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Plant and Soil 0: 1–9, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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Comparative study of soil properties under Chromolaena odorata, Pueraria phaseoloides and Calliandra callothyrsus L.-S. Koutika1,3, J.G. Meutem Kamga2 & B. Yerima2 1 IITA/HFEC,

P.O. Box 2008 (Messa) Yaound´e, Cameroon. 2 Dschang University, Faculty of Agronomy and Agricultural Sciences, P.O. Box 222, Dschang, Cameroon. 3 Corresponding author∗

Received . Accepted in revised form

Key words: C mineralisation, fallow types, nutrient concentrations, POM status

Abstract Fallows improve soil fertility and allow sustainable agriculture. Soil fertility was assessed under different types of fallow through pH, nutrient concentrations and particulate organic matter (POM) quantity and quality. The two year-fallows were under Chromolaena odorata, Calliandra calothyrsuso and Pueraria phaseoloides on a Typic Kandiudult. Soils were sampled from 0–10 cm and from 10–20 cm depth. The weight of POM was 2 mg g−1 of soil under Calliandra, 3.9 mg g−1 under Chromolaena and 3.7 mg g−1 under Pueraria in the 0–10 cm layer. The tPOM-C (proportion of C in the total POM) and tPOM-N (proportion of N in total POM) were 26.1% and 14.5% under Calliandra, 39.6% and 18.8% under Chromolaena and 37.0% and 16.7% under Pueraria. However, despite the improvement of soil fertility under Pueraria as compared to planted Calliandra, the effect of Pueraria on nutrient concentration and POM status remained similar to that of Chromolaena. Calliandra increased soil acidity and allowed a deterioration of nutrient concentration (Ca, K), ECEC and an impoverishment of POM status. Abbreviations: SOM – soil organic matter; ECEC – effective cation exchangeable capacity; POM – particulate organic matter; OMF – organo-mineral fraction; tPOM – total POM; tPOM-C – proportion of soil C in total POM; tPOM-N – proportion of soil N in total POM.

Introduction Soil fertility is related to the ability of the soil to supply nutrient elements and provide good chemical, physical and biological conditions for a growing crop. In the tropics, the soils in general have an inherently low fertility, which decline rapidly after the conversion from natural forest or natural fallow to agricultural systems (Serrao et al., 1979; Andreux et al., 1990; Martins et al., 1991). In the humid tropics, this rapid decline is due among others to the low buffering capacity of the majority of tropical soils, which mainly contain low activity clay and have low nutrient reserves. These ∗ E-mail: [email protected]

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soils are also susceptible to soil compaction, crusting, and erosion. In the humid forest zone, such as of southern Cameroon, the high temperatures (> 26 ◦ C) and precipitation (> 1500 mm) lead to a rapid mineralisation of organic matter material and the loss of nutrients through leaching, as already reported for other tropical soils (de Boissezon, 1973; Koutika et al., 1999a). In this area, it is a common practice to use the natural fallow to restore nutrient availability, suppress weeds, pests and diseases and to increase crop yields and cropping system sustainability. Nowadays, the long fallow periods (more than 5 years) have become economically unjustified because of demographic pressure, and a short fallow (2–3 years) is now more common (Prinz, 1986; Swift, 1986). Therefore, researchers introduced a planted fallow system to improve the efficiency of adopting a short

