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Effect of subsurface clay on nematode communities in a sandy soil
Applied Soil Ecology 19 (2002) 1–11
Effect of subsurface clay on nematode communities in a sandy soil Robert McSorley∗ , John J. Frederick Department of Entomology and Nematology, University of Florida, P.O. Box 110620, Gainesville, FL 32611-0620, USA Received 30 March 2001; accepted 7 August 2001
Abstract A variety of soil properties can directly or indirectly affect nematode community structure. The effects of subsurface clay content (at 20–40 cm depth) on nematodes in the surface layer (0–20 cm depth) of a sandy soil were examined in field experiments in Florida, USA. Plots were established in a site with a relatively uniform sandy upper soil layer (88–91% sand and 5–7% clay at 0–20 cm depth) but with varying levels of clay in the subsurface layer (3–35% clay at 20–40 cm depth). Nematode numbers in the surface soil layer were affected by the amount of clay in the subsurface layer. Population densities of a number of different nematode genera were greater in the surface layer of plots with 35% subsurface clay than in plots with 3% subsurface clay. Indices of nematode community structure were largely unaffected, since effects of subsurface clay were observed across all nematode groups. Most nematodes (70–80% of total numbers) occurred at 0–20 cm depth, although Teratocephalus was more common at 20–40 than at 0–20 cm. Subsurface clay content indirectly affected soil moisture and other environmental factors in the upper soil layer in which most nematodes reside. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Agroecosystems; Ecological indices; Nematode trophic groups; Soil nematodes; Soil texture; Vertical distribution
1. Introduction Recent work in nematode ecology has focused on understanding the factors that influence nematode community structure. Biological and physical characteristics of the ecosystem, such as the plant species present (DeGoede and Bongers, 1994; McSorley, 1997; Wasilewska, 1997a; Yeates, 1999) and properties related to soil moisture (McSorley, 1997; Porazinska et al., 1999; Todd et al., 1999) have major effects on nematode communities. However, many human activities also influence soil nematode communities (Wasilewska, 1989, 1995), particularly various ∗ Corresponding author. Tel.: +1-352-392-1901/ext. 137; fax: +1-352-392-0190. E-mail address: [email protected] (R. McSorley).
agricultural practices and intensity of agricultural practices (Ferris et al., 1996; Freckman and Ettema, 1993; McSorley and Frederick, 2000; Neher, 1999; Neher and Olson, 1999; Todd, 1996; Yeates and Bird, 1994). While some effects are evident on a regional basis (Neher et al., 1998), considerable heterogeneity occurs at the field level (McSorley, 1987). For example, the plant rhizosphere has such an important influence on most nematode groups that differences in nematode community structure are evident within a short distance from plant rows and root systems (McSorley and Frederick, 1996). Soil texture also has a major influence on nematode community structure, and the association of certain nematode genera with particular soil textures is well known (DeGoede and Bongers, 1994; Wallace, 1973; Yeates, 1984). Goodell and Ferris (1980) found
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that the observed pattern of nematode dispersion was related to differences in soil texture within the field. Subsurface soil texture is also important, since it can affect the vertical distribution of nematodes within a site (Brodie, 1976). The majority of nematodes in the soil community occur in the uppermost 15–20 cm of the soil profile (McSorley, 1987; Yeates, 1980), and most sampling schemes assay that depth, limiting the observed nematode community to that stratum. The influence of subsurface soil characteristics on nematode communities in the upper 15–20 cm of soil is unclear. The current study was conducted in a site with relatively constant soil texture near the surface but with variable levels of subsurface clay. The objective was to determine the effect of subsurface clay on nematode community structure in a sandy soil. In addition, the nematode communities at 0–20 and 20–40 cm depths were compared. 2. Materials and methods 2.1. Site description The study site was located on a former University of Florida agronomy farm in Alachua County (29◦ 40 N, 92◦ 30 W), about 10 km northwest of Gainesville, Florida. The climate is warm temperate, but not subtropical, with several frosts per year. Annual mean temperature is 20.9 ◦ C and annual mean precipitation is 1342 mm (Thomas et al., 1985). The soil type was Arredondo fine sand (loamy, siliceous hyperthermic Grossarenic Paleudults), typically consisting of 90–92% sand, 4–5% silt, and 4–6% clay in the upper 20 cm, with <2.0% organic matter and pH 5.6–5.9. Below 20 cm depth, the soil texture varied within this 1-ha site. During the spring of 1993, a cover crop of hairy vetch (Vicia villosa L.) growing on the site exhibited patchy, uneven growth, apparently related to the varying levels of subsurface clay. The vetch crop was mowed and disced in early May. On 9 June 1993, the site was planted with ‘Clemson Spineless’ okra (Hibiscus esculentus L.) in rows 0.76 m apart. The okra crop was maintained until late September, when crop residues were mowed and the site was disced. A cover crop of ‘Wrens Abruzzi’ rye (Secale cereale L.) was planted on 1 November 1993.
2.2. Plot design and sampling Two separate experiments with differing designs were conducted, one with the okra crop and one with the rye crop. For the okra experiment, the site was divided into six blocks, and two 1-m2 plots were established within each block. Within each block, one plot was selected with a high concentration and one with a low concentration of subsurface clay. The location of the plots was based on observations of growth of the previous crop (hairy vetch) and on preliminary sampling of soil texture. Soil texture in the surface layer (0–20 cm depth) of all plots was similar, regardless of the differences in clay content in the subsurface layer (20–40 cm depth). Therefore, for the okra experiment, the experimental design involved two treatments (high subsurface clay versus low subsurface clay) in a randomized complete block design with six replications. To prepare the rye experiment, 18 plots were systematically established over the entire site and checked for subsurface soil texture. Eight of these were selected as representing a range of four different subsurface clay levels, each replicated twice. In both experiments, a sample consisted of four soil cores (2.5 cm diameter × 20 cm deep) removed from a 10 cm × 10 cm area within each plot. Once a sample from the surface 0–20 cm was obtained, the surface (0–20 cm deep) soil remaining in 10 cm × 10 cm was removed, to facilitate collection of a second sample from 20 to 40 cm depth immediately beneath. After sampling, the remaining surface soil was placed back in the hole and the location marked. A subsequent sample collected later from the same plot was located at least 0.5 m away from the original sample location. The okra experiment was sampled on 15 June and 14 September 1993, and the rye experiment was sampled on 4 November 1993 and 19 January 1994. The heights of five okra plants per plot were measured on 26 July and 14 September 1993. The four cores comprising a soil sample were placed in a plastic bag, mixed, and stored at 10 ◦ C. Separate subsamples were removed for determination of soil texture and estimation of nematode population densities. To determine soil texture, a subsample was oven-dried, and 100 g of dry soil were analyzed by the Bouyoucos hydrometer method (Bowles, 1986). For nematode analysis, a 100 cm3 subsample of fresh soil suspended in 1.7 liter of water was sieved, and then nematodes and debris
R. McSorley, J.J. Frederick / Applied Soil Ecology 19 (2002) 1–11
were separated by sieving and centrifugation (Jenkins, 1964). Nematodes were identified to genus in most cases (except for Rhabditidae and Tylenchidae) and counted at intermediate magnification (150×), or higher as needed, on an inverted microscope. 2.3. Data analysis Based on recent literature on nematode feeding habits (Yeates et al., 1993), nematodes were assigned to five trophic groups: bacterivores, fungivores, herbivores, omnivores, and predators. The Tylenchidae (primarily Filenchus and Tylenchus) were considered fungivores (McSorley and Frederick, 1999). Percent composition by trophic group and several other indices of nematode community structure were calculated for each sample. These included Simpson’s index of dominance (Simpson, 1949) calculated at the genus and trophic group levels; diversity, calculated as the reciprocal of Simpson’s index (Freckman and Ettema, 1993; Porazinska et al., 1999) at the genus and trophic group levels; maturity index (MI) and plant-parasite index (PPI) as defined by Bongers (Bongers, 1990; Bongers and Bongers, 1998), and MI as refined by Yeates (1994); the ratio of fungivores to bacterivores (F/B); and Wasilewska’s (1994) ratio of decomposers to herbivores. The effects of subsurface clay content on population densities of nematodes and indices of nematode community structure were determined by analysis
3
of variance or contrasts as appropriate (Freed et al., 1991). The rye experiment was designed so that regression analysis could be used to relate nematode numbers to subsurface clay level, if analysis of variance results were significant. Nematode count data were transformed by log10 (x + 1) prior to analysis, but untransformed means are presented.
