briz de tierra Martiodrilus carimaguensis sp. nov. en la dinám- ica del nitrógeno de un oxisol de los Llanos Orientales de Co- lombia. Suelos Ecuat 27:235â241.
Biol Fertil Soils (1999) 30 : 20–28
Q Springer-Verlag 1999
ORIGINAL PAPER
T. Decaëns 7 A.F. Rangel 7 N. Asakawa 7 R.J. Thomas
Carbon and nitrogen dynamics in ageing earthworm casts in grasslands of the eastern plains of Colombia
Received: 22 October 1998
Abstract The effects of a large species of anecic earthworm, Martiodrilus carimaguensis Jiménez and Moreno, on soil C and N dynamics were investigated in a native savanna and a man-made pasture of the eastern plains of Colombia. We compared, across time (11 P months), the total C, total N, NH c 4 and NO 3 contents in the earthworm casts, the underlying soil and the adjacent soil. Additional sampling of root biomass and macrofauna was performed. In the two management systems, the total C and N contents were higher in casts (4.33–7.50%) than in the bulk soil (2.81–4.08%), showing that the earthworms selected food substrates with high organic contents. In general, C contents significantly increased during cast ageing (c100%), possibly because of CO2 fixation processes, dead root accumulation and/or macrofaunal activities in casts. In fresh –1 casts, NH c 4 levels were very high (294.20–233.98 mg g dry cast) when compared to the soil (26.96–73.95 mg g –1 dry soil), due to the intense mineralisation processes that occurred during the transit of soil and organic matter through the earthworm gut. During the first week of P cast ageing, NHc 4 levels sharply decreased, while NH3 levels showed successive peaks in the casts, the underlying soil and the adjacent soil. These results suggested the rapid production of NO P 3 by nitrification processes in the fresh casts, followed by diffusion to the nearby soil, first vertically, then horizontally. After 2 and NO P levels only weeks of cast ageing, NH c 4 3 showed slight variations, likely because of organic matter protection in stable dry casts. The root biomass was
T. Decaëns (Y) 1 Laboratoire d’Ecologie des Sols Tropicaux, IRD, 32 Av. Varagnat, F-93143 Bondy cedex, France A.F. Rangel 7 N. Asakawa 7 R.J. Thomas Unidad Suelos y Plantas, CIAT, AA 6713, Cali, Colombia Present address: 1 Laboratoire d’Ecologie, UFR Sciences, Université de Rouen, F-76821 Mont Saint Aignan Cedex, France e-mail: thibaud.decaens6univ-rouen.fr
higher (1.6–4.7 times) below the old earthworm casts. The ecological significance of these results is discussed. Key words Earthworms 7 Soil fertility 7 Soil organic matter dynamics 7 Nitrogen dynamics 7 Savanna
Introduction Tropical savannas cover 43% of the plains of Latin America, with an area of 243!10 6 ha. They are traditionally used for extensive cattle ranching of low productivity (Vera and Seré 1985; Rippstein et al. 1996). The expanse of land used for intensive agricultural production, however, has constantly increased during the past 20 years. Common agroecosystems range from pure grass or legume/grass-based improved pastures, to high input annual crops (Thomas et al. 1995). The intensive agriculture of these areas has important economical implications, and its environmental impacts are still little known. Large invertebrates, mostly earthworms, termites and ants, have been defined as “ecosystem engineers” (sensu Jones et al. 1994; see also Anderson 1995). In the humid tropics, where neither climatic conditions nor clay minerals can efficiently regulate mineralisation, regulation of soil processes by these organisms may become predominant (Lavelle and Martin 1992). Ecosystem engineers actively regulate the activity of soil microorganisms in the organo-mineral structures that they produce (e.g. earthworm casts, termite mounds or ant nests) and may have important effects on soil aggregation and the regulation of organic matter dynamics (Scheu 1987; Krietzschmar and Monestiez 1992; Marinissen and de Ruiter 1993; Bohlen and Edwards 1995; Blanchart et al. 1997). Earthworm effects on soil organic matter dynamics vary, depending on the scale of space and time that is considered (Lavelle et al. 1998). In the short-term, microbial activity is enhanced in the earthworm digestive
21
