm casts
_+SE
Sand (%) Silt (%) Clay (%) pH Organic C (%) Total N (%) Bray-l-P (mg kg-~ ) Exchangeable Ca (mmol ( + ) kg-~ ) Exchangeable Mg (mmol ( + ) kg- l ) Exchangeable Mn (mmol ( + ) kg -~ ) Exchangeable K ( mmol ( + ) kg- i ) Exchangeable Na (mmol ( + ) kg -~ ) Total acidity (mmol ( + ) kg- l ) Effective CEC (mmol ( + ) kg - t )
75.6 11.9 12.5 5.5 0.83 0.09 15.7 36.6 4.1 1.9 3.4 2.4 1.6 50.0
51.5 22.2 16.4 6.4 2.53 0.24 33.8 89.6 11.3 3.2 6.4 4.2 2.1 116.8
0.41 0.23 0.32 0.07 0.030 0.003 1.53 3.05 0.91 0.10 0.17 0.16 0.19 3.21
JMean of all cropping systems.
tem did not have any significant effect on the same soil properties. In relation to Rotation systems A and B, significantly higher (P < 0.01 ) values of organic C and total N were also observed in earthworm casts from Rotation system C, i.e. the three-component crop system, but not in adjacent soil (Table 4). C" N ratios of soil (9.2) were, however, significantly lower ( P < 0.01 ) than
60
N.R. HULUGALLEAND H.C. EZUMAH
TABLE 4 Effect of cropping system on organic C and total N contents of earthworm casts and soil in the 0-0.10 m depth Phases in rotation systems
Source
A~
Soil Earthworm Soil Earthworm Soil Earthworm Soil Earthworm Soil Earthworm Soil Earthworm
A2 Bi B2 C~ C2
cast cast cast cast cast cast
_+SE Between cropping systems Between sources Between sources for same cropping system Between sources for different cropping systems
Organic C (%)
Total N (%)
0.78 2.22 0.56 2.44 0.80 2.18 0.93 2.55 1.02 2.92 0.89 2.90
0.08 0.19 0.06 0.23 0.08 0.20 0.11 0.26 0.10 0.27 0.09 0.27
0.156 0.030 0.074 0.164
0.014 0.003 0.008 0.015
those of earthworm casts ( 10.50, _+SE = 0.06 ). Mulongoy and Bedoret ( 1989 ) in contrast reported that C : N ratios of earthworm casts obtained from Alfisols were significantly lower than those of adjacent soil. Soil organic C and total N values reported by these authors were, however, greater than the values observed in the present study. Furthermore their samples were obtained under perennial vegetation (>__6 years) whereas those of the present study were obtained below annual crops. Differences in soil properties and vegetation may, therefore, have contributed to the relative differences in C : N ratios. The higher values of silt, clay, exchangeable cations and effective CEC, and lower values of sand in earthworm casts may be caused by preferential ingestion of silt and clay particles by earthworms (Lee, 1985 ). Cropping system did not have any significant effect on bulk densities of either soil or earthworm casts. Bulk density of soil was, however, significantly lower ( P < 0.01 ) than that of earthworm casts. Bulk densities of earthworm casts, and soil in the 0-0.05 m and 0.05-0.10 m depths were 1.68, 1.40 and 1.48 Mg m -3, respectively ( + SE----0.025). The differences in bulk density were related to the silt and clay contents of soil and earthworm casts thus B D = 1.28 exp[5.55 X 1 0 - 3 ( S i + C ) ]
r2=0.62 ***, n = 4 8
61
CASSAVA-BASEDCROPPING SYSTEMS
where BD is mean bulk density in the 0-0.10 m depth (Mg m -3) and Si and C are silt and clay contents, respectively, in the 0-0.10 m depth (%). Mean weight diameter ( M W D ) of soil aggregates was not significantly affected by cropping system, but that of earthworm casts was ( P < 0.01 ) such that MWD increased as the number of component crops in a mixture increased (Table 5 ). Complex crop mixtures such as those used in the present study may provide organic and inorganic nutrients of high quality for the intestinal bacterial populations of the earthworm and hence, cause an increase in their numbers and activity. Secretions of intestinal bacteria perform a primary function in increasing the structural stability of earthworm casts (Swaby, 1950). Comparison of MWD of soil and earthworm casts also indicates that the latter was significantly greater ( P < 0.001 ) (Table 5 ). The differences in MWD of soil aggregates and earthworm casts observed in the present study were related primarily to their organic carbon contents (OC, %) thus MWD=0.99 exp (0.42 OC)
