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Distribution of Nematodes in Vertisol Aggregates Under a Permanent Pasture in Martinique

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Distribution of Nematodes in Vertisol Aggregates Under a Permanent Pasture in Martinique Applied Soil Ecology ELSEVIER Applied Soil Ecology 4 (1996) 193-200 Distribution of nematodes in vertisol aggregates under a permanent pasture in Martinique Patrick Quénéhervé, Jean-Luc Chotte * Laboratoire de Nhnatologie et Laboratoire de Matière Organique des Sols Tropicaux, Centre ORSTOM, B.P. 8006,97259 Fort de France Cedex, Martinique Accepted 13 May 1996 Abstract This study reports on the distribution of nematodes in different soil habitats under a permanent pasture of digitgrass (Digitaria decuinbens Stent. cv. 'Pangola') in Martinique. The objectives were (i) to evaluate a gentle fractionation method compatible with further soil nematode extraction and (ii) to assess the respective soil microhabitats of plant-feeding nematodes and free-living nematodes. This study indicated that gentle soil fractionation can effectively separate soil habitats and allow the recovery of associated nematodes. Plant-feeding nematodes were equally distributed between inter-aggregate pores, habitats constituted of aggregated fine silt + clay particles and roots + rhizosphere. Most of free-living nematodes (53%) resided in inter-aggre- gate pores. Irrespective of the food resource, densities of nematodes (number per gram of habitat) were similar in habitats coarser than 1000 pm (A5000, A2000, and A1000). Habitats with the finest soil (A200) were not favourable sites because of the rarity of roots (for plant-feeding nematodes) and physical constraints. Keywords: Nematode; Pasture; Soil aggregates; Soil fractionation; Soil habitats; Soil porosity; Vertisol 1. Introduction indicated that carbon concentration controls the dis- tribution of soil microorganisms. Griffin (1981) Soils consist of an assemblage of solid particles demonstrated the importance of water suction in and air-filled or water-filled pores of different sizes microbial metabolism. Hassink et al. (1993) found a and shapes where microorganisms reside according positive correlation between nematode biomass and to their feeding habits and physico-chemical con- the volume of pores 30-90 pm in diameter. Beare et straints. Relations between soil environment and or- al. (1995) developed a hierarchical model to describe ganisms have been widely reviewed (Smiles, 1988; the diversity of soil microhabitats in relation to Tisdall, 1991; Wolters, 1991; Lee and Foster, 1991; biogeochemical cycling. One example of these rela- Juma, 1993; Oades, 1993). Gray and Williams ( 1971 ) tions can be illustrated by the results obtained by Hassink et al. (1993) where they found that grazing of bacteria by bacterivorous nematodes might ex- plain differences in C/N ratios of soil organic matter * Corresponding author: Tel. (221) 32 18 46; Fax. (221) 32 16 75. in sandy soils and clay soils. 0929-1393/96/$15.00 Copyright O 1996 Elsevier Science B.V. A11 rights reserved. PII S0929-1393(96)001 18-7 194 P: Qrténéherué, J.-L. Chotre/Applied Soil Ecology 4 (1996) 193-200 Nematodes (free-living and plant parasitic) inhabit was dedicated to the determination of weight of the water-filled pore spaces. They occupy a specific habitats and the other to nematode enumeration. niche in the food chain. Plant-parasitic nematodes affect plant productivity. Free-living nematodes in- 2.2. Soil fluetionation teract with other microorganisms mainly through predation (Ingham et al., 1985). They are thought to Soil fractionation was carried out soon after soil be responsible for 30% of N mineralisation (Verhoef sampling. Each undisturbed soil sample was sub- and Brussaard, 1990). Nematodes have been referred merged in distilled water (250 ml per 100 g dry soil), to as biological indicators (Freckman, 1988; Elliott, sufficient to fill pores larger than 30 pm in diameter. 1994). However, despite the extensive literature on The samples were kept at 24°C for 36 h without soil nematodes and soil fertility, very little is known 1 agitation. Then, water in pores larger than 300 pm about the location of nematodes within soil aggre- in diameter (macropore water) was emptied by suc- gates. 11 tion (1 h at pF 1). To complement existing methods of evaluating For each replicate, the weight (80°C oven-dried relationships between soil structure and soil microor- basis) of unfractionated soil was determined on a ganisms (Jastrow and Miller, 1991; Darbyshire et al., subsample prior to soil fractionation. The samples 19931, this study investigated the performance of a were gently fractionated by hand and by wet sieving gentle physical soil fractionation in nematode numer- and sedimentation (Chotte et al., 1994). Care was ation in a vertisol with respect to soil aggregation. taken during soil fractionation to split the soil along This highly aggregated soil also provided us with a natural planes of weakness and to remove small good model to test the importance of the