tract, leading to increased organic matter mineralisation and the significant release of assimilable nutrients in fresh casts (Blair et al. 1994). In the long-term, ageing casts protect organic matter from further mineralisation processes as long as they retain their physical properties. Finally, earthworms seem to promote a rapid turnover of organic matter in the soil profile. More information, however, is still needed, since very few tropical species (mostly peregrine endogeic species) have been intensively studied so far in relation to soil processes (Fragoso et al. 1997). The aim of the present study was to describe shortand long-term dynamics of C and N in earthworm casts and in the surrounding superficial soil. Special attention was paid to the effects of Martiodrilus carimaguensis Jiménez and Moreno (Oligochaeta, Glossoscolecidae), the dominant surface-casting earthworm present at the study site. The experiments were conducted at the spatio-temporal scale of the surface casts produced by this species, in a native tropical savanna and a manmade pasture derived from savanna. The root biomass was measured to assess the plant response to cast deposition. The study was carried out at the CIAT-CORPOICA Research Station of Carimagua (4737bN, 71719bW), located in the phytogeographic unit of the well-drained isohyperthermic savannas of the eastern plains of Colombia. The climate is subhumid tropical with an annual mean temperature and rainfall of 26 7C and 2300 mm respectively, and a pronounced dry season from November to March. The native vegetation varies with topography: open savannas in the uplands (“altos” and “planos”), and gallery forests or flooding savannas in the low-lying areas (“bajos”). The soils are oxisols (Tropeptic Haplustox Isohyperthermic) in the uplands and ultisols (Ultic Aeric Plintaquox) in the low-lying areas. Both are characterised by favourable physical properties (aggregation, porosity, water retention), high acidity [pH(H2O)~5] and very low chemical fertility (Al saturation 1 80%, CEC~5 mEq 100 g –1).
large (up to 15 cm in height), tower like dejections that are usually produced during several days. In addition, this species adapts very well to some agroecosystems, and can therefore be considered as useful for sustainable agriculture (Jiménez et al. 1998b). The experiment started in May 1996, at the onset of the rainy season, during the peak of earthworm activity (Jiménez et al. 1998b). In both systems, 160 surface casts of M. carimaguensis, divided into four groups of 40 neighbouring casts, were identified individually with small metal plates. At this stage, only fresh and as yet small casts (i.e. at the early phase of deposition) were chosen. In doing this, we ensured the presence of earthworms in each of the marked casts, while avoiding any cumulative effects that might have resulted from their prolonged presence before the beginning of the experiment. In the pasture, each group of casts was protected from animal trampling by a metallic cage (2!2 m). Samples were taken 1, 2, 3, 7, and 14 days and 1, 2, 5, 6.5, 8 and 11 months after deposition of the casts. At each date, samples were randomly taken from each group of 40 casts. From each group the following samples were taken: (1) one cast, (2) the underlying soil (located directly below the cast), and (3) the adjacent soil (located 20 cm away from the cast; Fig. 1). The soil was sampled using a 5-cm-diameter and 5-cm-deep aluminium cylinder and kept at 4 7C.
Chemical analysis Mineral N was extracted by shaking 4 g of each fresh sample for 30 min with 40 ml of 1 M KCl solution. The suspensions were filtered and the filtrates kept at –15 7C before analysis. Colorimetric P methods were used to determine NH c 4 and NO 3 concentrations (Anderson and Ingram 1989). Total C and N were analysed on subsamples previously sieved through a 2-mm mesh. We used a colorimetric method after acid digestion to measure total C contents (Houba et al. 1988), and the standard Kjeldahl method to measure total N contents (Krom 1980).
Root and macroinvertebrate sampling Two more casts per group were sampled 0, 1, and 2 weeks and 1, 2, 3.5, 5, 6.5, 8 and 11 months after cast production to assess the biomass of the roots and macroinvertebrates (i.e. 1 2 mm) that were present inside them. Macroinvertebrates were sampled by hand-sorting. The casts were washed and sieved though a 0.053mm sieve for root extraction. Living roots were manually separated from dead roots and litter fragments according to their colour. Macroinvertebrates were hand-sorted and killed in 70% al-
Materials and methods Experimental plots The experiment was carried out in two different management systems on a well-drained upland oxisol: 1. A Trachypogon vestitus native savanna, protected from grazing for 4 years and managed traditionally by burning every year during the dry season. 2. A 3-year-old pasture of Brachiaria humidicola, Arachis pintoi, Stylosanthes capitata and Centrosema acutifolium, grazed by cattle with an average stocking rate of 2.0 animal units ha –1.