r2=0.76 ***, n=48
where MWD is mean weight diameter (in mm). Water infiltration was significantly affected by cropping system, with infiltration rates and transmissivities of intercropped plots being greater (Table 6 ). Sorptivity was not significantly affected by cropping system. The higher transmissivities and infiltration rates are primarily caused by increases in earthworm activity, and hence, the number of earthworm burrows in intercropped treatments. Increases in the number of earthworm burrows results in TABLE5 Effect of cropping system on aggregate stability of soil in the 0-0.10 m depth and earthworm casts (expressed as the mean weight diameter) Phases in rotation systems
Ai A2 Bl B2
CI C2 Mean _+SE Between cropping systems Between sources Between sources for same cropping system Between sources for different cropping systems
Mean weight diameter ( m m ) Soil
Earthworm cast
1.21 1.17 1.40 1.48 1.56 1.33 1.36
1.99 2.81 2.98 3.32 3.93 4.04 3.18
0.193 0.092 0.226 0.251
62
N.R. HULUGALLE AND H.C. EZUMAH
TABLE 6 Effect of cropping system on infiltration characteristics during September 1989 Phases in rotation systems
Sorptivity ~ (ram m i n - ~/2 )
Transmissivity / (mm m i n - t )
Infiltration rate ~ (mm m i n - ~)
A, A2 B, B2 C, C2 ±SE
8.76(2.17) 10.28(2.33) 18.54(2.92) 7.77(2.05) 7.85(2.06) 6.62(1.89) (0.296)
-0.51(2.25) 1.14(2.41) 1.02(2.40) 3.20(2.58) 1.02(2.40) 1.71(2.46) (0.048)
0.38(-0.98) 0.91(-0.09) 2.58 (0.95) 1.77 (0.57) 1.30 (0.26) 1.60 (0.47) (0.315)
~Values in parentheses are log transformed values. 2Values in parentheses are log transformed values of ( 10 + X ) where X is the untransformed value.
an increase in the number of water transmission pores (pore diameter > 50 gm ), and therefore of water in filtration ( Lal, 1983; Lee, 1985 ). CONCLUSIONS
Cassava-based cropping systems affected mean weight diameter, organic C and total N contents in earthworm casts such that significantly greater values of the above parameters were observed in casts from three-component crop mixtures than in casts from monocrops or two-component crop mixtures. Cassava-based cropping systems did not have any significant effect on soil physical and chemical properties, except for water infiltration, which was greater with intercropping in comparison with monocropping, although no significant differences were observed between two- and three-component crop mixtures. It appears from these results, therefore, that even after five cropping seasons, the changes in aggregate stability, organic C and total N levels of earthworm casts brought about by the three component crop mixtures used in this study were not reflected in the soil. Cropping systems may, however, affect soil fertility by influencing water-infiltration characteristics and hence, nutrient losses in runoff and erosion. In relation to soil, earthworm casts had higher silt and clay contents, bulk density, mean weight diameter, pH, organic C, total N, C: N ratio, Bray- l-P, exchangeable cations and effective CEC, and lower sand content.
REFERENCES Aina, P.O., Lai, R. and Taylor, G.S., 1979. Effects of vegetal cover on soil erosion on an Alfisol. In" R. Lal and D.J. Greenland (Editors), Soil Physical Properties and Crop Production in the Tropics. Wiley, Chichester, pp. 501-507.
