Production and nutrient content of earthworm casts in a tropical
agrisilvicultural system
L. Norgrove
a,b,*, S. Hauser
ba
Division of Life Sciences, King's College, University of London, London W8 7AH, UK
b
International Institute of Tropical Agriculture, Humid Forest Ecoregional Centre Mbalmayo, BP 2008 (Messa) Yaounde, Cameroon
Accepted 14 March 2000
Abstract
Earthworm surface cast production and nutrient turnover through casts were measured for 3 years in a 17-year-old timber plantation in southern Cameroon after selective reduction to two timber stand densities (TSDs) and understorey cropping with plantain and tannia. Neither understorey cropping nor timber stand density treatments had signi®cant eects upon cast production in any year. Mean cast production in cropped plots was 35.7 Mg haÿ1in the ®rst year, 34.9 Mg haÿ1 in year 2 and
30.1 Mg haÿ1 in year 3. This was 63%, 84%, and 65%, respectively, of cast production in undisturbed, unthinned timber plantation plots. In comparing cast nutrient concentrations between years 1, 2 and 3, there were highly signi®cant correlations for nearly all nutrients in control plots but fewer such correlations in the low TSD. In cropped plots, none of earthworm parameters was correlated with nutrient concentrations of slash or amounts of slash applied to the soil surface at establishment. There were positive correlations between cast N and litterfall N (kg haÿ1 yÿ1) in both year 2 (cast N = 0.49
litterfall N + 61.7,r20:46 and year 3 (cast N = 0.38litterfall N + 55.6,r20:42). The ratio of soil:cast nutrient concentrations were
related by negative power functions to soil nutrient concentrations for all nutrients and organic carbon. This suggests that earthworm communities adjust to lower soil nutrient concentrations by increasing their selectivity and thus produce relatively higher quality casts on poorer soil.72000 Elsevier Science Ltd. All rights reserved.
Keywords:Agroforestry; Southern Cameroon;Terminalia ivorensis; Timber stand density
1. Introduction
A central tenet in agroforestry research in the humid tropics is that trees improve or maintain soil fertility and ecosystem function (Sanchez et al., 1985; Ingram, 1990). Brown et al. (1994) hypothesised that soil
ferti-lity in tropical, derived systems is maintained `by
mimicking a forest ecosystem with perennials or peren-nial and annual mixtures'. However the mechanisms by which this is realised; maintaining soil organic matter levels, deep nutrient capture from subsoil layers,
tigh-ter nutrient cycling, reduction of topsoil acidity through base cation cycling, and shade from the tree canopy improving soil biological activity and nitrogen mineralisation, have not all been proven or only so under limited circumstances (Sanchez, 1995). The im-portance of earthworms in these mechanisms is con-sidered to be high (Swift et al., 1994; Lavelle et al.,
1998), as ecosystem engineers' (sensu Lavelle, 1994) in
litter transformation, and nutrient and organic carbon deposition (Hauser et al., 1997; Norgrove et al., 1998).
When trees are removed from land-use systems, such as when land is cleared, slashed, burned and tilled in slash and burn agriculture, earthworm density, diver-sity and activity are reduced. This has been shown in south-west Nigeria (Critchley et al., 1979; Cook et al., 1980; Hauser and Asawalam, 1998) where the
earth-worm fauna is dominated by Hyperiodrilus africanus
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* Corresponding author. IITA Cameroon, c/o Lambourn, 26 Ding-wall Road, Croydon CR9 3EE, UK. Tel.: +44-237-206-774; fax: +44-237-237-437.
(Beddard) and the epigeic Eudrilus eugeniae (Kinberg) (Madge, 1969); in the Peruvian Amazon (Lavelle and Pashanasi, 1989); in southern Cameroon (Norgrove et al., 1998) and in south east Mexico (Fragoso et al., 1993, 1995). When natural forests have been replaced by forest plantations, earthworm populations have been maintained or even increased (Blanchart and Julka, 1997) in some cases, yet reduced to lower levels in others (Zou and GonzaÂlez, 1997; GonzaÂlez et al., 1996). Zou and Bashkin (1998) showed that when pre-viously cropped land was reforested, earthworm popu-lations recovered; however, epigeic and anecic species, present in natural successional systems, remained absent.
