... such as litter decomposition (Witkamp and Crossley 1966; Voss- ..... Rev Entomol 29:25-46. Seastedt TR, Todd TC, James SW (1987) Experimental manipula-.
Biology and Fertility Biol Fertil Soils (1988) 7:46-52
°f S o i l s © Springer-Verlag1988
Soil faunal vs. fertilization effects on plant nutrition: results of a biocide experiment D.H. Wright and D.C. Coleman Department of Entomology, Universityof Georgia, Athens, GA 30602, USA
Summary. Intact cores of agricultural soil planted with Sorghum bicolor were treated with selective biocides or combinations of biocides to manipulate soil organisms. Half the replicates of each biocide treatment were also given N fertilizer. The plants were maintained in a greenhouse, where growth and nutrient content and soil-organism populations were monitored over 16 weeks. The plants responded strongly to fertilization, but showed weak and variable responses to the biocides, even though biocide treatments aimed at animal taxa effectively reduced their target groups. There were no strong interactions between faunal manipulations and fertilization, implying that there was little compensatory function of fauna in the absence of fertilizer. Conditions under which soil fauna are important in making mineral nutrients available to plants in the field need further investigation.
Relevant studies - major approaches
Key words: Nematodes - Microarthropods Agroecosystems - N mineralization - Sorghum bicolor
Colligative, or collective studies. Colligative studies attempt to maintain a complex and nearly natural ecosystem, and examine microbial-faunal interrelationships by manipulating the population of a target group of organisms. Physical exclusion, by means of appropriately sized mesh, and biocide treatment have been the most commonly used techniques for manipulating faunal populations. Both field and laboratory top-down studies have shown that soil fauna can affect decomposition rates, and that many species excrete N in usable forms (reviews by Crossley 1977; Coleman et al. 1983; Seastedt 1984; Norton 1985). For example, faunal effects on nutrient mineralization have been studied by Coleman and Cowley (1973), Edwards and Lofty (1978), and Anderson et al. (1985). Many studies have centered on measuring the effects of faunal exclusion on subsequent litter and soil processes, such as litter decomposition (Witkamp and Crossley 1966; Voss-
The influence of soil fauna on plant growth has consumed considerable time and effort in several research laboratories over the last 10 years (Coleman et al. 1977; Bh~tth et al. 1978; Elliott et al. 1979; B~t~tth et al. 1981; Clarholm 1985; Ingham et al. 1985). The "soil fauna" constitute a heterogeneous group, including microfauna and mesofauna (from 2 Ixm to 2 - 4 mm in length), with considerably different life-histories and activities. A series of reviews for and against faunal effects has been published (Seastedt 1984; Coleman 1985; Ingham et al. 1985), and this topic is only briefly discussed here. Offprint requests to: D.H. Wright
The individualistic approach. Research groups in Great Britain, Sweden, and the USA have pursued extensive studies on microbial-faunal interactions, their direct impact on microbial populations, and the indirect effect on nutrient availability to plants growing in the experimental incubation units or "microcosms" (Elliott et al. 1979; Anderson et al. 1981; Trofymow and Coleman 1982; Anderson et al. 1985; Ingham et al. 1985). These studies have taken a "bottom-up" approach, using highly controlled, often gnotobiotic systems in order to achieve replicable associations of microbial and faunal species. There is substantial evidence that soil mesofauna can liberate N and P from microbial biomass and make more N available for plant uptake in these situations (Cole et al. 1978; Anderson et al. 1981; Coleman et al. 1983; Clarholm 1985; Ingham et al. 1985).