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2 fallow, e.g., two years. The planted fallow may have a better potential to fix N2 (Vanlauwe et al., 1996; Tian et al., 1999). In fact, leguminous cover crops can be a promising planted fallow because potentially they can maintain soil N and build up soil organic matter (SOM). Besides beneficial effects of leguminous plants derived from their potential to fix N2 (LunaOrea et al., 1996), attention has been paid to introduce planted fallows such as Calliandra calothyrsus because of their high biomass (Duguma et al., 1994). In fact, Calliandra has been reported to be one of the most promising leguminous woody perennials for soil fertility management due to its adaptation to both acid and non-acid soils of the humid tropics (Duguma et al., 1994; MacQueen, 1993). Soil organic matter (SOM) quantity increases during the fallow period. SOM is considered to be the single most important indicator of soil quality (Larson and Pierce, 1991), because of its influence on the physical, chemical and biological properties of the soil (Karlen et al., 1997). In most tropical systems, the decomposition of SOM remains a major source of nutrients to the growing plant. In fact, SOM is associated with improvement or depletion of soil fertility, even though the relationship is complex and rarely proportional (Woomer et al., 1994). SOM status has been studied using several indicators. In some studies chemical parameters were used (Duxbury et al., 1989; Viterello et al., 1989); in others physical parameters (Christensen, 1992; Hassink, 1995; Koutika et al., 1999b). Moreover, as SOM changes slowly after the conversion from one system to another, its components such as particulate organic matter (POM) and specific respiration, that serve as early indicators of SOM changes, may be useful in the evaluation of soil quality (Sikora et al., 1996; Vanlauwe et al., 1999). As already found by several researchers, the fractionation of SOM into POM is necessary for a clear understanding of its distribution and contribution to the nutrient status (Cambardella and Elliot, 1992; Hassink, 1995; Vanlauwe et al., 1998). In an Ultisol located in southern Cameroon, Koutika et al. (2001) found an improvement of SOM quality through an increase in N content of POM under Pueraria compared to Chromolaena. In the present paper, after two years of fallow, the nutrient concentrations, C mineralization and POM status were determined in the 0–10 and 10–20 cm layers to compare soil fertility under Calliandra, Pueraria and Chromolaena fallow on a Typic Kandiudult. There were two hypotheses in the current study: (i) nutrient concentrations

and POM quantity and quality i.e., the POM N contents, will be higher and C mineralization lower under the planted fallow (Pueraria and Calliandra) than under the natural fallow (Chromolaena) and (ii) because of the high biomass and specific characteristics of Calliandra, nutrient concentrations and SOM status (POM and C mineralization) will be higher under the former than under Pueraria. The objectives of the study were: (1) to assess and compare the nutrient concentrations and SOM status under the three different fallow types; (2) to define the most appropriate fallow type for a sustainable agriculture in the studied humid forest zone.

Materials and methods Localisation and field history The sites were cleared in January 1994 at the Humid Forest Ecoregional Center of International Institute of Tropical Agriculture (HFEC/IITA), within the Mbalmayo Forest Reserve in the humid forest zone of southern Cameroon (3◦ 51 N and 11◦ 27 E). Average annual rainfall is 1513 mm in a bimodal distribution. Rains commence in March and end in early July, followed by a short dry season of six to eight weeks, then recommence in September and stop at the end of November. Average annual insolation is 1645 h. The soil is an isohyperthermic, Typic Kandiudult and is acid in the subsoil. The study was established in 1994 in an area dominated by Chromolaena odorata (L.) R. M. King & Robinson. The experiment was initially a randomised complete block designs with three blocks and one factor, at three levels: (1) low planting density, (2) high planting density and (3) high planting density plus interplanting of hedgerows of Calliandra calothyrsus. Plots were cropped for two consecutive years to a groundnut/maize/cassava/plantain intercrop. At the end of April 1996, after the final groundnut, maize and cassava harvest, plots were converted into three fallow types: (1) natural regrowth (mainly Chromolaena), (2) Pueraria phaseoloides by broadcasting 8 kg of seeds ha−1 , and (3) alleys of Calliandra calothyrsus already planted in 1994. At this time, the hedgerows Calliandra treatment were pruned and the plot weeded. Plots remained in fallow for two years. Neither fertilisers nor herbicides were used during the fallow period. At the end of the fallow period in February 1998, twelve places were marked in each plot,

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3 e.g., three lines separated each from another by 12 m. In each line four places separated by 6 m were selected. For the Calliandra treatment, soil samples were taken in the middle alley and under the hedgerows and mixed in the laboratory before analyses. The soil under Calliandra may be slightly different from those under Pueraria and Chromolaena (see the sand/clay ratios and the water-dispersed clay (WDC)).

dispersion, the suspension was wet-sieved to separate the 4000–2000 µm, 2000–250 µ, 250–53 µm, and 53–20 µm fractions. In the three larger fractions, the organic components obtained were coarse (4000–2000 µm), medium (2000–250 µm) and fine (250–53 µm). All the fractions were dried at 65 ◦ C. Organic matter C and total N in the coarse, medium, fine POM fractions and in the organo-mineral fraction (53–20 µm) were determined as described above.

