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Impact of land clearing methods and cropping systems on labile soil C and N pools in the humid zone Forest of Nigeria
Agriculture, Ecosystems and Environment 120 (2007) 250–258 www.elsevier.com/locate/agee
Impact of land clearing methods and cropping systems on labile soil C and N pools in the humid zone Forest of Nigeria I.K. Okore a,*, H. Tijani-Eniola b, A.A. Agboola b, E.A. Aiyelari b a
Rubber Research Institute of Nigeria, P.M B 1049, Benin City, Edo State, Nigeria b Department of Agronomy, University of Ibadan, Ibadan, Nigeria
Received 27 August 2005; received in revised form 28 June 2006; accepted 18 September 2006 Available online 7 November 2006
Abstract Labile soil C and N play vital roles in soil–plant nutrient dynamics, especially in the low input cropping system and are vulnerable to perturbation. Surface (0–0.15 m) soils from three land clearing methods (slash and burn, bulldozed non-windrowed and bulldozed windrowed) and each with two cropping systems (5-and 4-year cropping/2-year cassava fallow) were collected in the humid forest ecosystem of Nigeria. The soils were analysed for total C and N, microbial biomass C and N (SMB C and N), particulate organic matter C and N (POM C and N), water-soluble C, potentially mineralizable N (PMN) and mineral N. The size of the labile C and N and their relative contributions to the organic C and total N differed significantly among land clearing methods, irrespective of the cropping system. Soils under slash and burn had a significantly ( p > 0.05) higher particulate organic matter C, N (10.80 and 0.16 g kg1, respectively) and microbial biomass C and N (1.07 and 0.12 g kg1) compared to the bulldozed windrow, regardless of the cropping system. Four years cropping/2-year cassava fallow resulted in a significant higher labile C and N, relative to 5-year cropped plots across the land clearing methods. Effect of the treatments on the concentration of PMN and mineral N mirrored the SMB N and POM N. However, the quantity of most of the labile C and N pool and crop yield obtained from the slash and burn and bulldozed non-windrowed treatment did not differ significantly. Hence, bulldozed non-windrowed clearing could be a viable alternative to slash and burn in the case of large-scale farming in ensuring reduced losses of soil organic matter and nutrient during land clearing in the humid tropics. # 2006 Elsevier B.V. All rights reserved. Keywords: Soil fertility; Cropping system; Deforestation; Soil organic matter; Cassava fallow; Particulate organic matter
1. Introduction The humid forest eco-region of southern Nigeria witnessed a rapid conversion into agricultural use in the mid 1990s, following the establishment of the National Agricultural Land Development Authority (NALDA) by the then Federal Military Government. Most of the areas were cleared mechanically and could not sustain arable crop production even for three consecutive years without severe soil loss and degradation (Agboola et al., 1998). Mechanical land clearing has an adverse effect on soil physical and * Corresponding author. Tel.: +234 8035728268. E-mail address: [email protected] (I.K. Okore). 0167-8809/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2006.09.011
chemical conditions more especially in the humid forest ecosystem (Seubert et al., 1977; Hulugalle, 1994). However, the intensity of the effect varies with the method deployed, with those involving a combination of tree pushing and the root rake being more damaging than the shear blade clearing method alone (Lal, 1986). Traditionally, in the lowland humid tropics, the conversion of forest or fallow land into agricultural use involves felling and burning of the vegetation—manual clearing (Lal, 1986). The method ensures minimal soil disturbance and degradation (Seubert et al., 1977) and the cleared area can sustain economic crop yield for more than 4 years before being fallowed. However, the prohibitive amount of labour and time required to accomplish manual land clearing has
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made most people, especially large-scale farmers to opt for mechanized land clearing method without considering the consequences on the soil environment. Degradation in soil quality often associated with the conversion of primary or secondary forest into low input agroecosystem is mainly due to decline in soil organic matter (Sikora et al., 1996; Smith et al., 2000). The rapid initial losses are mainly the mineralization of the biologically labile or active pools: microbial biomass, particulate organic matter, soil carbohydrate and enzymes and water-soluble C (Martins et al., 1991; Lugo and Brown, 1993). Decreases of more than 47 and 24% in particulate matter and microbial biomass C have been recorded in less than a year after clearing of a secondary forest (Motavalli et al., 2000). Islam and Weil (2000) reported a labile C loss of about 67–167% in soils under 5–7 years continuous cropping. Their rate of losses is among other factors affected by land use and management strategies (Zak et al., 1993; Feller and Beare, 1997). Labile organic matter pools have a significant positive correlation with soil nutrient capital stock (Sikora et al., 1996; Haynes, 2000). Evaluation of changes in their quantity could be an early indicator of land use and management effect on the soil environment. The aim of this study was to evaluate the impact of three land clearing methods and subsequent uses on the soil labile C and N pools of the humid forest eco-region of southwestern Nigeria.
2. Materials and methods
251
Table 1 Monthly mean maximum, minimum (mm) and standard deviation of rainfall at Epemakinde, between 1980 and 2000 Month
Maximum
Minimum
Mean (n = 20)
S.D.
January February March April May June July August September October November December
31.2 133.0 223.9 225.0 284.1 337.9 442.7 211.5 379.2 375.9 145.8 56.4
5.3 16.0 8.5 53.6 174.4 185.9 102.7 39.9 171.5 159.2 19.6 7.4
20.9 34.5 90.9 153.7 217.2 265.7 317.0 127.1 256.4 212.8 55.8 9.1
12.9 23.0 82.8 80.1 54.8 52.8 120.1 65.3 192.1 204.4 125.9 51.5
n = 20.
Pycnanthus microcephalus (Beuth.), Staudtia stipitata (Warb.), Marathes glabra (Oliv.), Sacoglottis gabonensis (Baill), Elaeophorbia drupifera (Thonn.), Eribroma oblonga (Mast.) and Ceiba pentandra (Linn.) having a girth of between 0.25 and 0.30 m at 1.5 m height and the average tree density of >1000 ha1. The experimental consist of a factorial combination of three land clearing methods (slash and burn, bulldozed nonwindrowed and bulldozed windrowed) and two cropping systems (5 years maize/cassava inter crop and 4 years maize/ cassava intercrop followed by 2-year cassava fallow). A randomized complete block design was used with three replicates. Each of the experimental unit measured 0.49 ha.
2.1. Site description and field experiment A field experiment was established in 1994 at the project site of the Department of Agronomy, University of Ibadan located at the Ondo State Aforestation Project in Epemakinde to compare the conventional (slash and burn) and two mechanical (bulldozed non-windrowed and bulldozed windrowed) methods of bush clearing for arable cropping in the humid forest area of Nigeria. Epemakinde (48E, 68N) is characterized by a tropical humid rain forest climate. The rainfall pattern is bimodal, with a long (April–July) and short August–October) rainy period separated by a short period of dryness between late July and early August (Table 1). The temperature ranges between 26 and 28 8C with a relative humidity of 65–80%. The soil possesses a sandy loam in the upper layer over a well-drained sandy clay/clay loam sub soil (Agboola and Ogunkunle, 1993) which is classified in the USDA system as Typic Kandiudult (an ultisol). The soil textural analysis of samples from the adjacent matured high forest showed on average, 71.2 g 100 g1 sand, and 26.3 g 100 g1 silt and 2.5 g 100 g1 clay and bulk density was 1.06 g cm3 at 0– 0.15 m depth. Some of the chemical and biochemical properties of the soil are shown in Table 2. The vegetation at the beginning of the experiment was a matured high forest (>100 years old) with some identifiable tree species such as
Table 2 Some of the chemical and biochemical properties of the adjacent matured high forest soil (0–0.15 m) Chemical and biochemical properties
Value
Organic C (g kg1) Total N (g kg1) C/N ratio pH (H2O) Available P (mg kg1) Bray 1
49.78 3.25 18.39 6.40 8.20
Exchangeable cations (cmol kg1) Ca Mg K Na Total exchangeable acidity CEC Particulate organic matter C (g kg1) Particulate organic matter N (g kg1) Soil microbial biomass C (g kg1) Soil microbial biomass N (g kg1) Field moist water-soluble C (g kg1) Air-dried soil-soluble C (g kg1) Potentially mineralizable N (mg (kg1 day1)) NO3 N (mg kg1) NH4+ N (mg kg1) POM C/C org (%) SMB C/C org (%)
6.43 0.12 0.29 0.17 0.20 7.21 25.60 1.35 5.82 1.17 0.30 19.15 2.63 24.22 55.69 51.42 11.69
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The slash and burn clearing involved cutting the undergrowth and vines of the forest vegetation with machetes and felling of the trees at waist height with axes. Big trees were cut down with hand operated power chain saw. Following drying, the cut down biomass was burnt in situ and the ash evenly distributed within the plot. For the bulldozed non-windrowed clearing method, an A.D 83E Komatsu model bulldozer was used to knock down the trees. The knocked down trees were cut into logs of between 2 and 3 m long and rolled to the alley. Other debris like leaves; twigs and roots were also packed to the alley. The bulldozed windrowed clearing was accomplished with a similar bulldozer model, but the bulldozer after knocking down the trees, ripped out the roots and stumps with its shear blade and windrowed the knocked down trees, roots and other debris to the alley. The cleared plots were later subjected to different cropping systems before soil sampling for labile organic matter determination. The crops were sown on flat (without any form of tillage) and weeding was done manually, three times each year with hoe. There was no other form of burning in any of the plots during the cropping period except the initial burning in the slash and burn treatment. None of the treatments received any form of fertilizer.