While forest cover may be the most sustainable land-use in the humid tropics, it is unfeasible in areas with high human population densities; smallholder farmers need the land to meet their immediate need, food production. One compromise, postulated to maintain ecosystem function, and provide food and timber is combining silviculture with agriculture: grow-ing food crops between tree seedlgrow-ings at plantation establishment and then introducing a cropping phase at each routine thinning of the timber plantation, usually every 6 years. Cropping during timber
planta-tion establishment, known as `taungya', is viable and
well documented, yet there have been no studies on crop production later in the silvicultural cycle. Lower timber stand densities than those normally used in tim-ber plantations may be needed to allow sucient light and reduced competition for the understorey crop to produce acceptable yields. Any reduction in timber stand density may aect ecosystem functioning, earth-worm activity and nutrient cycling. There has been lit-tle research on the in¯uence of thinning upon nutrient cycling in plantations (Carlyle, 1995) and any change may depend on the crop management system used for the understorey crops.
Here we establish the eects of two timber stand
densities (192 and 64 trees haÿ1), imposed by selective
felling and various low-input crop management sys-tems, upon surface cast production and nutrient
depo-sition by earthworms in a 17-year-old timber
plantation with understorey crops in the humid forest zone of Cameroon for three years. Cast production was also monitored in undisturbed timber plantation control plots.
2. Materials and methods
2.1. Site
The experiment was established in April 1995 in a
17-year-oldTerminalia ivorensis A. Chev. plantation in
the Mbalmayo Forest Reserve in southern Cameroon
(38 51N and 118 27E). Initial land clearance in 1978
was manual (Lawson, 1995). The land is covered with semi-deciduous adult and young secondary forest. Alti-tude is 540 m above sea level. Average annual rainfall is 1513 mm in a bimodal distribution. Rains usually start in March and end in early July, followed by a short dry season of 6 to 8 weeks, then recommence in September and stop at the end of November. In the years described here, annual rainfall was 1327, 1586 and 1685 mm, respectively. Average annual insolation is 1645 h. The soil is classi®ed as a clayey, kaolinitic, isohyperthermic, Typic Kandiudult (Hulugalle and Ndi, 1993) and is acid in the subsoil. The earthworm fauna of the Reserve has not been fully described. However, in a preliminary assessment, four earthworm
species were found: Rosadrilus camerunensis Cognetti;
Eminoscolex lamani Michaelsen; Dichogaster munda-mensis (syn Diplothecodrilus mundamensis) Michaelsen;
Nematogenia panamaensis Eisen (Csuzdi pers. comm.). The majority of casts produced were globular; few were turret-shaped. Maximum diameter was approxi-mately 10 cm. Sub-surface casts have not been found in Mbalmayo after sampling down to 2 m in pits in 80 locations in this experiment and cropped ®elds with various degrees of soil compaction. It is thus assumed that here most of casts are egested on the soil surface (Norgrove and Hauser, 1998).
2.2. Design and establishment
The experiment had a factorial split-plot design with timber stand density (TSD) treatments as main-plots:
T. ivorensisat 192 trees haÿ1(high TSD), and; T. ivor-ensis at 64 trees haÿ1(low TSD). The experiment was
replicated in four blocks. Main-plots were 50 m50
m. Each main-plot was divided into four sub-plots, 25
m 25 m, containing four crop management
treat-ments: sole plantain (Musa spp. AAB sub-group
plan-tain `French' cv. `Essong', Musaceae) mulched with
the slashed material; sole tannia (Xanthosoma
sagittifo-lium (L.) Schott, Araceae), intercrop of plantain-tannia
mulched with the slashed material; and sole plantain, where the slashed material was burned before planting.
In addition, there was an undisturbed, non-cropped T.
ivorensis stand (control), not divided into sub-plots, in each of the four blocks.