3. Results 3.1. Okra experiment, effect of clay In this experiment, plots with high subsurface clay contained an average of 35.3% clay at 20–40 cm soil depth, while those with low subsurface clay averaged only 3.3% clay at that depth (Table 1). Soil texture in the upper layer (0–20 cm depth) of the soil profile was similar in plots with high subsurface clay and those with low subsurface clay. On 15 June at 0–20 cm depth, eight nematode taxa were more abundant (P ≤ 0.10) in plots with high subsurface clay than in those with low subsurface clay (Table 2). Total herbivores and total predators were also more abundant in high-subsurface-clay plots. The opposite trend occurred for one taxon (Nothotylenchus), which was more abundant in lowsubsurface-clay plots. On 14 September at 0–20 cm depth, 10 nematode taxa, as well as total bacterivores, fungivores,
Table 1 Soil texture and selected indices of nematode community structure at depths of 0–20 and 20–40 cm in plots with high or low levels of subsurface clay in okra experiment, 14 September 1993a Index measured
Percent of sand Percent of silt Percent of clay Nematode taxa/sample Percent of bacterivores Percent of fungivores Percent of herbivores Percent of omnivores Percent of predators a
0–20 cm
20–40 cm
High clay
Low clay
High clay
Low clay
90.5 4.0 5.5 19.5 19.8 11.2 62.3 5.1 1.1
91.2 (90.0–92.0) 4.2 (3.5–5.0) 4.7 (4.0–6.0) 18.2 (14–22) 28.5 (14.9–60.8) 13.2 (3.8–21.7) 48.3 (8.5–63.3) 5.5 (3.0–7.9) 2.8c (0.6–5.1)
58.0 6.7 35.3 14.3 21.4 27.0 44.8 2.4 3.3
93.8b (92.0–95.0) 2.8b (2.0–4.0) 3.3b (2.0–5.0) 10.7 (4–6) 42.3b (18.9–59.4) 4.6b (0–9.4) 43.0 (18.8–54.5) 1.5 (0–3.2) 5.6 (0–21.6)
(88.0–92.5) (3.5–4.5) (3.5–7.5) (16–25) (6.0–34.5) (4.0–18.2) (41.2–87.0) (1.4–10.9) (0–2.2)
Data are means of six replications, with ranges in parentheses. Means for high- and low-clay sites differ at P ≤ 0.01. c Means for high- and low-clay sites differ at P ≤ 0.10. b
(46.0–66.0) (4.0–10.0) (30.0–44.0) (10–18) (13.2–35.2) (12.0–49.2) (26.2–68.4) (0–6.4) (1.6–6.8)
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Table 2 Numbers of nematodes per 100 cm3 soil at depths of 0–20 and 20–40 cm in plots with high or low levels of subsurface clay in okra experimenta Nematode taxon
15 June 1993
14 September 1993
0–20 cm
0–20 cm
High clay Bacterivores Acrobeles Acrobeloides Cephalobus Cervidellus Chiloplacus Eucephalobus Panagrolaimus Prismatolaimus Rhabditidae Teratocephalus Zeldia
Low clay
High clay
Percent of total in upper 0–20 cm
20–40 cm Low clay
High clay
Low clay
62.0 110.8 47.2 67.0 3.0 61.2 13.2 6.5 366.8 0.8 55.8
79.8 65.2 26.0 106.2 7.5 98.5 1.8 2.0 144.5 0.2 5.0c
3.0 6.5 9.7 17.8 0.7 2.0 0 0.2 56.7 0.2 15.8
7.0 4.3 3.0 10.2c 0c 1.2 0.2 0.8 9.7 0 0.3d
1.3 2.5 2.3 2.5 0 0.2 0 1.3 7.5 0.2 2.5
0.8 2.2 0.8 3.3 0.2 0.2 0 0.3 4.5 0 0d
82.2b 69.9b 80.0b 82.8b 80.0 90.4b 100 37.5 84.7b 50.0 86.6
Total bacterivores
797.2
541.2
113.5
38.0c
21.2
12.7c
81.7b
Fungivores Aphelenchoides Aphelenchus Leptonchus Nothotylenchus Tylenchidae Fung. Dorylaimida
121.3 81.3 2.7 12.8 65.7 7.5
167.2 20.0e 0.8 31.7d 38.3 4.5
10.5 37.2 0.2 2.7 17.5 1.0
2.2 4.0d 0.2 3.0 6.8 1.7
1.3 19.2 0 1.5 3.5 0
0c 0.3e 0 0.2d 0.8 0.5
90.5b 67.9 100 77.3b 84.9b 84.2b
Total fungivores
291.3
269.8
70.0
17.8d
25.7
1.8e
76.1b
Herbivores Criconemella Helicotylenchus Meloidogyne Paratrichodorus Pratylenchus
1.0 92.0 27.0 51.8 8.7
2.2 10.2c 9.3 22.7d 1.7
18.3 26.8 320.5 23.7 6.0
8.7 0.7d 39.2d 4.7c 1.5
2.3 3.7 41.0 5.5 0.2
0.2 0.5c 5.8 7.5 0.3
91.5 86.8b 88.5b 68.5 93.8b
Total herbivores
181.5
58.8c
395.3
55.2e
53.5
14.8d
86.8b
Omnivores Aporcelaimellus Eudorylaimus Mesodorylaimus
14.3 24.3 6.8
4.2d 23.3 0.5e
2.0 17.7 1.3
1.2 3.3d 0c
0.2 2.5 0
0.2 0.3 0
90.4b 88.1b 100
Total omnivores
47.7
32.8
23.2
6.5e
3.5
0.5
88.1b
Predators Discolaimus Mononchus Mylonchulus Total predators Total nematodes a
0.8 5.8 5.7
1.7 0.5e 0e
1.7 2.7 0
1.0 0.3c 0
0 3.3 0
0 2.5 0
100b 34.0 –
12.8
6.0d
4.5
3.0
3.3
3.2
53.6
33.8e
83.8b
1344.2
920.0
690.7
122.7e
Data are means of six replications. Nematode numbers at 0–20 and 20–40 cm depth differ at P ≤ 0.10. c Nematode numbers in high- and low-clay sites differ at P ≤ 0.10. d Nematode numbers in high- and low-clay sites differ at P ≤ 0.05. e Nematode numbers in high- and low-clay sites differ at P ≤ 0.01. b
108.2
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herbivores, and omnivores, were more abundant (P ≤ 0.05) in the high-subsurface-clay plots (Table 2). The total number of nematodes was about five times greater (P ≤ 0.01) in high-clay plots than in low-clay plots. In addition to the nematode taxa shown, total numbers include occasional specimens of Alaimus, Anaplectus, Monhystera, Plectus, Wilsonema (bacterivores); Diphtherophora, Ditylenchus, Ecphyadophora, Neotylenchus, Psilenchus, Tylencholaimellus, Tylencholaimus (fungivores); Belonolaimus, Heterodera, Paratylenchus, Xiphinema (herbivores); Actinolaimus, Belondira, Enchodelus, Thonus (omnivores); Iotonchus, Miconchus, Nygolaimus, Triplya (predators). At 20–40 cm depth, where the differences in soil texture actually occurred, five taxa, as well as total numbers of several nematode trophic groups, were more abundant (P ≤ 0.10) in the plots with higher clay content. Although a variety of indices of nematode community structure were calculated, including diversity, dominance, and maturity indices, none of these was affected by clay level (data not shown). At 20–40 cm
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depth, percent of bacterivores was lower (P ≤ 0.01) and percent of fungivores was higher (P ≤ 0.01) in plots with high clay (Table 1). The heights of okra plants in plots with high clay (47.4 cm on 26 July; 58.0 cm on 14 September) were greater (P ≤ 0.10) than those from plots with low clay (20.7 cm on 26 July; 33.9 cm on 14 September). 3.2. Okra experiment, effect of depth On 14 September, a majority of nematode taxa and four of five trophic groups were more abundant (P ≤ 0.10) at 0–20 than at 20–40 cm depth (Table 2). A majority (83.8%) of the total nematodes occurred in the upper 0–20 cm. For most taxa, >80% of the individuals found occurred in the upper 0–20 cm of soil. The mean number of nematode taxa per sample at 0–20 cm depth (18.8) was greater (P ≤ 0.01) than at 20–40 cm depth (12.5). Several taxa, such as Prismatolaimus, Aphelenchus, Paratrichodorus, and Mononchus, as well as total predators, did not differ (P > 0.10) in numbers between 0–20 and 20–40 cm depths.