treatment was dispersed in 60 ml of 5 g1 of NaP(O3)6 by shaking overnight in a reciprocating shaker. The dispersed samples were sieved through 53 mm sieves. The slurry that passed through the 53 mm sieve was evaporated overnight in an oven at 50 8C. The oven-dried slurry was finally analysed for total N and organic C. The difference between the C and N values of the evaporated slurry and those of the nondispersed (whole soil) sample was considered as the particulate organic matter C and N. Soil microbial biomass C and N were determined by chloroform fumigation incubation method (Anderson and Ingram, 1989). A 10 g (oven dry weight equivalent) of field moist soil from each treatment was measured into each of two 50 ml glass beakers. The soil in one of the beakers was fumigated with 30 ml alcohol free chloroform in vacuum desiccators. After evaporating the chloroform by several repeated evacuation of the desiccators, the fumigated soils were incubated for 5 days. After the 5th day, 50 ml of 0.5 M K2SO4 was used to extract the dissolved organic C and N in the fumigated and un-fumigated samples. The extracted dissolved organic C was determined by titration with acidified Ferrous ammonium sulphate solution, while the dissolved N was determined by micro Kjeldahl method. Soil microbial biomass C and N were calculated as
2.2. Soil sampling and preparation for analysis
SMBC ¼ extracted C ðfumigatedÞ
A composite sample (0–0.15 m depth) consisting of 65 cores was collected in a grid pattern from within the 75 m 65 m experimental unit. Similar pattern was used in sampling the adjacent matured (>100 years) high forest. The samples from the various land clearing methods cropped 5 years were collected in May 1999, while those of the 4-year cropping/2-year cassava fallow were taken a year later (at the completion of the second year of the fallow). Coarse organic matter and plant roots were removed manually. Field moist samples were sieved (<2 mm) and stored at 4 8C for the determination of soil microbial biomass N and C, field moist soil water-soluble C, potentially mineralizable nitrogen and mineral nitrogen pools not latter than 14 days after collection. Sub samples were air-dried for the determination of particulate organic matter C and N. 2.3. Determination of total N, organic C, particulate organic C and N, soil microbial biomass C and N and water-soluble C Organic C content of the soil was measured by the method of Nelson and Sommer (1982) using the modified Mebius procedure involving the use of 0.5N K2Cr2O7, H2SO4 and 0.2N Fe (NH4)2(SO4)26H2O with N-phenylanthranilic indicator, while the total N was determined by Kjeldahl digestion and distillation procedure (Bremner, 1965). Soil particulate organic matter C and N were determined by the method of Cambardella and Elliott (1992). Briefly, 20 g of sub samples (>2 mm) from each
extracted C ðun-fumigatedÞ 2:64 SMBC ¼ extracted N ðfumigatedÞ extracted N ðun-fumigatedÞ 1:46 where 2.64 and 1.46 are constants for C and N, respectively (Brookes et al., 1985). Water-soluble C in field moist and air-dried soil subsamples was extracted by shaking and centrifuging 10 g of soil (oven dry weight) for 15 and 10 min, respectively, in 20 ml of distilled water (Haynes, 2000). The supernatants were analysed for dissolved organic C by dichromate oxidation procedure involving 30 min heating under reflux condition. 2.4. Potentially mineralizable and mineral N determination Potentially mineralizable N was determined by anaerobic incubation of sub-samples of fresh soil. 12 ml of distilled water was added to 5 g of soil and incubated for 7 days at 40 8C (Rowell, 1992). Ammonium-N was extracted by shaking for 1 h after adding 15 ml of 4 M KCl. Potentially mineralizable N was calculated as the difference in NH4+ N content of pre- and post-incubated samples divided by the number of days of incubation. Mineral N was extracted by shaking 10 g of sub samples of fresh soil in 100 ml of 2 M KCl for 1 h. Extracts were analysed for NH4+ and NO3 by automated colorimeter using a Technicon auto analyzer.
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2.5. Statistical analysis Statistical significance of treatment effects on whole soil C, total N, particulate organic matter C and N, soil microbial biomass C and N, water-soluble C, potentially mineralizable N and mineral N in the soil were determined by using twoway analysis of variance (ANOVA). Where a significant ( p > 0.05) treatment effect was found, LSD was calculated to compare treatment mean effect.
3. Results 3.1. Whole soil organic C, total N and C/N ratio The concentration of whole soil organic C, total N and C/ N ratio differed significantly ( p > 0.05) amongst soils under different land clearing methods and cropping systems (Table 3). Irrespective of the cropping system, the highest decline in organic C (31%) and total N (45%) among the land clearing methods compared to the adjacent mature high forest (Table 2) was recorded from the bulldozed windrowed
Mean LSD (0.05)
26.03 25.31
17.75
15.83
Lcm Cs: NS
3.3. Labile N pools: particulate organic matter N (POM N), microbial biomass N (SMB N), potential mineralizable N (PMN) and mineral N (NO3 N and NH4+ N)
Total N (g kg1) Slash and burn 2.10 Bulldozed non- 2.10 windrowed Bulldozed 1.43 windrowed Mean LSD (0.05)
1.88 Lcm: 0.32
C/N ratio Slash and burn 11.52 Bulldozed non- 10.85 windrowed Bulldozed 10.30 windrowed Mean LSD (0.05) NS: not significant.
Mean
5 years maize/ 4 years maize/cassava cassava intercrop followed by 2 intercrop years cassava fallow
19.97 Lcm: 4.71
10.91 Lcm: 0.94
3.2. Labile C pools: particulate organic matter C (POM C) soil microbial biomass C (SMB C) and water-soluble C
28.95 27.71
Cropping system (Cs)
Organic C (g kg1) Slash and burn 23.10 Bulldozed non- 22.90 windrowed Bulldozed 13.90 windrowed
clearing method. Among the cropping systems, a significantly higher levels of organic C (24.80 g kg1) and total N (2.04 g kg1) was obtain for soil cropped for 4 years/2 years cassava fallow compared to those of 5 years, across the land clearing methods. However, the values obtained from the slash and burn did not differ significantly from that recorded from the bulldozed non-windrowed under similar cropping system. The highest C/N ratio value (12.32), among the land clearing methods regardless of the cropping systems was obtained from the slash and burn soils, while the lowest (7.33) came from the bulldozed windrowed soil. Although, these values differed significantly ( p > 0.05) from each other, they did not differ from the values obtained from one obtained from the bulldozed non-windrowed treatment. The effect of cropping systems on the C/N ratio differed significantly; with the highest mean value being recorded from plots under 4-year cropping/2-year cassava fallow.
The particulate organic matter C (1.07 and 0.24 g kg1, respectively) obtained from slash and burn plot were more than two folds greater than those obtained from bulldozed windrowed plot, irrespective of the cropping system. Among the cropping systems, 4-year cropping/2-year cassava fallow plots had 61% more POM C and 41% more SMB C values than those under 5-year continuous cropping, with the highest value being recorded from slash and burn clearing method (Table 4). The concentration of field moist and airdried water-soluble C, across the land clearing methods mirrored the particulate organic matter and microbial biomass C. However, the effect of cropping system did not differ significantly.