Manual slashing of the understorey was started on 11 April, 1995 and the tree stand was mapped. TSDs were imposed on 27 April 1995 by felling the shorter or crippled trees ®rst, leaving the marketable timber trees. In the plots to be burned, slash was removed from the zones within 50 cm of standing tree trunks to prevent scorching. Plots were burned on 1 May 1995.
Plantain planting density was 1600 plants haÿ1 on a
2.5 m2.5 m square con®guration thus 100 plants per
haÿ1on a 0.7 m0.7 m square con®guration. Planting was from 2±6 May 1995. Plots were weeded by manual slashing with machetes approximately every four months.
2.3. Measurement of surface casting
Surface casting was measured in 0.5 m 0.5 m
frames (after Evans and Guild, 1947), with four frames per plot. Given the inherent heterogeneity within a tree stand in terms of distance from the trees, the two most frequent strata at the given densities were calculated as 7 m from one tree for the low density and within 8 m of four trees in the high density. Frames were ran-domly allocated within these strata. Areas within 2.5 m of plot borders were not used.
Any cast material found in frames before sampling started was discarded. All surface casts were collected from the frames twice per week from 1 June, 1995 until cast production ceased. Frames were checked for cast production after the ®rst rains in March 1996 and March 1997 and cast collection continued until casting
ceased. Casts were dried at 658C for 48 h after each
sampling and the dry weight per frame was recorded. From cropped plots, cast material was pooled by plot and by year. For the control plots, material was pooled by year for each frame separately. Casts were analysed for pH, organic C, total N and exchangeable (exch.) cations after the end of each casting season. Casts in the second year only were analysed for tex-ture.
2.4. Soil and mulch sampling
Soil was sampled in May 1995 at planting and after burning. Nine samples per sub-plot were taken on a rigid system at 3.5 m, 10.5 m, 17.5 m, 24.5 m and 31.5 m along the 35 m sub-plot diagonals. Mulch samples
were taken simultaneously from a 1.41 m 1.41 m
area at each sampling point. Soil was then sampled at ®ve points, at the corners and centre of the sampling area, and then bulked by depth: 0±10, 10±20, 20±30, 30±50, 50±70, and 70±100 cm. Samples at 0±10 cm and 10±20 cm depth were taken with soil cores (50 mm long and 50 mm diameter); deeper layers were sampled using an auger with an internal diameter of
25 mm. Soil and mulch samples were dried at 658C to
constant weight. Soil samples were passed through a 2.0 mm sieve. A sub-sample of each was ground to 0.5 mm and analysed for pH, organic C, total N and exch. cations. Due to cost constraints, only soil samples from plantain mulched, plantain burned and control plots were analysed. Mulch samples were bulked by sub-plot, ground to pass through a 0.5 mm sieve and retained for chemical analysis.
2.5. Litterfall sampling
Litterfall was collected for 2 years from September 1995 (Norgrove and Hauser, submitted). Litterfall was sampled in traps consisting of a circular metal frame
of 0.28 m2, mounted 1 m above the ground on a single
wooden prop. Eight litter-traps were set per main-plot. The traps were arranged at 14, 28, 42 and 56 m along the main-plots' 70 m diagonals. Litterfall was collected every week for 2 years and airdried. Samples were
bulked over 4 weeks, separated intoT. ivorensisleaves,
fruits, and non-T. ivorensis leaves and other material,
predominantly twigs. Branch fall (>5 cm diameter) was not sampled. Bulked samples were oven-dried at
658C for 4 days then weighed to 110ÿ4g resolution.
Data on litter is used here for correlations only.