porosphere aggregatesI adhering to roots. The different fractions and the aggregatusphere (Beare et al., 1995) in ne- were: over 5000 pm diameter (A5000); 2000-5000 matode distribution. pm (A2000); 1000-2000 pm (A1000); 200-1000 pm (A200); 50-200 pm (A50); 0-50 pm (AO). The weight of each habitat was determined on an oven- 2. Material and methods dried (80°@)basis. The weights of each aggregate- class were summed and compared with the weight of 2.1. Soil sampling unfractionated soil to test the efficiency of the frac- tionation. The fractionation schedule required 3 days for each set of five samples. Biological alterations, The study was conducted at Sainte-Anne, Mar- which would have occurred if soil samples had been tinique (Lesser Antilles: 14‘3”N, 62’34”W). The soil stored pending fractionation, prevented us from tak- was a vertisol (black earth) developed on volcanic ing more replicates. ash, pH 4.8, organic carbon 5.4% and total nitrogen The weight distribution of the habitats was com- 0.42%. This tropical area is characterised by an pared with the weight (8OOC oven-dried basis) of the annual rainfall of 1300 mm and a relative dry season simple mineral particles obtained by mechanical of 4 months (March-June). The soil was under a analysis after removal of organic cement by treat- 15-year-old pasture of digitgrass (Digitaria decum- ment with hydrogen peroxide (after Day, 1965). bens Stent, cv. ‘Pangola’). Five adjacent plots (1 Particles larger than 50 pm were obtained by siev- m2) with similar soil features, depth (1 m) and slope ing, and particles less than 50 pm were analysed by (on the top) were wetted to pF 2 (60 g water g-’ a laser diffraction analyser (Malvern, Mastersizer/E). soil) for 1 night prior to sampling, filling pores 1 smaller than 30 pm in diameter (Haines, 1927). The undisturbed soil top layer (3-8 cm) was 2.3. Nematode extraction and identijìcation i sampled with a soil ring sampler (5 cm diameter, volume 98 cm3, about 100 g of dry soil) in the root Nematodes were extracted from habitats larger zone of the digitgrass. Two replicates were taken in than 50 pm (Fig. 1) by the elutriation-sieving tech- the central area (625 cm2) of each plot. One replicate nique (Seinhorst, 1962). Roots were rinsed with tap P. Quinélierué, J.-L. Clrotte /Applied Soil Ecology 4 (1996) 193-200 195 Undisturbed soil top layer (3-8cm) (100 g e.d.w.) immersed in 250 ml of distilled water 24°C for 36 hours suction for hour 1 , Macropore water I atpF 1 Sieving1- Rhizosphere Roots /( I ROOtS LYrl Habitat > 5,000 pm I 2,000Pm1 I - Habitat 2,000-5,000,um I ( Habitat 1,000-2,000pm ) I - I 200pm I -( Habitat200-1,000pm ) I I (' Babitat 50-200pm ) I - I, Centrifugation 10 min (2,000 g) -c CaCl,O.OSM \(- Habitat 0-50 pm I Fig. 1. Habitats analysed for nematodes. water (rhizosphere fraction) prior to nematode ex- habitat. Specific identifications were performed on traction in a mist-chamber for 2 weeks (Seinhorst, fixed nematodes only for plant-feeding nematodes. 1950). Nematodes were collected from liquid frac- tions, i.e. macropore water and rhizosphere fractions, on a bank of four 50-pm sieves, allowing total 3. Results recovery of the nematodes present (Seinhorst, 1956). Similarly, soil sieving at 50 pm was repeated four 3.1. Weight distribution of aggregates times so that nematodes in habitats less than 50 pm (AO) were not assayed. No weight loss occurred on soil fractionation Nematode abundance was expressed as number since weight recoveries after soil fractionation ap- per 100 g of unfractionated soil or per gram of I proximated lOq% (e.g. 100.17%, 99.55%, 100.89%, 196 P. Qu6néherue', J.-L. Chotte/Applied Soil Ecology 4 (1996) 193-200 ^-- 3.2. Distribution of nematodes among habitats so Plant-feeding and free-living nematodes extracted from unfractionated soil were almost equally divided between plant-feeding (1300 individuals per 100 g of unfractionated soil) and free-living nematodes (1250 individuals per 100 g of unfractionated soil). Plant-feeding nematodes comprised five different 30 species. The two dominant species were the endopar- 20 asite Pratylenchus zeae and the ectoparasite Ty- Zenchus sp., representing 73.1% and 26.7%, respec- 10 tively, of the total plant-feeding nematodes. Three O R A5000 A2000 AlOOO A200 A50 AO other species (less than 0.2%) were observed only Habitat(*) 0 two diffcrcnt Letters indicated a significative difference (non parmetricU Mann-Whiiuey test, p c 0.05) * R: Roots A5000 Habitat > 5,000 pm A200 Habitat200-1,OOOpm A. Plant-feeding nematodes A2000 Habitat 2,000-5,OOOp A50 Habiht 50-2OOpm A1000: Habitat 1.000-2.000pm AO: HabitateSOpm Fig. 2. Weight distribution of the fractions after soil fractionation and the simple mineral particles after mechanical analysis (results fa\ I per 100 g of unfractionated soil). 98.27%, and 101.09%). The Kruskal-Wallis test in- Li dicated that habitat weight differed significantly (P R+&i Asboa A?& Alb A200 A$ < 0.001). Comparison of mean weight among frac- tions (Mann-Whitney non-parametric U test) re- .- B.
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