Table 3 Impact of land clearing methods and cropping system on whole soil organic C, total N and C/N ratio Land clearing method (Lcm)
253
24.80 Cs: 3.34
2.45 2.15
2.28 2.11
1.51
1.47
2.04 Cs: NS
Lcm Cs: NS
13.07 12.89
12.32 11.87
11.68
7.33
12.55 Cs: NS
Lcm Cs: NS
As indicated in Table 5, land clearing methods effect on labile N pools differed significantly ( p > 0.05). The POM N ranged from a mean of 0.61 g N kg1 soils in the slash and burn plot to 0.25 g N kg1 soil in bulldozed windrowed plot. The value obtained from the bulldozed windrowed treatment represented a decline of 81% from that of the matured high forest and the highest decline recorded among the land clearing methods. The POM N content of the soil across the cropping systems did not differ significantly, although, 4year cropping/2-year cassava fallow resulted in a 53% more POM N compared to 5-year continuous cropping treatment across the land clearing methods. The mean level of SMB N (1.01 g N kg1 soil) recorded from the slash and burn was significantly higher than those of the bulldozed windrowed plots, irrespective of the cropping system. The order of decline in SMB N amongst the cropping systems relative to the value recorded from the adjacent
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Table 4 Impact of land clearing methods and cropping system on labile soil C (g kg1) Land clearing method (Lcm)
Particulate organic matter C Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05) Soil microbial biomass C Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05) Field moist soil water-soluble C Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05) Air-dried soil water-soluble C Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05)
Cropping system (Cs)
Mean
5 years maize/cassava intercrop
4 years maize/cassava intercrop followed by 2 years cassava fallow
7.70 5.40 3.10
13.75 7.60 4.80
10.80 6.50 4.00
5.40 Lcm: 1.40
8.70 Cs: 2.90
Lcm Cs: NS
0.88 0.91 0.28
1.25 1.10 0.56
1.07 1.01 0.42
0.69 Lcm: 0.5
0.97 Cs: 0.1
Lcm Cs: NS
0.20 0.14 0.07
0.27 0.17 0.08
0.24 0.16 0.08
0.14 Lcm: 0.06
0.17 Cs: NS
Lcm Cs: NS
3.81 3.97 2.01
3.337 3.37 2.33
3.57 3.67 2.17
3.26 Lcm: 1.43
3.01 Cs: NS
Lcm Cs: NS
NS: not significant.
matured high forest (Table 2) was 5-year cropping > 4-year cropping/2-year cassava fallow. The potentially mineralizable N (PMN) content of the soil under different land clearing methods and cropping systems were relatively low relative to the concentration observed in the adjacent matured high forest. Among the cropping systems, soil under 4-year cropping/2-year cassava fallow doubled the amount of PMN recorded in 5-year continuous cropping. The mineral nitrogen of the adjacent soil under matured high forest was found to be 24.22 and 55.69 mg kg1 for the NO3 N and NH4+ N, respectively (Table 2). The nitrate nitrogen (NO3–N) concentration in the slash and burn soil was 16 and 82% higher than those of the bulldozed non-windrowed and the bulldozed windrowed plots in that order, regardless of the cropping system. The NO3 N level in 5-year cropped plots was 33% lower than that of the 4-year cropping/2-year cassava fallow. The effect of the treatment of NH4+ N followed the same trend as NO3 N. 3.4. Proportion of organic C and total N present as particulate organic matter C, N and microbial biomass C and N The impact of land clearing methods on the proportion of organic C and total N present as particulate organic
matter C, N and microbial biomass C and N differed significantly. But the effect of cropping system differed significantly only on the proportion of total N in the form of particulate organic matter N. Land clearing method and cropping system interaction effect on the proportion of total N present as microbial biomass N was significant (Table 6). In the slash and burn clearing method, the POM C/C organic ratio ranged from 47% in the 4-year cropping/2-year cassava fallow to 37% in the 5year continuously cropped plots and these were the highest values recorded amongst the treatments. The treatment effect on the SMB C/C organic ratio and the SMB N/total N ratios mirrored the POM C/C organic ratio. 3.5. Effect of land clearing methods on crop yield As shown in Table 7, plots cleared by slash and burn gave a consistent significant higher maize grain and cassava fresh tuber yields than those cleared by bulldozed windrowed for the 5 years. The maize grain yield recorded from the slash and burn clearing method was 19% higher than ones from bulldozed non-windrowed treatment on the average, while in the case of cassava, it was 18%.
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Table 5 Impact of land clearing methods and cropping system on labile soil N Land clearing method (Lcm)
Cropping system (Cs)
Mean
5 years maize/cassava intercrop
4 years maize/cassava intercrop followed by 2 years cassava fallow
0.45 0.36 0.20
0.76 0.50 0.30
0.61 0.43 0.25
0.34 Lcm: 0.26
0.52 Cs: NS
Lcm Cs: NS
0.11 0.10 0.02
0.12 0.09 0.05
0.12 0.10 0.04
0.07 Lcm: 0.06
0.09 Cs: NS
Lcm Cs: NS
0.85 0.40 0.23
1.16 1.12 0.70
1.01 0.76 0.47
0.49 Lcm: 0.50
0.99 Cs: 0.32
Lcm Cs: NS
N (mg kg ) Slash and burn Bulldozed non-windrowed Bulldozed windrowed
16.77 13.52 7.49
22.42 20.19 3.98
19.60 16.86 10.74
Mean LSD (0.05)
12.59 Lcm: 5.57
18.86 Cs: 2.22
Lcm Cs: NS
20.65 20.03 12.91
28.90 24.59 23.78
24.78 22.31 18.35
17.86 Lcm: 5.20
25.76 Cs: 6.07
Lcm Cs: NS
Particulate organic matter N (g kg1) Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05) 1
Soil microbial biomass N (g kg ) Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05) 1
Partially mineralizable N (mg kg Slash and burn Bulldozed non-windrowed Bulldozed windrowed
1
day )
Mean LSD (0.05) NO3+
1
NH4+ N (mg kg1) Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05) NS: not significant.
4. Discussion Results obtained from this study showed that the conversion of the humid forest ecosystem into arable land had a profound negative impact on the soil labile C and N pools, irrespective of the clearing method and subsequent cropping system. The observed reduction in the relative
sizes of labile C and N pools across the land clearing methods and cropping systems compared with the level obtained in the adjacent matured high forest soil might not be unconnected with the drastic reduction in the total soil organic C and N. Labile SOM pools are fractions or components of the total SOM. Franzluebbers et al. (1994) and Haynes (2000) observed that as the whole soil organic C
Table 6 Mean square from analysis of variance for the proportion (in percent) of organic C and total N as particulate organic matter C, N and soil microbial biomass C and N Source of variation
Land clearing method (Lcm) Cropping system (Cs) Lcm Cs Error ns: not significant. * Significant at 0.05.
d.f.