2.6. Soil and cast analysis
pH was determined in a water suspension at a 2:5
soil:water ratio. Exchangeable (exch.) Ca2+, Mg2+,
K+ and total P were extracted by the Mehlich-3
pro-cedure (Mehlich, 1984). Cations were determined by atomic absorption spectrophotometry and P by the malachite green colorimetric procedure (Motomizu et al., 1983). Organic C was determined by Heanes' improved chromic acid digestion and spectrophoto-metric procedure (Heanes, 1984). Total N was deter-mined using the Kjeldahl method for digestion and ammonium electrode determination (Bremner and Tabatabai, 1972). Soil and cast texture was determined using Day's procedure (Day, 1965)
2.7. Litter and mulch analyses
Samples were ground to 0.5 mm. Samples were digested according to Novozamsky et al. (1983). Total N was determined with an ammonium sensitive elec-trode (Powers et al., 1981). Total Ca, Mg, and K were analysed by atomic absorption spectrophotometry.
2.8. Statistical analysis
Rohlf, 1995). Exploratory analysis revealed that sample variances of cast amounts and nutrient concen-trations were not correlated with the means so analyses were performed on untransformed data. Dierences
with probabilities of P<0:05 are mentioned with the
probability class P<0:05, P<0:01, P<0:001 in
brackets.
3. Results
3.1. Quantities of casts
Cast collection started on 1 June in y1, after the start of the casting season. Casting ceased on 18 December, 1995, 29 weeks later. In y2, cast production started on 21 March and ceased on 6 December, 38 weeks later. In y3, cast production started on 19 March and ceased on 8 January the following calendar year, 42 weeks later.
Cast production was greater in the control than in
the cropped plots in y1 and y3 P<0:05), however,
there was no signi®cant dierence in y2 (Table 1). There was no signi®cant dierence in cast production between TSD treatments in any year and in the cumu-lative cast production over all years. There were no signi®cant dierences in cast production between crop management treatments in any year. Cast production in cropped plots averaged 35.7, 34.9 and 30.1 Mg
haÿ1across treatments in y1, y2 and y3, respectively.
Cast production was lower (Wilks' Lambda < 0.05) in the control in y2 than in y1 however remained stable in cropped plots. Cast production increased (Wilks' Lambda < 0.05) in the control from y2 to y3,
how-ever, decreased in cropped plots (Wilks' Lambda < 0.05).
3.2. Cast texture
There were no signi®cant dierences between TSD or between crop management treatments in the percen-tages of sand, clay and silt in casts. Across all treat-ments, averages were 62% sand, 24% clay, and 14% silt.
3.3. Nutrient concentrations in casts
There were no dierences in cast pH between TSD or between crop management treatments within any year (Table 2). There were no signi®cant dierences in
total N, organic C, exch. Ca2+ and Mg2+
concen-trations between TSD or between crop management
treatments within any year. Exchangeable K+
concen-tration in casts from the forest control was lower than
from cropped plots in y1 P<0:05), y2 P<0:05and
y3 P<0:01). In y3, casts from low TSD treatments
had higher K+concentrations P<0:001 than those
from high TSD treatments. There were no dierences in cast nutrient concentrations between crop manage-ment treatmanage-ments.
From y1 to y2 there were decreases (all Wilks'
Lambda <0.001) in total N, organic C, exch. Ca2+
and Mg2+ cast concentrations in all treatments. There
were no signi®cant changes in K+ concentrations
between y1 and y2. From y2 to y3, there were no
sig-ni®cant dierences in total N, exch. Ca2+ and Mg2+
cast concentrations. Organic carbon concentration in
casts increased (Wilks' Lambda < 0.001) yet K+
con-centration decreased (Wilks' Lambda < 0.001) from y2 to y3.
Correlations over time for cast amounts and nutri-ent concnutri-entrations in casts, separated by main-plot treatments, are in Table 3. Correlations between them are in Table 4.
3.4. Amounts of nutrients deposited in casts
In y1, more total N P<0:01), Corg P<0:05 and
Mg2+ P<0:05 were deposited in casts in the forest
control than in the cropped plots (Table 5). In y2, none of these dierences was signi®cant. In y3, more
Corg P<0:05 and Mg2+ P<0:05 were deposited
in casts in the forest control than in the cropped plots. There were no other dierences. Neither TSD nor cropped treatments had any signi®cant eects on amounts of any nutrient deposited in surface casts within any year.