Experimental design Among the eight earthworm species recorded in the native savanna at the study site (Jiménez et al. 1998a), M. carimaguensis was chosen because it is the only one that has a significant casting activity at the soil surface. Casts produced by this species are
Fig. 1 Description of the sampling procedure. At each sampling, date, three samples were taken from each group of casts: (1) one cast, (2) the underlying soil and (3) the adjacent soil
22 cohol. Both roots and invertebrates were oven-dried at 75 7C for 48 h and weighed. Additional samples (other than the identified casts) were taken randomly in the two systems to assess the effect of casts on root biomass in the soil. They consisted of soil samples located: (1) in an area free of casts, (2) below fresh casts, and (3) below dry casts. For each location, eight samples were taken using a 10cm-diameter and 10-cm-deep aluminium cylinder. Roots were sampled, dried and weighed as described below.
contents than the adjacent soil (Table 1, Fig. 2). Afterwards, the water content of casts sharply decreased between 7 and 30 days of ageing. After 30 days, when compared to the soil, casts constantly had lower moisture contents which showed higher variation. The decrease in water content observed about 8 months after the beginning of the experiment corresponded to the middle of the dry season.
Statistical treatments Data were transformed before analysis to reduce the asymmetry of the frequency distribution. Normalisation of data was obtained using the Box-Cox transformation (Sokal and Rohlf 1995): yp(x dP1)/d. The d parameters were estimated using the program VerNorm 3.0 from the “R package” developed by Legendre and Vaudor (1991). Three-way ANOVAs were performed with system, sample origin (i.e. casts, underlying soil and adjacent soil) and cast age as the fixed main effects. These analyses were performed for NH c 4 , –1 NO P dry soil), total C 3 and inorganic N concentrations (mg g and N concentration (% of dry soil), C:N ratio and moisture contents (%). Seven analyses were performed and each analysis involved seven tests (three main effects and four interactions). The Bonferroni procedure for nested tests (Cooper 1968) was used to ensure against statistical error. The adjusted 0.05, 0.01 and 0.001 significance levels were: 0.001 [p0.05/(7!7)], 0.0002 [p0.01/ (7!7)] and 0.00002 [p0.001/(7!7)], respectively. Two-way ANOVAs were performed for the root and macroinvertebrate biomass in casts (g dry matter g –1 dry cast). The fixed main effects were system and cast age. The Bonferroni procedure for nested tests was used and the adjusted 0.05, 0.01 and 0.001 significance levels were: 0.0125 [p0.05/(2!2)], 0.003 [p0.01/(2!2)] and 0.0003 [p0.001/(2!2)]. Additional comparisons of average data values were performed respectively by using a fisher PLSD test.
N dynamics as affected by earthworm activity High NH c 4 levels were observed in the casts of M. carimaguensis during the first month of cast ageing (5 and 15 times higher than in the bulk soil in 1-day-old casts, respectively, in the savanna and the pasture; Table 1, concentration Fig. 3a,b). After this period the NH c 4 fell to a minimum and constant value similar to the one
Results and discussion Earthworm behaviour Most of the earthworms were active in the same galleries during at least the first week following the onset of cast production. This confirmed previous ecological studies of M. carimaguensis, which concluded that this species has a semi-sedentary behaviour (Jiménez et al. 1998b). As a result, in the two systems, casts had fresh parts during all this period, and had higher moisture
Fig. 2 Evolution over time of the moisture content in the soil and the casts of the two studied systems
P Table 1 Three-way ANOVAs for the NHc 4 , NO3 , inorganic N, total C and total N contents, the C : N ratio and the moisture content. The F-ratio and error mean square are presented. Each test
was significant at the Bonferroni-corrected probability [overall probability/(n of variable!n of tests)] for overall significance levels of 0.05, 0.01 and 0.001. NS Not significant
Source
df
NHc 4
NOP 3
Inorganic N
Total C
Total N
C : N ratio
Humidity
System (A) Sample origin (B) AB Cast age (C) AC BC ABC Error mean squares
1 2 2 10 10 20 20 197
7.24 NS 24.22** 7.97* 30.75** 1.81 NS 5.45** 1.42 NS 2.45
46.48** 1.42 NS 4.49 NS 16.39** 4.50** 2.92** 1.06 NS 0.17
8.48 NS 24.91** 9.05** 32.22** 1.69 NS 5.15** 1.45 NS 2.48
434.91*** 404.95*** 41.20** 45.35** 18.49** 3.33** 3.64** 0.04
75.64** 242.49*** 18.37** 2.22 NS 3.53** 1.22 NS 0.91 NS 197.71
203.66*** 23.55** 2.74 NS 84.05** 29.54** 3.96** 3.95** 0.35
26.38** 1.37 NS 4.81 NS 28.59** 3.74** 7.10** 1.88 NS 2.91
* P~0.05, ** P~0.01, *** P~0.001
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P Fig. 3 Evolution over time of NH c 4 and NO 3 contents in the soil and the casts of the Brachiaria humidicola/Arachis pintoi pasture (a, c) and the native savanna (b, d)