47
brinck et al. 1979; Santos et al. 1981; Parker et al. 1984). The effects attributable to soil mesofauna, specifically microarthropods, are somewhat variable, but generally result in a 20% - 30% slower decomposition rate when the fauna are physically or chemically excluded (Seastedt 1984). Recent multiple biocide studies have taken a foodweb emphasis. A field study of several months examined the effects of biocide exclusion of various groups of soil organisms but yielded less marked results than those targeted at litter bags. Ingham et al. (1986) applied four biocides: a nematicide (carbofuran), an acaricide (cygon = dimethoate), a fungicide (captan), and a vesicular-arbuscular mycorrhizae-suppressant (penta-chloronitrobenzene), in soil microplots on a short-grass prairie. Only the nematicide, which reduced total nematode populations by 6 0 % - 80%, significantly increased plant production and N content in the short-grass vegetation over the first 1 - 3 months of the 6-month study (cf. Stanton et al. 1981). Seastedt et al. (1987) reported that plant production in tallgrass prairie was insensitive to applications of isofenphos and carbaryl, although some accumulation of dead root material occurred following isofenphos treatment during the first year of study, implying a reduced rate of decomposition. Simulation models of microbial-faunal interactions and soil processes also often predict a stimulation of nutrient mineralization by fauna, suggesting that fauna may be responsible for more than 20% of net N mineralization (Hunt et al. 1987). The present study was a greenhouse experiment designed to test whether soil fauna affect the nutrition of a crop plant, grain sorghum, and whether fertilization bypasses or overwhelms any faunal effects. We manipulated the soil organisms in naturally occurring communities using biocides. In view of the numerous empirical and modelling results cited above concerning the effects of fauna on nutrient mineralization, we expected to see declines in plant growth or nutrition in response to faunal reductions.
Methods Experimental design and setting. The experiment was a completely randomized three-way factorial, with five biocide treatments x two fertilization treatments × five sampling dates. Two hundred 1-1itre soil cores were taken from the boundary between 3-year-old tilled and no-till plots at the University of Georgia's Horseshoe Bend Research Area, Athens, Georgia. The soil was a typic Rhodudult, a well-drained sandy loam. Further site details are given in Stinner et al. (1984) and Hendrix et al. (1986). The intact cores were put into waxed-cardboard cartons with holes punched in the bottoms, and placed in a greenhouse. Temperature and humidity were moderated by an evaporative cooler. Greenhouse air temperatures ranged from 23°-43 °C during the study period, which was hot and sunny. The
Table 1. Biocide treatments Treatment
Target group
Application rate (g/m 2 active ingredient)
Captan Carbofuran Naphthalene Carbofuran + Naphthalene None
Fungi Nematodes Arthropods Mesofauna (Control)
23 2.8 100 2.8 + 100 -
photoperiod was maintained at 14 h, using fluorescent lighting until the natural photoperiod reached this length. On 24 April 1986 (day 0) each pot was planted with three or four untreated grain sorghum seeds (Sorghum bicolor, Funks 522), and half the pots were fertilized with NH4NO 3 at a rate equivalent to 95 kg N/ha. The plants were later thinned to one plant per plot, and weeds were removed periodically. In a few pots no sorghum plants germinated; these pots were dropped from the analysis. The plants were given 75 ml tap water every other day, or daily when temperatures were high. The biocide treatments, first applied on day 12, were as follows: captan (fungicide), carbofuran (nematicide), naphthalene (insecticide/acaricide), carbofuran + naphthalene, or none (control) (Table 1). Captan and naphthalene (in the naphthalene and carbofuran + naphthalene treatments) were applied again on day 54. Carbofuran was not applied twice. Biocides were selected on the basis of the previous experience of other workers at Horseshoe Bend (Coleman et al. 1988) and on literature reports (Ingham and Coleman 1984; Ingham et al. 1986) in order to substantially reduce target groups with a minimum of non-target effects. Captan has been reported to control saprotrophic fungi, with little effect on vesicular-arbuscular mycorrhizae (Ingham 1985; Ingham et al. 1986). Sorghum grown at Horseshoe Bend supports vesicular-arbuscular mycorrhizae. Carbofuran was selected as an effective nematicide despite the possibility of non-target effects. Carbofuran has also been used as an insecticide and an acaricide (Cohick 1975; Homeyer 1975; Ingham and Coleman 1984), and is toxic to earthworms (Parmelee 1987) The carbofuran + naphthalene treatment was designed as a general soil animal reduction treatment. A preliminary trial using unfertilized soil defaunated by autoclaving showed that captan and carbofuran had no significant phytotoxic or phytostimulatory effects. Naphthalene vapors proved toxic to young sorghum plants when kept in a closed container (cartons in the preliminary trial were covered with plastic wrap), and field studies at Horseshoe Bend have revealed some inhibition of root growth of rye (Secale cereale) by naphthalene (Wei-Xin Cheng 1986, personal communication; cf. Williams and Wiegert 1971). However, ventilation in the greenhouse was ample, and we observed no negative effects of naphthalene on sorghum growth or nutrient content. Both captan and carbofuran contain N (Table 1). At the application rates used, the captan treatment added the equivalent of 10.7 kg N/ha per application (total 21.4 kg N/ha during the experiment), while the carbofuran treatment added the equivalent of 1.8 kg N/ha. Naphthalene contains no N.