Less Corg (Wilks' Lambda < 0.01), Ca2+ (Wilks'
Lambda < 0.01) and Mg2+ (Wilks' Lambda < 0.01)
were deposited in casts in y2 than in y1 in all treat-Table 1
Quantities of casts produced in the high and low timber stand den-sity (TSD) treatments and the uncropped, undisturbed control from 1995 to 1997a
y1 y2 y3 n
Plantain, mulched 38.0 (6.4) 41.0 (5.7) 35.3 (6.2) 8
Plantain, burned 35.0 (4.4) 35.2 (5.6) 28.8 (3.3) 8
Tannia 40.5 (4.4) 34.7 (5.3) 34.8 (5.0) 8
Intercrop 29.1 (4.6) 28.5 (4.3) 21.7 (4.3) 8
P(crop) 0.44ns 0.40ns 0.17ns
High TSD 39.7 (3.4) 38.8 (4.1) 33.7 (3.9) 16
Low TSD 31.7 (3.5) 30.9 (3.0) 26.6 (3.0) 16
P(TSD) 0.13ns 0.13ns 0.14ns
Mean TSD 35.7 34.9 30.1
Control 56.2 (5.4) 41.6 (5.6) 46.1 (9.7) 4
P(con v TSD) 0.013 0.39ns 0.031
P(interaction) 0.40ns 0.48ns 0.56ns
a
ments. There was no signi®cant dierence in the
amount of K+ deposited in casts between y1 and y2.
From y2 to y3, there were decreases (Wilks' Lambda
< 0.001) in the amount of K+ deposited in all
treat-ments. There were no signi®cant dierences in the
amounts of total N, exch. Ca2+ or Mg2+ deposited
between y2 and y3.
3.5. Comparison of cast and topsoil chemical properties
The average nutrient concentrations of topsoil (0±10
cm depth) across the site were 1.67 mg gÿ1 total N,
17.7 mg gÿ1 Corg, 0.42 mg gÿ1exch. Ca2+, 0.133 mg
gÿ1Mg2+and 0.082 mg gÿ1K+. Average cast
concen-trations were 1.84 for total N, 1.93 for Corg, 3.07 for
exch. Ca2+, 2.39 for exch. Mg2+ and 1.65 for exch.
K+ times higher than average topsoil (0±10 cm)
con-centrations. There were negative, power function
re-lationships between the cast:soil ratio and soil
concentrations at 0±10 and at 10±20 cm depth (Fig. 1a±e)
3.6. Relationships between casts and slash and litterfall
There were no signi®cant correlations P>0:1
between cast production, nutrient concentration or nutrient amounts in casts in the ®rst year and amounts of slash, nutrient concentrations or nutrient amounts in the slash. There were no signi®cant correlations
P>0:1 between cast production and litterfall
amounts in y2 and y3. N concentrations in litterfall and casts were not signi®cantly correlated (Fig. 2a). However, amounts of N in litterfall and amounts of N deposited in casts were correlated in both y1 and y2 (Fig. 2b).