CASSAVA-BASED CROPPING SYSTEMS
63
Ikeorgu, J.E.G., Ezumah, H.C. and Wahua, T.A.T., 1983. Soil moisture and soil temperature variations under a cassava/melon/okra intercropping system. Paper presented at the 6th Symposium of the International Society for Tropical Root Crops, 20-25 February, 1983, Lima, Peru. Klute, A. (Editor), 1986. Methods of Soil Analysis, Part 1: Physical and Mineralogical Methods. Am. Soc. Agron., Madison, WI, 1188 pp. Lal, R., 1983. No-till Farming: Soil and Water Conservation and Management in the Humid and Subhumid Tropics. Monograph No. 2, IITA, Ibadan, Nigeria, 64 pp. Lal, R. and Akinremi, O.O., 1983. Physical properties of earthworm casts and surface soil as influenced by management. Soil Sci., 135:114-122. Lavelle, P., 1975. Consommation annuelle de terre par une population naturelle de vers de terre (Millsonia anomala Omodeo, Acanthodrilae-Oligochetes) dans la savane de Lamto, C6te d'Ivoire. Rev. Ecol. Biol. Sol., 12:11-24. Lee, K., 1985. Earthworms: Their Ecology and Relationships with Soils and Land Use. Academic Press, Sydney, 411 pp. Madge, D.S., 1969. Field and laboratory studies on the activities of two species of tropical earthworms. Pedobiologia, 9:188-214. Moormann, F.R., Lal, R. and Juo, A.S.R., 1975. The Soils of IITA. IITA Techn. Bull. No. 3, IITA, Ibadan, Nigeria, 48 pp. Mulongoy, K. and Bedoret, A., 1989. Properties of worm casts and surface soils under various plant covers in the humid tropics. Soil Biol. Biochem., 21: 197-203. Okigbo, B., 1978. Cropping systems and related research in Africa. Occasional Publication Series OT-1, Association for the Advancement of Agricultural Science in Africa. AAASA, Addis Ababa, Ethiopia, 81 pp. Page, A.L., Miller, R.H. and Keeney, D.R. (Editors), 1982. Methods of Soil Analysis, Part 2: Chemical and Microbiological Properties. Am. Soc. Agron., Madison, WI, 1159 pp. Philip, J.R., 1957. The theory of infiltration: 4. Sorptivity and algebraic infiltration equations. Soil Sci., 84: 257-264. Swaby, R.J., 1950. The influence of earthworms on soil aggregation. J. Soil Sci., 1: 195-197. Terry, E.R., Doku, E.V., Arene, O.B. and Mahungu, N.M. (Editors), 1984. Tropical Root Crops: Production and Uses in Africa: IDRC, Ottawa, Canada, 231 pp. Terry, E.R., Akoroda, M.O. and Arene, O.B. (Editors), 1987. Tropical Root Crops: Root Crops and the African Food Crisis. IDRC, Ottawa, Canada, 197 pp.
55
Elsevier Science Publishers B.V., Amsterdam
Effects of cassava-based cropping systems on physico-chemical properties of soil and earthworm casts in a tropical Alfisol N.R. Hulugalle* and H.C. Ezumah International Institute of Tropical Agriculture, PMB 5320, Oyo Road, lbadan (Nigeria) (Accepted for publication 7 August 1990)
ABSTRACT Hulugalle, N.R. and Ezumah, H.C., 1991. Effects of cassava-based cropping systems on physicochemical properties of soil and earthworm casts in a tropical Alfisol. Agric. Ecosystems Environ., 35: 55-63. The effects of cassava-based intercropping systems and rotations on physical and chemical properties of earthworm casts and the adjacent soil were studied on an Oxic Paleustalf in south-western Nigeria. Earthworm activity was greater with intercropping although it was not significantly affected by the number of component crops in a mixture. The particle size distribution, bulk density, exchangeable cations, Bray-I-P, pH and effective cation exchange capacity (CEC) of soil and earthworm casts did not differ among the cassava-based cropping systems investigated. Greatest values of mean weight diameter, organic C and total N were observed in earthworm casts from three component crop mixtures, although the adjacent soil was not similarly affected. The results suggest that although the cropping system changed some physico-chemical properties of earthworm casts, similar changes did not occur in the adjacent soil. Water infiltration into the soil was, however, increased by intercropping and may be related to earthworm activity. Cropping system may, therefore, influence soil fertility indirectly by changing water infiltration characteristics and hence, nutrient losses in runoff and erosion. In relation to soil, earthworm casts had higher silt and clay contents, bulk density, mean weight diameter, pH, Bray-l-P, Organic C, total N, C : N ratio, exchangeable cations and effective CEC, and lower sand content.