Soil sampling and standard soil analyses Statistical data analysis As described above, twelve samples were collected from each plot with an auger (twelve places in each plot) at 0–10 and 10–20 cm depths. Soils were air-dried, crushed and passed through a 4-mm sieve before analysis. Soil texture was determined by the method of chemical dispersion using hexametaphosphate (soil to liquid ratio,1:20, wt/wt). Ca2+ , Mg2+ , K+ and P were extracted by the Mehlich-3 procedure (Mehlich, 1984). Cations were determined by atomic absorption spectrophotometry and P by the malachite green colorimetric procedure (Motomizu et al., 1983). Soil pH was determined in water and in 1 M KCl at a soil to solution ratio of 2:5. Organic C was determined by chromic acid digestion and spectrophotometric procedure (Heanes, 1984). Total N was determined using the Kjeldahl method for digestion and ammonium electrode determination (Bremner and Tabatabai, 1972; Nelson and Sommers, 1972).

All studied parameters were subjected to statistical analysis by PROC MIXED mixed procedures in SAS (1989). Significant differences between treatments mentioned in the text are significant at P < 0.05.

Results Soil texture and acidity The lowest pH-H2 O and pH-KCl were found under Calliandra in the 0–10 cm layer (Table 1).
pH (the difference between pH-H2O and pH-KCl) was lowest under Pueraria. In the 10–20 cm layer, pH-H2 O and pH-KCl values were higher under Pueraria than under Calliandra and Chromolaena fallow (Table 1). C, N contents and nutrient concentrations

C mineralization Ten g of pre-incubated soil, moistened with 2 mL of deionised water was collected in a 250 mL plastic bottle and plastic lids containing 10 mL of NaOH (0.1 M) were placed into the plastic bottles (to capture the CO2 ), and incubated at 28 ◦ C. After 1, 2, 4, and 6 weeks, soil was harvested and then CO−2 3 in the NaOH was determined by titration with 0.1 M HCl on the Radiometer PHM 82 Standard pH meter using a TT 80 Titrator Radiometer and ABV 80 Autoburette Radiometer. Soil organic matter fractionation SOM was fractionated according to the procedure described by Koutika et al., (2001). Soils were dried at 65 ◦ C for 12 h and 100 g of dry soil was dispersed in 100 mL of Na-hexametaphosphate-Na-carbonate solution and 400 mL distilled water by shaking for 16 h on an end-over-end shaker at 140 rev.min−1. After

In the 0–10 cm layer, C and N contents were higher under Chromolaena than under Calliandra and Pueraria (Table 2). In the 10–20 cm layer, the C content remained higher under Chromolaena than under Calliandra and Pueraria. The lowest C/N ratio was found under Pueraria. The lowest available P concentrations were found under Calliandra in both depths (Table 2). Soil exchangeable Ca concentration was lower under Calliandra than under Chromolaena at 0–10 cm depth, whereas in the 10–20 cm layer exchangeable Ca was lower under Calliandra than under Pueraria (Table 2). In the 10–20 cm layer, exchangeable Mg was significantly higher under Calliandra than under Pueraria. Exchangeable K concentrations were lower under Calliandra than under Chromolaena and Pueraria at 0– 10 cm depth. ECEC was higher under Chromolaena than under Calliandra in both layers, while no difference in ECEC was found between Chromolaena and Pueraria (Table 2).

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4 Table 1. Particle size distribution, water-dispersed clay (WDC) and pH of soil under Calliandra, Chromolaena and Pueraria after 2 years of fallow. (SED = standard error deviation) Fallow types

Sand (%)

Silt (%)

Clay (%)

Sand/clay

WDC (%)

pHH2O

pHKcl


pH

0–10 cm Calliandra Chromolaena Pueraria SED

62.3 59.5 62.8 0.6

16.0 15.4 15.8 0.2

21.7 25.1 21.4 0.5

2.96 2.44 3.01 0.09

9.2 10.6 8.7 0.3

5.94 6.16 6.21 0.07

5.14 5.39 5.57 0.08

0.80 0.70 0.65 0.03

10–20 cm Calliandra Chromolaena Pueraria SED

57.2 55.9 57.4 0.7

13.8 13.5 14.0 0.2

29.0 30.6 28.6 0.7

2.04 1.87 2.06 0.07

7.8 9.8 8.8 0.5

5.51 5.54 5.92 0.08

4.49 4.47 4.88 0.09

1.02 1.07 1.04 0.03

Table 2. Organic C, total N, available P and exchangeable cation concentrations under Calliandra, Chromolaena and Pueraria. (SED = standard error deviation) Fallow types