Table 2
Nutrient concentrations of casts in main plot treatments (mg gÿ1
)a
totN Corg Ca
2+
Mg2+ K+ pH (H2O)
y1
High TSD 3.13 (0.18) 34.7 (1.50) 1.19 (0.09) 0.292 (0.018)) 0.125 (0.006) 5.71 (0.07)
Low TSD 3.01 (0.16) 34.5 (1.62) 1.02 (0.07) 0.282 (0.014) 0.142 (0.009) 5.65 (0.10)
P(TSD) 0.72ns 0.93ns 0.43ns 0.75ns 0.32ns 0.49ns
Control 3.36 (0.40) 34.1 (4.73) 1.11 (0.29) 0.280 (0.049) 0.086 (0.014) 5.71 (0.11)
P(con v TSD) 0.47ns 0.90ns 0.98ns 0.85ns 0.020 0.83ns
y2
High TSD 2.36 (0.08) 28.9 (0.77) 0.79 (0.06) 0.225 (0.014) 0.122 (0.006) 5.74 (0.05)
Low TSD 2.41 (0.07) 26.8 (0.77) 0.72 (0.04) 0.196 (0.009) 0.125 (0.006) 5.93 (0.10)
P(TSD) 0.52ns 0.11ns 0.55ns 0.15ns 0.69ns 0.18ns
Control 2.52 (0.27) 28.8 (2.71) 0.70 (0.20) 0.211 (0.035) 0.089 (0.009) 5.79 (0.10)
P(con v TSD) 0.45ns 0.62ns 0.67ns 0.99ns 0.021 0.16ns
y3
High TSD 2.51 (0.06) 32.0 (0.67) 0.78 (0.06) 0.195 (0.011) 0.084 (0.004) 5.58 (0.08)
Low TSD 2.55 (0.05) 31.1 (0.51) 0.67 (0.04) 0.207 (0.006) 0.114 (0.006) 5.78 (0.06)
P(TSD) 0.64ns 0.39ns 0.92ns 0.41ns 0.0001 0.95ns
Control 2.34 (0.21) 31.5 (2.33) 0.67 (0.16) 0.209 (0.035) 0.069 (0.007) 5.60 (0.07)
P(con v TSD) 0.16ns 0.98ns 0.27ns 0.75ns 0.0031 0.59ns
a
Mean with standard error in brackets.n16 for TSD treatments,n4 for control.P<0:05,P<0:01,P<0:001.
Table 3
Pearson correlation coecients for cast parameters within TSD and control treatments between years n16)a
Parameter Low High Con
y1/y2 y2/y3 y1/y3 y1/y2 y2/y3 y1/y3 y1/y2 y2/y3 y1/y3
Mg haÿ1 Cast 0.65 0.67 0.80 0.82 0.93 0.88 ns 0.74 0.58
mg gÿ1
totN ns 0.51 ns ns ns ns 0.90 0.73 0.89
mg gÿ1 Corg ns 0.88
ns 0.64 0.68 ns 0.88 0.82 0.88
mg gÿ1 Ca2+ 0.70 0.76 0.55 0.94 0.89 0.87 0.95 0.92 0.97
mg gÿ1 Mg2+ ns 0.64 ns 0.90 0.84 0.83 0.88 0.94 0.92
mg gÿ1 K+ ns ns ns 0.71 0.91 0.64 0.73 0.57 ns
4. Discussion
Cast production and nutrient deposition were higher in this agrisilvicultural system than in other systems previously studied in the Mbalmayo Forest Reserve. Cast production at two secondary forest sites was 6.8
and 12.4 Mg haÿ1 from mid-May to December 1994
(Norgrove et al., 1998). This may re¯ect dierences in
sites or that T. ivorensis plantations are conducive for
earthworm activity. No comparable data on casting in tree plantations have been found. GonzaÂlez et al. (1996) found earthworm densities were 50% lower in tree plantations than in secondary forest in Puerto Rico. Zou and Bashkin (1998) compared earthworm numbers and biomass between sugarcane ®elds and abandoned sugar cane ®elds at various times since abandonment that had either been (a) converted to eucalypt plantations, or, (b) abandoned to natural suc-cessional vegetation. Earthworms were absent in the
sugar cane ®elds. Densities (number mÿ2) of endogeic
earthworms increased over time in both natural succes-sion and eucalypt systems and there were no signi®cant dierences in density between systems of similar age. There were small numbers of anecic and epigeic worms in the natural succession system, but none in the euca-lypt system, possibly because of the low palatability of eucalyptus litter (Lee, 1985). Gilot et al. (1995), in a
pseudoreplicated experiment, compared earthworm
biomass between rubber (Hevea brasiliensis)
planta-tions 5, 10, 20 and 30 years of age and a secondary forest in Ivory Coast on an Oxisol. Earthworm bio-mass was comparable between the secondary forest, the 10 and 20-year-old plantations, with lower den-sities in the 5- and 30-year-old plantations. In another pseudoreplicated experiment, Mboukou-Kimbatsa et al. (1998), working in Congo-Brazzaville on coastal sandy soils, found higher earthworm biomasses in a
13-year-old Acacia auriculiformis plantation than in a
Table 4
Pearson correlation coecients between amounts of casts produced in each sampling period (Mg haÿ1
yÿ1
) and cast nutrient and organic carbon concentrations (mg gÿ1) n16)a
Low TSD High TSD Control
y1 y2 y3 y1 y2 y3 y1 y2 y3
totN ns ns ns ns ns ÿ0.55 ÿ0.79 ns ÿ0.69
Corg ns ns ns ns ns ns ÿ0.80 ns ÿ0.66
Ca2+ ns ns ns ns ÿ0.56 ÿ0.56 ÿ0.73 ns ÿ0.58
Mg2+ ns ns ns ns ns ns ÿ0.62 ns ÿ0.57
K+ ns ÿ0.72 ns ns ÿ0.50 ns ÿ0.65 ÿ0.54 ns
aP<0:05,P<0:01,P<0:001.