2 1 2 10
Organic C as (%)
Total N as (%)
Particulate organic matter C
Soil microbial biomass C
Particulate organic matter N
Soil microbial biomass N
325.48* 75.32 ns 3.59 ns 74.98
4.30* 2.61 ns 0.77 ns 0.97
240.95* 374.11* 3.15 ns 51.91
11.09* 1.16 ns 9.59* 1.51
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Table 7 Mean annual maize grain and cassava fresh tuber yields (t ha1) as affected by land clearing methods between 1994 and 1999 Land clearing method Maize grain yield Slash and burn Bulldozed non-windrowed Bulldozed windrowed LSD (0.05) Cassava fresh tuber yield Slash and burn Bulldozed non-windrowed Bulldozed windrowed LSD (0.05)
1994/1995
1995/1996
1996/1997
1997/1998
1998/1999
Pooled mean
4.10 3.80 3.60
3.50 3.20 2.70
3.06 2.33 2.09
3.10 2.43 2.13
2.95 2.24 1.96
3.34 2.80 2.50
0.23
0.45
0.65
0.88
0.64
–
38.92 31.85 29.36
35.10 27.31 26.14
33.62 30.36 24.91
32.80 29.33 20.85
33.56 28.28 23.31
6.59
5.30
4.99
6.34
4.20
pool increases or decreases due to changes in C input to the soil, the relative size of the labile C pools also increases or decreases. The decline in whole soil organic C observed across the land clearing methods after 5-year cropping (54 and 72% in slash and burn and bulldozed windrowed treatments, respectively) relative to the value in the matured high forest, is on the average higher than the reported average rate of decline (3.3% per annum) in the humid forest zone (Nye and Greenland, 1960). However, the mean annual decline recorded in the slash and burn plot (10.8% year1) is quite close to the range (9.7% year1) recorded by Martins et al. (1991) in a low input agro-ecosystem after forest clearing, burning and cropping for 5 years in the Amazon. The observed drastic decline in organic C in this study relative to those reported elsewhere (Martins et al., 1991; Hulugalle, 1994; Islam and Weil, 2000) could be linked to the existence of a greater proportion of the total organic C of the experimental site in the POM C fraction as reflected in the adjacent matured high forest and the soil texture (sandy loam). Relatively higher rate of losses in POM C is often encountered in coarse textured soils and POM C is always the first to be depleted upon the conversion of native forest into agricultural use or any form of perturbation (Feller and Beare, 1997). Hence, its relative abundance in a particular ecosystem affects the rate at which soil C could be depleted or conserved upon the removal or the restoration of the native vegetation. Relatively, lower range of POM C in bulldozed windrowed plots, either under 5- or 4-year cropping/2year cassava fallow than the slash and burn and bulldozed non-windrowed plots under similar cropping system could be due to the displacement of the top soil through scraping and extraction of the plant root during the clearing and windrowed operation. Disruption of macro-aggregates do occur during mechanical clearing (Alegre et al., 1989); when the aggregate structure has broken down part of the mechanisms that protect soil organic matter components from biological degradation are inactivated. On the contrary, the higher level of POM C in the slash and burn compared to other land clearing methods, irrespective of
34.80 29.43 24.91 –
their use, could be attributed to higher C inputs resulting from the left over roots during land clearing. Although, soil heating during burning (depending on the intensity and period of burning) is believed to cause the death of fine roots. Okore et al. (2001) observed a massive and rapid reemergence of juvenile plants from the root portions of cut down trees in slash and burn cleared plots. The activities of living roots and decomposition of dead ones have been found to contribute to soil C pools (Van Noordwijik and Brouwer, 1997). The concentration of SMB C across the land clearing methods and cropping systems mirrored those of the POM C. This demonstrates the fact that microbial biomass C is often limited in size by the availability of other labile C pools which serve as substrate and is sensitive to variation in land use and soil management options. It follows that the lower level of microbial biomass C in the bulldozed windrowed relative to the other land clearing methods, across the cropping systems, may partly be explained by the lower level organic C orchestrated by the higher loss of POM C in that treatments. The ratio of microbial biomass C to total organic C gives an insight into the soil C level. Any change in ratio from that of a natural undisturbed soil reflects either loss or accumulations of soil C (Anderson and Domsch, 1989). Therefore, the observed decrease in the ratio across the land clearing methods and cropping system compared to the adjacent matured high forest soil indicate that the conversion of the humid forest ecosystem into arable land resulted to a drastic loss of soil C. The higher ratios recorded among the 4-year cropping/2-year cassava fallow plots, relative to the 5-year continuously cropped plots, irrespective of the clearing method can be ascribed to the presence of more organic substrate in the 2-year cassava fallow plots. This conformed to the findings of Cerri et al. (1985) and Feller and Beare (1997). Water-soluble C (dissolved organic matter) originates as leachates from decomposing above ground litters, exudates of living roots and soil microbes and decomposition of roots and soil microbes (Haynes, 2000). Hence, a soil management system that retains more labile organic C as observed among the slash and burn plots will tend to have higher
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water-soluble C contents. These observations are in conformity with the report of Kalbitz et al. (2000). The response of the soil total and labile N concentration to the various land clearing methods and cropping systems mirrored those of the total and labile C pools, with mechanical land clearing inflicting a more negative effect. The observed higher N values in the slash and burn soil compared to the mechanically cleared soils are in line with the observation of Seubert et al. (1977). The higher availability of particulate organic matter and microbial biomass N in the slash and burn soils across the cropping systems may have been responsible for the higher potentially mineralizable N recorded from that treatment than others. A more rapid N release from particulate organic matter (coarse) N fraction than the silt and clay N fractions of organic matter have been reported in Nigeria (Vanlauwe et al., 1998). The concentration of mineral N in the soil across the land clearing methods and cropping systems was relatively lower than that of the adjacent soil under matured high forest. This could be as a result of the exposure of the soil in the cleared area to elevated temperature and moisture, which may have enhanced N mineralization and subsequent losses through leaching and uptake by crops all through the years of cropping. The consistently higher crop yield recorded from the slash and burn treatments may be due to relative favourable soil environment as reflected in the level of labile organic matter contents of the treatment compared to others. Lal (1992) made similar observation. Further research would be required to quantify the contribution of left over above-andbelow grow ground phytomass to the soil organic C and N stock under the slash and burn clearing methods which tends to conserve soil labile C and N than the mechanical method in the humid tropics, though some similarities were observed between the slash and burn and bulldozed windrowed plots.
5. Conclusion The results obtained from this study indicate that the degree of losses (compared to the adjacent matured high forest) in labile soil C and N pools upon the conversion of a humid tropical forest to arable land depends on the method deployed in removing the natural vegetation. We observed that the magnitude of losses was in the order, bulldozed windrowed > bulldozed non-windrowed > slash and burn. While amongst the cropping systems, it was 5-year cropping > 4-yearcropping/2-year cassava fallow. However, the quantity of most of the labile C and N pools and crop yield obtained in the slash and burn and bulldozed non-windrowed treatments did not differ significantly. This tend to suggest that the non scraping of the top soil and retention of below-ground phytomass like roots in both land clearing methods may have resulted to the non significant variation in their effects. We could
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then say that any land clearing method that does not remove the top soil and retains some of the below-ground phytomass may be a sure way of reducing labile soil C and N losses upon the transformation of a humid into arable land. Consequently, due to the high cost and drudgery associated with slash and burn clearing method (Couper et al., 1981) as a viable alternative to mechanized land clearing for large-scale farming; the result obtained from this study suggest that bulldozed non-windrowed (mechanical) clearing method could be an option to slash and burn in ensuring reduced organic matter losses and drudgery. The study further indicates that allowing of fast growing biannual or semi-perennial crops as fallow could help in rebuilding the lost organic matter.
References Agboola, A.A., Ogunkunle, A.O., 1993. Site characterization at Epemakinde, Ondo State, Nigeria. Technical Report on Land Development for sustainable Agriculture in Africa. IBSRAM/AFRICALAND Network, Bangkok, Thailand, pp.120–131. Agboola, A.A., Ndaeyo, N.U., Okore, I.K., 1998. Soil fertility management alternatives to inorganic fertilizer use. In: Babalola, O., Babaji, G.A., Mustapha, S. (Eds.), Proceeding of the 24th Annual Conference of the Soil Science Society of Nigeria on Soil Management for Sustainable Agriculture and Environmental Harmony. pp. 1–8. Alegre, J.C., Sanchez, P.A., Palm, C.A., Perez, J.M., 1989. Comparative soil dynamics under different management options. In: Candle, N. (Ed.), Trop Soils Technical Report, 1986–1987. North Carolina State University, Raleigh, NC, USA, pp. 102–198. Anderson, T.H., Domsch, K.H., 1989. Carbon link between microbial biomass and organic matter. In: Meguscar, F., Gantar, M. (Eds.), Proceedings of the 4th International Symposium on Microbial Ecology, Slovene Society for Microbiology, Ljubljana, pp. 467–471. Anderson, J.M., Ingram, J.S.I., 1989. Tropical Soil Biology and Fertility: A Hand book of Methods. CAB International, Wallingford, UK, pp. 68–71. Bremner, J.M., 1965. Inorganic forms of nitrogen. In: Black, C.A. (Ed.), Methods of Soil Analysis. Part 2. Agronomy Monograph 9. ASA, Madison, pp. 117–123. Brookes, P.C., Landam, A., Pruden, G., Jenkinson, D.S., 1985. Chloroform fumigation and the release of soil nitrogen: a rapid direct extraction method to measure microbial biomass nitrogen in soil. Soil Biol. Biochem. 17, 837–838. Cambardella, C.A., Elliott, E.T., 1992. Particulate soil organic matter changes across a grassland cultivation sequence. Soil. Sci. Soc. Am. J. 56, 777–783. Cerri, C., Feller, C., Balesdent, J., Victoria, R., Plenecassagne, A., 1985. Application dutracage Isoto opique naturelau 13 C a l0 etude de la matiere organique danles sols. CR Acad. Sci. Paris 30011 (9), 423–428. Couper, D.C., Lal, R., Claassen, S.L., 1981. Land clearing and development for Agricultural purposes in Western Nigeria. In: Lal, R., Russell, E.W. (Eds.), Tropical Agricultural Hydrology. John Wiley & Sons, Chichester, UK, pp. 119–130. Feller, C., Beare, M.H., 1997. Physical control of soil organic matter dynamics in the tropics. Georderma 79, 69–116. Franzluebbers, A.J., Hons, F.N., Zuberer, D.A., 1994. Long-term changes in soil carbon and nitrogen pools in wheat management systems. Soil Sci. Soc. Am. J. 58, 1639–1645. Haynes, R.J., 2000. Labile organic matter as an indicator of organic matter quality in arable and pastoral soils in New Zealand. Soil Biol. Biochem. 32, 211–219.