C (%)

N (%)

C/N ppm

P cmol.kg−1

Ca cmol.kg−1

Mg cmol.kg−1

K cmol.kg−1

ECEC

0–10 cm Calliandra Chromolaena Pueraria SED

2.15 2.35 2.19 0.07

0.14 0.16 0.15 0.00

15.4 15.0 15.4 0.4

3.13 3.97 4.39 0.49

3.9 5.2 4.7 0.5

1.00 1.02 0.9 0.04

0.11 0.17 0.16 0.00

5.0 6.4 5.8 0.5

10–20 cm Calliandra Chromolaena Pueraria SED

0.99 1.14 0.89 0.04

0.07 0.07 0.06 0.00

15.5 15.4 14.0 0.4

0.59 1.08 0.93 0.10

1.9 2.3 2.4 0.2

0.66 0.51 0.47 0.03

0.06 0.06 0.09 0.00

2.7 3.2 3.0 0.2

C mineralization There was no difference in C mineralisation within the three fallow types in the 0–10 cm layer (Figure 1a). In the 10–20 cm layer, C decomposition was lower under Calliandra during the 4 weeks of incubation (Figure 1b), while C decomposition was not different in soil under Chromolaena and Pueraria. Particulate organic matter status In both layers, the tPOM dry matter content, i.e., the sum of the concentrations of the coarse, medium and fine POM fractions, was lower under Calliandra than under Chromolaena and Pueraria (Figure 2a and b). The tPOM-C and tPOM-N (Figure 2c, d, e, and f) were also lowest under Calliandra in both layers. At

0–10 cm depth, the content of cPOM (coarse POM), mPOM (medium POM) and fPOM (fine POM) was lower under Calliandra than under Chromolaena and Pueraria (Figure 3a, c, e). In the 10–20 cm layer, the content of cPOM was lower under Pueraria (Figure 3b), while the contrary was found for fPOM (Figure 3f). The weight of mPOM at 10–20 cm depth was lowest in Calliandra (Figure 3d). The proportion of cPOM-C in soil was highest under Chromolaena in the 0–10 cm layer, while the mPOM-C was unaffected by the fallow type (Table 3). The proportion of fPOM-C and the OMF-C in soil C were lower under Calliandra than under the two other fallows. The proportion of cPOM-N in soil N was highest under Chromolaena (0–10 cm) and under Calliandra (10– 20 cm). The proportion of mPOM-N in soil N was not

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Figure 1. C mineralisation at 28 ◦ C during 4 weeks in 0–10 cm (a) and 10–20 cm (b) depth.

different between fallows. The proportion of fPOM-N in soil N was lowest under Calliandra in both layers. Generally, the proportion of OMF-N in soil N was higher than of any of the POM fractions and particularly high in the 10–20 cm layer. In both layers, the lowest value was found under Chromolaena fallow.

Discussion In the current study, there is an improvement of soil acidity under Chromolaena and Pueraria fallow, and a deterioration under Calliandra. In pasture systems of 7 to 17 years old located in the eastern Brazilian Amazon, the relationship between WDC/TC and
pH was used as an indicator of acid functional group status (Koutika et al., 1997). In the layer of 0–10 cm depth, Pueraria fallow decreases soil acidity as indicated by a decrease of the
pH value compared to Chromolaena and Calliandra fallow. In contrary, Calliandra increased soil acidity compared to Chromolaena and Pueraria as indicated by an increase in the
pH value in the 0–10 cm layer. This result may indicate a decrease in organic acid functional groups under Chromolaena and Pueraria and an increase in acid functional groups under Calliandra in the 0–10 cm layer. The soil acidity improvement under Pueraria was also found earlier by other authors. Tian et al. (1999) found that Pueraria maintains or improves soil properties in the soils of Southwest Nigeria. In the current study, it appears that Chromolaena is a good fallow option in the humid forest zone, according to soil acidity and nutrient concentrations. Kanmegne et al. (1999) had shown that Chromolaena residues decomposed quickly and lead to an improvement of soil properties. There was no difference on POM status between Pueraria and Chromolaena fallow in the current study, even though Koutika et al.