Table 5
Amounts of nutrients and organic carbon deposited in surface casts (kg haÿ1
yÿ1
)a
totN Corg Ca
2+
Mg2+ K+
y1
High TSD 121.1 (10.2) 1339.2 (102.7) 45.6 (4.3) 11.24 (1.00) 4.87 (0.45)
Low TSD 94.1 (12.8) 1081.8 (136.8) 31.5 (3.8) 8.61 (0.93) 4.35 (0.44)
P(TSD) 0.086ns 0.11ns 0.097ns 0.13ns 0.41ns
Control 177.7 (9.9) 1785.8 (114.7) 54.6 (9.9) 14.38 (1.47) 4.50 (0.51)
P(con v TSD) 0.0047 0.020 0.088ns 0.038 0.91ns
y2
High TSD 91.7 (9.9) 1104.0 (115.1) 28.7(3.0) 8.41 (0.95) 4.53 (0.44)
Low TSD 73.2 (6.7) 838.0 (89.4) 22.0(2.4) 5.89 (0.55) 3.66 (0.29)
P(TSD) 0.10ns 0.059ns 0.20ns 0.088ns 0.079ns
Control 102.4 (12.0) 1166.6 (115.4) 27.6 (8.0) 8.47 (1.83) 3.60 (0.36)
P(con v TSD) 0.24ns 0.34ns 0.71ns 0.43ns 0.49ns
y3
High TSD 82.9 (8.6) 1068.8 (119.1) 24.5 (2.74) 6.35 (0.81) 2.74 (0.30)
Low TSD 67.5 (7.4) 824.2 (90.4) 20.4 (2.07) 5.45 (0.56) 2.94 (0.28)
P(TSD) 0.080ns 0.079ns 0.24ns 0.35ns 0.64ns
Control 104.6 (4.9) 1420.4 (93.7) 29.3 (6.76) 9.24 (1.35) 3.07 (0.20)
P(con v TSD) 0.067ns 0.026 0.19ns 0.026 0.70ns
a
30-year-old secondary forest, however biomass in a
16-year-old Pinus caribaea Morelet plantation was lower
than in the forest.
In the present experiment, slashing the plantation undergrowth and cropping signi®cantly reduced earth-worm cast production to 63% in y1, and 65% in y3, yet not signi®cantly in y2 to 84% of the control plot
levels. In an analogous experiment in a 6-year-old T.
ivorensis plantation, activity declined to 50% of the non-cropped control (Norgrove and Hauser, 1999) in the ®rst year. Other previous work in Mbalmayo showed a more severe decline to 25% of the forest level after burning, tillage and cropping (Norgrove et al., 1998), and similar severe declines were reported by
Cook et al. (1980) working with cowpea (Vigna ungui-culata (L.) Walp cv. Prima) and by Hauser and Asa-walam (1998) working with maize and cassava in Nigeria. In the present experiment, the land was not tilled and remained partially shaded and this might explain the less detrimental impact of cropping here. Nutrient concentrations of casts decreased from y1 to y2 in all treatments, including the control, yet remained stable from y2 and y3. In August of y1 only, the T. ivorensistrees were completely defoliated by an
Epicerura sp. caterpillar (Lepidoptera: Notodontidae), however, new leaf ¯ush occurred in late August (L. A. Norgrove, unpub. PhD thesis, London University, 1999). Thus, the nutrients in the canopy were depos-ited on the soil surface as nutrient-rich caterpillar frass, which may have been selectively ingested by earthworms, increasing the nutrient concentrations in the casts in y1.