Table 3 Soil texture and selected indices of nematode community structure at depths of 0–20 and 20–40 cm in plots with various levels of subsurface clay in rye experimenta Index measured
0–20 cm
20–40 cm
3% clay
6% clay
12.5% clay
22% clay
3% clay
6% clay
12.5% clay
22% clay
4 November 1993 Percent of sand Percent of silt Percent of clay Taxa/sample Percent of bacterivores Percent of fungivores Percent of herbivores Percent of omnivores Percent of predators
91.0 4.2 4.8 20.0 44.8 34.8 16.8 2.0 0.5
91.0 2.8 6.2 26.0 46.5 19.1 29.0 2.0 2.4
90.0 4.0 6.0 26.5 37.0 24.9 32.1 3.5 0.8
88.2 4.2 7.5 25.0c 38.6 18.2c 38.4 3.0 1.0
93.0 4.0 3.0 21.5 43.9 33.0 16.2 3.7 1.4
90.2 3.8 6.0 20.0 40.9 14.4 39.4 3.0 1.0
80.5 6.5 12.5 20.0 31.0 22.8 41.0 2.9 0.8
75.0b 3.0 22.0b 18.5 19.2 27.2 51.8 0.8 0.6
19 January 1994 Taxa/sample Percent of bacterivores Percent of fungivores Percent of herbivores Percent of omnivores Percent of predators
21.5 60.0 28.8 7.6 2.1 1.0
25.5 48.3 29.3 17.8 0.8 2.3
24.0 51.3 34.8 10.1 1.6 0.6
27.5 41.2 30.4 25.0 1.5 1.0
22.0 67.2 16.2 10.4 3.4 0.8
21.0 44.0 17.1 33.2 3.0 1.6
23.0 54.6 23.2 19.5 1.2 1.0
19.5 40.4 22.4 34.0 2.1d 0.2
a
Data are means of two replications. At a given depth, means among clay levels differ at P ≤ 0.01. c At a given depth, means among clay levels differ at P ≤ 0.10. d At a given depth, means among clay levels differ at P ≤ 0.05. b
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Table 4 Numbers of nematodes per 100 cm3 soil at depths of 0–20 and 20–40 cm in plots with various levels of subsurface clay in rye experiment, 4 November 1993a Nematode taxon
0–20 cm 3% clay
20–40 cm 6% clay
12.5% clay
18.0 49.5 20.5 24.5 2.0 2.0 1.5 1.0 52.0 0 0
28.5 93.5 19.0 19.5 3.0 2.0 0.5 4.5 134.5 0.5 0.5
33.0 36.0 13.0 16.5 2.0 7.0 1.5 0.5 110.5 1.0 1.5
38.5 101.5c 13.0 22.0 3.5 1.0 5.0 2.0 193.5 1.5 6.5
24.0 36.0 16.5 22.5 0 2.0 0 1.5 47.0 1.5 1.0
8.0 32.5 11.0 20.0 1.0 1.0 0 2.0 74.5 4.5 0
7.5 10.5 6.5 11.5 0 1.0 0 2.5 18.0 3.5 0
1.0 9.0 7.5 3.0 0 0.5 0 5.0 14.0 0.5 0
74.4b 76.1b 61.2 59.1b 91.3b 72.7 100b 42.1 76.2b 23.1b 92.9
Total bacterivores
174.0
316.0
228.0
393.5
157.0
157.0
64.0
43.0
72.5b
Fungivores Aphelenchoides Aphelenchus Leptonchus Nothotylenchus Tylenchidae Fung. Dorylaimida
68.0 40.0 0 6.5 16.0 2.5
51.0 40.0 2.0 2.0 27.0 14.5
76.0 32.0 2.5 15.0 17.5 6.5
105.0 57.5 4.5d 5.5 35.0 6.0
61.5 39.0 1.0 2.0 17.0 6.0
6.5 19.0 0 0 19.0 3.0
3.0 10.0 1.5 0 25.5 3.5
2.5 47.5 0.5 0.5e 11.5 10.0
80.3b 59.5 75.0 92.1b 56.7 56.7
Total fungivores
133.5
135.0
149.0
209.0
126.0
49.0
42.0
72.0
68.4b
Herbivores Criconemella Helicotylenchus Meloidogyne Paratrichodorus Pratylenchus
0 0 21.5 23.5 7.0
0 3.5 87.0 89.5 9.5
35.5 0.5 70.5 70.0 2.5
337.5 61.5 179.5 42.0 20.0
10.0 0 18.5 11.5 3.0
0 0 139.0 67.5 9.5
3.0 0 5.5 81.5 2.0
1.5 5.0 46.5 41.5 10.5
96.3 92.9 63.1 52.6 60.9
Total herbivores
56.0
189.0
181.5
642.0
44.0
216.0
92.0
105.5
70.0
Omnivores Aporcelaimellus Eudorylaimus Mesodorylaimus
1.5 5.5 0
2.0 10.5 0.5
4.0 15.0 0
0 18.5 21.5d
1.5 11.5 0
0 8.5 0.5
0 5.0 0
0 2.0 0.5
83.3 64.7 95.6
Total omnivores
8.0
13.0
20.5
41.0
13.5
10.0
5.5
2.5
72.4b
0 1.5 0
5.5 7.5 1.5
2.0 1.5 1.5
4.5d 4.0 3.0
0 3.0 0
0 4.0 0
0 2.0 0
0 1.5 0
1.5
15.5
5.0
11.5c
3.5
4.0
2.0
1.5
75.3b
376.5
675.5
593.5
350.0
440.0
208.5
225.5
70.7b
Bacterivores Acrobeles Acrobeloides Cephalobus Cervidellus Chiloplacus Eucephalobus Panagrolaimus Prismatolaimus Rhabditidae Teratocephalus Zeldia
Predators Discolaimus Mononchus Mylonchulus Total predators Total nematodes
22% clay
1308.0
a
Data are means of two replications. Nematode numbers at 0–20 and 20–40 cm depth differ c At a given depth, means among clay levels differ at P d At a given depth, means among clay levels differ at P e At a given depth, means among clay levels differ at P b
at P ≤ 0.10. ≤ 0.10. ≤ 0.01. ≤ 0.05.
3% clay
6% clay
12.5% clay
22% clay
Percent of total in upper 0–20 cm
100b 55.3 100b
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Table 5 Numbers of nematodes per 100 cm3 soil at depths of 0–20 and 20–40 cm in plots with various levels of subsurface clay in rye experiment, 19 January 1994a Nematode taxon
0–20 cm
20–40 cm 6% clay
36.0 180.0 39.0 12.5 1.0 10.0 1.5 1.0 94.5 0 13.5
37.0 148.5 51.5 12.5 2.0 14.5 27.5 2.5 144.0 3.5 1.5
103.5 113.5 45.5 25.0 6.0 19.5 0 0.5 232.0 1.0 17.0
102.5 155.0 25.0 30.0 1.5 98.0 3.0c 2.0 89.5 0.5 4.5
17.0 138.0 11.5 13.5 1.0 5.0 0.5 1.0 17.5 2.0 1.5
4.5 87.0 33.0 4.0 0 1.5 0 5.0 25.5 6.5 0
24.5 101.5 12.0 21.5 0 8.5 0.5 0.5 59.0 4.5 3.0
26.5 46.5 5.5 16.5 1.0 3.0 0 3.5 27.5 16.5 0
79.4b 61.5b 72.2 59.0 84.0b 88.8b 97.0b 37.5 81.2b 14.5b 89.0b
Total bacterivores
390.5
456.5
567.5
513.5
210.5
170.0
240.0
153.0
71.4b
Fungivores Aphelenchoides Aphelenchus Leptonchus Nothotylenchus Tylenchidae Fung. Dorylaimida
139.5 32.0 1.0 2.5 26.5 5.0
177.5 60.5 2.0 20.5 13.0 10.0
137.0 51.5 1.0 28.0 169.5 4.0
172.5 53.5 4.5 18.0c 136.5 21.5
2.5 18.0 0 2.0 21.5 2.0
6.5 32.5 1.0 0 18.0 8.5
7.0 31.0 1.0 2.5 50.5 8.0
17.5 15.5 0.5 0.5 17.5 1.5c
94.9b 67.1b 77.3b 93.2b 76.3 66.9
Total fungivores
205.5
282.5
391.5
402.5
46.0
66.0
101.5
53.5
82.8b
Herbivores Criconemella Helicotylenchus Meloidogyne Paratrichodorus Pratylenchus
0 0 4.5 41.5 2.0
0 24.0 86.0 48.0 0
5.0 0 5.5 54.0 1.0
28.5 50.0 20.5 119.0d 5.0
1.0 1.5 1.5 26.0 0
0 6.0 75.0 45.5 0
9.0 0 15.5 51.0 1.0
4.0 9.0 4.0 55.0 1.0
70.5 81.8 54.8 59.7 80.0
Total herbivores
48.5
158.5
76.0
228.5d
30.5
126.5
79.5
73.0d
62.3
Omnivores Aporcelaimellus Eudorylaimus Mesodorylaimus
1.0 13.5 0
2.0 4.5 0.5
0.5 12.0 0.5
1.0 13.0 0.5
0.5 9.5 0.5
0 9.0 2.5
0.5 4.0 0
0.5 6.0 0
75.0b 60.1 33.3
Total omnivores
15.5
7.5
13.0
14.5
10.5
11.5
5.5
7.0
59.4
0 4.5 0
1.0 16.5 4.5
0.5 4.0 1.0
3.0d 3.0c 3.5
0 3.0 0
2.5 3.0 0.5
1.5 3.0 0
0 1.0 0
52.9 73.7b 94.7b
4.5
22.0
5.5
9.5
3.0
6.0
4.5
1.0
74.1b
669.0
940.5
1068.5
1177.5
305.5
384.5
433.5
290.0
73.2b
Bacterivores Acrobeles Acrobeloides Cephalobus Cervidellus Chiloplacus Eucephalobus Panagrolaimus Prismatolaimus Rhabditidae Teratocephalus Zeldia
Predators Discolaimus Mononchus Mylonchulus Total predators Total nematodes a
12.5% clay
22% clay
Data are means of two replications. Nematode numbers at 0–20 and 20–40 cm depth differ at P ≤ 0.10. c At a given depth, means among clay levels differ at P ≤ 0.05. d At a given depth, means among clay levels differ at P ≤ 0.10. b
3% clay
6% clay
12.5% clay
22% clay
Percent of total in upper 0–20 cm
3% clay
8
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3.3. Rye experiment, effect of clay As planned, the plots in this experiment differed (P ≤ 0.01) in their content of clay and sand at 20–40 cm depth, while no differences (P > 0.10) were evident at 0–20 cm (Table 3). On 4 November at 0–20 cm depth, differences among clay levels (P ≤ 0.10) were observed for four taxa (Acrobeloides, Leptonchus, Mesodorylaimus, Discolaimus) and for total predators (Table 4). Attempts to develop regression equations relating numbers of these nematodes to subsurface clay level were unsuccessful (P > 0.10). However, when numbers of nematodes at the lowest subsurface clay level (3%) were contrasted with those at the highest subsurface clay level (22%), significantly more (P ≤ 0.10) nematodes were found in the highest subsurface clay plots for seven taxa (Acrobeles, Acrobeloides, Leptonchus, Helicotylenchus, Eudorylaimus, Mesodorylaimus, Discolaimus) and for total omnivores, total predators, and total nematodes. At 0–20 cm depth, differences among subsurface clay levels (P ≤ 0.10) occurred on 4 November in number of taxa per sample and percent of fungivores (Table 3). However, none of the calculated indices of community structure, such as maturity indices, diversity, dominance, or F/B ratios were affected by subsurface clay level (data not shown). On 19 January at 0–20 cm depth, differences among subsurface clay levels (P ≤ 0.10) were observed for five taxa (Panagrolaimus, Nothotylenchus, Paratrichodorus, Discolaimus, Mononchus) and for total herbivores (Table 5). Numbers of nematodes at the highest and lowest subsurface clay levels differed (P ≤ 0.10) for Nothotylenchus, Helicotylenchus, Paratrichodorus, Discolaimus, and total herbivores. Relatively few effects of subsurface clay level on nematode taxa were observed at 20–40 cm depth (Tables 4 and 5). At this depth, numbers of herbivores on 19 January were greater (P ≤ 0.10) in plots with 22% clay subsurface than in plots with 3% subsurface clay (Table 5). 3.4. Rye experiment, effect of depth As in the okra experiment, most taxa and trophic groups were more abundant at 0–20 than at 20–40 cm (Tables 4 and 5). A majority of all nematodes
recovered occurred in the upper 0–20 cm on 4 November (70.7%) and on 19 January (73.2%). The mean number of taxa per sample was greater (P ≤ 0.05) at 0–20 (24.4 on 4 November, 24.6 on 19 January) than at 20–40 cm (20.0 on 4 November, 21.4 on 19 January). Taxa that did not differ (P ≤ 0.10) in numbers between 0–20 and 20–40 cm depths included Prismatolaimus, Meloidogyne, Paratrichodorus, and Eudorylaimus on both sampling dates (Tables 4 and 5). Teratocephalus was more abundant (P ≤ 0.05) at 20–40 than at 0–20 cm on both sampling dates, with only 14.5–23.1% of its population present in the upper 0–20 cm.