(2001) found that Pueraria increased the N content of POM, compared to natural regrowth mainly composed of Chromolaena in an Ultisol. In addition, Koutika et al. (2002) found an improvement of soil acidity under Pueraria compared to Chromolaena in soils with low soil acidity. In that study, the decrease in pH values under Pueraria was found mainly in the 0–10 cm zone, probably due to the short period of fallow (2 years), as incorporation in the deeper layers of newly added organic matter increases with age of vegetation cover. Morover, in soils with high soil acidity and Al saturation in the humid forest zone, Chromolaena seems a more adapted fallow than Pueraria according to nutrient concentrations and POM status (Koutika et al., 2002). From the previous findings and the current study, it appears that Pueraria performed better or similar than Chromolaena in soils with low soil acidity. Therefore, in the current study, the POM status of Pueraria is not different from that of Chromolaena probably because of the low soil acidity of the studied Typic Kandiudult. However, in the soils with chemical constraints (high soil acidity and Al saturation), Chromolaena does better than Pueraria (Koutika et al., 2002). The C decomposition rate is lower under Calliandra than under Chromolaena and Pueraria in the 10–20 cm layer. The higher decomposition rate under Chromolaena and Pueraria may have involved the high POM weight and the increase in nutrient concentrations relative to Calliandra. This indicates that when the nutrient concentrations and the POM weight increased, the C decomposition is faster. The low nutrient concentrations and POM quantity and quality under Calliandra may be due to the fact that its biomass is more in the woody parts than in the leaves. In addition, the important root system of Calliandra contributes to stock nutrient elements. Besides

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Figure 2. The weight of tPOM: (a) in 0–10 cm and (b) in 10–20 cm; tPOM-C at: (c) in 0–10 cm and (d) in 10–20 cm; tPOM-N at: (e) in 0–10 cm and (f) in 10–20 cm depth.

its high living biomass (two times higher than that of Chromolaena and Pueraria), its beneficial effect for apiculture, animal fodder and weed suppression, and its adaptation to poor soils (Nadiar, 1979; Palmer and Schlink, 1992; Duguma et al., 1994), Calliandra induced a low soil fertility compared to Chromolaena and Pueraria. The low soil fertility under Calliandra is characterised by a deterioration of soil acidity and an impoverishment of the nutrient concentrations and the POM status in the studied Typic Kandiudult of the humid forest zone. Other studies conducted in other soil types in southern Cameroon confirmed the deterioration of soil fertility under Calliandra com-

pared to natural fallow of two different ages (Koutika et al., unpublished data). However, Calliandra fallow is not always inferior to Chromolaena and Pueraria fallow. On very poor soils, when Pueraria and Chromolaena will not grow, Calliandra sometimes does better (Duguma et al., 1994, 1998). This study shows that POM, the more active part of SOM, is underlying the concept that even when no significant or only small changes occur in total C and N after cover vegetation changes, POM does change (Sikora et al., 1996; Vanlauwe et al., 1999; Koutika et al., 2001). Thus, POM fractionation might be used to evaluate soil quality in the fallow systems of the

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Figure 3. The weight of cPOM in: (a) 0–10 cm and (b) 10–20 cm layer; mPOM: (c) 0–10 cm and (d) 10–20 cm; fPOM in: (e) 0–10 cm and (f) 10–20 cm. Table 3. The proportion of total soil C and N in cPOM, mPOM, fPOM and OMF in soil under Calliandra, Chromolaena and Pueraria after 2 years of fallow. (SED = standard error deviation) Fallow types

coarse POMC medium POM C fine POM C OMF C coarse POM N medium POM N fine POM N OMF N (%) (%) (%) (%) (%) (%) (%) (%)

0–10 cm Calliandra Chromolaena Pueraria SED

3.7 7.7 5.0 0.6

14.2 17.4 16.7 1.9

8.3 14.5 15.3 1.0

6.0 10.4 8.6 1.4

1.7 2.8 1.6 0.2

7.2 7.2 6.9 0.8

5.7 8.8 8.4 0.9

13.8 11.7 16.0 2.9

10–20 cm Calliandra Chromolaena Pueraria SED

1.7 2.5 1.7 0.5

6.4 7.7 7.2 1.2

3.1 7.3 8.6 0.9

9.6 19.9 10.9 1.5

2.1 1.3 1.1 0.6

2.8 2.5 4.4 0.9

2.0 5.7 3.5 0.7

29.9 25.7 32.4 5.9

humid forest zone, where usually living biomass is burnt and SOM is assessed by aboveground vegetal material. This study is also indicates that after a short fallow period (2 years) there is usually no difference in total C and N, whereas differences were found in the most active part of SOM, POM, which may indicating the trend of soil quality under an appropriate fallow type.