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Hulugalle, N.R., 1994. Long term effect of land clearing methods, tillage systems and cropping system on surface soil properties of a tropical alfisol SW Nigeria. Soil Use Manage. 6 (3), 137–144. Islam, K.R., Weil, R.R., 2000. Land use effect on soil quality in a tropical forestecosystem of Bangladesh. Agric. Ecosyst. Environ. 79, 17–27. Kalbitz, K., Solinger, S., Park, J.H., Michalzik, B., Matzner, E., 2000. Controls on the dynamics of dissolved organic matter in soils: a review. Soil Sci. 164 (4), 277–304. Lal, R., 1986. Different methods of land clearing for agricultural purposes in the tropics. In: Lal, R., Sanchez, P.A., Cumming, R.W. (Eds.), Land Clearing and Development in the Tropics. Balkkema/Rotterdam, pp. 55–66. Lal, R., 1992. Tropical Agricultural Hydrology and Sustainability of Agricultural Systems: A Ten Year Watershed Management Project in Southwestern Nigeria. Ohio State University, Columbus. Lugo, A.E., Brown, S., 1993. Management of tropical soil as sinks or sources of atmospheric carbon. Plant Soil 149, 27–41. Martins, P.F., Cerri, C.C., Volkoff, B., Andreux, E., Chauvel, A., 1991. Consequences of clearing and tillage of natural Amazonian ecosystem. Forest Ecol. Manage. 38, 273–282. Motavalli, P.P., Discekichi, H., Kuhu, J., 2000. The impact of land clearing and agricultural practices on soil organic C fractions and CO2 efflux in the Northern Guam aquifer. Agric. Ecosyst. Environ. 79, 17–27. Nelson, D.W., Sommer, L.E., 1982. Total carbon, organic carbon and organic matter. In: Page, A.L. (Ed.), Methods of Soil Analysis. Part 2. Agronomy Monograph 9, 2nd ed. ASA and SSSA, Madison, WI, pp. 539–594.
Nye, P.H., Greenland, D.J., 1960. The Soil Under Shifting Cultivation. Commonwealth Bureau of Soil, Farnham Royal Techn. Comm., pp. 51– 156. Okore, I.K., Tijani-Eniola, H., Agboola, A.A., Nwagwu, F.A., 2001. Effect of different land management methods on weed flora dynamics at Lisagbede, Southwestern Nigeria. Nigerian J. Weed Sci. 14, 25–30. Rowell, D.L., 1992. Soil Science: Methods and Application. Longman, London. Seubert, C.E., Sanchez, P.A., Valvarde, C., 1977. Effect of land clearing methods on soil properties of an ultisol and crop performance in the Amazon jungle of Peru. Trop. Agric. (Trinidad) 54 (4), 307–321. Sikora, L.J., Yakovchenko, V., Cambardella, C.A., Doran, J.W., 1996. Assessing soil quality by testing organic matter. In: Magdoff, F.R., Tabatabai, M.A., Hanlon, E.A. (Eds.), Soil Organic Matter: Analysis and Interpretation, vol. 46. SSSA Speci. Pub., pp. 41–50. Smith, O.H., Petersen, G.W., Needdman, B.A., 2000. Environmental indicators of agro-ecosystem. Adv. Agron. 69, 75–97. Van Noordwijik, M., Brouwer, G., 1997. Roots as sink and source of nutrient and carbon in agricultural systems. In: Brussard, L., Ferrera-Cerrto, R. (Eds.), Soil Ecology in Sustainable Agricultural Systems. Lewis Publ., New York, pp. 71–90. Vanlauwe, B., Sanginga, R., Merckx, R., 1998. Soil organic matter dynamics after addition of nitrogen-15-labeled leucaena and dactyladenia residue. Soil Sci. Soc. Am. J. 62, 461–466. Zak, D.R., Grigal, D.F., Ohman, L.F., 1993. Kinetics of microbial respiration and nitrogen mineralization in Great Lake forests. Soil Sci. Soc. Am. J. 57, 1100–1106.
Impact of land clearing methods and cropping systems on labile soil C and N pools in the humid zone Forest of Nigeria I.K. Okore a,*, H. Tijani-Eniola b, A.A. Agboola b, E.A. Aiyelari b a
Rubber Research Institute of Nigeria, P.M B 1049, Benin City, Edo State, Nigeria b Department of Agronomy, University of Ibadan, Ibadan, Nigeria
Received 27 August 2005; received in revised form 28 June 2006; accepted 18 September 2006 Available online 7 November 2006
Abstract Labile soil C and N play vital roles in soil–plant nutrient dynamics, especially in the low input cropping system and are vulnerable to perturbation. Surface (0–0.15 m) soils from three land clearing methods (slash and burn, bulldozed non-windrowed and bulldozed windrowed) and each with two cropping systems (5-and 4-year cropping/2-year cassava fallow) were collected in the humid forest ecosystem of Nigeria. The soils were analysed for total C and N, microbial biomass C and N (SMB C and N), particulate organic matter C and N (POM C and N), water-soluble C, potentially mineralizable N (PMN) and mineral N. The size of the labile C and N and their relative contributions to the organic C and total N differed significantly among land clearing methods, irrespective of the cropping system. Soils under slash and burn had a significantly ( p > 0.05) higher particulate organic matter C, N (10.80 and 0.16 g kg1, respectively) and microbial biomass C and N (1.07 and 0.12 g kg1) compared to the bulldozed windrow, regardless of the cropping system. Four years cropping/2-year cassava fallow resulted in a significant higher labile C and N, relative to 5-year cropped plots across the land clearing methods. Effect of the treatments on the concentration of PMN and mineral N mirrored the SMB N and POM N. However, the quantity of most of the labile C and N pool and crop yield obtained from the slash and burn and bulldozed non-windrowed treatment did not differ significantly. Hence, bulldozed non-windrowed clearing could be a viable alternative to slash and burn in the case of large-scale farming in ensuring reduced losses of soil organic matter and nutrient during land clearing in the humid tropics. # 2006 Elsevier B.V. All rights reserved. Keywords: Soil fertility; Cropping system; Deforestation; Soil organic matter; Cassava fallow; Particulate organic matter
1. Introduction The humid forest eco-region of southern Nigeria witnessed a rapid conversion into agricultural use in the mid 1990s, following the establishment of the National Agricultural Land Development Authority (NALDA) by the then Federal Military Government. Most of the areas were cleared mechanically and could not sustain arable crop production even for three consecutive years without severe soil loss and degradation (Agboola et al., 1998). Mechanical land clearing has an adverse effect on soil physical and * Corresponding author. Tel.: +234 8035728268. E-mail address: [email protected] (I.K. Okore). 0167-8809/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2006.09.011
chemical conditions more especially in the humid forest ecosystem (Seubert et al., 1977; Hulugalle, 1994). However, the intensity of the effect varies with the method deployed, with those involving a combination of tree pushing and the root rake being more damaging than the shear blade clearing method alone (Lal, 1986). Traditionally, in the lowland humid tropics, the conversion of forest or fallow land into agricultural use involves felling and burning of the vegetation—manual clearing (Lal, 1986). The method ensures minimal soil disturbance and degradation (Seubert et al., 1977) and the cleared area can sustain economic crop yield for more than 4 years before being fallowed. However, the prohibitive amount of labour and time required to accomplish manual land clearing has
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made most people, especially large-scale farmers to opt for mechanized land clearing method without considering the consequences on the soil environment. Degradation in soil quality often associated with the conversion of primary or secondary forest into low input agroecosystem is mainly due to decline in soil organic matter (Sikora et al., 1996; Smith et al., 2000). The rapid initial losses are mainly the mineralization of the biologically labile or active pools: microbial biomass, particulate organic matter, soil carbohydrate and enzymes and water-soluble C (Martins et al., 1991; Lugo and Brown, 1993). Decreases of more than 47 and 24% in particulate matter and microbial biomass C have been recorded in less than a year after clearing of a secondary forest (Motavalli et al., 2000). Islam and Weil (2000) reported a labile C loss of about 67–167% in soils under 5–7 years continuous cropping. Their rate of losses is among other factors affected by land use and management strategies (Zak et al., 1993; Feller and Beare, 1997). Labile organic matter pools have a significant positive correlation with soil nutrient capital stock (Sikora et al., 1996; Haynes, 2000). Evaluation of changes in their quantity could be an early indicator of land use and management effect on the soil environment. The aim of this study was to evaluate the impact of three land clearing methods and subsequent uses on the soil labile C and N pools of the humid forest eco-region of southwestern Nigeria.
2. Materials and methods
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Table 1 Monthly mean maximum, minimum (mm) and standard deviation of rainfall at Epemakinde, between 1980 and 2000 Month
Maximum
Minimum
Mean (n = 20)
S.D.