INTRODUCTION
Cassava (Manihot esculenta (Crantz.) is one of the major food crops in the h u m i d and sub-humid zones ofsub-Saharan Africa (Terry et al., 1984, 1987 ). The total area planted to cassava in Africa is 7.48 million ha (52.9% of world area) (Terry et al., 1987). A major proportion of this cassava ( > 9 5 % ) is grown as an intercrop with crops such as maize (Zea mays), sorghum *Address correspondence to: IITA, c / o Ms. M. Larkin, L.W. Lambourne and Co., 26 Dingwall Road, Croydon CR9 3EE, Gt. Britain.
0167-8809/91/$03.50
© 1 9 9 1 - Elsevier Science Publishers B.V.
56
N.R. HULUGALLE AND H.C. EZUMAH
(Sorghum bicolor), yam (Dioscorea alata and D. rotundata), cocoyam ( Xanthosoma sagittifolium ) and melon ( Citrullus lanatus ) ( Okigbo, 1978; Terry et al., 1984, 1987 ). Research work in the past has concentrated primarily on yield and other crop growth aspects of such cassava-based cropping systems (Okigbo, 1978; Terry et al., 1984, 1987 ). The effects of cassava-based cropping systems on the soil environment have been little studied. The few studies conducted have shown that in relation to monocropping, intercropping cassava increases water infiltration and decreases water runoff and soil erosion (Aina et al., 1979). This has been ascribed to protection of the soil surface from the compactive effects of high intensity rainfall (Aina et al., 1979) and greater earthworm activity (Lal, 1983 ). Increased earthworm activity results in a high proportion of water transmission pores which in turn lead to greater water infiltration rates and hence, lower erosion (Lee, 1985). Increases in soil water content and decreases in soil temperature also occur concurrently in cassava-based intercropping systems (Ikeorgu et al., 1983). In addition, soil fertility decreases are reported to occur at a lower rate, particularly in crop mixtures of three or more species (Okigbo, 1978). Lower soil erosion and water runoff, and hence, lower nutrient losses are likely to be the primary cause for the lower rate of fertility decrease in such cropping systems. A secondary cause may be the accumulation of higher levels of nutrients in earthworm casts voided in intercropping systems of three or more species. The breakdown of these casts over time could result in the release of nutrients to the adjacent soil (Lee, 1985 ). Hence, long-term management of soil fertility in cassavabased cropping systems would appear to require detailed information on the physical and chemical characteristics of earthworm casts voided therein. The objective of the present study, therefore, was to quantify the effect of cassavabased cropping systems on physico-chemical properties of earthworm casts and adjacent soil in a tropical Alfisol. MATERIALS AND METHODS
The experiment was conducted at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria ( 7 ° 30' N, 3 ° 54' E). The IITA is located in the forest-savannah mosaic zone of south-western Nigeria. Mean annual rainfall is 1250 m m . Rains commence in late March and continue until late July. Following a short dry spell of approximately one m o n t h the rains recommence in late August and continue until early November. The dry season lasts from November to March. The soil at the experimental site is a kaolinitic, skeletal, isohyperthermic, Oxic Paleustalf (Ferric Luvisol) belonging to the Ibadan series with a sandy loam topsoil and a sandy clay subsoil ( M o o r m a n et al., 1975). Particle size analysis showed that the surface 0.15 m consisted of 71% sand, 12% silt and
CASSAVA-BASEDCROPPING SYSTEMS
57