The impact of fallow types on soil quality strongly depends on soil types. The results found in the current study do confirm neither the first nor the second hypotheses. The effect of Pueraria was equal to that of Chromolaena in the present study showing that it is not necessary to replace Chromolaena by Pueraria, even though the beneficial effect of Pueraria on N content of POM was found comparable to Chromolaena

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8 elsewhere (Koutika et al., 2001, 2002). Since other studies have shown that Chromolaena does better or is equal to Pueraria in acid soils, Chromolaena may be used as fallow, rather than to be considered as weed. Since soil properties deteriorated under Calliandra compared to Chromolaena and Pueraria, it is not a good option to use Calliandra as fallow in the studied Typic Kandiudult. According to the values of nutrient concentration and POM status, it seems that for sustainable agriculture in the studied humid forest area, the use of fallow (planted or natural) appears to be insufficient.

Acknowledgements Authors thank IITA staff for physical, chemical and SOM analyses.

References Andreux F, Cerri C C, Eduardo de P B and Choné Th 1990 Humus contents and transformations in native and cultivated soils. Sci. Total Environ. 90, 249–265. Boissezon P de 1973 Les matières organiques des sols ferrallitiques. In La matière organique et la vie dans les sols ferrallitiques. T4 pp. 10–55, ORSTOM Bondy. Bremner J M and Tabatabai M A 1972 Use of an ammonia electrode for determination of ammonium in Kjeldahl. Analysis 3, 159– 165. Cambardella C A and Elliot E T 1992 Particulate soil organic matter changes across a grassland cultivation sequence. Soil Sci. Soc. Am. J. 56, 777–783. Christensen B T 1992 Physical fractionation of soil organic matter in a primary particle size and density separates. Adv. Soil Sci. 20, 1–89. Duguma B, Tonye J, Kanmegne J, Manga T and Enoch T 1994 Growth of ten multipurpose tree species on acid soils of Sangmelima, Cameroon. Agro. Systems 27, 107–119. Duguma B and Mollet M 1998 Early growth of Calliandra calothyrsus (Meissner) provenance’s in the acid soils of southern Cameroon. Agro. Systems 40, 283–296. Duxbury J M, Smith M S and Doran J W 1989 Soil organic matter as a source and a sink of plant nutrients. In Dynamics of soil organic matter in tropical ecosystems. Eds. D C Coleman et al. pp. 33–67. Univ. of Hawaii Press, Honolulu. Hassink J 1995 Decomposition rate constants of size and density fractions of soil organic matter. Soil Sci. Soc. Am. J. 53, 773– 778. Heanes D L 1984 Determination of total C in soils by an improved chromic acid digestion and spectrometric procedure. Comm. Soil Sci. Plant Anal. 15, 1191–1213. Kanmegne J, Duguma B, Henrot J and Isirimah N O 1999 Soil fertility enhancement by planted tree-fallow species in the humid lowlands of Cameroon. Agro. Systems 46, 239–249. Karlen D L, Mausbach M J, Doran J W, Glie R G, Harris R F and Schuman G E 1997 Soil quality: A concept definition, and framework for evaluation. Soil Sci. Soc. Am. J. 61, 4–10.