January February March April May June July August September October November December
31.2 133.0 223.9 225.0 284.1 337.9 442.7 211.5 379.2 375.9 145.8 56.4
5.3 16.0 8.5 53.6 174.4 185.9 102.7 39.9 171.5 159.2 19.6 7.4
20.9 34.5 90.9 153.7 217.2 265.7 317.0 127.1 256.4 212.8 55.8 9.1
12.9 23.0 82.8 80.1 54.8 52.8 120.1 65.3 192.1 204.4 125.9 51.5
n = 20.
Pycnanthus microcephalus (Beuth.), Staudtia stipitata (Warb.), Marathes glabra (Oliv.), Sacoglottis gabonensis (Baill), Elaeophorbia drupifera (Thonn.), Eribroma oblonga (Mast.) and Ceiba pentandra (Linn.) having a girth of between 0.25 and 0.30 m at 1.5 m height and the average tree density of >1000 ha1. The experimental consist of a factorial combination of three land clearing methods (slash and burn, bulldozed nonwindrowed and bulldozed windrowed) and two cropping systems (5 years maize/cassava inter crop and 4 years maize/ cassava intercrop followed by 2-year cassava fallow). A randomized complete block design was used with three replicates. Each of the experimental unit measured 0.49 ha.
2.1. Site description and field experiment A field experiment was established in 1994 at the project site of the Department of Agronomy, University of Ibadan located at the Ondo State Aforestation Project in Epemakinde to compare the conventional (slash and burn) and two mechanical (bulldozed non-windrowed and bulldozed windrowed) methods of bush clearing for arable cropping in the humid forest area of Nigeria. Epemakinde (48E, 68N) is characterized by a tropical humid rain forest climate. The rainfall pattern is bimodal, with a long (April–July) and short August–October) rainy period separated by a short period of dryness between late July and early August (Table 1). The temperature ranges between 26 and 28 8C with a relative humidity of 65–80%. The soil possesses a sandy loam in the upper layer over a well-drained sandy clay/clay loam sub soil (Agboola and Ogunkunle, 1993) which is classified in the USDA system as Typic Kandiudult (an ultisol). The soil textural analysis of samples from the adjacent matured high forest showed on average, 71.2 g 100 g1 sand, and 26.3 g 100 g1 silt and 2.5 g 100 g1 clay and bulk density was 1.06 g cm3 at 0– 0.15 m depth. Some of the chemical and biochemical properties of the soil are shown in Table 2. The vegetation at the beginning of the experiment was a matured high forest (>100 years old) with some identifiable tree species such as
Table 2 Some of the chemical and biochemical properties of the adjacent matured high forest soil (0–0.15 m) Chemical and biochemical properties
Value
Organic C (g kg1) Total N (g kg1) C/N ratio pH (H2O) Available P (mg kg1) Bray 1
49.78 3.25 18.39 6.40 8.20
Exchangeable cations (cmol kg1) Ca Mg K Na Total exchangeable acidity CEC Particulate organic matter C (g kg1) Particulate organic matter N (g kg1) Soil microbial biomass C (g kg1) Soil microbial biomass N (g kg1) Field moist water-soluble C (g kg1) Air-dried soil-soluble C (g kg1) Potentially mineralizable N (mg (kg1 day1)) NO3 N (mg kg1) NH4+ N (mg kg1) POM C/C org (%) SMB C/C org (%)
6.43 0.12 0.29 0.17 0.20 7.21 25.60 1.35 5.82 1.17 0.30 19.15 2.63 24.22 55.69 51.42 11.69
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The slash and burn clearing involved cutting the undergrowth and vines of the forest vegetation with machetes and felling of the trees at waist height with axes. Big trees were cut down with hand operated power chain saw. Following drying, the cut down biomass was burnt in situ and the ash evenly distributed within the plot. For the bulldozed non-windrowed clearing method, an A.D 83E Komatsu model bulldozer was used to knock down the trees. The knocked down trees were cut into logs of between 2 and 3 m long and rolled to the alley. Other debris like leaves; twigs and roots were also packed to the alley. The bulldozed windrowed clearing was accomplished with a similar bulldozer model, but the bulldozer after knocking down the trees, ripped out the roots and stumps with its shear blade and windrowed the knocked down trees, roots and other debris to the alley. The cleared plots were later subjected to different cropping systems before soil sampling for labile organic matter determination. The crops were sown on flat (without any form of tillage) and weeding was done manually, three times each year with hoe. There was no other form of burning in any of the plots during the cropping period except the initial burning in the slash and burn treatment. None of the treatments received any form of fertilizer.
treatment was dispersed in 60 ml of 5 g1 of NaP(O3)6 by shaking overnight in a reciprocating shaker. The dispersed samples were sieved through 53 mm sieves. The slurry that passed through the 53 mm sieve was evaporated overnight in an oven at 50 8C. The oven-dried slurry was finally analysed for total N and organic C. The difference between the C and N values of the evaporated slurry and those of the nondispersed (whole soil) sample was considered as the particulate organic matter C and N. Soil microbial biomass C and N were determined by chloroform fumigation incubation method (Anderson and Ingram, 1989). A 10 g (oven dry weight equivalent) of field moist soil from each treatment was measured into each of two 50 ml glass beakers. The soil in one of the beakers was fumigated with 30 ml alcohol free chloroform in vacuum desiccators. After evaporating the chloroform by several repeated evacuation of the desiccators, the fumigated soils were incubated for 5 days. After the 5th day, 50 ml of 0.5 M K2SO4 was used to extract the dissolved organic C and N in the fumigated and un-fumigated samples. The extracted dissolved organic C was determined by titration with acidified Ferrous ammonium sulphate solution, while the dissolved N was determined by micro Kjeldahl method. Soil microbial biomass C and N were calculated as
2.2. Soil sampling and preparation for analysis
SMBC ¼ extracted C ðfumigatedÞ
A composite sample (0–0.15 m depth) consisting of 65 cores was collected in a grid pattern from within the 75 m 65 m experimental unit. Similar pattern was used in sampling the adjacent matured (>100 years) high forest. The samples from the various land clearing methods cropped 5 years were collected in May 1999, while those of the 4-year cropping/2-year cassava fallow were taken a year later (at the completion of the second year of the fallow). Coarse organic matter and plant roots were removed manually. Field moist samples were sieved (<2 mm) and stored at 4 8C for the determination of soil microbial biomass N and C, field moist soil water-soluble C, potentially mineralizable nitrogen and mineral nitrogen pools not latter than 14 days after collection. Sub samples were air-dried for the determination of particulate organic matter C and N. 2.3. Determination of total N, organic C, particulate organic C and N, soil microbial biomass C and N and water-soluble C Organic C content of the soil was measured by the method of Nelson and Sommer (1982) using the modified Mebius procedure involving the use of 0.5N K2Cr2O7, H2SO4 and 0.2N Fe (NH4)2(SO4)26H2O with N-phenylanthranilic indicator, while the total N was determined by Kjeldahl digestion and distillation procedure (Bremner, 1965). Soil particulate organic matter C and N were determined by the method of Cambardella and Elliott (1992). Briefly, 20 g of sub samples (>2 mm) from each
extracted C ðun-fumigatedÞ 2:64 SMBC ¼ extracted N ðfumigatedÞ extracted N ðun-fumigatedÞ 1:46 where 2.64 and 1.46 are constants for C and N, respectively (Brookes et al., 1985). Water-soluble C in field moist and air-dried soil subsamples was extracted by shaking and centrifuging 10 g of soil (oven dry weight) for 15 and 10 min, respectively, in 20 ml of distilled water (Haynes, 2000). The supernatants were analysed for dissolved organic C by dichromate oxidation procedure involving 30 min heating under reflux condition. 2.4. Potentially mineralizable and mineral N determination Potentially mineralizable N was determined by anaerobic incubation of sub-samples of fresh soil. 12 ml of distilled water was added to 5 g of soil and incubated for 7 days at 40 8C (Rowell, 1992). Ammonium-N was extracted by shaking for 1 h after adding 15 ml of 4 M KCl. Potentially mineralizable N was calculated as the difference in NH4+ N content of pre- and post-incubated samples divided by the number of days of incubation. Mineral N was extracted by shaking 10 g of sub samples of fresh soil in 100 ml of 2 M KCl for 1 h. Extracts were analysed for NH4+ and NO3 by automated colorimeter using a Technicon auto analyzer.
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2.5. Statistical analysis Statistical significance of treatment effects on whole soil C, total N, particulate organic matter C and N, soil microbial biomass C and N, water-soluble C, potentially mineralizable N and mineral N in the soil were determined by using twoway analysis of variance (ANOVA). Where a significant ( p > 0.05) treatment effect was found, LSD was calculated to compare treatment mean effect.