4. Discussion and conclusions The direct effects of clay content on nematode population levels are best assessed at 20–40 cm depth in this experiment, since that depth is where the differences in subsurface clay content actually occurred. The fact that some nematode taxa were more abundant in high-subsurface-clay soil than in low-subsurface-clay soil is not surprising, since many nematodes show distinct preferences for sandy or clay soil (Brodie, 1976). However, in the current experiments, even the soils with the highest levels of subsurface clay still contained 58% sand, and could not technically be considered clay soils. Therefore, differences in nematode densities between high- and low-clay soils observed here may not be as great as in other soils. Although direct effects from clay content were limited, the indirect effects of subsurface clay content on nematodes at 0–20 cm depth were numerous. At this depth, all plots had a similar high sand (means ranged from 88.2 to 91.2% across both experiments) and low clay (4.7–7.5%) content. Yet within this relatively uniform layer of sandy soil, a number of nematode taxa representing a range of trophic groups were affected by the varying clay content of the underlying soil layer. The observed effects of subsurface clay were greater in the okra experiment than in the rye experiment. The rye experiment was designed to assess effects across a range of subsurface clay levels, but the number of replications within each clay level was low. The intermediate clay levels contributed additional variability, possibly obscuring trends. When only the highest and lowest subsurface clay levels were contrasted, more
R. McSorley, J.J. Frederick / Applied Soil Ecology 19 (2002) 1–11
differences were apparent. Perhaps most important however, is that the difference between highest (mean: 22.0%; range: 22–22%) and lowest (mean: 3.0%; range: 3–3%) subsurface clay levels in the rye experiment was less than that between the high clay (mean: 35.3%; range: 30–44%) and low clay (mean: 3.3%; range: 2–5%) levels in the okra experiment. Therefore, effects on nematodes were observed more frequently in the okra experiment, in which the differences between subsurface clay treatments were greater. Many different nematode taxa were more abundant in the top layer (0–20 cm) of soil with high clay beneath than in the top layer of soil with low clay beneath, especially in the okra experiment. Clay content influences other important soil properties such as water holding capacity and nutrient availability (Brady and Weil, 1996; Wallace, 1973). In the current study, it is presumed that such effects, resulting from subsurface clay, may extend into the upper soil layers as well. Within the upper 0–20 cm of soil, soil moisture in the plots with high clay beneath (8.1%) was greater (P ≤ 0.01) than in plots with low clay beneath (5.6%), as measured on 15 June 1993. Okra plants were larger in the high-clay plots than in the low-clay plots, offering more roots and resources to nematode communities in the high-clay plots. It is interesting that populations of Meloidogyne incognita, a serious pest of okra (Netscher and Sikora, 1990), were higher in plots with high clay beneath, yet plant growth was also better in these plots than in plots with low clay beneath. Increased soil water-holding capacity can improve tolerance of susceptible crops to root–knot nematodes (McSorley and Gallaher, 1995), which may have occurred in the present study as well. Although a number of different nematode taxa were affected by subsurface clay, no effects could be detected with commonly-used indices of nematode community structure. This was probably because subsurface clay levels did not consistently affect one particular group of nematodes, but rather, affected several selected taxa in all trophic groups. For example, maturity indices have performed well in situations where succession or environmental disturbance have affected nematode colonizers and persisters in different ways (Bongers and Bongers, 1998; Wasilewska, 1994, 1997b). In the current study, however both colonizers (e.g. Aphelenchus, Zeldia) and persisters
9
(e.g. Aporcelaimellus, Mononchus) were affected by subsurface clay levels in similar ways. As changes in the decomposition process occur, changes in the ratio of nematode fungivores to bacterivores (F/B ratio) are anticipated (McSorley and Frederick, 1999). But in the present study, since both bacterivores and fungivores were similarly affected by subsurface clay, no significant effects on F/B were observed. It is becoming evident that indices of community structure cannot be applied indiscriminately to all situations. In another recent study (McSorley and Frederick, 2000), many different nematode taxa showed strong seasonal responses that were not consistently reflected in maturity indices and several other indices. As in the present study, a variety of diverse taxa were similarly affected, and so no mathematical response in the indices should have been anticipated. Optimal performance of an index depends on differential responses of taxa appropriate to the situation for which the index was intended. The current study also provided some information on vertical distribution of the nematode community in this site. Although most nematodes are reported to occur in the upper 20 cm of the soil profile (McSorley, 1987; Yeates, 1980), a few genera were reported to have much of their population distributed below 20 cm (Boag et al., 1987; Yeates et al., 1983). In the current study, genera such as Paratrichodorus and Prismatolaimus did not differ in abundance between 0–20 and 20–40 cm depths. Teratocephalus represented an extreme case in that >75% of the population (present in the profile from 0 to 40 cm) was found at 20–40 cm depth. The opposite trend occurred with most other nematodes, with 70–80% of total nematode numbers recovered from 0 to 20 cm. Thus for the majority of nematodes in the soil community, a sample of 0–20 cm depth should be adequate. Sampling deeper than 20 cm requires additional effort (Boag et al., 1987), which may be difficult to justify unless objectives require a critical assessment of deep-dwelling genera such as Teratocephalus. The sampling and evaluation of nematode communities is complex, since individual genera are influenced by a variety of different physical and biological factors. Many of these factors can vary within the microhabitat, where they can directly affect nematodes (DeGoede and Bongers, 1994; McSorley, 1997; Yeates, 1999). In the current study, subsurface
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clay indirectly influenced a number of nematode taxa residing at a different layer within the soil profile.
Acknowledgements The authors thank Kip Malcolm for technical assistance and Nancy Sanders for manuscript preparation. This work was supported by the Florida Agricultural Experiment Station, and this paper is Florida Agricultural Experiment Station Journal Series No. R-08070. References Boag, B., Brown, D.J.F., Topham, P.B., 1987. Vertical and horizontal distribution of virus-vector nematodes and implications for sampling procedures. Nematologica 33, 83–96. Bongers, T., 1990. The maturity index: an ecological measure of environmental disturbance based on nematode species composition. Oecologia 83, 14–19. Bongers, T., Bongers, M., 1998. Functional diversity of nematodes. Appl. Soil Ecol. 10, 239–251. Bowles, J.E., 1986. Engineering Properties of Soils and Their Measurement. McGraw-Hill, New York. Brady, N.C., Weil, R.R., 1996. The Nature and Property of Soils. Prentice-Hall, Upper Saddle River, NJ. Brodie, B.B., 1976. Vertical distribution of three nematode species in relation to certain soil properties. J. Nematol. 8, 243–247. DeGoede, R.G.M., Bongers, T., 1994. Nematode community structure in relation to soil and vegetation characteristics. Appl. Soil Ecol. 1, 29–44. Ferris, H., Venette, R.C., Lau, S.S., 1996. Dynamics of nematode communities in tomatoes grown in conventional and organic farming systems, and their impact on soil fertility. Appl. Soil Ecol. 3, 161–175. Freckman, D.W., Ettema, C.H., 1993. Assessing nematode communities in agroecosystems of varying human intervention. Agric. Ecosyst. Environ. 45, 239–261. Freed, R., Eisensmith, S.P., Goetz, S., Reicosky, D., Smail, V.W., Wohlberg, P., 1991. User’s Guide to MSTAT-C. Michigan State University, East Lansing, MI. Goodell, P., Ferris, H., 1980. Plant-parasitic nematode distributions in an alfalfa field. J. Nematol. 12, 136–141. Jenkins, W.R., 1964. A rapid centrifugal-flotation technique for separating nematodes from soil. Plant Dis. Rep. 48, 692. McSorley, R., 1987. Extraction of nematodes and sampling methods. In: Brown, R.H., Kerry, B.R. (Eds.), Principles and Practice of Nematode Control in Crops. Academic Press, Sydney, pp. 13–47. McSorley, R., 1997. Relationship of crop and rainfall to soil nematode community structure in perennial agroecosystems. Appl. Soil Ecol. 6, 147–159. McSorley, R., Gallaher, R.N., 1995. Cultural practices improve crop tolerance to nematodes. Nematropica 25, 53–60.