observed in soil (Fig. 3a,b). Three significant and transient peaks of NO P 3 were successively observed in the casts, the underlying and the adjacent soil (Table 1, Fig. 3c,d). Finally, inorganic N excesses largely disappeared from the casts and the surrounding soil. These results reflected the high rate of mineralisation that occurs in fresh earthworm casts. In fact, microbial activities and mineralisation are known to be greatly enhanced during the transit of soil through the gut and in fresh casts (Barois et al. 1987). This process, plus the addition of urine in the posterior part of the digestive tract, may explain the high levels of NHc 4 observed in the fresh casts. During the first month, these levels were maintained by the continuous excretion of deposits on the same casts. Then, the earthworms abandoned their burrows and the NH c 4 concentration progressively decreased. This decrease could be attributed to the production of NO P 3 in fresh casts via nitrification processes (Scheu 1987; Lavelle and Martin 1992). Rapid diffusion into the soil would explain why no accumulation of NO P 3 was observed in the casts. Hence, the three transient peaks of NOP 3 could be interpreted as the results of: (1) fast production of NO P 3 in the casts, (2) rapid diffusion of this NO P 3 into the water flowing through the gallery, (3) slower lateral diffusion with the water running off the cast and soil surface (Fig. 4). The increasing intensity of the 3 peaks of NOP 3 may have reflected a stimulation of a nutrient-dependent microflora in the neighbouring soil. Finally, inorganic N excesses largely disappeared from the casts and the surrounding soil, certainly as the result of root up-
take, denitrification processes, immobilisation in the soil microbial biomass or losses by leaching (Syers et al. 1979; Elliott et al. 1990; Lavelle and Martin 1992). After this first period of fast N dynamics, the casts rapidly dried at the soil surface (Fig. 2). Dry earthworm casts are known to be stable, rather compact and impermeable aggregates (Shipitalo and Protz 1989; Blan-
Fig. 4 N dynamics in earthworm casts and the nearby soil. The arrows indicate the direction of water flow, numbers refer to a chronological order: (1) high NH c 4 release in fresh casts plus NO P 3 production, (2) vertical drainage in the gallery, (3) lateral runoff from the cast and the soil surface, (4) vertical leaching in the soil profile, (5) denitrification plus root and microbial uptake, (6) N immobilisation in dry casts
24
chart et al. 1993; Guggenberger et al. 1996), which efficiently protect the organic matter they contain from decomposition processes (Martin 1991; Lavelle and Martin 1992). In our experiment, however, slight increases P in NH c 4 and NO 3 concentrations were observed between 30 and 195 days after the beginning of the experiment, simultaneously in the casts and soil of the two systems (Fig. 3). These increases occurred during the “veranillo” (i.e. a short dry event in the middle of the wet season) and could be attributed to alternations of dry and wet conditions that were likely to favour sucP cessively NH c 4 and NO 3 production in the soil (Birch 1964). Since surface casts were exposed to higher moisture fluctuations, they showed higher amplitudes in the production of inorganic N. In both systems, average values of inorganic N (i.e. P NH c 4 plus NO 3 ) were significantly higher in the casts than in soil, while no significant differences were observed between the underlying and the adjacent soil (Table 1, Fig. 5). During the study period, estimated surface cast production by M. carimaguensis at the study site was, respectively, 9 and 54 t ha –1 year –1 in the savanna and in the pasture (based on a density of 0.3 individuals m –2 in the savanna and 1.6 individuals m –2 in the pasture; unpublished data). This species also produces underground casts, and the overall production of casts (surface plus below ground) was estimated at 14 t dry casts ha –1 year –1 in the savanna and 114 t ha –1 year –1 in the pasture (calculated after Rangel et al. 1998). Thus, 3 and 34 kg ha –1 year –1 inorganic N may be released in fresh casts of M. carimaguensis, respectively, in the savanna and the pasture. This represents a significant contribution to the overall N budget of these agroecosystems. For example, in the case of the pasture, inorganic N release from casts is equivalent to ;22% of the total annual N uptake by grasses (about 155 kg ha –1 year –1; Fisher and Kerridge 1996). This is also equivalent to ;48% of the total N inputs generally used in upland rice monocultures (70 kg ha –1 year –1; Thomas et al. 1995). The estimates presented here were consistent with other estimates of earthworm impact on
N mineralisation (James 1991; Martin 1991; Curry et al. 1995). The global contribution of earthworms to N available to plants may have been even higher, due to the presence of significant populations of other species (Jiménez et al. 1998a), and to the production of mineral N through other processes such as urine or cutaneous mucus release, and decomposition of earthworms dead bodies (Scheu 1994).