3. Results 3.1. Whole soil organic C, total N and C/N ratio The concentration of whole soil organic C, total N and C/ N ratio differed significantly ( p > 0.05) amongst soils under different land clearing methods and cropping systems (Table 3). Irrespective of the cropping system, the highest decline in organic C (31%) and total N (45%) among the land clearing methods compared to the adjacent mature high forest (Table 2) was recorded from the bulldozed windrowed
Mean LSD (0.05)
26.03 25.31
17.75
15.83
Lcm Cs: NS
3.3. Labile N pools: particulate organic matter N (POM N), microbial biomass N (SMB N), potential mineralizable N (PMN) and mineral N (NO3 N and NH4+ N)
Total N (g kg1) Slash and burn 2.10 Bulldozed non- 2.10 windrowed Bulldozed 1.43 windrowed Mean LSD (0.05)
1.88 Lcm: 0.32
C/N ratio Slash and burn 11.52 Bulldozed non- 10.85 windrowed Bulldozed 10.30 windrowed Mean LSD (0.05) NS: not significant.
Mean
5 years maize/ 4 years maize/cassava cassava intercrop followed by 2 intercrop years cassava fallow
19.97 Lcm: 4.71
10.91 Lcm: 0.94
3.2. Labile C pools: particulate organic matter C (POM C) soil microbial biomass C (SMB C) and water-soluble C
28.95 27.71
Cropping system (Cs)
Organic C (g kg1) Slash and burn 23.10 Bulldozed non- 22.90 windrowed Bulldozed 13.90 windrowed
clearing method. Among the cropping systems, a significantly higher levels of organic C (24.80 g kg1) and total N (2.04 g kg1) was obtain for soil cropped for 4 years/2 years cassava fallow compared to those of 5 years, across the land clearing methods. However, the values obtained from the slash and burn did not differ significantly from that recorded from the bulldozed non-windrowed under similar cropping system. The highest C/N ratio value (12.32), among the land clearing methods regardless of the cropping systems was obtained from the slash and burn soils, while the lowest (7.33) came from the bulldozed windrowed soil. Although, these values differed significantly ( p > 0.05) from each other, they did not differ from the values obtained from one obtained from the bulldozed non-windrowed treatment. The effect of cropping systems on the C/N ratio differed significantly; with the highest mean value being recorded from plots under 4-year cropping/2-year cassava fallow.
The particulate organic matter C (1.07 and 0.24 g kg1, respectively) obtained from slash and burn plot were more than two folds greater than those obtained from bulldozed windrowed plot, irrespective of the cropping system. Among the cropping systems, 4-year cropping/2-year cassava fallow plots had 61% more POM C and 41% more SMB C values than those under 5-year continuous cropping, with the highest value being recorded from slash and burn clearing method (Table 4). The concentration of field moist and airdried water-soluble C, across the land clearing methods mirrored the particulate organic matter and microbial biomass C. However, the effect of cropping system did not differ significantly.
Table 3 Impact of land clearing methods and cropping system on whole soil organic C, total N and C/N ratio Land clearing method (Lcm)
253
24.80 Cs: 3.34
2.45 2.15
2.28 2.11
1.51
1.47
2.04 Cs: NS
Lcm Cs: NS
13.07 12.89
12.32 11.87
11.68
7.33
12.55 Cs: NS
Lcm Cs: NS
As indicated in Table 5, land clearing methods effect on labile N pools differed significantly ( p > 0.05). The POM N ranged from a mean of 0.61 g N kg1 soils in the slash and burn plot to 0.25 g N kg1 soil in bulldozed windrowed plot. The value obtained from the bulldozed windrowed treatment represented a decline of 81% from that of the matured high forest and the highest decline recorded among the land clearing methods. The POM N content of the soil across the cropping systems did not differ significantly, although, 4year cropping/2-year cassava fallow resulted in a 53% more POM N compared to 5-year continuous cropping treatment across the land clearing methods. The mean level of SMB N (1.01 g N kg1 soil) recorded from the slash and burn was significantly higher than those of the bulldozed windrowed plots, irrespective of the cropping system. The order of decline in SMB N amongst the cropping systems relative to the value recorded from the adjacent
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Table 4 Impact of land clearing methods and cropping system on labile soil C (g kg1) Land clearing method (Lcm)
Particulate organic matter C Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05) Soil microbial biomass C Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05) Field moist soil water-soluble C Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05) Air-dried soil water-soluble C Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05)
Cropping system (Cs)
Mean
5 years maize/cassava intercrop
4 years maize/cassava intercrop followed by 2 years cassava fallow
7.70 5.40 3.10
13.75 7.60 4.80
10.80 6.50 4.00
5.40 Lcm: 1.40
8.70 Cs: 2.90
Lcm Cs: NS
0.88 0.91 0.28
1.25 1.10 0.56
1.07 1.01 0.42
0.69 Lcm: 0.5
0.97 Cs: 0.1
Lcm Cs: NS
0.20 0.14 0.07
0.27 0.17 0.08
0.24 0.16 0.08
0.14 Lcm: 0.06
0.17 Cs: NS
Lcm Cs: NS
3.81 3.97 2.01
3.337 3.37 2.33
3.57 3.67 2.17
3.26 Lcm: 1.43
3.01 Cs: NS
Lcm Cs: NS
NS: not significant.
matured high forest (Table 2) was 5-year cropping > 4-year cropping/2-year cassava fallow. The potentially mineralizable N (PMN) content of the soil under different land clearing methods and cropping systems were relatively low relative to the concentration observed in the adjacent matured high forest. Among the cropping systems, soil under 4-year cropping/2-year cassava fallow doubled the amount of PMN recorded in 5-year continuous cropping. The mineral nitrogen of the adjacent soil under matured high forest was found to be 24.22 and 55.69 mg kg1 for the NO3 N and NH4+ N, respectively (Table 2). The nitrate nitrogen (NO3–N) concentration in the slash and burn soil was 16 and 82% higher than those of the bulldozed non-windrowed and the bulldozed windrowed plots in that order, regardless of the cropping system. The NO3 N level in 5-year cropped plots was 33% lower than that of the 4-year cropping/2-year cassava fallow. The effect of the treatment of NH4+ N followed the same trend as NO3 N. 3.4. Proportion of organic C and total N present as particulate organic matter C, N and microbial biomass C and N The impact of land clearing methods on the proportion of organic C and total N present as particulate organic
matter C, N and microbial biomass C and N differed significantly. But the effect of cropping system differed significantly only on the proportion of total N in the form of particulate organic matter N. Land clearing method and cropping system interaction effect on the proportion of total N present as microbial biomass N was significant (Table 6). In the slash and burn clearing method, the POM C/C organic ratio ranged from 47% in the 4-year cropping/2-year cassava fallow to 37% in the 5year continuously cropped plots and these were the highest values recorded amongst the treatments. The treatment effect on the SMB C/C organic ratio and the SMB N/total N ratios mirrored the POM C/C organic ratio. 3.5. Effect of land clearing methods on crop yield As shown in Table 7, plots cleared by slash and burn gave a consistent significant higher maize grain and cassava fresh tuber yields than those cleared by bulldozed windrowed for the 5 years. The maize grain yield recorded from the slash and burn clearing method was 19% higher than ones from bulldozed non-windrowed treatment on the average, while in the case of cassava, it was 18%.
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Table 5 Impact of land clearing methods and cropping system on labile soil N Land clearing method (Lcm)
Cropping system (Cs)
Mean
5 years maize/cassava intercrop
4 years maize/cassava intercrop followed by 2 years cassava fallow
0.45 0.36 0.20
0.76 0.50 0.30
0.61 0.43 0.25
0.34 Lcm: 0.26
0.52 Cs: NS
Lcm Cs: NS
0.11 0.10 0.02
0.12 0.09 0.05
0.12 0.10 0.04
0.07 Lcm: 0.06
0.09 Cs: NS
Lcm Cs: NS
0.85 0.40 0.23
1.16 1.12 0.70
1.01 0.76 0.47
0.49 Lcm: 0.50
0.99 Cs: 0.32
Lcm Cs: NS
N (mg kg ) Slash and burn Bulldozed non-windrowed Bulldozed windrowed
16.77 13.52 7.49
22.42 20.19 3.98
19.60 16.86 10.74
Mean LSD (0.05)
12.59 Lcm: 5.57
18.86 Cs: 2.22
Lcm Cs: NS
20.65 20.03 12.91
28.90 24.59 23.78
24.78 22.31 18.35
17.86 Lcm: 5.20
25.76 Cs: 6.07
Lcm Cs: NS
Particulate organic matter N (g kg1) Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05) 1
Soil microbial biomass N (g kg ) Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05) 1
Partially mineralizable N (mg kg Slash and burn Bulldozed non-windrowed Bulldozed windrowed
1
day )
Mean LSD (0.05) NO3+
1
NH4+ N (mg kg1) Slash and burn Bulldozed non-windrowed Bulldozed windrowed Mean LSD (0.05) NS: not significant.