McSorley, R., Frederick, J.J., 1996. Nematode community structure in rows and between rows of a soybean field. Fund. Appl. Nematol. 19, 251–261. McSorley, R., Frederick, J.J., 1999. Nematode population fluctuations during decomposition of specific organic amendments. J. Nematol. 31, 37–44. McSorley, R., Frederick, J.J., 2000. Short-term effects of cattle grazing on nematode communities in Florida pastures. Nematropica 30, 211–221. Neher, D.A., 1999. Nematode communities in organically and conventionally managed agricultural soils. J. Nematol. 31, 142–154. Neher, D.A., Olson, R.K., 1999. Nematode communities in soils of four farm cropping management systems. Pedobiologia 43, 430–438. Neher, D.A., Easterling, K.N., Fiscus, D., Campbell, C.L., 1998. Comparison of nematode communities in agricultural soils of North Carolina and Nebraska. Ecol. Application 8, 213–223. Netscher, C., Sikora, R.A., 1990. Nematode parasites of vegetables. In: Luc, M., Sikora, R.A., Bridge, J. (Eds.), Plant-Parasitic Nematodes in Subtropical and Tropical Agriculture. CAB International, Wallingford, UK, pp. 237–283. Porazinska, D.L., Duncan, L.W., McSorley, R., Graham, J.H., 1999. Nematode communities as indicators of status and processes of a soil ecosystem influenced by agricultural management practices. Appl. Soil Ecol. 13, 69–86. Simpson, E.H., 1949. Measurement of diversity. Nature 163, 688. Thomas, B.P., Cummings, E., Wittstruck, W.H., 1985. Soil survey of Alachua County, Florida. Soil Conservation Service, US Department of Agriculture, Washington, DC. Todd, T.C., 1996. Effects of management practices on nematode community structure in tallgrass prairie. Appl. Soil. Ecol. 3, 235–246. Todd, T.C., Blair, J.M., Milliken, G.A., 1999. Effects of altered soil–water availability on a tallgrass prairie nematode community. Appl. Soil Ecol. 13, 45–55. Wallace, H.R., 1973. Nematode Ecology and Plant Disease. Edward Arnold, London. Wasilewska, L., 1989. Impact of human activities on nematode communities in terrestrial ecosystems. In: Clarholm, M., Bergstrom, L. (Eds.), Ecology of Arable Land. Kluwer Academic Publishers, Dordrecht, pp. 123–132. Wasilewska, L., 1994. The effect of age of meadows on succession and diversity in soil nematode communities. Pedobiologia 38, 1–11. Wasilewska, L., 1995. Directions of changes in communities of soil nematodes in man-disturbed ecosystems. Acta Zool. Fennica 196, 271–274. Wasilewska, L., 1997a. The relationship between the diversity of soil nematode communities and the plant species richness of meadows. Ekol. Polska 45, 719–732. Wasilewska, L., 1997b. Soil invertebrates as bioindicators, with special reference to soil-inhabiting nematodes. Russian J. Nematol. 5, 113–126. Yeates, G.W., 1980. Populations of nematode genera in soils under pasture. III. Vertical distribution at eleven sites. New Zealand J. Agric. Res. 23, 117–128.
R. McSorley, J.J. Frederick / Applied Soil Ecology 19 (2002) 1–11 Yeates, G.W., 1984. Variation in soil nematode diversity under pasture with soil and year. Soil Biol. Biochem. 16, 95–102. Yeates, G.W., 1994. Modification and quantification of the nematode maturity index. Pedobiologia 38, 97–101. Yeates, G.W., 1999. Effects of plants on nematode community structure. Annu. Rev. Phytopathol. 37, 127–149. Yeates, G.W., Bird, A.F., 1994. Some observations on the influence of agricultural practices on the nematode faunae of some South Australian soils. Fund. Appl. Nematol. 17, 133–145.
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Yeates, G.W., Stannard, R.E., Barker, G.M., 1983. Vertical distribution of nematode populations in Horotiu soils. New Zealand Soil Bureau Science Report 60, Department of Scientific and Industrial Research, Lower Hutt, New Zealand. Yeates, G.W., Bongers, T., de Goede, R.G.M., Freckman, D.W., Georgieva, S., 1993. Feeding habits in soil nematode families and genera—an outline for soil ecologists. J. Nematol. 25, 315–331.
Effect of subsurface clay on nematode communities in a sandy soil Robert McSorley∗ , John J. Frederick Department of Entomology and Nematology, University of Florida, P.O. Box 110620, Gainesville, FL 32611-0620, USA Received 30 March 2001; accepted 7 August 2001
Abstract A variety of soil properties can directly or indirectly affect nematode community structure. The effects of subsurface clay content (at 20–40 cm depth) on nematodes in the surface layer (0–20 cm depth) of a sandy soil were examined in field experiments in Florida, USA. Plots were established in a site with a relatively uniform sandy upper soil layer (88–91% sand and 5–7% clay at 0–20 cm depth) but with varying levels of clay in the subsurface layer (3–35% clay at 20–40 cm depth). Nematode numbers in the surface soil layer were affected by the amount of clay in the subsurface layer. Population densities of a number of different nematode genera were greater in the surface layer of plots with 35% subsurface clay than in plots with 3% subsurface clay. Indices of nematode community structure were largely unaffected, since effects of subsurface clay were observed across all nematode groups. Most nematodes (70–80% of total numbers) occurred at 0–20 cm depth, although Teratocephalus was more common at 20–40 than at 0–20 cm. Subsurface clay content indirectly affected soil moisture and other environmental factors in the upper soil layer in which most nematodes reside. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Agroecosystems; Ecological indices; Nematode trophic groups; Soil nematodes; Soil texture; Vertical distribution
1. Introduction Recent work in nematode ecology has focused on understanding the factors that influence nematode community structure. Biological and physical characteristics of the ecosystem, such as the plant species present (DeGoede and Bongers, 1994; McSorley, 1997; Wasilewska, 1997a; Yeates, 1999) and properties related to soil moisture (McSorley, 1997; Porazinska et al., 1999; Todd et al., 1999) have major effects on nematode communities. However, many human activities also influence soil nematode communities (Wasilewska, 1989, 1995), particularly various ∗ Corresponding author. Tel.: +1-352-392-1901/ext. 137; fax: +1-352-392-0190. E-mail address: [email protected] (R. McSorley).
agricultural practices and intensity of agricultural practices (Ferris et al., 1996; Freckman and Ettema, 1993; McSorley and Frederick, 2000; Neher, 1999; Neher and Olson, 1999; Todd, 1996; Yeates and Bird, 1994). While some effects are evident on a regional basis (Neher et al., 1998), considerable heterogeneity occurs at the field level (McSorley, 1987). For example, the plant rhizosphere has such an important influence on most nematode groups that differences in nematode community structure are evident within a short distance from plant rows and root systems (McSorley and Frederick, 1996). Soil texture also has a major influence on nematode community structure, and the association of certain nematode genera with particular soil textures is well known (DeGoede and Bongers, 1994; Wallace, 1973; Yeates, 1984). Goodell and Ferris (1980) found
0929-1393/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 9 - 1 3 9 3 ( 0 1 ) 0 0 1 6 7 - 6
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that the observed pattern of nematode dispersion was related to differences in soil texture within the field. Subsurface soil texture is also important, since it can affect the vertical distribution of nematodes within a site (Brodie, 1976). The majority of nematodes in the soil community occur in the uppermost 15–20 cm of the soil profile (McSorley, 1987; Yeates, 1980), and most sampling schemes assay that depth, limiting the observed nematode community to that stratum. The influence of subsurface soil characteristics on nematode communities in the upper 15–20 cm of soil is unclear. The current study was conducted in a site with relatively constant soil texture near the surface but with variable levels of subsurface clay. The objective was to determine the effect of subsurface clay on nematode community structure in a sandy soil. In addition, the nematode communities at 0–20 and 20–40 cm depths were compared. 2. Materials and methods 2.1. Site description The study site was located on a former University of Florida agronomy farm in Alachua County (29◦ 40 N, 92◦ 30 W), about 10 km northwest of Gainesville, Florida. The climate is warm temperate, but not subtropical, with several frosts per year. Annual mean temperature is 20.9 ◦ C and annual mean precipitation is 1342 mm (Thomas et al., 1985). The soil type was Arredondo fine sand (loamy, siliceous hyperthermic Grossarenic Paleudults), typically consisting of 90–92% sand, 4–5% silt, and 4–6% clay in the upper 20 cm, with <2.0% organic matter and pH 5.6–5.9. Below 20 cm depth, the soil texture varied within this 1-ha site. During the spring of 1993, a cover crop of hairy vetch (Vicia villosa L.) growing on the site exhibited patchy, uneven growth, apparently related to the varying levels of subsurface clay. The vetch crop was mowed and disced in early May. On 9 June 1993, the site was planted with ‘Clemson Spineless’ okra (Hibiscus esculentus L.) in rows 0.76 m apart. The okra crop was maintained until late September, when crop residues were mowed and the site was disced. A cover crop of ‘Wrens Abruzzi’ rye (Secale cereale L.) was planted on 1 November 1993.