Sampling and analysis. Four replicates per treatment combination were destructively sampled on days 26, 46, 67, 88, and 116. The following response variables were measured: plant height and flowering stage; root, shoot, and inflorescence dry weight; shoot and inflorescence N and P content; bacterial density in soil, total soil fungi, nematode abundance in four trophic categories, and soil microarthropod abundance in eight taxonomic categories. Bacteria
48 were estimated by direct counts using fluorescent isothiocyanate stain (Babiuk and Paul 1970). Fungi were counted with modification of the Jones and Mollison (1948) technique. Soil subsamples weighing 5 g were extracted on Baermann funnels (Van Gundy 1982) for 72 h, and the nematodes were collected on a 26-~tm mesh sieve and counted as described in Wright (1988). Microarthropods were extracted from a 4.7-cm diameter×5-cm deep subcore using a modified Tullgren high-gradient extractor similar to that of Merchant and Crossley (1970), and were counted using the method of Wright (1988). Total N and P in plant tissues were measured by microKjeldahl digestion and colorimetry (Bremner and Mulvaney 1982; Tecator 1981, 1983, 1984). Analyses of variance and other statistical analyses were performed using StatView 512+ ® on a microcomputer. Figures presented here are arithmetic means, with significance tests based on the appropriately transformed data. In most cases a logarithmic transformation was used, but for microarthropods, which were strongly clumped, an inverse hyberbolic sine-square root transformation was required to achieve homogeneous variances (Kempthorne 1952). Fisher's protected least significant difference was used to compare overall treatment means (main effects) with the control, and Bonferroni's t (Neter and Wasserman 1974, p. 480-482) was used to compare treatment means with the control within sample dates.
with our prior expectations about the role of fauna in nutrient availability. In other words, if fauna significantly enhance nutrient turnover, there should be evidence of greater nutrient stress in unfertilized plants subjected to carbofuran + naphthalene treatment. However, this effect was quite weak in the present study. The plants treated with carbofuran + naphthalene also showed a non-significant tendency toward delayed flowering (P~0.07).
8
g 4 3 2 1 0
~ 20
40
~ 80
60
t
i
100
120
DAY
Results Plant response
As expected, the sorghum plants responded strongly to N fertilization with significant increases in the biomass of all plant components: roots, shoots, and inflorescences (Fig. 1), and also a general increase in the top:root ratio. Tissue N concentrations in aboveground parts were initially higher in fertilized than in unfertilized plants (Fig. 2), and fertilized plants were taller and flowered earlier. Tissue P concentrations were lower in fertilized plants, but overall, fertilized plants contained larger absolute amounts of P above ground than unfertilized plants, due to their greater size (P_< 0.001 in all comparisons listed above except flowering stage, for which P = 0.011). In contrast, the plants showed weaker responses to soil biocide treatments. Captan applications had the greatest effect on the sorghum biomass, resulting in higher shoot, above-ground (Fig. 3) and total weights, and in higher top:root ratios. Naphthalene also had a positive effect on the plants, producing marginally significant increases in shoot and root (but not inflorescence) weights ( P ~ 0.05) which summed to a significant difference in total weight (P>0.05). The only negative effect observed, and the only significant biocide-fertilizer interaction, was a reduction in the top:root ratio in unfertilized carbofuran + naphthalene treated plants (P