Fig. 5 Mean values of inorganic N in the soil and the casts of the two studied systems (different letters indicate significant differences at P~0.05)
Fig. 6 Mean values of total C (a), total N (b) and C:N ratio (c) in the soil and the casts of the two studied systems (different letters indicate significant differences at P~0.05). ppm mg g –1
C accumulation in earthworm structures In the soils that had not been recently in contact with earthworms, the total C content was significantly higher in the pasture than in the savanna, while no significant differences were recorded for the total N contents and the C:N ratio (Fig. 6). This was consistent with other
25
Fig. 7 Evolution over time of the total C (a, b) and N contents (c, d), and the C:N ratio (e, f) in the soil and the casts of the Brachiaria humidicola/Arachis pintoi pasture (a, c, e) and the native savanna (b, d, f)
studies showing that legume/grass pastures, when sown on savanna, generally increased to a large extent the level of soil organic C, while organic N remained the same (Thomas et al. 1995). The progressive increase in the C content and C:N ratio observed in the pasture soil (Fig. 7a,e) was probably due to the presence of the cages used to avoid grazing pressure on the vegetation, as this may have promoted leaf litter production and the accumulation of fresh organic matter in the soil. Compared to the bulk soil, casts produced in the two systems had significantly higher total C contents (1.5–1.9 times higher) and total N (1.4–1.6 times higher; Fig. 6). Similar results have been reported in other studies, and can be explained by the capacity of M. carimaguensis to select food substrates with high organic contents (Rangel et al. 1998c; Jiménez et al. 1998b). We also observed significant quantities of recognisable plant debris, ranging from 1% to 3% of the total dry weight of the casts, on average, in the savanna and the pasture (data not presented). This result confirmed the ability of M. carimaguensis to ingest a mixture of soil
and fresh litter, and that this species truly belongs to the anecic ecological category (sensu Bouché 1977). This could explain why casts produced in the pasture, with a high availability of palatable legume litter, had significantly higher levels of total C and N, when compared to those of the savanna where plant debris was scarce. The concentrations of total N were rather constant during the entire ageing process of casts (Table 1, Fig. 7c,d). More surprising was the continuous and significant increase in C observed in casts during their ageing (Fig. 7a,b). This increase was highly significant in both systems (;c100%), and may be explained by a combination of several factors, the relative importances of which are still unknown: 1. Fixation of atmospheric C02 by autotrophic microorganisms (e.g. algae or nitrification microorganisms) may have been enhanced in casts, at least when moisture conditions were suitable for their activity (Vinceslas-Akpa and Loquet 1997). 2. The biomass of roots never reached a high level in casts (Table 2, Fig. 8a), but adverse humidity and temperature variations may have resulted in rapid root turnover and the retention of dead roots in the ageing faeces. 3. Cast-dwelling macroinvertebrates were present from the first month of the experiment (Table 2,
26 Table 2 Two way ANOVAs for the root, macrofaunal and microorganism biomass. The F-ratio and error mean square are presented. Each test was significant at the Bonferroni-corrected Source
System (A) Cast age (B) AB Error mean squares
probability [overall probability/(n of variable!n of tests)] for overall significance levels of 0.05, 0.01 and 0.001
df
1 9 9 140
Biomass Root
Macrofaunal
Microorganism
42.47*** 4.07*** 3.77** 6.23E9
3.07 NS 2.74* 0.88 NS 1.88E14
25.80*** 5.39** 1.55 NS 2.83E5
* P~0.05, ** P~0.01, *** P~0.001