17% clay. Effective cation exchange capacity (CEC) ( 1N ammonium acetate extractable) was 108.4 mmol ( + ) kg -1, with 69.2, 22.1, 11.1, 4.8 and 1.2 mmol ( + ) kg-1 being contributed by Ca, Mg, K, Na and total acidity ( A I + H ) , respectively. Soil pH ( H 2 0 ) , Bray-l-P and total N were 6.7, 73.7 mg kg- 1 and 0.22%, respectively. Three cassava-based rotation systems in two phases were used in the trial, which had been established since 1985. Table 1 summarizes the crop combinations, rotations, their respective chronologies and populations. The experimental design was a randomized complete block design with four replications. Plot size was 15 m × 5 . 5 m. Prior to commencement of the trial in 1985 the site was disc-harrowed to a depth of 0.20 m. Thereafter no-tillage (Lal, 1983 ) was used in all treatments except when yam was planted, when ridges 0.30 m high and 0.75 m apart were constructed. Weeds were controlled by spraying 'Galex' (metabromouran + metolachlor) at a rate of 3 kg a.i. ha- ~ before planting. Fertilizer was applied at planting to all plots at rates of 45 kg N, 60 kg P and 60 kg K hain the forms of calcium ammonium nitrate, single superphosphate and muriate of potash. Soil bulk density was determined on undisturbed cores, 50 mm high and 51 TABLE 1
Crop combinations, rotations, and their respective chronologies and populations during the study ( + = intercrop; - = rotation )
Rotation systems
Phases Years 1985
Plant population ( m -2) 1986
Cycle 1 A
A~
Cassava
1987
Cycle 2 Maize-
Cassava
cowpea A2
Maize-
Cassava
cowpea B
B~ B2
C
C~
C2
1988
1989
Cycle 3 Maize-
Cassava
cowpea Maize-
Cassava
cowpea
Maize-
cowpea
Cassava = Cowpea = Cassava = Cowpea =
1; Maize = 5.3 6.2 1; Maize = 5.3 6.2
Maize+ cassava Yam+
Yam+ cowpea Maize+
Maize+ cassava Yam+
Yam+ cowpea Maize+
cowpea
cassava
cowpea
cassava
Maize+ M a i z e = 4; C a s s a v a = 1 cassava Yam = 1; Cowpea = 6.2 C a s s a v a + M a i z e = 4 ; C a s s a v a = 1; cowpea Yam = 1; Cowpea = 6.2
Maize+ cassava+
Maize+ cassava+
Maize+ cassava+
Maize+ cassava+
Maize+ cassava+
melon
okra
melon
okra
melon
Maize + cassava+
Maize + cassava+
Maize + cassava+
Maize + cassava+
Maize + cassava+
okra
melon
okra
melon
cowpea
M a i z e = 4; C a s s a v a = 1; M e l o n = 2; O k r a = 3.0 Maize = 4; Cassava = 1; Melon =2; O k r a = 3.0 Cowpea = 6.2
58
N.R. HULUGALLE AND H.C. EZUMAH
m m in diameter, taken at r a n d o m from five locations in the 0-0.05 m and 0.05-0.10 m depth of each plot. Composite soil samples collected at r a n d o m in the 0-0.10 m depth from 10 locations in each plot were air-dried, ground and analyzed for particle size distribution (hydrometer m e t h o d ) (Klute, 1986), organic carbon (dichromate oxidation), pH ( 1 : 1 soil:water suspension), total N (Kjeldahl analysis), Bray-l-P and 1N a m m o n i u m acetate-extractable Ca, Mg, Mn, K and Na, and 1N KCl-extractable total acidity (A1 + H ) (Page et al., 1982 ). Aggregate stability was determined by wet sieving air-dried, unground samples and expressed as the mean weight diameter of the aggregates (Klute, 1986 ). Soil sampling for physical and chemical analyses was carried out during September 1989. Earthworm activity was measured as the mass of earthworm casts per unit area of a 10 m × 3 m area in each plot during November 1988 and September 1989. The worm casts collected were the columnar casts of Hyperiodrilus africanus (Madge, 1969), which was the predominant earthworm species at the experimental site. Except for bulk density, the previously described physical and chemical analyses were also performed on earthworm casts collected during September 1989. Bulk density of earthworm casts collected during the same period was determined by the water displacement m e t h o d (Lal and Akinremi, 1983 ). Water infiltration was measured during September with a double-ring infiltrometer at one location in each plot. The results of the infiltration tests were analyzed according to Philip's equation (1957) to compute water sorptivity and transmissivity. RESULTS AND DISCUSSION
Earthworm activity Earthworm activity measured as the mass of earthworm casts per unit area did not differ significantly between years, but was greater ( P < 0.01 ) with intercropping (Table 2). There were no significant differences between twoand three-component crop systems. Phase 2, especially for the two and three crop mixtures tended to amass more earthworm casts than the monocrop sequence, though the difference was not significant. Increase in casting activity arises from cooler and wetter soil in intercropped systems (Ikeorgu et al., 1983). Supra-optimal soil temperatures for earthworm activity occur frequently in monocropped cassava systems (Lavelle, 1975; Ikeorgu et al., 1983 ).