The high degree of correlation of cast carbon and nutrient concentrations between years in the control plots can be interpreted as an indicator for a relatively stable system, with tree litter as the dominant input, and in which changes of conditions cause a uniform
response by earthworms in relation to nutrient concen-trations in their casts (Table 4). As the control system received no inputs of slash or weed residues, the amount of available food and the nutrient concen-trations therein were more stable for 3 years, contri-buting to close correlations. On the contrary, in the low TSD, there were greater year to year ¯uctuations and thus fewer correlations. This might have been caused by the greater diversity of inputs. Large amounts of slash were applied at establishment and nutrients from the slash were released. Further, there was a lower nutrient uptake into tree biomass yet a higher uptake into weeds (L. A. Norgrove, unpub. PhD thesis, London University, 1999). After weeding, the fast decomposing weed residue (Norgrove and Hauser, 1999 submitted) might provide food sources, causing the worms to ingest materials of dierent qual-ity and quantqual-ity than in the control, thus contributing to less strong correlations. The situation in the high TSD would have been intermediate. The relatively low level of disturbance of cropping under a higher timber stand density and the lower input of food sources from decomposing weeds apparently retain conditions
more similar to the control and thus a higher degree of correlation.
There were marked inverse relationships between amounts of casts and their nutrient concentrations in the control plots. In cropped treatments, these re-lationships disappeared, presumably because of the in-itial application of nutrients with the slash and further ¯uctuations in nutrient inputs during the cropping phase, i.e. through slashed weeds and crop residues. However, the continuation of cast production, albeit at a lower level, suggests that earthworms are able to adapt to these changes in nutrient ¯ux in their environ-ment. Indeed, even in the low TSD cropped system, where litterfall plays a less important role, strong re-lationships between litterfall N input and cast N amounts persist (Fig. 2b). Zou (1993), working in tro-pical tree plantations in Hawaii, found that earthworm abundance was correlated positively with litterfall N content.
Ratios of cast to soil nutrient concentrations were
comparable to those reported by Lal and De
Vleeschauwer (1982) and Hauser (1993) on an Al®sol in south west Nigeria and by Barois et al. (1987) from south America. Cast nutrient and organic carbon centrations were more dependent upon initial soil con-ditions than upon cropping or TSD treatment. The ratio of soil:cast nutrient concentrations were related by negative power functions to soil nutrient concen-trations for all nutrients and organic carbon. Thus, within this range of soil nutrient concentrations, earth-worms adjust to lower soil concentrations by increas-ing their selectivity and thus produce relatively higher quality casts on poorer soil. Hauser and Asawalam (1998) found similar relationships between cast to soil ratios and soil nutrient concentrations in short fallows and cropped ®elds on Al®sols in south western Nigeria.
In conclusion, the disturbance through cropping, even when nearly all trees were retained, caused a re-duction in surface cast prore-duction. The rere-duction in cast production was less than that found in other sys-tems without tree retention. However, the tree density to which plots were thinned and the crop management treatment had little direct impact upon cast production and nutrient turnover. Rather, initial soil chemical properties and input amounts and qualities were more important.
Acknowledgements
We thank the soil physics sta at IITA Mbalmayo, particularly J. Nkem-Ndi, who helped with the estab-lishment of the experiment. Thanks to M.Engueng for the routine collection of casts, J.-P. Edzoa for textural analyses and R. Ndango, IITA chemistry lab manager,
for the plant and soil analyses. L. Norgrove was sup-ported by a King's College London Association scho-larship.
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