Fig. 8b) and may have contributed to the increase in the C concentration by accumulating organic material and/or producing C-rich faecal pellets (5.2–10.2% of C; unpublished data). A possible effect of the concentration of organic C in the casts and burrows of M. carimaguensis was the build-up of a rather active, but physically protected, C pool which was probably released concurrently with the disintegration of the casts (Lavelle and Martin 1992; Guggenberger et al. 1996). Due to the large quantity of soil excreted by earthworms as casts, earthworm-induced C accumulation in stable aggregates may be considerable. We estimated this quantity at 0.6 t ha –1 year –1 in the savanna and 8.6 t ha –1 year –1 in the pasture. A part of this C (83% and 62% in the savanna and the pasture, respectively) corresponded to the C increase in fresh casts compared with soil, and may have been due to the selective ingestion by earthworms of organic-rich food substrates. The remaining C was due
Fig. 8 Evolution over time of root (a) and macrofaunal biomass (b) in the casts of the two studied systems
to the increases in the C concentrations that occurred after the casts had been produced. The total quantity of C concentrated in the casts of M. carimaguensis represented 2% and 30% of the total soil C in the top 10 cm (based on a bulk density of 1.0 g cm –3), respectively, in the savanna and the pasture. Effect of casting activity on surface root growth Though roots were found in very low quantities inside casts, the root biomass in the upper soil layer of the pasture significantly responded to the presence of casts at the soil surface (Fig. 9). Compared with the control soil without casts, this parameter increased by a factor of 2 below recent casts and a factor of 5 when located under aged casts. In the savanna, the same trend was observed, although it was not statistically significant. Positive effects of earthworms on plant growth have been widely documented, especially in short-term studies on plants grown in pots (see review by Lavelle 1997; Brown et al. 1999). They are the result of several mechanisms, some of them supported by the present study. Casting activity by M. carimaguensis first enhances the P mobilisation of nutrients (e.g. of NH c 4 and NO 3 in our study) that are not generally available in the soil (Rangel et al. 1998c). Moreover, as this species is most-
Fig. 9 Average root biomass in the 0–15 cm superficial soil layer as affected by the presence of fresh and dry casts on the surface (different letters indicate significant differences at P~0.05)
27
ly active at the beginning of the wet season (Jiménez et al. 1998b), the timing of earthworm-induced mineralisation may coincide with high nutrient requirements of plants. In conclusion, the present study confirmed the hypothesis of contrasting effects of earthworm activities on soil organic matter, according to the time-scale considered (Martin 1991; Lavelle and Martin 1992; Lavelle et al. 1998). Casts of M. carimaguensis may be considered as microsites of short-term mineral N production and medium-term soil organic matter accumulation. Inorganic N production occurs during the period of cast production, and there is evidence of rapid diffusion of the NO P 3 produced into the surrounding soil. A buildup of C pool occurs gradually during cast ageing, maybe under the influence of other organisms such as autotrophic microorganisms, small invertebrates and also plant roots. Modifications in the location and dynamics of organic resources through the production of earthworm casts may be considered as examples of the effects of earthworms’ “engineering activity” (sensu Jones et al. 1994). In the light of the large quantities of soil processed in some ecosystems by the total earthworm population (e.g. in some pastures of Carimagua, the biomass of M. carimaguensis was up to 2 times higher than in the pasture of this study; Jiménez et al. 1998b), the global effects on soil fertility and plant production must be extensive. This may be of great relevance in the context of soil organic matter management, which is a fundamental step in improving agroecosystem sustainability and decreasing CO2 emissions to the atmosphere (Woomer et al. 1994; Tiessen et al. 1994). Acknowledgements The authors thank DK Friesen and CG Melendez (CIAT) for technical support, M Vinceslas-Akpa and M Loquet (University of Rouen), and P Lavelle, L Mariani and JP Rossi (IRD) for making useful suggestions on a first version of this paper.
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