Physical and chemical properties In relation to soil in the 0-0.10 m depth, silt and clay contents, and chemical properties except total acidity were significantly greater ( P < 0.001 ) and sand content lower ( P < 0.001 ) in earthworm casts (Table 3 ). Cropping sys-
59
CASSAVA-BASEDCROPPING SYSTEMS TABLE 2 Effect of cropping system on mass of earthworm casts per unit area Sampling time
November 1988 September 1989 Phase means 2 Rotation means
Mean
Mass of earthworm casts (t ha-~ )
Aj
A2
Bi
B2
CI
C2
0.10 0.16 0.25
0.44 0.41 0.30
0.75 1.14 0.78
1.46 0.81 1.30
0.37 1.24 0.76
0.94 1.15 1.09
+SE Between years Between rotation means Between phases
0.92
1.04
0.28
0.68 0.82
0.094 0.125 0.217
2AI in 1988 carries the same crop as A2 in 1989. The phase mean is therefore the average of the 2 years. Phase means for systems B and C were calculated following the same procedure. TABLE3 Comparative sand, silt and clay contents, and chemical properties of earthworm casts and soil in the 0-0.10 m depth Soil property
Soil
Earthworm casts
_+SE
Sand (%) Silt (%) Clay (%) pH Organic C (%) Total N (%) Bray-l-P (mg kg-~ ) Exchangeable Ca (mmol ( + ) kg-~ ) Exchangeable Mg (mmol ( + ) kg- l ) Exchangeable Mn (mmol ( + ) kg -~ ) Exchangeable K ( mmol ( + ) kg- i ) Exchangeable Na (mmol ( + ) kg -~ ) Total acidity (mmol ( + ) kg- l ) Effective CEC (mmol ( + ) kg - t )
75.6 11.9 12.5 5.5 0.83 0.09 15.7 36.6 4.1 1.9 3.4 2.4 1.6 50.0
51.5 22.2 16.4 6.4 2.53 0.24 33.8 89.6 11.3 3.2 6.4 4.2 2.1 116.8
0.41 0.23 0.32 0.07 0.030 0.003 1.53 3.05 0.91 0.10 0.17 0.16 0.19 3.21
JMean of all cropping systems.
tem did not have any significant effect on the same soil properties. In relation to Rotation systems A and B, significantly higher (P < 0.01 ) values of organic C and total N were also observed in earthworm casts from Rotation system C, i.e. the three-component crop system, but not in adjacent soil (Table 4). C" N ratios of soil (9.2) were, however, significantly lower ( P < 0.01 ) than
60
N.R. HULUGALLEAND H.C. EZUMAH
TABLE 4 Effect of cropping system on organic C and total N contents of earthworm casts and soil in the 0-0.10 m depth Phases in rotation systems
Source
A~
Soil Earthworm Soil Earthworm Soil Earthworm Soil Earthworm Soil Earthworm Soil Earthworm
A2 Bi B2 C~ C2
cast cast cast cast cast cast
_+SE Between cropping systems Between sources Between sources for same cropping system Between sources for different cropping systems
Organic C (%)
Total N (%)
0.78 2.22 0.56 2.44 0.80 2.18 0.93 2.55 1.02 2.92 0.89 2.90
0.08 0.19 0.06 0.23 0.08 0.20 0.11 0.26 0.10 0.27 0.09 0.27
0.156 0.030 0.074 0.164
0.014 0.003 0.008 0.015
those of earthworm casts ( 10.50, _+SE = 0.06 ). Mulongoy and Bedoret ( 1989 ) in contrast reported that C : N ratios of earthworm casts obtained from Alfisols were significantly lower than those of adjacent soil. Soil organic C and total N values reported by these authors were, however, greater than the values observed in the present study. Furthermore their samples were obtained under perennial vegetation (>__6 years) whereas those of the present study were obtained below annual crops. Differences in soil properties and vegetation may, therefore, have contributed to the relative differences in C : N ratios. The higher values of silt, clay, exchangeable cations and effective CEC, and lower values of sand in earthworm