4. Discussion Results obtained from this study showed that the conversion of the humid forest ecosystem into arable land had a profound negative impact on the soil labile C and N pools, irrespective of the clearing method and subsequent cropping system. The observed reduction in the relative
sizes of labile C and N pools across the land clearing methods and cropping systems compared with the level obtained in the adjacent matured high forest soil might not be unconnected with the drastic reduction in the total soil organic C and N. Labile SOM pools are fractions or components of the total SOM. Franzluebbers et al. (1994) and Haynes (2000) observed that as the whole soil organic C
Table 6 Mean square from analysis of variance for the proportion (in percent) of organic C and total N as particulate organic matter C, N and soil microbial biomass C and N Source of variation
Land clearing method (Lcm) Cropping system (Cs) Lcm Cs Error ns: not significant. * Significant at 0.05.
d.f.
2 1 2 10
Organic C as (%)
Total N as (%)
Particulate organic matter C
Soil microbial biomass C
Particulate organic matter N
Soil microbial biomass N
325.48* 75.32 ns 3.59 ns 74.98
4.30* 2.61 ns 0.77 ns 0.97
240.95* 374.11* 3.15 ns 51.91
11.09* 1.16 ns 9.59* 1.51
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Table 7 Mean annual maize grain and cassava fresh tuber yields (t ha1) as affected by land clearing methods between 1994 and 1999 Land clearing method Maize grain yield Slash and burn Bulldozed non-windrowed Bulldozed windrowed LSD (0.05) Cassava fresh tuber yield Slash and burn Bulldozed non-windrowed Bulldozed windrowed LSD (0.05)
1994/1995
1995/1996
1996/1997
1997/1998
1998/1999
Pooled mean
4.10 3.80 3.60
3.50 3.20 2.70
3.06 2.33 2.09
3.10 2.43 2.13
2.95 2.24 1.96
3.34 2.80 2.50
0.23
0.45
0.65
0.88
0.64
–
38.92 31.85 29.36
35.10 27.31 26.14
33.62 30.36 24.91
32.80 29.33 20.85
33.56 28.28 23.31
6.59
5.30
4.99
6.34
4.20
pool increases or decreases due to changes in C input to the soil, the relative size of the labile C pools also increases or decreases. The decline in whole soil organic C observed across the land clearing methods after 5-year cropping (54 and 72% in slash and burn and bulldozed windrowed treatments, respectively) relative to the value in the matured high forest, is on the average higher than the reported average rate of decline (3.3% per annum) in the humid forest zone (Nye and Greenland, 1960). However, the mean annual decline recorded in the slash and burn plot (10.8% year1) is quite close to the range (9.7% year1) recorded by Martins et al. (1991) in a low input agro-ecosystem after forest clearing, burning and cropping for 5 years in the Amazon. The observed drastic decline in organic C in this study relative to those reported elsewhere (Martins et al., 1991; Hulugalle, 1994; Islam and Weil, 2000) could be linked to the existence of a greater proportion of the total organic C of the experimental site in the POM C fraction as reflected in the adjacent matured high forest and the soil texture (sandy loam). Relatively higher rate of losses in POM C is often encountered in coarse textured soils and POM C is always the first to be depleted upon the conversion of native forest into agricultural use or any form of perturbation (Feller and Beare, 1997). Hence, its relative abundance in a particular ecosystem affects the rate at which soil C could be depleted or conserved upon the removal or the restoration of the native vegetation. Relatively, lower range of POM C in bulldozed windrowed plots, either under 5- or 4-year cropping/2year cassava fallow than the slash and burn and bulldozed non-windrowed plots under similar cropping system could be due to the displacement of the top soil through scraping and extraction of the plant root during the clearing and windrowed operation. Disruption of macro-aggregates do occur during mechanical clearing (Alegre et al., 1989); when the aggregate structure has broken down part of the mechanisms that protect soil organic matter components from biological degradation are inactivated. On the contrary, the higher level of POM C in the slash and burn compared to other land clearing methods, irrespective of
34.80 29.43 24.91 –
their use, could be attributed to higher C inputs resulting from the left over roots during land clearing. Although, soil heating during burning (depending on the intensity and period of burning) is believed to cause the death of fine roots. Okore et al. (2001) observed a massive and rapid reemergence of juvenile plants from the root portions of cut down trees in slash and burn cleared plots. The activities of living roots and decomposition of dead ones have been found to contribute to soil C pools (Van Noordwijik and Brouwer, 1997). The concentration of SMB C across the land clearing methods and cropping systems mirrored those of the POM C. This demonstrates the fact that microbial biomass C is often limited in size by the availability of other labile C pools which serve as substrate and is sensitive to variation in land use and soil management options. It follows that the lower level of microbial biomass C in the bulldozed windrowed relative to the other land clearing methods, across the cropping systems, may partly be explained by the lower level organic C orchestrated by the higher loss of POM C in that treatments. The ratio of microbial biomass C to total organic C gives an insight into the soil C level. Any change in ratio from that of a natural undisturbed soil reflects either loss or accumulations of soil C (Anderson and Domsch, 1989). Therefore, the observed decrease in the ratio across the land clearing methods and cropping system compared to the adjacent matured high forest soil indicate that the conversion of the humid forest ecosystem into arable land resulted to a drastic loss of soil C. The higher ratios recorded among the 4-year cropping/2-year cassava fallow plots, relative to the 5-year continuously cropped plots, irrespective of the clearing method can be ascribed to the presence of more organic substrate in the 2-year cassava fallow plots. This conformed to the findings of Cerri et al. (1985) and Feller and Beare (1997). Water-soluble C (dissolved organic matter) originates as leachates from decomposing above ground litters, exudates of living roots and soil microbes and decomposition of roots and soil microbes (Haynes, 2000). Hence, a soil management system that retains more labile organic C as observed among the slash and burn plots will tend to have higher
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water-soluble C contents. These observations are in conformity with the report of Kalbitz et al. (2000). The response of the soil total and labile N concentration to the various land clearing methods and cropping systems mirrored those of the total and labile C pools, with mechanical land clearing inflicting a more negative effect. The observed higher N values in the slash and burn soil compared to the mechanically cleared soils are in line with the observation of Seubert et al. (1977). The higher availability of particulate organic matter and microbial biomass N in the slash and burn soils across the cropping systems may have been responsible for the higher potentially mineralizable N recorded from that treatment than others. A more rapid N release from particulate organic matter (coarse) N fraction than the silt and clay N fractions of organic matter have been reported in Nigeria (Vanlauwe et al., 1998). The concentration of mineral N in the soil across the land clearing methods and cropping systems was relatively lower than that of the adjacent soil under matured high forest. This could be as a result of the exposure of the soil in the cleared area to elevated temperature and moisture, which may have enhanced N mineralization and subsequent losses through leaching and uptake by crops all through the years of cropping. The consistently higher crop yield recorded from the slash and burn treatments may be due to relative favourable soil environment as reflected in the level of labile organic matter contents of the treatment compared to others. Lal (1992) made similar observation. Further research would be required to quantify the contribution of left over above-andbelow grow ground phytomass to the soil organic C and N stock under the slash and burn clearing methods which tends to conserve soil labile C and N than the mechanical method in the humid tropics, though some similarities were observed between the slash and burn and bulldozed windrowed plots.
5. Conclusion The results obtained from this study indicate that the degree of losses (compared to the adjacent matured high forest) in labile soil C and N pools upon the conversion of a humid tropical forest to arable land depends on the method deployed in removing the natural vegetation. We observed that the magnitude of losses was in the order, bulldozed windrowed > bulldozed non-windrowed > slash and burn. While amongst the cropping systems, it was 5-year cropping > 4-yearcropping/2-year cassava fallow. However, the quantity of most of the labile C and N pools and crop yield obtained in the slash and burn and bulldozed non-windrowed treatments did not differ significantly. This tend to suggest that the non scraping of the top soil and retention of below-ground phytomass like roots in both land clearing methods may have resulted to the non significant variation in their effects. We could
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then say that any land clearing method that does not remove the top soil and retains some of the below-ground phytomass may be a sure way of reducing labile soil C and N losses upon the transformation of a humid into arable land. Consequently, due to the high cost and drudgery associated with slash and burn clearing method (Couper et al., 1981) as a viable alternative to mechanized land clearing for large-scale farming; the result obtained from this study suggest that bulldozed non-windrowed (mechanical) clearing method could be an option to slash and burn in ensuring reduced organic matter losses and drudgery. The study further indicates that allowing of fast growing biannual or semi-perennial crops as fallow could help in rebuilding the lost organic matter.
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