2.2. Plot design and sampling Two separate experiments with differing designs were conducted, one with the okra crop and one with the rye crop. For the okra experiment, the site was divided into six blocks, and two 1-m2 plots were established within each block. Within each block, one plot was selected with a high concentration and one with a low concentration of subsurface clay. The location of the plots was based on observations of growth of the previous crop (hairy vetch) and on preliminary sampling of soil texture. Soil texture in the surface layer (0–20 cm depth) of all plots was similar, regardless of the differences in clay content in the subsurface layer (20–40 cm depth). Therefore, for the okra experiment, the experimental design involved two treatments (high subsurface clay versus low subsurface clay) in a randomized complete block design with six replications. To prepare the rye experiment, 18 plots were systematically established over the entire site and checked for subsurface soil texture. Eight of these were selected as representing a range of four different subsurface clay levels, each replicated twice. In both experiments, a sample consisted of four soil cores (2.5 cm diameter × 20 cm deep) removed from a 10 cm × 10 cm area within each plot. Once a sample from the surface 0–20 cm was obtained, the surface (0–20 cm deep) soil remaining in 10 cm × 10 cm was removed, to facilitate collection of a second sample from 20 to 40 cm depth immediately beneath. After sampling, the remaining surface soil was placed back in the hole and the location marked. A subsequent sample collected later from the same plot was located at least 0.5 m away from the original sample location. The okra experiment was sampled on 15 June and 14 September 1993, and the rye experiment was sampled on 4 November 1993 and 19 January 1994. The heights of five okra plants per plot were measured on 26 July and 14 September 1993. The four cores comprising a soil sample were placed in a plastic bag, mixed, and stored at 10 ◦ C. Separate subsamples were removed for determination of soil texture and estimation of nematode population densities. To determine soil texture, a subsample was oven-dried, and 100 g of dry soil were analyzed by the Bouyoucos hydrometer method (Bowles, 1986). For nematode analysis, a 100 cm3 subsample of fresh soil suspended in 1.7 liter of water was sieved, and then nematodes and debris
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were separated by sieving and centrifugation (Jenkins, 1964). Nematodes were identified to genus in most cases (except for Rhabditidae and Tylenchidae) and counted at intermediate magnification (150×), or higher as needed, on an inverted microscope. 2.3. Data analysis Based on recent literature on nematode feeding habits (Yeates et al., 1993), nematodes were assigned to five trophic groups: bacterivores, fungivores, herbivores, omnivores, and predators. The Tylenchidae (primarily Filenchus and Tylenchus) were considered fungivores (McSorley and Frederick, 1999). Percent composition by trophic group and several other indices of nematode community structure were calculated for each sample. These included Simpson’s index of dominance (Simpson, 1949) calculated at the genus and trophic group levels; diversity, calculated as the reciprocal of Simpson’s index (Freckman and Ettema, 1993; Porazinska et al., 1999) at the genus and trophic group levels; maturity index (MI) and plant-parasite index (PPI) as defined by Bongers (Bongers, 1990; Bongers and Bongers, 1998), and MI as refined by Yeates (1994); the ratio of fungivores to bacterivores (F/B); and Wasilewska’s (1994) ratio of decomposers to herbivores. The effects of subsurface clay content on population densities of nematodes and indices of nematode community structure were determined by analysis
3
of variance or contrasts as appropriate (Freed et al., 1991). The rye experiment was designed so that regression analysis could be used to relate nematode numbers to subsurface clay level, if analysis of variance results were significant. Nematode count data were transformed by log10 (x + 1) prior to analysis, but untransformed means are presented.
3. Results 3.1. Okra experiment, effect of clay In this experiment, plots with high subsurface clay contained an average of 35.3% clay at 20–40 cm soil depth, while those with low subsurface clay averaged only 3.3% clay at that depth (Table 1). Soil texture in the upper layer (0–20 cm depth) of the soil profile was similar in plots with high subsurface clay and those with low subsurface clay. On 15 June at 0–20 cm depth, eight nematode taxa were more abundant (P ≤ 0.10) in plots with high subsurface clay than in those with low subsurface clay (Table 2). Total herbivores and total predators were also more abundant in high-subsurface-clay plots. The opposite trend occurred for one taxon (Nothotylenchus), which was more abundant in lowsubsurface-clay plots. On 14 September at 0–20 cm depth, 10 nematode taxa, as well as total bacterivores, fungivores,
Table 1 Soil texture and selected indices of nematode community structure at depths of 0–20 and 20–40 cm in plots with high or low levels of subsurface clay in okra experiment, 14 September 1993a Index measured
Percent of sand Percent of silt Percent of clay Nematode taxa/sample Percent of bacterivores Percent of fungivores Percent of herbivores Percent of omnivores Percent of predators a
0–20 cm
20–40 cm
High clay
Low clay
High clay
Low clay
90.5 4.0 5.5 19.5 19.8 11.2 62.3 5.1 1.1
91.2 (90.0–92.0) 4.2 (3.5–5.0) 4.7 (4.0–6.0) 18.2 (14–22) 28.5 (14.9–60.8) 13.2 (3.8–21.7) 48.3 (8.5–63.3) 5.5 (3.0–7.9) 2.8c (0.6–5.1)
58.0 6.7 35.3 14.3 21.4 27.0 44.8 2.4 3.3
93.8b (92.0–95.0) 2.8b (2.0–4.0) 3.3b (2.0–5.0) 10.7 (4–6) 42.3b (18.9–59.4) 4.6b (0–9.4) 43.0 (18.8–54.5) 1.5 (0–3.2) 5.6 (0–21.6)
(88.0–92.5) (3.5–4.5) (3.5–7.5) (16–25) (6.0–34.5) (4.0–18.2) (41.2–87.0) (1.4–10.9) (0–2.2)
Data are means of six replications, with ranges in parentheses. Means for high- and low-clay sites differ at P ≤ 0.01. c Means for high- and low-clay sites differ at P ≤ 0.10. b
(46.0–66.0) (4.0–10.0) (30.0–44.0) (10–18) (13.2–35.2) (12.0–49.2) (26.2–68.4) (0–6.4) (1.6–6.8)
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Table 2 Numbers of nematodes per 100 cm3 soil at depths of 0–20 and 20–40 cm in plots with high or low levels of subsurface clay in okra experimenta Nematode taxon
15 June 1993
14 September 1993
0–20 cm
0–20 cm
High clay Bacterivores Acrobeles Acrobeloides Cephalobus Cervidellus Chiloplacus Eucephalobus Panagrolaimus Prismatolaimus Rhabditidae Teratocephalus Zeldia
Low clay
High clay
Percent of total in upper 0–20 cm
20–40 cm Low clay
High clay
Low clay
62.0 110.8 47.2 67.0 3.0 61.2 13.2 6.5 366.8 0.8 55.8
79.8 65.2 26.0 106.2 7.5 98.5 1.8 2.0 144.5 0.2 5.0c
3.0 6.5 9.7 17.8 0.7 2.0 0 0.2 56.7 0.2 15.8
7.0 4.3 3.0 10.2c 0c 1.2 0.2 0.8 9.7 0 0.3d
1.3 2.5 2.3 2.5 0 0.2 0 1.3 7.5 0.2 2.5
0.8 2.2 0.8 3.3 0.2 0.2 0 0.3 4.5 0 0d
82.2b 69.9b 80.0b 82.8b 80.0 90.4b 100 37.5 84.7b 50.0 86.6
Total bacterivores
797.2
541.2
113.5
38.0c
21.2
12.7c
81.7b
Fungivores Aphelenchoides Aphelenchus Leptonchus Nothotylenchus Tylenchidae Fung. Dorylaimida
121.3 81.3 2.7 12.8 65.7 7.5
167.2 20.0e 0.8 31.7d 38.3 4.5
10.5 37.2 0.2 2.7 17.5 1.0
2.2 4.0d 0.2 3.0 6.8 1.7
1.3 19.2 0 1.5 3.5 0
0c 0.3e 0 0.2d 0.8 0.5
90.5b 67.9 100 77.3b 84.9b 84.2b
Total fungivores
291.3
269.8
70.0
17.8d
25.7
1.8e
76.1b
Herbivores Criconemella Helicotylenchus Meloidogyne Paratrichodorus Pratylenchus
1.0 92.0 27.0 51.8 8.7
2.2 10.2c 9.3 22.7d 1.7
18.3 26.8 320.5 23.7 6.0
8.7 0.7d 39.2d 4.7c 1.5
2.3 3.7 41.0 5.5 0.2
0.2 0.5c 5.8 7.5 0.3
91.5 86.8b 88.5b 68.5 93.8b
Total herbivores
181.5
58.8c
395.3
55.2e
53.5
14.8d
86.8b
Omnivores Aporcelaimellus Eudorylaimus Mesodorylaimus
14.3 24.3 6.8
4.2d 23.3 0.5e
2.0 17.7 1.3
1.2 3.3d 0c
0.2 2.5 0
0.2 0.3 0
90.4b 88.1b 100
Total omnivores
47.7
32.8
23.2
6.5e
3.5
0.5
88.1b
Predators Discolaimus Mononchus Mylonchulus Total predators Total nematodes a
0.8 5.8 5.7
1.7 0.5e 0e
1.7 2.7 0
1.0 0.3c 0
0 3.3 0
0 2.5 0
100b 34.0 –
12.8
6.0d
4.5
3.0
3.3
3.2
53.6
33.8e
83.8b
1344.2
920.0
690.7
122.7e
Data are means of six replications. Nematode numbers at 0–20 and 20–40 cm depth differ at P ≤ 0.10. c Nematode numbers in high- and low-clay sites differ at P ≤ 0.10. d Nematode numbers in high- and low-clay sites differ at P ≤ 0.05. e Nematode numbers in high- and low-clay sites differ at P ≤ 0.01. b
108.2
R. McSorley, J.J. Frederick / Applied Soil Ecology 19 (2002) 1–11
herbivores, and omnivores, were more abundant (P ≤ 0.05) in the high-subsurface-clay plots (Table 2). The total number of nematodes was about five times greater (P ≤ 0.01) in high-clay plots than in low-clay plots. In addition to the nematode taxa shown, total numbers include occasional specimens of Alaimus, Anaplectus, Monhystera, Plectus, Wilsonema (bacterivores); Diphtherophora, Ditylenchus, Ecphyadophora, Neotylenchus, Psilenchus, Tylencholaimellus, Tylencholaimus (fungivores); Belonolaimus, Heterodera, Paratylenchus, Xiphinema (herbivores); Actinolaimus, Belondira, Enchodelus, Thonus (omnivores); Iotonchus, Miconchus, Nygolaimus, Triplya (predators). At 20–40 cm depth, where the differences in soil texture actually occurred, five taxa, as well as total numbers of several nematode trophic groups, were more abundant (P ≤ 0.10) in the plots with higher clay content. Although a variety of indices of nematode community structure were calculated, including diversity, dominance, and maturity indices, none of these was affected by clay level (data not shown). At 20–40 cm
5
depth, percent of bacterivores was lower (P ≤ 0.01) and percent of fungivores was higher (P ≤ 0.01) in plots with high clay (Table 1). The heights of okra plants in plots with high clay (47.4 cm on 26 July; 58.0 cm on 14 September) were greater (P ≤ 0.10) than those from plots with low clay (20.7 cm on 26 July; 33.9 cm on 14 September). 3.2. Okra experiment, effect of depth On 14 September, a majority of nematode taxa and four of five trophic groups were more abundant (P ≤ 0.10) at 0–20 than at 20–40 cm depth (Table 2). A majority (83.8%) of the total nematodes occurred in the upper 0–20 cm. For most taxa, >80% of the individuals found occurred in the upper 0–20 cm of soil. The mean number of nematode taxa per sample at 0–20 cm depth (18.8) was greater (P ≤ 0.01) than at 20–40 cm depth (12.5). Several taxa, such as Prismatolaimus, Aphelenchus, Paratrichodorus, and Mononchus, as well as total predators, did not differ (P > 0.10) in numbers between 0–20 and 20–40 cm depths.