casts may be caused by preferential ingestion of silt and clay particles by earthworms (Lee, 1985 ). Cropping system did not have any significant effect on bulk densities of either soil or earthworm casts. Bulk density of soil was, however, significantly lower ( P < 0.01 ) than that of earthworm casts. Bulk densities of earthworm casts, and soil in the 0-0.05 m and 0.05-0.10 m depths were 1.68, 1.40 and 1.48 Mg m -3, respectively ( + SE----0.025). The differences in bulk density were related to the silt and clay contents of soil and earthworm casts thus B D = 1.28 exp[5.55 X 1 0 - 3 ( S i + C ) ]
r2=0.62 ***, n = 4 8
61
CASSAVA-BASEDCROPPING SYSTEMS
where BD is mean bulk density in the 0-0.10 m depth (Mg m -3) and Si and C are silt and clay contents, respectively, in the 0-0.10 m depth (%). Mean weight diameter ( M W D ) of soil aggregates was not significantly affected by cropping system, but that of earthworm casts was ( P < 0.01 ) such that MWD increased as the number of component crops in a mixture increased (Table 5 ). Complex crop mixtures such as those used in the present study may provide organic and inorganic nutrients of high quality for the intestinal bacterial populations of the earthworm and hence, cause an increase in their numbers and activity. Secretions of intestinal bacteria perform a primary function in increasing the structural stability of earthworm casts (Swaby, 1950). Comparison of MWD of soil and earthworm casts also indicates that the latter was significantly greater ( P < 0.001 ) (Table 5 ). The differences in MWD of soil aggregates and earthworm casts observed in the present study were related primarily to their organic carbon contents (OC, %) thus MWD=0.99 exp (0.42 OC)
r2=0.76 ***, n=48
where MWD is mean weight diameter (in mm). Water infiltration was significantly affected by cropping system, with infiltration rates and transmissivities of intercropped plots being greater (Table 6 ). Sorptivity was not significantly affected by cropping system. The higher transmissivities and infiltration rates are primarily caused by increases in earthworm activity, and hence, the number of earthworm burrows in intercropped treatments. Increases in the number of earthworm burrows results in TABLE5 Effect of cropping system on aggregate stability of soil in the 0-0.10 m depth and earthworm casts (expressed as the mean weight diameter) Phases in rotation systems
Ai A2 Bl B2
CI C2 Mean _+SE Between cropping systems Between sources Between sources for same cropping system Between sources for different cropping systems
Mean weight diameter ( m m ) Soil
Earthworm cast
1.21 1.17 1.40 1.48 1.56 1.33 1.36
1.99 2.81 2.98 3.32 3.93 4.04 3.18
0.193 0.092 0.226 0.251
62
N.R. HULUGALLE AND H.C. EZUMAH
TABLE 6 Effect of cropping system on infiltration characteristics during September 1989 Phases in rotation systems
Sorptivity ~ (ram m i n - ~/2 )
Transmissivity / (mm m i n - t )
Infiltration rate ~ (mm m i n - ~)
A, A2 B, B2 C, C2 ±SE
8.76(2.17) 10.28(2.33) 18.54(2.92) 7.77(2.05) 7.85(2.06) 6.62(1.89) (0.296)
-0.51(2.25) 1.14(2.41) 1.02(2.40) 3.20(2.58) 1.02(2.40) 1.71(2.46) (0.048)
0.38(-0.98) 0.91(-0.09) 2.58 (0.95) 1.77 (0.57) 1.30 (0.26) 1.60 (0.47) (0.315)
~Values in parentheses are log transformed values. 2Values in parentheses are log transformed values of ( 10 + X ) where X is the untransformed value.