Koutika L S, Bartoli F, Andreux F, Cerri C C, Burtin G, Chone Th and Philippy R 1997 Organic matter dynamics and aggregation in soils under rain forest and pastures of increasing age in the eastern Amazon Basin. Geoderma 76, 87–112. Koutika L S, Andreux F, Hassink J, Choné Th and Cerri C C 1999a Characterisation of soil organic matter in the topsoils under rain forest and pastures in the eastern Brazilian Amazon Basin. Biol. Fert. Soils 29, 309–313. Koutika L S, Chone T, Andreux F, Burtin G and Cerri C C 1999b Factors influencing carbon decomposition of topsoils from the Brazilian Amazon Basin. Biol. Fert. Soils 28, 436–438. Koutika L S, Hauser S and Henrot J 2001 Soil organic matter in natural regrowth, Pueraria phaseoloides and Mucuna pruriens fallow. Soil Biol. Biochem. 33, 1095–1101. Koutika L S, Sanginga N, Vanlauwe B and Weise S 2002 Chemical properties and soil organic matter assessment in fallow systems in the forest margins benchmark. Soil Biol. Biochem. 34, 757– 765. Larson W E and Pierce F J 1991 Conservation and enhancement of soil quality. In Evaluation of sustainable management in the developing world. IBSRAM Proc.121 (2) pp. 175–203. Int. Board for Soil Res. Man. Bangkok. Luna-Orea P, Wagger M G and Gumpertz M L 1996 Decomposition and nutrient release dynamics of two tropical legume cover crop. Agro. J. 88, 758–764. MacQueen J D 1993 Calliandra Series Racemosae: Taxonomic Information; OFI seed collections; Trial design. Oxford Forestry Institute, Department of Plant Sciences, University of Oxford, 157 pp. Martins P F S, Cerri C C, Volkoff B, Andreux F and Chauvel A 1991 Consequences of clearing and tillage on the soil of a natural Amazonian ecosystem. For. Ecol. Man. 38, 273–282. Mehlich N W 1984 Mehlich 3 soil test extractant: a modification of the Mehlich 2 extractant. Comm. Soil Sci. Plant Anal. 15, 1409– 1416. Motomizu S, Wakimoto P and Toei K 1983 Spectrophotometric determination of phosphate in river waters with molybdate and malachite green. Anal. 108, 361–367. Nadiar S 1979 Knowing the land of honey, Gunung arca. Duta Rimba 5(33), 15–19 (cited in MacQueen, 1993). Nelson D W and Sommers L E 1972 A simple digestion procedure for estimation of ammonium in Kjeldahl soils. J. Environ. Qual. 1, 423–425. Palmer B and Schlink A C 1992 The effect of drying on the intake and rate of digestion of shrub legume Calliandra calothyrsus. Trop. Grassland 26, 89–93. Prinz D 1986 Increasing the productivity of smallholder farming system by introduction of planted fallow. Plant Res. Develop. 24, 76–85. SAS 1989 SAS Stat User’s Guide, Version 6, Fourth Edition, Volume 2. Cary NC. SAS Institute Inc. 846 pp. Serrao E A S, Falesi I C, da Veiga J B and Texeira Neto J F 1979 Productivity of cultivated pastures on low fertility soils of the Amazon region of Brazil. In Pasture production on acid soils of the tropics. Eds. P A Sanchez and T E Tergas. pp. 195–225. CIAT, Cali, Colombia. Sikora L J, Yakovchenko V, Cambardella C A and Doran J W 1996 Assessing soil quality by testing organic matter. In Soil organic matter analysis and interpretation. Eds. Magdoff F R et al. pp. 41–50. SSSA Spec. 46 Publ. Madison. Swift M J 1986 Enhancement of tropical fertility: The role of biological research. Annual Report ISRIC, Wageningen, the Netherlands.

P5272279.tex; 22/03/2004; 8:52; p.8

9 Tian G, Kolowale G O, Salako F K and Kang B T 1999. An improved cover crop fallow system for sustainable management of low activity clay soils of the tropics. Soil Sci. 1649, 671–682. Vanlauwe B, Nwoke O C, Sanginga N and Merckx R 1996 Impact of residue quality on the C and N mineralisation of leaf and root residues of three agroforestry species. Plant Soil 183, 221–231. Vanlauwe B, Sanginga N and Merckx R 1998. Soil organic matter dynamics after additing of Nitrogen −15-labelled Leucaena and Dactyladenia residues. Soil Sci. Soc. Am. J. 62, 461–466. Vanlauwe B, Nwokoe O C, Sanginga N and Merckx R 1999 Evaluation of methods for measuring microbial biomass C and N and

their relationship with soil organic matter particle size classes for soils from the West-African moist savanna zone. Soil Biol. Biochem. 31,1071–1082. Viterello V A, Cerri C C, Andreux F, Feller C and Victoria R L 1989 Organic matter and natural carbon distribution in forested and cultivated oxisols. Soil Sci. Soc. Am. J. 53, 773–778. Woomer P L, Martin A, Albrecht A, Resck D V S and Scharpenseel H W 1994 The importance and management of soil organic matter in the tropics. In The Biological Management of Tropical Soil Fertility. Eds. P L Woomer and M J Swift. pp. 47–79. A Wiley-Sayce Publication, UK.

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