Table 3 Soil texture and selected indices of nematode community structure at depths of 0–20 and 20–40 cm in plots with various levels of subsurface clay in rye experimenta Index measured
0–20 cm
20–40 cm
3% clay
6% clay
12.5% clay
22% clay
3% clay
6% clay
12.5% clay
22% clay
4 November 1993 Percent of sand Percent of silt Percent of clay Taxa/sample Percent of bacterivores Percent of fungivores Percent of herbivores Percent of omnivores Percent of predators
91.0 4.2 4.8 20.0 44.8 34.8 16.8 2.0 0.5
91.0 2.8 6.2 26.0 46.5 19.1 29.0 2.0 2.4
90.0 4.0 6.0 26.5 37.0 24.9 32.1 3.5 0.8
88.2 4.2 7.5 25.0c 38.6 18.2c 38.4 3.0 1.0
93.0 4.0 3.0 21.5 43.9 33.0 16.2 3.7 1.4
90.2 3.8 6.0 20.0 40.9 14.4 39.4 3.0 1.0
80.5 6.5 12.5 20.0 31.0 22.8 41.0 2.9 0.8
75.0b 3.0 22.0b 18.5 19.2 27.2 51.8 0.8 0.6
19 January 1994 Taxa/sample Percent of bacterivores Percent of fungivores Percent of herbivores Percent of omnivores Percent of predators
21.5 60.0 28.8 7.6 2.1 1.0
25.5 48.3 29.3 17.8 0.8 2.3
24.0 51.3 34.8 10.1 1.6 0.6
27.5 41.2 30.4 25.0 1.5 1.0
22.0 67.2 16.2 10.4 3.4 0.8
21.0 44.0 17.1 33.2 3.0 1.6
23.0 54.6 23.2 19.5 1.2 1.0
19.5 40.4 22.4 34.0 2.1d 0.2
a
Data are means of two replications. At a given depth, means among clay levels differ at P ≤ 0.01. c At a given depth, means among clay levels differ at P ≤ 0.10. d At a given depth, means among clay levels differ at P ≤ 0.05. b
6
R. McSorley, J.J. Frederick / Applied Soil Ecology 19 (2002) 1–11
Table 4 Numbers of nematodes per 100 cm3 soil at depths of 0–20 and 20–40 cm in plots with various levels of subsurface clay in rye experiment, 4 November 1993a Nematode taxon
0–20 cm 3% clay
20–40 cm 6% clay
12.5% clay
18.0 49.5 20.5 24.5 2.0 2.0 1.5 1.0 52.0 0 0
28.5 93.5 19.0 19.5 3.0 2.0 0.5 4.5 134.5 0.5 0.5
33.0 36.0 13.0 16.5 2.0 7.0 1.5 0.5 110.5 1.0 1.5
38.5 101.5c 13.0 22.0 3.5 1.0 5.0 2.0 193.5 1.5 6.5
24.0 36.0 16.5 22.5 0 2.0 0 1.5 47.0 1.5 1.0
8.0 32.5 11.0 20.0 1.0 1.0 0 2.0 74.5 4.5 0
7.5 10.5 6.5 11.5 0 1.0 0 2.5 18.0 3.5 0
1.0 9.0 7.5 3.0 0 0.5 0 5.0 14.0 0.5 0
74.4b 76.1b 61.2 59.1b 91.3b 72.7 100b 42.1 76.2b 23.1b 92.9
Total bacterivores
174.0
316.0
228.0
393.5
157.0
157.0
64.0
43.0
72.5b
Fungivores Aphelenchoides Aphelenchus Leptonchus Nothotylenchus Tylenchidae Fung. Dorylaimida
68.0 40.0 0 6.5 16.0 2.5
51.0 40.0 2.0 2.0 27.0 14.5
76.0 32.0 2.5 15.0 17.5 6.5
105.0 57.5 4.5d 5.5 35.0 6.0
61.5 39.0 1.0 2.0 17.0 6.0
6.5 19.0 0 0 19.0 3.0
3.0 10.0 1.5 0 25.5 3.5
2.5 47.5 0.5 0.5e 11.5 10.0
80.3b 59.5 75.0 92.1b 56.7 56.7
Total fungivores
133.5
135.0
149.0
209.0
126.0
49.0
42.0
72.0
68.4b
Herbivores Criconemella Helicotylenchus Meloidogyne Paratrichodorus Pratylenchus
0 0 21.5 23.5 7.0
0 3.5 87.0 89.5 9.5
35.5 0.5 70.5 70.0 2.5
337.5 61.5 179.5 42.0 20.0
10.0 0 18.5 11.5 3.0
0 0 139.0 67.5 9.5
3.0 0 5.5 81.5 2.0
1.5 5.0 46.5 41.5 10.5
96.3 92.9 63.1 52.6 60.9
Total herbivores
56.0
189.0
181.5
642.0
44.0
216.0
92.0
105.5
70.0
Omnivores Aporcelaimellus Eudorylaimus Mesodorylaimus
1.5 5.5 0
2.0 10.5 0.5
4.0 15.0 0
0 18.5 21.5d
1.5 11.5 0
0 8.5 0.5
0 5.0 0
0 2.0 0.5
83.3 64.7 95.6
Total omnivores
8.0
13.0
20.5
41.0
13.5
10.0
5.5
2.5
72.4b
0 1.5 0
5.5 7.5 1.5
2.0 1.5 1.5
4.5d 4.0 3.0
0 3.0 0
0 4.0 0
0 2.0 0
0 1.5 0
1.5
15.5
5.0
11.5c
3.5
4.0
2.0
1.5
75.3b
376.5
675.5
593.5
350.0
440.0
208.5
225.5
70.7b
Bacterivores Acrobeles Acrobeloides Cephalobus Cervidellus Chiloplacus Eucephalobus Panagrolaimus Prismatolaimus Rhabditidae Teratocephalus Zeldia
Predators Discolaimus Mononchus Mylonchulus Total predators Total nematodes
22% clay
1308.0
a
Data are means of two replications. Nematode numbers at 0–20 and 20–40 cm depth differ c At a given depth, means among clay levels differ at P d At a given depth, means among clay levels differ at P e At a given depth, means among clay levels differ at P b
at P ≤ 0.10. ≤ 0.10. ≤ 0.01. ≤ 0.05.
3% clay
6% clay
12.5% clay
22% clay
Percent of total in upper 0–20 cm
100b 55.3 100b
R. McSorley, J.J. Frederick / Applied Soil Ecology 19 (2002) 1–11
7
Table 5 Numbers of nematodes per 100 cm3 soil at depths of 0–20 and 20–40 cm in plots with various levels of subsurface clay in rye experiment, 19 January 1994a Nematode taxon
0–20 cm
20–40 cm 6% clay
36.0 180.0 39.0 12.5 1.0 10.0 1.5 1.0 94.5 0 13.5
37.0 148.5 51.5 12.5 2.0 14.5 27.5 2.5 144.0 3.5 1.5
103.5 113.5 45.5 25.0 6.0 19.5 0 0.5 232.0 1.0 17.0
102.5 155.0 25.0 30.0 1.5 98.0 3.0c 2.0 89.5 0.5 4.5
17.0 138.0 11.5 13.5 1.0 5.0 0.5 1.0 17.5 2.0 1.5
4.5 87.0 33.0 4.0 0 1.5 0 5.0 25.5 6.5 0
24.5 101.5 12.0 21.5 0 8.5 0.5 0.5 59.0 4.5 3.0
26.5 46.5 5.5 16.5 1.0 3.0 0 3.5 27.5 16.5 0
79.4b 61.5b 72.2 59.0 84.0b 88.8b 97.0b 37.5 81.2b 14.5b 89.0b
Total bacterivores
390.5
456.5
567.5
513.5
210.5
170.0
240.0
153.0
71.4b
Fungivores Aphelenchoides Aphelenchus Leptonchus Nothotylenchus Tylenchidae Fung. Dorylaimida
139.5 32.0 1.0 2.5 26.5 5.0
177.5 60.5 2.0 20.5 13.0 10.0
137.0 51.5 1.0 28.0 169.5 4.0
172.5 53.5 4.5 18.0c 136.5 21.5
2.5 18.0 0 2.0 21.5 2.0
6.5 32.5 1.0 0 18.0 8.5
7.0 31.0 1.0 2.5 50.5 8.0
17.5 15.5 0.5 0.5 17.5 1.5c
94.9b 67.1b 77.3b 93.2b 76.3 66.9
Total fungivores
205.5
282.5
391.5
402.5
46.0
66.0
101.5
53.5
82.8b
Herbivores Criconemella Helicotylenchus Meloidogyne Paratrichodorus Pratylenchus
0 0 4.5 41.5 2.0
0 24.0 86.0 48.0 0
5.0 0 5.5 54.0 1.0
28.5 50.0 20.5 119.0d 5.0
1.0 1.5 1.5 26.0 0
0 6.0 75.0 45.5 0
9.0 0 15.5 51.0 1.0
4.0 9.0 4.0 55.0 1.0
70.5 81.8 54.8 59.7 80.0
Total herbivores
48.5
158.5
76.0
228.5d
30.5
126.5
79.5
73.0d
62.3
Omnivores Aporcelaimellus Eudorylaimus Mesodorylaimus
1.0 13.5 0
2.0 4.5 0.5
0.5 12.0 0.5
1.0 13.0 0.5
0.5 9.5 0.5
0 9.0 2.5
0.5 4.0 0
0.5 6.0 0
75.0b 60.1 33.3
Total omnivores
15.5
7.5
13.0
14.5
10.5
11.5
5.5
7.0
59.4
0 4.5 0
1.0 16.5 4.5
0.5 4.0 1.0
3.0d 3.0c 3.5
0 3.0 0
2.5 3.0 0.5
1.5 3.0 0
0 1.0 0
52.9 73.7b 94.7b
4.5
22.0
5.5
9.5
3.0
6.0
4.5
1.0
74.1b
669.0
940.5
1068.5
1177.5
305.5
384.5
433.5
290.0
73.2b
Bacterivores Acrobeles Acrobeloides Cephalobus Cervidellus Chiloplacus Eucephalobus Panagrolaimus Prismatolaimus Rhabditidae Teratocephalus Zeldia
Predators Discolaimus Mononchus Mylonchulus Total predators Total nematodes a
12.5% clay
22% clay
Data are means of two replications. Nematode numbers at 0–20 and 20–40 cm depth differ at P ≤ 0.10. c At a given depth, means among clay levels differ at P ≤ 0.05. d At a given depth, means among clay levels differ at P ≤ 0.10. b
3% clay
6% clay
12.5% clay
22% clay
Percent of total in upper 0–20 cm
3% clay
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R. McSorley, J.J. Frederick / Applied Soil Ecology 19 (2002) 1–11
3.3. Rye experiment, effect of clay As planned, the plots in this experiment differed (P ≤ 0.01) in their content of clay and sand at 20–40 cm depth, while no differences (P > 0.10) were evident at 0–20 cm (Table 3). On 4 November at 0–20 cm depth, differences among clay levels (P ≤ 0.10) were observed for four taxa (Acrobeloides, Leptonchus, Mesodorylaimus, Discolaimus) and for total predators (Table 4). Attempts to develop regression equations relating numbers of these nematodes to subsurface clay level were unsuccessful (P > 0.10). However, when numbers of nematodes at the lowest subsurface clay level (3%) were contrasted with those at the highest subsurface clay level (22%), significantly more (P ≤ 0.10) nematodes were found in the highest subsurface clay plots for seven taxa (Acrobeles, Acrobeloides, Leptonchus, Helicotylenchus, Eudorylaimus, Mesodorylaimus, Discolaimus) and for total omnivores, total predators, and total nematodes. At 0–20 cm depth, differences among subsurface clay levels (P ≤ 0.10) occurred on 4 November in number of taxa per sample and percent of fungivores (Table 3). However, none of the calculated indices of community structure, such as maturity indices, diversity, dominance, or F/B ratios were affected by subsurface clay level (data not shown). On 19 January at 0–20 cm depth, differences among subsurface clay levels (P ≤ 0.10) were observed for five taxa (Panagrolaimus, Nothotylenchus, Paratrichodorus, Discolaimus, Mononchus) and for total herbivores (Table 5). Numbers of nematodes at the highest and lowest subsurface clay levels differed (P ≤ 0.10) for Nothotylenchus, Helicotylenchus, Paratrichodorus, Discolaimus, and total herbivores. Relatively few effects of subsurface clay level on nematode taxa were observed at 20–40 cm depth (Tables 4 and 5). At this depth, numbers of herbivores on 19 January were greater (P ≤ 0.10) in plots with 22% clay subsurface than in plots with 3% subsurface clay (Table 5). 3.4. Rye experiment, effect of depth As in the okra experiment, most taxa and trophic groups were more abundant at 0–20 than at 20–40 cm (Tables 4 and 5). A majority of all nematodes