an increase in the number of water transmission pores (pore diameter > 50 gm ), and therefore of water in filtration ( Lal, 1983; Lee, 1985 ). CONCLUSIONS
Cassava-based cropping systems affected mean weight diameter, organic C and total N contents in earthworm casts such that significantly greater values of the above parameters were observed in casts from three-component crop mixtures than in casts from monocrops or two-component crop mixtures. Cassava-based cropping systems did not have any significant effect on soil physical and chemical properties, except for water infiltration, which was greater with intercropping in comparison with monocropping, although no significant differences were observed between two- and three-component crop mixtures. It appears from these results, therefore, that even after five cropping seasons, the changes in aggregate stability, organic C and total N levels of earthworm casts brought about by the three component crop mixtures used in this study were not reflected in the soil. Cropping systems may, however, affect soil fertility by influencing water-infiltration characteristics and hence, nutrient losses in runoff and erosion. In relation to soil, earthworm casts had higher silt and clay contents, bulk density, mean weight diameter, pH, organic C, total N, C: N ratio, Bray- l-P, exchangeable cations and effective CEC, and lower sand content.
REFERENCES Aina, P.O., Lai, R. and Taylor, G.S., 1979. Effects of vegetal cover on soil erosion on an Alfisol. In" R. Lal and D.J. Greenland (Editors), Soil Physical Properties and Crop Production in the Tropics. Wiley, Chichester, pp. 501-507.
CASSAVA-BASED CROPPING SYSTEMS
63
Ikeorgu, J.E.G., Ezumah, H.C. and Wahua, T.A.T., 1983. Soil moisture and soil temperature variations under a cassava/melon/okra intercropping system. Paper presented at the 6th Symposium of the International Society for Tropical Root Crops, 20-25 February, 1983, Lima, Peru. Klute, A. (Editor), 1986. Methods of Soil Analysis, Part 1: Physical and Mineralogical Methods. Am. Soc. Agron., Madison, WI, 1188 pp. Lal, R., 1983. No-till Farming: Soil and Water Conservation and Management in the Humid and Subhumid Tropics. Monograph No. 2, IITA, Ibadan, Nigeria, 64 pp. Lal, R. and Akinremi, O.O., 1983. Physical properties of earthworm casts and surface soil as influenced by management. Soil Sci., 135:114-122. Lavelle, P., 1975. Consommation annuelle de terre par une population naturelle de vers de terre (Millsonia anomala Omodeo, Acanthodrilae-Oligochetes) dans la savane de Lamto, C6te d'Ivoire. Rev. Ecol. Biol. Sol., 12:11-24. Lee, K., 1985. Earthworms: Their Ecology and Relationships with Soils and Land Use. Academic Press, Sydney, 411 pp. Madge, D.S., 1969. Field and laboratory studies on the activities of two species of tropical earthworms. Pedobiologia, 9:188-214. Moormann, F.R., Lal, R. and Juo, A.S.R., 1975. The Soils of IITA. IITA Techn. Bull. No. 3, IITA, Ibadan, Nigeria, 48 pp. Mulongoy, K. and Bedoret, A., 1989. Properties of worm casts and surface soils under various plant covers in the humid tropics. Soil Biol. Biochem., 21: 197-203. Okigbo, B., 1978. Cropping systems and related research in Africa. Occasional Publication Series OT-1, Association for the Advancement of Agricultural Science in Africa. AAASA, Addis Ababa, Ethiopia, 81 pp. Page, A.L., Miller, R.H. and Keeney, D.R. (Editors), 1982. Methods of Soil Analysis, Part 2: Chemical and Microbiological Properties. Am. Soc. Agron., Madison, WI, 1159 pp. Philip, J.R., 1957. The theory of infiltration: 4. Sorptivity and algebraic infiltration equations. Soil Sci., 84: 257-264. Swaby, R.J., 1950. The influence of earthworms on soil aggregation. J. Soil Sci., 1: 195-197. Terry, E.R., Doku, E.V., Arene, O.B. and Mahungu, N.M. (Editors), 1984. Tropical Root Crops: Production and Uses in Africa: IDRC, Ottawa, Canada, 231 pp. Terry, E.R., Akoroda, M.O. and Arene, O.B. (Editors), 1987. Tropical Root Crops: Root Crops and the African Food Crisis. IDRC, Ottawa, Canada, 197 pp.