recovered occurred in the upper 0–20 cm on 4 November (70.7%) and on 19 January (73.2%). The mean number of taxa per sample was greater (P ≤ 0.05) at 0–20 (24.4 on 4 November, 24.6 on 19 January) than at 20–40 cm (20.0 on 4 November, 21.4 on 19 January). Taxa that did not differ (P ≤ 0.10) in numbers between 0–20 and 20–40 cm depths included Prismatolaimus, Meloidogyne, Paratrichodorus, and Eudorylaimus on both sampling dates (Tables 4 and 5). Teratocephalus was more abundant (P ≤ 0.05) at 20–40 than at 0–20 cm on both sampling dates, with only 14.5–23.1% of its population present in the upper 0–20 cm.
4. Discussion and conclusions The direct effects of clay content on nematode population levels are best assessed at 20–40 cm depth in this experiment, since that depth is where the differences in subsurface clay content actually occurred. The fact that some nematode taxa were more abundant in high-subsurface-clay soil than in low-subsurface-clay soil is not surprising, since many nematodes show distinct preferences for sandy or clay soil (Brodie, 1976). However, in the current experiments, even the soils with the highest levels of subsurface clay still contained 58% sand, and could not technically be considered clay soils. Therefore, differences in nematode densities between high- and low-clay soils observed here may not be as great as in other soils. Although direct effects from clay content were limited, the indirect effects of subsurface clay content on nematodes at 0–20 cm depth were numerous. At this depth, all plots had a similar high sand (means ranged from 88.2 to 91.2% across both experiments) and low clay (4.7–7.5%) content. Yet within this relatively uniform layer of sandy soil, a number of nematode taxa representing a range of trophic groups were affected by the varying clay content of the underlying soil layer. The observed effects of subsurface clay were greater in the okra experiment than in the rye experiment. The rye experiment was designed to assess effects across a range of subsurface clay levels, but the number of replications within each clay level was low. The intermediate clay levels contributed additional variability, possibly obscuring trends. When only the highest and lowest subsurface clay levels were contrasted, more
R. McSorley, J.J. Frederick / Applied Soil Ecology 19 (2002) 1–11
differences were apparent. Perhaps most important however, is that the difference between highest (mean: 22.0%; range: 22–22%) and lowest (mean: 3.0%; range: 3–3%) subsurface clay levels in the rye experiment was less than that between the high clay (mean: 35.3%; range: 30–44%) and low clay (mean: 3.3%; range: 2–5%) levels in the okra experiment. Therefore, effects on nematodes were observed more frequently in the okra experiment, in which the differences between subsurface clay treatments were greater. Many different nematode taxa were more abundant in the top layer (0–20 cm) of soil with high clay beneath than in the top layer of soil with low clay beneath, especially in the okra experiment. Clay content influences other important soil properties such as water holding capacity and nutrient availability (Brady and Weil, 1996; Wallace, 1973). In the current study, it is presumed that such effects, resulting from subsurface clay, may extend into the upper soil layers as well. Within the upper 0–20 cm of soil, soil moisture in the plots with high clay beneath (8.1%) was greater (P ≤ 0.01) than in plots with low clay beneath (5.6%), as measured on 15 June 1993. Okra plants were larger in the high-clay plots than in the low-clay plots, offering more roots and resources to nematode communities in the high-clay plots. It is interesting that populations of Meloidogyne incognita, a serious pest of okra (Netscher and Sikora, 1990), were higher in plots with high clay beneath, yet plant growth was also better in these plots than in plots with low clay beneath. Increased soil water-holding capacity can improve tolerance of susceptible crops to root–knot nematodes (McSorley and Gallaher, 1995), which may have occurred in the present study as well. Although a number of different nematode taxa were affected by subsurface clay, no effects could be detected with commonly-used indices of nematode community structure. This was probably because subsurface clay levels did not consistently affect one particular group of nematodes, but rather, affected several selected taxa in all trophic groups. For example, maturity indices have performed well in situations where succession or environmental disturbance have affected nematode colonizers and persisters in different ways (Bongers and Bongers, 1998; Wasilewska, 1994, 1997b). In the current study, however both colonizers (e.g. Aphelenchus, Zeldia) and persisters
9
(e.g. Aporcelaimellus, Mononchus) were affected by subsurface clay levels in similar ways. As changes in the decomposition process occur, changes in the ratio of nematode fungivores to bacterivores (F/B ratio) are anticipated (McSorley and Frederick, 1999). But in the present study, since both bacterivores and fungivores were similarly affected by subsurface clay, no significant effects on F/B were observed. It is becoming evident that indices of community structure cannot be applied indiscriminately to all situations. In another recent study (McSorley and Frederick, 2000), many different nematode taxa showed strong seasonal responses that were not consistently reflected in maturity indices and several other indices. As in the present study, a variety of diverse taxa were similarly affected, and so no mathematical response in the indices should have been anticipated. Optimal performance of an index depends on differential responses of taxa appropriate to the situation for which the index was intended. The current study also provided some information on vertical distribution of the nematode community in this site. Although most nematodes are reported to occur in the upper 20 cm of the soil profile (McSorley, 1987; Yeates, 1980), a few genera were reported to have much of their population distributed below 20 cm (Boag et al., 1987; Yeates et al., 1983). In the current study, genera such as Paratrichodorus and Prismatolaimus did not differ in abundance between 0–20 and 20–40 cm depths. Teratocephalus represented an extreme case in that >75% of the population (present in the profile from 0 to 40 cm) was found at 20–40 cm depth. The opposite trend occurred with most other nematodes, with 70–80% of total nematode numbers recovered from 0 to 20 cm. Thus for the majority of nematodes in the soil community, a sample of 0–20 cm depth should be adequate. Sampling deeper than 20 cm requires additional effort (Boag et al., 1987), which may be difficult to justify unless objectives require a critical assessment of deep-dwelling genera such as Teratocephalus. The sampling and evaluation of nematode communities is complex, since individual genera are influenced by a variety of different physical and biological factors. Many of these factors can vary within the microhabitat, where they can directly affect nematodes (DeGoede and Bongers, 1994; McSorley, 1997; Yeates, 1999). In the current study, subsurface
10
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clay indirectly influenced a number of nematode taxa residing at a different layer within the soil profile.
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